2009. "Rapid Measurement of Emissions from Military Aircraft Turbine Engines by Downstream Extractive Sampling of Aircraft on the Ground: Results for C-130 and F-15 Aircraft." Atmospheric Environment 43(16):2612-2622. doi:10.1016/j.atmosenv.2009.02.012 Abstract Aircraft emissions affect air quality on scales from local to global. About 10% of the aviation fuel used in the U.S. is consumed by military aircraft, and emissions from this source are facing increasingly stringent environmental regulations, so improved methods for quickly and accurately determining emissions from existing and new engines are needed. This paper reports results of a study to advance the methods used for detailed characterization of military aircraft emissions, and provides emission factors for two aircraft; the F-15 fighter and the C-130 cargo plane. The new approach employs a strategy of outdoor ground-level sampling downstream behind operational military aircraft. This permits rapid change-out of the aircraft so that engines can be tested quickly on operational aircraft Measurements were made at throttle settings from idle to afterburner using a simple extractive probe in the dilute exhaust. Emission factors determined using this approach agree very well with those from the traditional method of extractive sampling at the exhaust exit. Emission factors are reported for CO2, CO, NO, NOx, and more than 60 hazardous and/or reactive organic gases. Intra-engine and engine to engine variability were assessed. For both engines, the effect of engine power on emissions was as expected, with higher power leading to reduced emission factors for CO and organic gases and higher emission factors for nitrogen oxides. At afterburner power, the F-15 engine yielded higher emission factors for CO and many organics and lower NOx emission factors compared with the military power throttle setting. The C-130 turboprop engine generally produced higher CO andorganic emissions and lower NOx emissions per unit of fuel consumed than the F-15 engines. Comparison of the emissions of nine hazardous air pollutants from these two engines with emissions from nine other aircraft engines also is discussed.
2009. "Molecular Characterization of Biomass Burning Aerosols Using High Resolution Mass Spectrometry." Analytical Chemistry 81(4):1512-1521. doi:10.1021/ac8020664 Abstract Chemical characterizations of atmospheric aerosols is a serious analytical challenge because of the complexity of particulate matter analyte composed of a large number of compounds with a wide range of molecular structures, physico-chemical properties, and reactivity. In this study chemical composition of biomass burning organic aerosol (BBOA) samples is characterized by high resolution electrospray ionization mass spectrometry (ESI/MS). Accurate mass measurement combined with Kendrick analysis allowed us to assign elemental composition for hundreds of compounds in the range of m/z values of 50-1000. ESI/MS spectra of different BBOA samples contain a variety of distinct, sample specific, characteristic peaks that can be used as unique markers for different types of biofuels. Our results indicate that a significant number of high-MW organic compounds in BBOA samples are highly oxidized polar species that can be efficiently detected using ESI/MS but are difficult to observe using the conventional GCMS analysis of aerosol samples. The average O:C ratios obtained for each of the BBOA samples studied in this work are in a strikingly good agreement with the previously reported values obtained using STXM/NEXAFS. The degree of unsaturation of detected organic compounds shows a clear decrease with increase in the molecular weight of the anyalyte molecules. The decrease is particularly pronounced for the samples containing a large number of CH2-based homologous series.
2009. "Hygroscopic Properties of CH3SO3Na, CH3SO3NH4, (CH3SO3)2Mg and (CH3SO3)2Ca Particles Studied by Micro-FTIR Spectroscopy." Journal of Physical Chemistry A 113(8):1531-1538. Abstract The hygroscopic behavior of CH3SO3Na, CH3SO3NH4, (CH3SO3)2Mg and (CH3SO3)2Ca particles as a function of relative humidity (RH) has been studied using microscopic Fourier transform infrared (micro-FTIR) spectroscopy. The approach used exposure of substrate deposited, ~1 μm dry-size particles to humidified nitrogen followed by micro-FTIR spectroscopy over a selected sample area. The results show that CH3SO3Na particles undergo characteristic phase transitions at deliquescence relative humidity (DRH) of 71% and efflorescence relative humidity (ERH) of ~40%. In contrast, CH3SO3NH4, (CH3SO3)2Mg and (CH3SO3)2Ca particles do not undergo phase transitions and exhibit continuous, reversible uptake and evaporation of water under the influence of changing RH. The extent of water uptake is quantified and presented as water-to-solute ratios (WSR) in particles as a function of RH. The WSR values are determined from the integrated absorbance of the water and the solute-specific bands in IR spectra recorded at different RH.
2009. "Molecular Characterization of Nitrogen Containing Organic Compounds in Biomass Burning Aerosols Using High Resolution Mass Spectrometry." Environmental Science & Technology 43(10):3764-3771. doi:10.1021/es803456n Abstract Although nitrogen-containing organic compounds (NOC) are important components of atmospheric aerosols, little is known about their chemical compositions. Here we present detailed characterization of the NOC constituents of biomass burning aerosol (BBA) samples using high resolution electrospray ionization mass spectrometry (ESI/MS). Accurate mass measurements combined with MS/MS fragmentation experiments of selected ions were used to assign molecular structures to individual NOC species. Our results indicate that N-heterocyclic alkaloid compounds - species naturally produced by plants and living organisms - comprise a substantial fraction of NOC in BBA samples collected from test burns of five biomass fuels. High abundance of alkaloids in test burns of ponderosa pine - a widespread tree in the western U.S. areas frequently affected by large scale fires - suggests that N-heterocyclic alkaloids in BBA can play a significant role in dry and wet deposition of fixed nitrogen in this region.
2009. "Time-Resolved Molecular Characterization of Limonene/Ozone Aerosol using High-Resolution Electrospray Ionization Mass Spectrometry." Physical Chemistry Chemical Physics. PCCP (11):7931-7942. doi:10.1039/b905288g Abstract Molecular composition of limonene/O3 secondary organic aerosol (SOA) was investigated using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) as a function of reaction time. SOA was generated by ozonation of D-limonene in a reaction chamber and sampled at different time intervals using a cascade impactor. The SOA samples were extracted into acetonitrile and analyzed using a HR-ESI-MS instrument with a resolving power of 100,000 (m/Δm). The resulting mass spectra provided detailed information about the extent of oxidation inferred from the O:C ratios, double bond equivalency (DBE) factors, and aromaticity indexes (AI) in hundreds of identified individual SOA species.
2009. "Mexico City Aerosol Analysis during MILAGRO using High Resolution Aerosol Mass Spectrometry at the Urban Supersite (T0). Part 1: Fine Particle Composition and Organic Source Apportionment." Atmospheric Chemistry and Physics 9(17):6633-6653. Abstract Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and complementary instrumentation. Mass concentrations, diurnal cycles, and size distributions of inorganic and organic species are similar to results from the CENICA supersite in April 2003 with organic aerosol (OA) comprising about half of the fine PM mass. Positive Matrix Factorization (PMF) analysis of the high resolution OA spectra identifies three major components: chemically-reduced urban primary emissions (hydrocarbon-like OA, HOA), oxygenated OA (OOA, mostly secondary OA or SOA), and biomass burning OA (BBOA) that correlates with levoglucosan and acetonitrile. BBOA includes several very large plumes from regional fires and likely also some refuse burning.
2008. "High-Resolution Mass Spectrometric Analysis of Secondary Organic Aerosol Produced by Ozonation of Limonene." Physical Chemistry Chemical Physics. PCCP 10(7):1009-1022. doi:10.1039/b712620d Abstract Secondary organic aerosol (SOA) particles formed from the ozone-initiated oxidation of limonene are characterized by high-resolution electrospray ionization mass spectrometry in both the positive and negative ion modes. The mass spectra reveal a large number of both monomeric (m/z < 300) and oligomeric (m/z > 300) products of oxidation. A combination of high resolving power (m/Δm ~60,000) and Kendrick mass defect analysis makes it possible to unambiguously determine the composition for hundreds of individual compounds in SOA samples. Van Krevelen analysis shows that the SOA compounds are heavily oxidized, with average O:C ratios of 0.43 and 0.50 determined from the positive and negative ion mode spectra, respectively. An extended reaction mechanism for the formation of the first generation SOA molecular components is proposed. The mechanism includes known isomerization and addition reactions of the carbonyl oxide intermediates generated during the ozonation of limonene, and numerous isomerization pathways for alkoxy radicals resulting from the decomposition of unstable carbonyl oxides. The isomerization reactions yield numerous products with a progressively increasing number of alcohol and carbonyl groups, whereas C-C bond scission reactions in alkoxy radicals shorten the carbon chain. Together these reactions yield a large number of isomeric products with broadly distributed masses. A qualitative agreement is found between the number and degree of oxidation of the predicted and measured reaction products in the monomer range.
2008. "Characterization of Aerosols Containing Zn, Pb, and Cl from an Industrial Region of Mexico City." Environmental Science & Technology 42(19):7091-7097. doi:10.1021/es7030483 Abstract During the March, 2006 MILAGRO campaign, measurements in the Northern Mexico City Metropolitan Area revealed the frequent appearance of particles with a characteristically high content of internally mixed Zn, Pb, Cl, and P. A comprehensive study of the chemical and physical properties of these particles was performed using a complementary combination of aerosol measurement techniques. Individual particles were analyzed using Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) and Computer Controlled Scanning Electron Microscopy/Energy Dispersive X-Ray spectroscopy (CCSEM/EDX). Proton Induced X-Ray Emission (PIXE) analysis of bulk aerosol samples provided time-resolved mass concentrations of individual elements. The PIXE measurements indicated that Zn is more strongly correlated with Cl than with any other element and that Zn concentrations are higher than other non-ferrous transition metals. The Zn- and Pb - containing particles have both spherical and non-spherical morphologies. Many metal rich particles had needle-like structures and were found to be composed of ZnO and/or Zn(NO3)2·6H2O as indicated by scanning transmission x-ray microscopy/near edge X-ray absorption spectroscopy (STXM/NEXAFS). The Zn and Pb rich particles were primarily in the submicron size range and internally mixed with elemental carbon. The unique chemical associations most closely match signatures acquired for garbage incineration. This unique combination of complementary analytical techniques has allowed for a comprehensive evaluation of Zn- and Pb- containing particles in a complex urban environment, highlighting unique characteristics that give powerful insight into their origin.
2008. "Kinetics of Heterogeneous Reaction of CaCO3 Particles with Gaseous HNO3 Over a Wide Range of Humidity." Journal of Physical Chemistry A 112(7):1561-1571. doi:10.1021/jp076169H Abstract Heterogeneous reaction kinetics of gaseous nitric acid (HNO3) with calcium carbonate (CaCO3) particles was investigated using the Particle-on-Substrate Stagnation Flow Reactor (PS-SFR). The technique utilizes the exposure of substrate deposited, isolated, and narrowly dispersed particles to a gas mixture of HNO3/H2O/N2 followed by microanalysis of individual reacted particles using computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX). The first series of experiment was conducted at atmospheric pressure, room temperature and constant relative humidity (40%) with a median dry particle diameter pD = 0.85 μm, particle loading densities 2×104 ≤ Ns ≤ 6×106 cm–2 and free stream HNO3 concentrations of 7, 14 and 25 ppb. The apparent, pseudo first-order rate constant for the reaction was determined from oxygen enrichment in individual particles as a function of particle loading. Quantitative treatment of the data using a diffusion-kinetic model yields lower limit to the net reaction probability γnet ≥ 0.06 (×3/÷2). In the second series of experiments, HNO3 uptake on CaCO3 of the same particle size was examined over a wide range of relative humidity, from 10 to 80%. The lower limit for the net reaction probability was found to increase with an increase in the relative humidity, from γnet ≥ 0.003 at RH = 10% to 0.21 at 80%.
2008. "Hygroscopic Behavior of Substrate-Deposited Particles Studied by micro-FT-IR Spectroscopy and Complementary Methods of Particle Analysis." Analytical Chemistry 80(3):633-642. doi:10.1021/ac701638r Abstract The application of Microscopic Fourier Transform Infrared (micro-FTIR) spectroscopy combined with complementary methods of particle analysis is demonstrated here for investigations of phase transitions and hygroscopic growth of micron-sized particles. The approach utilizes the exposure of substrate-deposited, isolated particles to humidified nitrogen inside a sample cell followed by micro-FTIR spectroscopy over selected sample area. Phase transitions of NaCl, sea salt, NaNO3 and (NH)4SO4 particles are monitored with this technique to evaluate its utility and applicability for particle hydration studies. The results are found in excellent agreement with literature data in terms of (a) reliable and reproducible detection of deliquescence and efflorescence phase transitions, (b) quantitative measurements of water-to-solute ratios in particles as a function of relative humidity, and (c) changes in the IR spectra resulting from phase transitions and changing relative humidity. Additional methods of particle analysis are employed to complement and assist in the interpretation of particle hygroscopicity data obtained from micro-FTIR measurements. The analytical approach and the experimental setup presented here are relatively simple, inexpensive, readily available, and therefore may be practical for hydration studies of environmental particles collected in both laboratory and field studies.
2008. "Comparative Analysis of Urban Atmospheric Aerosol by Particle-Induced X-ray Emission (PIXE), Proton Elastic Scattering Analysis (PESA), and Aerosol Mass Spectrometry (AMS)." Environmental Science & Technology 42(17):6619-6624. doi:10.1021/es800393e Abstract A multifaceted approach to atmospheric aerosol analysis is often desirable in field studies where an understanding of technical comparability among different measurement techniques is essential. Herein we report quantitative intercomparisons of Particle-Induced X-ray Emission (PIXE) and Proton Elastic Scattering Analysis (PESA), performed off-line under vacuum, with analysis by Aerosol Mass Spectrometry (AMS) carried out in real-time during the MCMA-2003 Field Campaign in the Mexico City Metropolitan Area. Good agreement was observed for mass concentrations of PIXE-measured sulfur (assuming it was dominated by SO42−) and AMS-measured sulfate during the most of the campaign. PESA-measured hydrogen mass was separated into sulfate H and organic H mass fractions assuming the only major contributions were (NH4)2SO4 and organic compounds. Comparison of the organic H mass with AMS organic aerosol measurements indicates that about 75% of the mass of these species evaporated under vacuum. However ~25% of the organics does remain under vacuum, which is only possible with low vapor pressure compounds, and which supports the presence of high molecular weight and/or highly oxidized organics consistent with atmospheric aging. Approximately 10% of the chloride detected by AMS was measured by PIXE, possibly in the form of metal-chloride complexes, while the majority of Cl was likely present as more volatile species including NH4Cl. This is the first comparison of PIXE/PESA and AMS, and to our knowledge also the first report of PESA hydrogen measurements for urban organic aerosols.
2008. "Chemical Speciation of Sulfur in Marine Cloud Droplets and Particles: Analysis of Individual Particles from the Marine Boundary Layer Over the California Current." Journal of Geophysical Research. D. (Atmospheres) 113(D4):D04209. doi:10.1029/2007/JD008954 Abstract Detailed chemical speciation of the dry residue particles from individual cloud droplets and interstitial aerosol collected during the Marine Stratus Experiment (MASE) was performed using a complementary combination of microanalysis techniques. Techniques include computer controlled scanning electron microscopy with energy dispersed analysis of X-rays (CCSEM/EDX), time-of-flight secondary ionization mass spectrometry (TOFSIMS), and scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). Samples were collected at the ground site located in Point Reyes National Seashore, approximately 1 km from the coast. This manuscript focuses on the analysis of individual particles sampled from an air mass that originated over the open ocean and then passed through the area of the California current located along the northern California coast. Based on composition, morphology, and chemical bonding information, two externally mixed, distinct classes of sulfur containing particles were identified: chemically modified (aged) sea salt particles and secondary formed sulfate particles. The results indicate substantial heterogeneous replacement of chloride by methanesulfonate (CH3SO3 -) and non-sea salt sulfate (nss-SO4 2-) in sea-salt particles with the characteristic ratios of CH3SO3 −/nss-SO4 2−> 0.6. Although this value seems too high for a mid-latitude site, our model calculations suggest that high CH3SO3 -/nss-SO4 2- ratios are expected during the early stages of dimethyl sulfide (DMS) oxidation when CH3SO3H forms more rapidly than H2SO4.
2008. "Applications of Lagrangian Dispersion Modeling to the Analysis of Changes in the Specific Absorption of Elemental Carbon." Atmospheric Chemistry and Physics 8(5):1377-1389. Abstract We use a Lagrangian dispersion model driven by a mesoscale model with four-dimensional data assimilation to simulate the dispersion of elemental carbon (EC) over a region encompassing Mexico City and its surroundings, the study domain for the 2006 MAX-MEX experiment, which was a component of the MILAGRO campaign. The results are used to identify periods when biomass burning was likely to have had a significant impact on the concentrations of elemental carbon at two sites, T1 and T2, downwind of the city, and when emissions from the Mexico City Metropolitan Area (MCMA) were likely to have been more important. They are also used to estimate the median ages of EC affecting the specific absorption of light, aABS, at 870 nm as well as to identify periods when the urban plume from the MCMA was likely to have been advected over T1 and T2. Values of aABS at T1, the nearer of the two sites to Mexico City, were smaller at night and increased rapidly after mid-morning, peaking in the mid-afternoon. The behavior is attributed to the coating of aerosols with substances such as sulfate or organic carbon during daylight hours, but such coating appears to be limited or absent at night. Evidence for this is provided by scanning electron microscope images of aerosols collected at three sampling sites. During daylight hours the values of aABS did not increase with aerosol age for median ages in the range of 1-4 hours. There is some evidence for absorption increasing as aerosols were advected from T1 to T2 but the statistical significance of that result is not strong.
2008. "Characterization and Acid-Mobilization Study of Iron-Containing Mineral Dust Source Materials." Journal of Geophysical Research. D. (Atmospheres) 113(D5):Art. No. D05202. doi:10.1029/2007JD009332 Abstract Processes that solubilize the iron in mineral dust aerosols may increase the amount of iron supplied to ocean surface waters, and thereby stimulate phytoplankton productivity. It was recently proposed that mixing of mineral dusts with SO2 and HNO3 produces extremely acidic environments that favor the formation of bioavailable Fe(II). Here, four authentic mineral dust source materials (Saudi Beach sand (SB), Inland Saudi sand (IS), Saharan Sand (SS) and China Loess (CL)) and one commercial reference material (Arizona Test Dust (AZTD)) were spectroscopically characterized, and their dissolution at pH 1 was examined in aqueous batch systems. Spectroscopic analyses indicated that the bulk and near-surface region of all samples possessed similar elemental compositions and that iron was unevenly distributed among dust 10 particles. Mössbauer spectroscopy revealed Fe(III) in all samples, although SB, CL and AZTD also contained appreciable Fe(II). Both Fe(II) and Fe(III) were primarily substituted into aluminosilicates, although CL, AZTD and IS also contained Fe(III) oxides. Total iron solubility (defined as the summed concentration of dissolved Fe(II) and Fe(III) measured after 24 h) ranged 14 between 4-12% of the source materials’ iron content, but did not scale with either the surface area or the iron content of the samples. This suggests that other factors such as iron speciation and mineralogy may play a key role in iron solubility. Also, the elevated nitrate concentrations encountered from nitric acid at pH 1 suppressed dissolution of Fe(II) from AZTD, CL and SB particles, which we propose results from the surface-mediated, non-photochemical reduction of nitrate by Fe(II).
2008. "Probing Emissions of Military Cargo Aircraft: Description of a Joint Field Measurement Strategic Environmental Research and Development Program." EM: Air & Waste Management Association's magazine for environmental managers 58(6):787-796. Abstract To develop effective air quality control strategies for military air bases, there is a need to accurately quantify these emissions. In support of the Strategic Environmental Research and Development Program project, the PM and gaseous emissions from two T56 engines on a parked C-130 aircraft were characterized at the Kentucky Air National Guard base in Louisville, KY. Conventional and research-grade instrumentation and methodology were employed in the field campaign during the first week of October 2005. Particulate emissions were sampled at the engine exit plane and at 15 meters downstream. In addition, remote sensing of the gaseous species was performed via spectroscopic techniques at 5 and 15 m downstream of the engine exit. It was found that PM mass and number concentrations measured at 15-m downstream locations, after dilution corrected, generally agreed well with those measured at the engine exhaust plane; however, higher variations were observed in the far-field after natural dilution of the downstream measurements was accounted for. Using CO2-normalized data we demonstrated that gas species measurements by extractive and remote sensing techniques agreed reasonably well.
2008. "The Effect of Solvent on the Analysis of Secondary Organic Aerosol Using Electrospray Ionization Mass Spectrometry." Environmental Science & Technology 42(19):7341-7346. doi:10.1021/es801226w Abstract Solvent-analyte reactions in organic aerosol (OA) extracts prepared for analysis by electrospray ionization mass spectrometry (ESI-MS) were examined. Secondary organic aerosol (SOA) produced by ozonation of d-limonene as well as several test organic chemicals with functional groups typical for OA constituents were dissolved and stored in methanol, d3-methanol, acetonitrile, and d3-acetonitrile to investigate the extent and relative rates of reactions between analyte and solvent. High resolution ESI-MS showed that reactions of carbonyls with methanol produce significant amounts of hemiacetals and acetals on time scales ranging from several minutes to several days, with the reaction rates increasing in acidified solutions. Carboxylic acid groups were observed to react with methanol resulting in the formation of esters. In contrast, acetonitrile extracts showed no evidence of reactions with analyte molecules, suggesting that acetonitrile is the preferred solvent for SOA extraction. The use of solvent-analyte reactivity as an analytical chemistry tool for the improved characterization of functional groups in complex organic mixtures was also demonstrated. Direct comparison between ESI mass spectra of the same SOA samples extracted in reactive (methanol) versus non-reactive (acetonitrile) solvents was used to estimate the relative fractions of ketones (38%), aldehydes (6%), and carboxylic acids (55%) in d-limonene SOA.
2007. "Effect of Hydrophobic Primary Organic Aerosols on Secondary Organic Aerosol Formation from Ozonolysis of α-Pinene." Geophysical Research Letters 34(20):Paper # L20803. doi:10.1029/2007GL030720 Abstract Semi-empirical secondary organic aerosol (SOA) models typically assume a well-mixed organic aerosol phase even in the presence of hydrophobic primary organic aerosols (POA). This assumption significantly enhances the modeled SOA yields as additional organic mass is made available to absorb greater amounts of oxidized secondary organic gases than otherwise. We investigate the applicability of this critical assumption by measuring SOA yields from ozonolysis of α-pinene (a major biogenic SOA precursor) in a smog chamber in the absence and in the presence of dioctyl phthalate (DOP) and lubricating oil seed aerosol. These particles serve as surrogates for urban hydrophobic POA. The results show that these POA did not enhance the SOA yields. If these results are found to apply to other biogenic SOA precursors, then the semi-empirical models used in many global models would predict significantly less biogenic SOA mass and display reduced sensitivity to anthropogenic POA emissions than previously thought.
2007. "Kinetic Study of Heterogeneous Reaction of Deliquesced NaCI Particles with Gaseous HNO3 Using Particle-on-Substrate Stagnation Flow Reactor Approach." Journal of Physical Chemistry A 111(40):10026-10043. doi:10.1021/jp072005p Abstract Heterogeneous reaction kinetics of gaseous nitric acid with deliquesced sodium chloride particles were investigated with a novel Particle-on-Substrate Stagnation Flow Reactor (PS-SFR) approach under conditions, including particle size, relative humidity and reaction time, directly relevant to the atmospheric chemistry of sea salt particles. Particles deposited onto an electron microscopy grid substrate were exposed to the reacting gas at atmospheric pressure and room temperature by impingement via a stagnation flow inside the reactor. The reactor design and choice of flow parameters were guided by computational fluid dynamics results to ensure uniformity of the diffusion flux to all particles undergoing reaction. The chloride depletion in the particles was followed by computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX). The validity of the current approach was examined first by conducting experiments with median dry particle diameter = 0.82 μm, 80% relative humidity, particle loading densities 4×104 ≤ Ns ≤ 7×106 cm–2 and free stream HNO3 concentrations 2, 7 and 22 ppb. Upon deliquescence the droplet diameter approximately doubles. The apparent, pseudo first-order rate constant determined in these experiments varied with particle loading and HNO3 concentration in a manner consistent with a diffusion-kinetic analysis reported earlier (Laskin, A.; Wang, H.; Robertson, W. H.; Cowin, J. P.; Ezell, M. J.; Finlayson-Pitts, B. J. J. Phys. Chem. A 2006, 110, 10619). The intrinsic, second-order rate constant was obtained as kII = 5.7×10–15 cm3molecule–1s–1 in the limit of zero particle loading and by assuming that the substrate is inert to HNO3. Under this loading condition the experimental, net reaction uptake coefficient was found to be γnet = 0.11 with an uncertainty factor of 3. Additional experiments examined the variations of HNO3 uptake on pure NaCl, a sea salt-like mixture of NaCl and MgCl2 (Mg-to-Cl molar ratio of 0.114) and real sea salt particles as a function of relative humidity. Results show behavior of the uptake coefficient to be similar for all three types of salt particles with ~ 0.9 μm over the relative humidity range of 20-80%. Well over 0.2 for sea salt. Below the efflorescence relative humidity the uptake coefficient rapidly declines with decreasing RH for all three sea salt types, and it does so without exhibiting a sudden shutoff of reactivity. The uptake of HNO, and uptake on both sea salt and sea salt-like mixture was faster than on pure NaCl. The uptake of HNOon pure NaCl particles was also examined over the particle size range of 0.57 ≤ ≤ 1.7 μm (1.1 ≤ Gaseous HNO3 uptake coefficient peaks around a relative humidity of 55%, with γnet 3 on sea salt particles was more rapid than that on the mixture of NaCl and MgCl23 ≤ 3.4 μm) under a constant relative humidity of 80%. The uptake coefficient decreases monotonically with an increase in particle size. Application of a resistance model of reaction kinetics and reactant diffusion over a single particle suggests that, over the range of particle size studied, the uptake is largely controlled by gaseous reactant diffusion from the free stream to the particle surface. In addition, a combined consideration of uptake coefficients obtained in the present study and those previously reported for substantially smaller droplets (~ 0.1 μm) (Saul, T. D.; Tolocka, M. P.; Johnston, M. V. J. Phys. Chem. A 2006, 110, 7614) suggests that the peak reactivity occurs at a droplet diameter of ~0.7 μm, which is immediately below the size at which sea salt aerosols begin to notably contribute to light scattering.
2007. "Correlations Between Optical, Chemical and Physical Properties of Biomass Burn Aerosols." Geophysical Research Letters 34:Art. No. L18806. doi:10.1029/2007GL030502 Abstract Single scattering albedo (ω) and Angstrom absorption coefficient (αap) values are measured at 405, 532 and 870 nm for aerosols generated during controlled laboratory combustion of twelve wildland fuels. Considerable fuel dependent variation in these optical properties is observed at these wavelengths. Complementary microspectroscopy techniques are used to elucidate spatially resolved local chemical bonding, carbon-to-oxygen atomic ratios, percent of sp2 hybridization (graphitic nature), elemental composition, particle size and morphology. These parameters are compared directly with the corresponding optical properties for each combustion product, facilitating an understanding of the fuel dependent variability observed. Results indicate that combustion products can be divided into three categories based on chemical, physical and optical properties. Only materials displaying a high degree of sp2 hybridization, with chemical and physical properties characteristic of ‘soot’ or black carbon, exhibit ω and αap values that indicate a high light absorbing capacity.
2006. "Characterization of Ambient Aerosols in Mexico City during the MCMA-2003 Campaign with Aerosol Mass Spectrometry. Results from the CENICA Supersite." Atmospheric Chemistry and Physics 6:925-946. Abstract An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, while another was deployed in the Aerodyne Mobile Laboratory (AML) during the Mexico City Metropolitan Area field study (MCMA-2003) from March 29-May 4, 2003 to investigate particle concentrations, sources, and processes. This is the first of a series of papers reporting the AMS results from this campaign. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 μm (NR PM1) with high time and size resolution. For the first time, we report field results from a beam width probe, which was used to study the shape and mixing state of the particles and to quantify potential losses of irregular particles due to beam broadening inside the AMS. Data from this probe show that no significant amount of irregular particles was lost due to excessive beam broadening. A comparison of the CENICA and AML AMSs measurements is presented, being the first published intercomparison between two quadrupole AMSs. The speciation, and mass concentrations reported by the two AMSs compared well. In order to account for the refractory material in the aerosol, we also present measurements of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from PIXE analysis of filters. Comparisons of (AMS + BC + soil) mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a Tapered Element Oscillating Microbalance (TEOM) and a DustTrack Aerosol Monitor) are also presented. The comparisons show that the (AMS + BC + soil) mass concentration during MCMC-2003 is a good approximation to the total PM₂․₅ mass concentration.
2006. "Analysis of Individual Environmental Particles Using Modern Methods of Electron Microscopy and X-Ray Microanalysis." Journal of Electron Spectroscopy and Related Phenomena 150(2-3):260-274. Abstract Understanding the composition of particles in the atmosphere is critical because of their health effects and their direct and indirect effects on radiative forcing, and hence on climate. In this manuscript, we demonstrate the utility of single particle off-line analysis to investigate the chemistry of individual atmospheric particles using modern, state-of-the-art electron microscopy and time-of-flight secondary ionization mass spectrometry techniques. We show that these methods provide specific, detailed data on particle composition, chemistry, morphology, phase and internal structure. This information is crucial for evaluating hygroscopic properties of aerosols, understanding aerosol aging and reactivity, and correlating the characteristics of aerosols with their optical properties. The manuscript presents a number of analytical advances in methods of electron probe particle analysis along with a brief review of a number of the research projects carried out in the authors’ laboratory on the chemical characterization of environmental particles. The obtained data offers a rich set of qualitative and quantitative information on the particle chemistry, composition and the mechanisms of gas-particle interactions which are of high importance to atmospheric processes involving particulate matter and air pollution.
2006. "A New Approach to Determining Gas-Particle Reaction Probabilities and Application to the Heterogeneous Reaction of Deliquesced Sodium Chloride Particles with Gas-Phase Hydroxyl Radicals." Journal of Physical Chemistry A 110(36):10619-10627. doi:10.1021/jp063263+ Abstract The reaction kinetics for gaseous hydroxyl radicals (OH) with deliquesced sodium chloride particles (NaClaq) were investigated using a novel experimental approach. The technique utilizes the exposure of substrate-deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) capability. Experiments were performed at room temperature and atmospheric pressure with deliquesced NaCl particles in the micron size range at 70-80% RH and with OH concentrations in the range of 1 to 7×109 cm-3. The apparent, pseudo first-order rate constant for the reaction was determined from measurements of changes in the chloride concentration of individual particles upon reaction with OH as a function of the particle loading on the substrate. Quantitative treatment of the data using a model that incorporates both diffusion and reaction kinetics yields a lower-limit to the net reaction probability of γnet > 0.1, with an overall uncertainty of a factor of two.
2006. "Aerosol Composition and Source Apportionment in the Mexico City Metropolitan Area with PIXE/PESA/STIM and Multivariate Analysis." Atmospheric Chemistry and Physics 6(12):4591-4600. Abstract Aerosols play an important role in the atmosphere but are poorly characterized, particularly in urban areas like the Mexico City Metropolitan Area (MCMA). The chemical composition of urban particles must be known to assess their effects on the environment, and specific particulate emissions sources should be identified to establish ef- 5 fective pollution control standards. For these reasons, samples of particulate matter _2.5 µm (PM2.5) were collected during the MCMA-2003 Field Campaign for elemental and multivariate analyses. Proton-Induced X-ray Emission (PIXE), Proton-Elastic Scattering Analysis (PESA) and Scanning Transmission Ion Microscopy (STIM) techniques were done to determine concentrations of 19 elements from Na to Pb, hydrogen, and 10 total mass, respectively. The most abundant elements from PIXE analysis were S, Si, K, Fe, Ca, and Al, while the major emissions sources associated with these elements were industry, wind-blown soil, and biomass burning. Wind trajectories suggest that metals associated with industrial emissions came from northern areas of the city whereas soil aerosols came from the southwest and increased in concentration during 15 dry conditions. Elemental markers for fuel oil combustion V and Ni correlated with a large SO2 plume to suggest an anthropogenic, rather than volcanic, emissions source. By subtracting major components of soil and sulfates determined by PIXE analysis from STIM total mass measurements, we estimate that approximately 50% of PM2.5 consisted of carbonaceous material.
2005. "Hydrophilic Properties of Aged Soot." Geophysical Research Letters 32:LO1807. Abstract The global presence of soot has significant effects on regional and global climate, as well as human health. Influence of soot on radiation budget, rain formation and heterogeneous chemistry, and its residence time in the atmosphere are largely dependent on its ability to interact with water. While freshly emitted soot is extremely hydrophobic oxidation during aging causes soot to become more hydrophilic. Laboratory studies demonstrate that aged soot attracts and retains water, and can be efficiently removed from the troposphere by entrapment in existing liquid clouds droplets or by activation as cloud condensation nuclei.
2005. "Heterogeneous Chemistry of Individual Mineral Dust Particles with Nitric Acid. A Combined CCSEM/EDX, ESEM AND ICP-MS Study." Journal of Geophysical Research. D. (Atmospheres) 110(D10):D10208. Abstract The heterogeneous chemistry of individual dust particles from four authentic dust samples with gas-phase nitric acid was investigated in this study. Morphology and compositional changes of individual particles as they react with nitric acid were observed using conventional scanning electron microscopy with energy dispersive analysis of X-rays (SEM/EDX) and computer controlled SEM/EDX. Environmental Scanning Electron Microscopy (ESEM) was utilized to investigate the hygroscopic behavior of mineral dust particles reacted with nitric acid. Differences in the reactivity of mineral dust particles from these four different dust source regions with nitric acid were observed. Mineral dust from source regions containing high levels of calcium, namely China loess dust and Saudi coastal dust, were found to react to the greatest extent.
2005. "Direct Observation of Completely Processed Calcium Carbonate Dust Particles." Faraday Discussions 130:453 - 468. doi:10.1039/b417366j Abstract This study presents, for the first time, field evidence of complete, irreversible processing of solid calcium carbonate (calcite)-containing particles and quantitative formation of liquid calcium nitrate particles apparently as a result of heterogeneous reaction of calcium carbonate-containing mineral dust particles with gaseous nitric acid. Formation of nitrates from individual calcite and sea salt particles was followed as a function of time in aerosol samples collected at Shoresh, Israel. Morphology and compositional changes of individual particles were observed using conventional scanning electron microscopy with energy dispersive analysis of X-rays (SEM/EDX) and computer controlled SEM/EDX. Environmental scanning electron microscopy (ESEM) was utilized to determine and demonstrate the hygroscopic behavior of calcium nitrate particles found in some of the samples. Calcium nitrate particles are exceptionally hygroscopic and deliquesce even at very low relative humidity (RH) of 9-11% which is lower than typical atmospheric environments. Transformation of non-hygroscopic dry mineral dust particles into hygroscopic wet aerosol may have substantial impacts on light scattering properties, the ability to modify clouds and heterogeneous chemistry.
2005. "Erratum To "Heterogeneous Chemistry of Individual Mineral Dust Particles From Different Dust Source Regions: The Importance of Particle Mineralogy" [Atmos.Environ.38 (36)(2004)6253 –6261]." Atmospheric Environment 39(2005):395. Abstract The publisher regrets that there was an error in the information in Table 1 incorrectly identifies the mineral dust and their corresponding composition. The corrected table is given below. In addition, the second full sentence below the table should read “Smaller amounts ≤1% of other elements such as V and Ti were also observed but are not listed in Table 1 and is the reason why the percentages do not total to 100 in Table 1.” In the original article it states that K was not included in Table 1 but clearly it is.
2005. "Processing of Soot in an Urban Environment: Case Study from the Mexico City Metropolitan Area." Atmospheric Chemistry and Physics 5:3033-3043. Abstract Chemical composition, size, and mixing state of atmospheric particles are critical in determining their e ffects on the environment. There is growing evidence that soot aerosols play a particularly important role in both climate and human health, but still relatively little is known of their physical and chemical nature. In addition, the atmo- 5 spheric residence times and removal mechanisms for soot are neither well understood nor adequately represented in regional and global climate models. To investigate the effect of locality and residence time on properties of soot and mixing state in a polluted urban environment, particles of diameter 0.2–2.0 µm were collected in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003 field campaign from various sites 10 within the city. Individual particle analysis by di fferent electron microscopy methods coupled with energy dispersed X-ray spectroscopy, and secondary ionization mass spectrometry show that freshly-emitted soot particles become rapidly processed in the MCMA. Whereas fresh particulate emissions from mixed-tra ffic are almost entirely carbonaceous, consisting of soot aggregates with liquid coatings suggestive of unburned 15 lubricating oil and water, ambient soot particles which have been processed for less than a few hours are heavily internally mixed, primarily with ammonium sulfate. Single particle analysis suggests that this mixing occurs through several mechanisms that require further investigation. In light of previously published results, the internally-mixed nature of processed soot particles is expected to a ffect heterogeneous chemistry on 20 the soot surface, including interaction with water during wet-removal.
2005. "Optical, Physical and Chemical Properties of Tar Balls Observed During the Yosemite Aerosol Characterization Study." Journal of Geophysical Research. D. (Atmospheres) 110:Art. No. D21210. doi:10.1029/2004JD005728 Abstract The Yosemite Aerosol Characterization Study of summer 2002 (YACS) occurred during an active fire season in the western U. S., and provided an opportunity to investigate many unresolved issues related to the radiative effects of biomass burning aerosols. Single particle analysis was performed on field collected aerosol samples using an array of electron microscopy techniques. Amorphous carbon spheres, or “tar balls”, were present in samples collected during episodes of high particle light scattering coefficients that occurred during the peak of a smoke/haze event. The highest concentrations of light-absorbing carbon from a dual-wavelength aethalometer (λ = 370 and 880 nm) occurred during periods when the particles were predominantly tar balls, indicating they do absorb light in the UV and near-IR range of the solar spectrum. Closure experiments of mass concentrations and light scattering coefficients during periods dominated by tar balls did not require any distinct assumptions of organic carbon molecular weight correction factors, density, or refractive index compared to periods dominated by other types of organic carbon aerosols. Measurements of the hygroscopic behavior of tar balls using an environmental SEM indicate that tar balls do not exhibit deliquescence, but do uptake some water at high (~83 %) relative humidity. The ability of tar balls to efficiently scatter and absorb light, and to absorb water has important implications for their role in regional haze and climate fence.
2004. "Response to Comments on "Reactions at Interfaces as a Source of Sulfate Formation in Sea-Salt Particles"." Science 303(5658):1. Abstract Our paper (1) demonstrated that the oxidation of chloride at the interface will be a new source of alkalinity that can modulate the acidification of particles during the day when gaseous OH is present. We proposed that this modulation of the acidity of sea-salt particles could potentially affect the rate of uptake and oxidation of SO2 [S(IV)], which is very sensitive to pH.
2004. "Heterogeneous Chemistry of Individual Mineral Dust Particles from Different Dust Source Regions: The Importance of Particle Mineralogy." Atmospheric Environment 38(36):6253-6261. Abstract The heterogeneous chemistry of individual dust particles from four different dust source regions is investigated on a particle-by-particle basis using state-of-the-art scanning electron microscopy techniques including computer-controlled scanning electron microscopy/computer-controlled X-ray analysis (CCSEM/EDX). Morphology and compositional changes of individual particles as they react with nitric acid are observed. Clear differences in the reactivity of mineral dusts from these four different dust regions with nitric acid could be observed. Mineral dust from source regions containing high levels of calcium, such as those found in parts of China and Saudi Arabia, are found to react to the greatest extent. Calcium containing minerals, such as calcite (CaCO3) and dolomite (CaMg(CO3)2), react to form nitrate salt whereas other calcium containing minerals such as gypsum (CaSO4·2H2O) do not react. The importance of particle chemical composition and mineralogy in the heterogeneous chemistry of mineral dust aerosols is definitively borne out in this study of individual dust particles.
2004. "Sodium Nitrate Particles: Physical and Chemical Properties During Hydration and Dehydration, and Implications for Aged Sea Salt Aerosols." Journal of Aerosol Science 35(7):869-887. Abstract Experiments probing the phase and behavior of NaNO3 particles at different relative humidities, important for elucidating the role these play in the chemistry and radiative properties of marine regions, are presented. Changes in NaNO3 particles during hydration were studied using environmental scanning electron mircoscopy (ESEM) and conventional SEM coupled with energy dispersive X-ray analysis (SEM/EDX). Mixtures of NaNO3 and NaCI, which are typical of partially processed sea salt particles, were also studied. Complementary studies using long path FTIR were carried out to determine the extent of water association with NaNO3 aerosols, and for comparison, NaC1, MgC12, and NH4NO3, as a function of relative humidity. The combination of these techniques shows that NaNO3 particles exist as unusual metastable, amorphous solids at low relative humidity that undergo continuous hygroscopic growth with increasing relative humidity. While other evidence for this phenomenon has been reported, this is the first direct observation using ESEM.
2004. "TOF-SIMS Analysis of Sea Salt Particles: Imaging and Depth Profiling in the Discovery of an Unrecognized Mechanism for pH Buffering." Applied Surface Science 231-2(Sp. Iss. SI):520-523. Abstract As part of a broader effort at understanding the chemistry of sea salt particles, we have performed time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis of individual sea salt particles deposited on a transmission electron microscopy (TEM) grid. Environmental scanning electron microscopy (ESEM) and TOF-SIMS analysis have, in conjunction with OH exposure studies, led to the discovery of an unrecognised buffering mechanism in the uptake and oxidation of SO2 in sea salt particles in the marine boundary layer. This chemistry may resolve several discrepancies in the atmospheric chemistry literature. Several challenges during the acquisition and interpretation of both imaging and depth profiling data on specific particles on the TEM grid identified by the ESEM were overcome. A description of the analysis challenges and the solutions ultimately developed to them is presented here, along with an account of how the TOF-SIMS data were incorporated into the overall research effort. Several issues unique to the analysis of high aspect ratio particles are addressed.[1]
2003. "Time-Resolved Aerosol Collector for CCSEM/EDX Single Particle Analysis." Aerosol Science and Technology 37(3):246-260. Abstract An automated Time Resolved Aerosol Collector (TRAC) has been developed for sequential sampling of field-collected aerosols for laboratory-based Computer Controlled Scanning Electron Microscopy/Energy Dispersed X-ray (CCSEM/EDX) single particle analysis. The collector is optimized for use of grid-supported 20 nm carbon films as deposition substrates. The carbon films have low enough X-ray background to permit EDX analysis down to 0.1-0.2 ?m particles, including detection of low-Z elements: C, N, & O. The TRAC provides unattended sampling onto a set of 151 individual grids, at sequential time intervals as short as 1 min. After collection, the samples are sealed and refrigerated pending analysis. The utility of the TRAC-CCSEM/EDX approach is exemplified using the aerosol samples collected during the Texas 2000 Air Quality Studies (Aug. 15 ? Sept. 15, 2000). We are able to quantitatively follow the time evolution in the relative contribution of non-volatile particles such as ammonium sulfate, mineral dust, sea salt, carbonaceous, etc. in the aerosol make up. The results show, among other things, the diurnal cycles in appearance of fine carbonaceous & ammonium sulfate particles & substantial mixing/coating of mineral particles with ammonium sulfates.
2003. "Reactions at Interfaces As a Source of Sulfate Formation In Sea-Salt Particles." Science 301(5631):340-344. Abstract Understanding the formation of sulfate particles in the troposphere is critical because of their health effects and their direct and indirect effects on radiative forcing, and hence on climate.
2003. "The Transformation of Solid Atmospheric Particles into Liquid Droplets Through Heterogeneous Chemistry: Laboratory Insights into the Processing of Calcium Containing Mineral Dust Aerosol in the Troposphere." Geophysical Research Letters 30(3):1148. doi:10.1029/2002GL016563 Abstract [1] Individual calcium carbonate particles reacted with gas- phase nitric acid at 293 K have been followed using Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis as a function of time and relative humidity (RH). The rate of calcium carbonate to calcium nitrate conversion is significantly enhanced in the presence of water vapor. The SEM images clearly show that solid CaCO3 particles are converted to spherical droplets as the reaction proceeds. The process occurs through a two-step mechanism involving the conversion of calcium carbonate into calcium nitrate followed by the deliquescence of the calcium nitrate product. The change in phase of the particles and the significant reactivity of nitric acid and CaCO3 at low RH are a direct result of the deliquescence of the product at low RH. This is the first laboratory study to show the phase transformation of solid particles into liquid droplets through heterogeneous chemistry.
2003. "Probing Heterogeneous Chemistry of Individual Atmospheric Particles Using Scanning Electron Microscopy and Energy-Dispersive X-ray Analysis." Analytical Chemistry 75(19):5170-5179. Abstract In this paper, we demonstrate the utility of single-particle analysis to investigate the chemistry of isolated, individual particles of atmospheric relevance such as NaCl, sea salt, CaCO3, and SiO2. A variety of state-of-th-art scanning electron microscopy techniques, including environmental scanning electon microscopy and computer-controlled scanning electron microscopy/energy-dispersive X-ray analysis, were utilized for monitoring and quantifying phase transitions of individual particles, morphology, and compositional changes of individual particles as they react with nitric acid.
2002. "Quantitative Time-Resolved Monitoring of Nitrate Formation in Sea Salt Particles Using a CCSEM/EDX Single Particle Analysis"." Environmental Science and Technology 36(23):4948-4955. Abstract Progress of the chlorine depletion and nitrate enrichment in individual sea salt particles was detected as a function of time using aerosol samples collected during the TexAQS 2000 experiment. We demonstrate that the time-resolved collection approach coupled with the automated EDX single particle analysis made it possible to follow in great detail the time evolution of sea salt particles within a diverse aerosol mixture. Using a custom built Time Resolved Aerosol Collector (TRAC), particulate samples were taken sequentially on grid-supported 20 nm carbon films with a time resolution of 10 minutes between two consecutive samples. The samples were analyzed in the laboratory using Computer Controlled Scanning Electron Microscopy with Energy-Dispersed analysis of X-rays (CCSEM/EDX). Between midnight of 08/16/00 and the early morning of 08/17/00, a steady, particularly sea salt rich aerosol was observed at the measurement site, which later showed the effects of atmospheric processing. During the night of 08/17/00 the sea salt particles were almost unprocessed, having elemental composition close to that of seawater. By 12 noon, the evolving atmosphere was able to completely convert them, predominantly to sodium nitrate particles. During the next night, this process had nearly stopped and fairly virgin sea salt particles appeared again.
2002. "On Deposition Efficiency of Point-To-Plate Electrostatic Precipitator." Journal of Aerosol Science 33(3):405-409. Abstract The deposition efficiency of a point-to-plate electrostatic precipitator onto a target substrate was studied in this work. The efficiency when measured in the traditional way, by the loss of particles from the exiting air, was like in many others such devices, typically high (80-90%). However the actual useful deposition onto the target substrate was never more than 50% and sometime as low as a few percent only. Therefore, special caution should be taken in design and testing of electrostatic precipitators for quantitative sampling of aerosols.
2001. "Automated Single-Particle SEM/EDX Analysis of Submicron Particles down to 0.1 mu m." Analytical Chemistry 73(5):1023-1029. Abstract An automated single particle SEM/EDX analysis of aerosols deposited onto grid supported carbon film of 15-25 nm thickness is demonstrated. Use of the carbon film gives exceptionally low background in the SEM/EDAX and allows satisfied automated analysis of particles down to 0.1 mm size, including analysis of low-Z elements (C, O and N). In this work, six lab-generated 0.1-2 mm aerosols were tested for their elemental composition. The EDAX yields reasonably accurate quantitative results featuring all the elements presented in the tested compounds, namely C, O, N, Na, S, Al, Si and Cl. Furthermore, the carbon film has very low backscattered electron (BSE) yield compared to that from the particle, so in the BSE mode the particle image is seen with very high contrast. This greatly improves quality and speed of the automated mapping of particles by SEM prior to EDAX. The presented approach is believed to be a significant improvement over the automated single particle analysis in the electron microprobe based techniques, which usually provide limited information on low-Z elements and usually could not go below ~0.5 mm.