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A large and increasing fraction of the planet’s population lives in megacities, especially in the developing world. These large metropolitan areas generally have very high levels of both gaseous and particulate air pollutants that have severe impacts on human health, ecosystem viability, and climate on local, regional, and even continental scales. Emissions fluxes and ambient pollutant concentration distributions are generally poorly characterized for large urban areas even in developed nations. Much less is known about pollutant sources and concentration patterns in the faster growing megacities of the developing world. New methods of locating and measuring pollutant emission sources and tracking subsequent atmospheric chemical transformations and distributions are required. Measurement modes utilizing an innovative van based mobile laboratory equipped with a suite of fast response instruments to characterize the complex and “nastier” chemistry of the urban boundary layer are described. Instrumentation and measurement strategies are illustrated with examples from the Mexico City and Boston metropolitan areas. It is shown that fleet average exhaust emission ratios of formaldehyde (HCHO), acetaldehyde (CH3CHO) and benzene (C6H6) are substantial in Mexico City, with gasoline powered vehicles emitting higher levels normalized by fuel consumption. NH3 exhaust emissions from newer light duty vehicles in Mexico City exceed levels from similar traffic in Boston. A mobile conditional sampling air sample collection mode designed to collect samples from intercepted emission plumes for later analysis is also described.

[1] An Aerodyne aerosol mass spectrometer (AMS) was deployed at the Pittsburgh Environmental Protection Agency Supersite from 7 to 22 September 2002 as part of the Pittsburgh Air Quality Study (PAQS). The main objectives of this deployment were to characterize the concentrations, size distributions, and temporal variations of nonrefractory (NR) chemical species in submicron particles (approximately PM1) and to further develop and evaluate the AMS. Reasonably good agreement was observed on particle concentrations, composition, and size distributions between the AMS data and measurements from collocated instruments (given the difference between the PM1 and PM2.5 size cuts), including TEOM, semicontinuous sulfate, 2-hour- and 24-hour-averaged organic carbon, SMPS, 4-hour-averaged ammonium, and micro-orifice uniform deposit impactor. Total NR-PM1 mass concentration in Pittsburgh accumulates over periods of several days punctuated with rapid cleaning due to rain or air mass changes. Sulfate and organics are the major NR-PM1 components while the concentrations of nitrate and chloride are generally low. Significant amounts of ammonium, which most of the time are consistent with sulfate present as ammonium sulfate, are also present in particles. However, there are periods when the aerosols are relatively acidic and more than 50% of sulfate is estimated to be in the form of ammonium bisulfate. No major enhancement of the organic concentration is observed during these acidic periods, which suggests that acid-catalyzed SOA formation was not an important process during this study. Size distributions of particulate sulfate, ammonium, organics, and nitrate vary on timescales of hours to days, showing unimodal, bimodal and even trimodal characteristics. The accumulation mode (peaking around 350–600 nm in vacuum aerodynamic diameter for the mass distributions) and the ultrafine mode (

[1] During the field campaign of the Study of Houston Atmospheric Radical Precursors/Surface-Induced Oxidation of Organics in the Troposphere (SHARP/SOOT) in Houston, Texas, a suite of aerosol instruments was deployed to directly measure a comprehensive set of aerosol properties, including the particle size distribution, effective density, hygroscopicity, and light extinction and scattering coefficients. Those aerosol properties are employed to quantify the mixing state and composition of ambient particles and to gain a better understanding of the formation and transformation of fine particulate matter in this region. During the measurement period, aerosols are often internally mixed, with one peak in the effective density distribution at 1.55 ± 0.07 g cm−3, consistent with a population composed largely of sulfates and organics. Episodically, a second mode below 1.0 g cm−3 is identified in the effective density distributions, reflecting the presence of freshly emitted black carbon (BC) particles. The measured effective density demonstrates a clear diurnal cycle associated with primary emissions from transportation and photochemical aging, with a minimum during the morning rush hour, increasing from 1.4 to 1.5 g cm−3 on average over 5 h, and remaining nearly constant throughout the afternoon. The average BC concentration derived from light-absorption measurements is 0.31 ± 0.22 µg m−3, and the average measured particle single scattering albedo is 0.94 ± 0.04. When elevated BC concentrations are observed, typically during the morning rush hours, single scattering albedo decreases, with a smallest measured value of about 0.7. Aerosol hygroscopicity measurements indicate that larger particles (e.g., 400 nm) are more hygroscopic than smaller particles (e.g., 100 nm). The measurements also reveal discernable meteorological impacts on the aerosol properties. After a frontal passage, the average particle effective density decreases, the average BC concentration increases, and the aerosol size distribution is dominated by new particle formation.

[1] Two Aerodyne aerosol mass spectrometers (AMSs) were deployed at Trinidad Head on the north Californian coast during the National Oceanographic and Atmospheric Administration Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) experiment, to study the physiochemical properties of submicron aerosol particles within the Pacific marine boundary layer. One AMS was modified to allow the study of sea salt-based particles, while the other used a temperature cycling system on its inlet. The reported loadings increased by a factor of 2 when the temperature approached the dew point, which is due to the inlet performance and has implications for other AMS experiments and applications. The processed data were compared with those of a particle into liquid sampler-ion chromatograph and showed that the ammonium, sulfate and organic fractions of the particles were consistently found within a single, normally acidic, accumulation mode at around 300 - 400 nm. However, when influenced by land-based sources, vehicle emissions and increased ammonium loadings were seen. The concentrations of nitrate in the accumulation mode were low, but it was also found within sea salt particles in the coarse mode and can be linked to the displacement of chloride. The organic fraction showed a high degree of chemical ageing and evidence of nitrogen-bearing organics was also observed. The particulate organic data were compared to the volatile organic carbon data derived from an in-situ gas chromatograph-mass spectrometer-flame ionization detector and relationships were found between the gas and particle phase chemicals in both the overall concentrations and the levels of oxidation.

The rate constant for electron attachment to SO3 is 3±1×10−9 cm3 s−1 at 300 K, measured in helium gas at pressures from 53 to 160 Pa (0.4 to 1.2 torr). The sole product of attachment is SO−3 under these conditions. The same rate constant and ionic product were obtained at 400 and 505 K. The measurements were carried out using a flowing‐afterglow Langmuir‐probe apparatus.

[1] In the paper ‘‘Quantitative sampling using an Aerodyne aerosol mass spectrometer: 1. Techniques of data interpretation and error analysis’’ by J. D. Allan et al. (Journal of Geophysical Research, 108(D3), 4090, doi:10.1029/2002JD002358, 2003), the authors of the Jimenez et al. [2003] reference are incorrect. The correct reference is as follows: Jimenez, J. L., J. T. Jayne, Q. Shi, C. E. Kolb, D. R. Worsnop, I. Yourshaw, J. H. Seinfeld, R. C. Flagan, X. Zhang, K. A. Smith, J. W. Morris, and P. Davidovits, Ambient aerosol sampling using the Aerodyne aerosol mass spectrometer, J. Geophys. Res., 10.1029/ 2001JD001213, in press, 2003. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D9, 4283, doi:10.1029/2003JD001607, 2003

The Aerodyne Aerosol Mass Spectrometer (AMS) has been designed to measure size-resolved mass distributions and total mass loadings of volatile and semivolatile chemical species in/on submicron particles. This paper describes the application of this instrument to ambient aerosol sampling. The AMS uses an aerodynamic lens to focus the particles into a narrow beam, a roughened cartridge heater to vaporize them under high vacuum, and a quadrupole mass spectrometer to analyze the vaporized molecules. Particle size is measured via particle time-of-flight. The AMS is operated in two modes: (1) a continuous mass spectrum mode without size information; and (2) a size distribution measurement mode for selected m/z settings of the quadrupole. Single particles can also be detected and sized if they have enough mass of a chemical component. The AMS was deployed at a ground sampling site near downtown Atlanta during August 1999, as part of the Environmental Protection Agency/Southern Oxidant Study Particulate Matter “Supersite” experiment, and at a suburban location in the Boston area during September 1999. The major observed components of the aerosol at both sites were sulfate and organics with a minor fraction of nitrate, consistent with prior studies and colocated instruments. Different aerosol chemical components often had different size distributions and time evolutions. More than half of the sulfate mass was contained in 2% of the ambient particles in one of the sampling periods. Trends in mass concentrations of sulfate and nitrate measured with the AMS in Atlanta compare well with those measured with ion chromatography-based instruments. A marked diurnal cycle was observed for aerosol nitrate in Atlanta. A simple model fit is used to illustrate the integration of data from several chemical components measured by the AMS together with data from other particle instruments into a coherent representation of the ambient aerosol.

[1] In part 1 of this series, techniques for generating quantitative information on fine airborne particulate-size and chemically resolved mass concentration from an Aerodyne aerosol mass spectrometer were introduced. Presented here are the results generated using these techniques from sampling U. K. urban air with such an instrument in Edinburgh during October 2000 and in Manchester during July 2001 and January 2002. Data on the total mass concentrations and size-resolved mass distributions of nitrate, sulfate, and organic compounds were obtained for all three campaigns and compared with data from other sources, including a micro-orifice uniform deposit impactor, total particle numbers, CO and NOx concentrations, local wind speed and temperature, and back trajectory analysis. All three locations showed evidence for emissions from local transport, with a mass modal aerodynamic diameter of around 100-200nm. This mode was dominated by hydrocarbons showing little evidence of oxidization. The three sites also exhibited a larger mode consisting of inorganic chemicals and oxidized organics, which appeared to be governed by sources external to the cities and showed evidence of internal mixing. The mass modal aerodynamic diameter varied between approximately 200-500 nm during the winter and 500-800 nm during the summer. The summer also showed an increased mass loading without an increase in total particle number. Evidence of material building up and ageing in the atmospheric surface layer during periods of low wind speeds was also observed.

Received 22 March 2002; revised 2 July 2002; accepted 5 August 2002; published 4 February 2003. [1] The aerosol mass spectrometer (AMS), manufactured by Aerodyne Research, Inc., has been shown to be capable of delivering quantitative information on the chemical composition and size of volatile and semivolatile fine airborne particulate matter with high time resolution. Analytical and software tools for interpreting the data from this instrument and generating meaningful, quantitative results have been developed and are presented here with a brief description of the instrument. These include the conversion of detected ion rates from the quadrupole mass spectrometer during the mass spectrum (MS) mode of operation to atmospheric mass concentrations of chemical species (in m gm � 3 ) by applying calibration data. It is also necessary to correct for variations in the electron multiplier performance, and a method involving the measurement of the instrument’s response to gas phase signals is also presented. The techniques for applying particle velocity calibration data and transforming signals from time of flight (TOF) mode to chemical mass distributions in terms of aerodynamic diameter (dM/dlog(Da) distributions) are also presented. It is also possible to quantify the uncertainties in both MS and TOF data by evaluating the ion counting statistics and variability of the background signal, respectively. This paper is accompanied by part 2 of this series, in which these methods are used to process and analyze AMS results on ambient aerosol from two U.K. cities at different times of the year. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0394 Atmospheric Composition and Structure: Instruments and techniques; 0399 Atmospheric Composition and Structure: General or miscellaneous; KEYWORDS: aerosols, chemical composition, mass spectrometry, analysis techniques

During the Atlanta Supersite Project, four particle mass spectrometers were operated together for the first time: NOAA's Particle Analysis by Laser Mass Spectrometer (PALMS), University of California at Riverside's Aerosol Time-of-Flight Mass Spectrometer (ATOFMS), University of Delaware's Rapid Single-Particle Mass Spectrometer II (RSMS-II), and Aerodyne's Aerosol Mass Spectrometer (AMS). Although these mass spectrometers are generally classified as similar instruments, they clearly have different characteristics due to their unique designs. One primary difference is related to the volatilization/ionization method: PALMS, ATOFMS, and RSMS-II utilize laser desorption/ionization, whereas particles in the AMS instrument are volatilized by impaction onto a heated surface with the resulting components ionized by electron impact. Thus mass spectral data from the AMS are representative of the ensemble of particles sampled, and those from the laser-based instruments are representative of individual particles. In addition, the AMS instrument cannot analyze refractory material such as soot, sodium chloride, and crustal elements, and some sulfate or water-rich particles may not always be analyzed with every laser-based instrument. A main difference among the laser-based mass spectrometers is that the RSMS-II instrument can obtain size-resolved single particle composition information for particles with aerodynamic diameters as small as 15 nm. The minimum sizes analyzed by ATOFMS and PALMS are 0.2 and about 0.35 μm, respectively, in aerodynamic diameter. Furthermore, PALMS, ATOFMS, and RSMS-II use different laser ionization conditions. Despite these differences the laser-based instruments found similar individual particle classifications, and their relative fractions among comparable sized particles from Atlanta were broadly consistent. Finally, the AMS measurements of the nitrate/sulfate mole ratio were highly correlated with composite measurements (r^2 = 0.93). In contrast, the PALMS nitrate/sulfate ion ratios were only moderately correlated (r^2 ∼ 0.7).

An Aerodyne aerosol mass spectrometer (AMS) was deployed during the Aerosol Characterization Experiment-Asia (ACE-Asia) field campaign on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft to measure the size-resolved chemical composition of submicron aerosols in the outflow from eastern Asia. Research flights were carried out from 31 March to 1 May 2001 in an area that covered 127°E–135°E and 32°N–38°N. Valid data from the AMS were obtained during 15 out of a total of 19 research flights. During the mission the AMS measured distinct layers (from the boundary layer to ∼3700 m) of submicron aerosols composed of sulfate, ammonium, and organics as the major nonrefractory components, separated by layers with much lower aerosol concentrations. Sulfate and organics mass concentrations of up to 10 μg m^(−3) and 13 μg m^(−3), respectively, were measured in some pollution layers. Back-trajectory analysis shows that the polluted layers originated in urban and industrial areas of China and Korea. The mass-weighed size distribution of the submicron sulfate was relatively constant from day to day and layer to layer, with an aerodynamic diameter mode of 400–500 nm and a width (full width half maximum) of about 450 nm in most of the layers. On the days with low influence of dust in the aerosol outflow, as indicated by other instruments aboard the Twin Otter, the total mass of nonrefractory aerosols estimated by the AMS correlated well with total volume of aerosols measured by a differential mobility analyzer.

Biogenically produced reduced sulfur compounds from the marine environment, deliver a sulfur burden to the atmosphere which is about half as large as that due to sulfur oxides produced by fossil fuel combustion. The multiphase chemical processes for these species must be understood in order to evaluate the relative roles of biogenic and combustion produced sulfur oxides over the oceans. The aim of the studies funded by the subject DOE grant is to measure parameters governing the heterogeneous chemistry of the species occurring in the marine environment. During the past year, uptake studies for the sulfur species MSA, DMSO, DMSO{sub 2}, DMS, OCS, CS{sub 2}, H{sub 2}S, and CH{sub 3}SH have been finalized. Studies of the reactive uptake of Cl{sub 2} and Br{sub 2} by Br{sup -} and I{sup -} solutions as a function of temperature have been completed. The uptake of O{sub 3} by aqueous NaI solutions has also been studied for the purpose of comparison. We have begun co-deposition studies and have obtained some preliminary results for the codeposition with ozone of DMS, DMSO, DMSO{sub 2} and MSA. For the next phase of the work, a new horizontal bubbler apparatus was designed and built and construction to improve the detection sensitivity of the apparatuses was begun. Altogether during 1994, 8 articles have been accepted for publication and 2 Ph.D. dissertations have been submitted and approved.

Sulfur trioxide (SO3) has long been known to react with water to produce sulfuric acid (H2S04). It has been commonly assumed that the gas phase reaction in the Earth`s atmosphere between SO3 and water vapor to produce sulfuric acid vapor is an important step in the production of sulfuric acid aerosol particles. The kinetics of the gas phase reaction of SO3 with water vapor have previously been studied by Castleman and co-workers, Wang et al and Reiner and Arnold. Each of these studies was carried out in a flow reactor, with the first two studies performed at low pressure (1-10 Torr) and the latter from approx. 30 to 260 Torr. Each of these studies measured SO3 decays over a range of H2O vapor levels, obtaining data consistent with interpreting the reaction of gaseous SO3 and H2O as a bimolecular process. It is not clear why previous experimental studies failed to observe a nonlinear dependence of SO3 consumption on water vapor concentration. It is probable that sufficient water dimer exists in much of the Earth`s atmosphere to allow dimer reactions to participate in sulfuric acid vapor formation.

Rate constants and products were measured for over 50 ion-molecule reactions involving SO3 or ions formed from SO3. All negative ions studied reacted with SO3 at or near the collisional rate by a variety of processes including charge transfer, O− transfer, ligand exchange, and clustering. Ambient negative ions tended to form SO− 4 or SO− 4 core ions. A chemical ionization detection scheme for monitoring SO3 in the atmosphere is presented based on CO−3 (H2O)n = 0–1 or NO−3 (H2O)n = 0–1 as the primary ions. The proposed scheme can be used in conjunction with previous schemes for monitoring SO2 and H2SO4 concentrations to measure all three gases simultaneously. The measurements also confirm that the ion chemistry used in separate studies to monitor the reaction of SO3 with H2O is correct and rapid and that the reaction of SO3 with positive ion water clusters does not contribute to H2SO4 production in the atmosphere.

The authors report the laboratory determination of the temperature-dependent mass accommodation coefficients of SO{sub 2} and H{sub 2}O{sub 2} on aqueous surfaces over the range 260-292 K. Mass accommodation kinetics involve fundamental chemical interactions of liquid surfaces about which little is known. Uptake of SO{sub 2} and H{sub 2}O{sub 2} by cloud droplets is believed to be critical to S(IV) oxidation in the troposphere. Our experimental method combines a monodisperse train of droplets (200 {mu}m in diameter) and a low-pressure flow reactor. Uptake rates of trace gases are determined by measuring changes in trace gas number density as a function of exposed liquid surface area. Experiments with systematic variation of water vapor and rare gas partial pressures permit deconvolution of gas diffusion and temperature-dependent mass accommodation. In the case of SO{sub 2}, variation of droplet-gas interactions on millisecond time scales resolved pH-dependent saturation effects at the aqueous surface. Results for SO{sub 2} show {gamma}{sub 273} = 0.11 {plus minus} 0.02 with no significant temperature variation. H{sub 2}O{sub 2} shows a strong temperature dependence consistent with an attractive well depth of at least 26 kJ mol{sup {minus}1} with {gamma}{sub 273} = 0.18 {plus minus} 0.02. These results are discussed in termsmore » of the different aqueous solubilities of SO{sub 2} and H{sub 2}O{sub 2}.« less