Abstract. The volatilities of different chemical species in ambient aerosols are important but remain poorly characterized. The coupling of a recently developed rapid temperature-stepping thermodenuder (TD, operated in the range 54–230°C) with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) during field studies in two polluted megacities has enabled the first direct characterization of chemically-resolved urban particle volatility. Measurements in Riverside, CA and Mexico City are generally consistent and show ambient nitrate as having the highest volatility of any AMS aerosol species while sulfate showed the lowest volatility. Total organic aerosol (OA) showed volatility intermediate between nitrate and sulfate, with an evaporation rate of 0.6% K−1 near ambient temperature, although OA dominates the residual species at the highest temperatures. Different types of OA were characterized with marker ions, diurnal cycles, and positive matrix factorization (PMF) and show significant differences in volatility. Reduced hydrocarbon-like OA (HOA, a surrogate for primary OA, POA), oxygenated OA (OOA, a surrogate for secondary OA, SOA), and biomass-burning OA (BBOA) separated with PMF were all determined to be semi-volatile. The most aged OOA-1 and its dominant ion, CO2+, consistently exhibited the lowest volatility, with HOA, BBOA, and associated ions for each among the highest. The similar or higher volatility of HOA/POA compared to OOA/SOA contradicts the current representations of OA volatility in most atmospheric models and has important implications for aerosol growth and lifetime. Our results strongly imply that all OA types should be considered semivolatile in models. The study in Riverside identified organosulfur species (e.g. CH3HSO3+ ion, likely from methanesulfonic acid), while both studies identified ions indicative of amines (e.g. C5H12N+) with very different volatility behaviors than inorganic-dominated ions. The oxygen-to-carbon ratio of OA in each ambient study was shown to increase both with TD temperature and from morning to afternoon, while the hydrogen-to-carbon ratio showed the opposite trend.

The uptake of gas-phase ammonia by aqueous surfaces was measured as a function of temperature, gas liquid interaction time, and pH in the range 0-13. Uptake measurements at low pH yielded values of the mass accommodation coefficient (R) as a function of temperature. The mass accommodation coefficient increases as the temperature decreases, from 0.08 at 290 K to 0.35 at 260 K. Time dependence of the uptake yielded values for the Henry's law constant. Uptake measurements at high pH indicate that an ammonia surface complex is formed at the interface. Codeposition studies in which an aqueous surface, initially at pH ) 4, was simultaneously exposed to both gas-phase ammonia and SO 2 were also performed. In such a codeposition experiment, the species entering the liquid neutralize each other and as a result the uptake of each species is enhanced. Modeling calculations indicate that the uptake of each species is in accord with bulk liquid-phase kinetics. IntroductionAmmonia in the atmosphere originates primarily from ground sources including decaying organic matter and chemical fertilizers. Significant amounts of NH 3 (0.1-100 ppbv) are found in both clean and polluted atmospheres as well as in cloud and fog droplets. 1 Since ammonia is the only soluble base found in the atmosphere in significant quantities, it plays a principal role in neutralizing acidic aerosols (H 2 SO 4 , HNO 3 , and HCl) converting them to new nonvolatile or semivolatile components; (NH 4 ) 2 SO 4 , NH 4 HSO 4 , NH 4 NO 3 , NH 4 Cl. 2 The process of neutralization influences the aqueous oxidation rates of S(IV) species. A recent study by Meng et al. 3 found that atmospheric ammonia is an important precursor for aerosol formation in the Los Angeles area.Gas-phase reactions involving NH 3 are slow. 4 Tropospheric lifetime for reaction with OH radical for example, is typically about 3 months, and tropospheric photolysis is negligible. 5 Therefore, uptake by aerosols and liquid droplets is the principal tropospheric sink for gaseous ammonia and heterogeneous interactions of NH 3 are of significant interest to atmospheric chemists.The uptake of gas phase ammonia by water has been previously studied in a limited range of acidities by Ponche et al. 6 at 17°C, and Bongartz et al. 7 at 25°C. We have completed a series of NH 3 -liquid water and the NH 3 -sulfuric acid uptake measurements in two independent studies using separate droplet train apparatuses. The water studies were done as a function of pH (0-13) and temperature in the range 20°C to -10°C. The sulfuric acid studies were done in the range 10 to 70 wt % H 2 -SO 4 and as a function of temperature in the range 20°C to -25°C. The time dependence of the uptake was measured by varying the gas-liquid interaction time from 2 to 15 ms. Uptake measurements yielded values of the mass accommodation coefficient (R) and provided information about interactions of

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Abstract. The heterogeneous reactions C1ONO 2 + H20 • HOC1 + HNO3 (1), C1ONO2 + HC1 • C12 + HNO3 (2), and HOC1 + HC1 • C12 + H20 (3)on stratospheric aerosols convert C1ONO2 and HC1 to photo-labile species, producing reactive C1 and C10 which are responsible for catalyzing ozone destruction in the lower stratosphere. The extent of the resulting ozone loss mirrors the steep negative temperature dependence of these reactions, which strongly depend on the solubility of C1ONO2, HC1, and HOC1, and on the activity of H20. Predicting the effect of these heterogeneous processes throughout the stratosphere requires detailed modeling of liquid phase solubility, diffusion, and reaction kinetics. A series of recent experiments from a number of laboratories have refined measurements of liquid diffusion coefficients, HC1 and HOC1 solubilities, and the reactivity of C1ONO 2 + H20, C1ONO2 + HC1 and HC1 + HOC1 on liquid films, droplets, and aerosols. On the basis of those measurements we present a phenomenological uptake model in which parameterizations of C1ONO2, HC1, and HOC1 heterogeneous kinetics appropriate for stratospheric H2SO4/H20 aerosols are addressed. In this model we suggest that under high acid concentration conditions both HOC1 and C1ONO2 are protonated before they react with HC1. Data for all three reactions in concentrated H2SO 4 solution indicate an acid-catalyzed reaction channel, which had previously been inferred for C1ONO2 hydrolysis. This updated parameterization is most significant at relatively high temperatures above 205 K which produce H2SO 4 aerosols of >60 acid wt%, where the acid-catalyzed reaction channels dominate. The comparisons between our new formulation and other recent formulations are presented. Since most stratospheric aerosols are primarily sulfuric acid, detailed knowledge of the processes governing these heterogeneous reactions over the stratospherically relevant acid concentration ranges of--•40 to 80 wt% H2SO 4 is required to understand stratospheric ozone chemistry. These reaction probabilities are strongly dependent on the temperature and water vapor partial pressure, which in turn determine the H2SO4/H20 condensed phase concentration ratio and solubility of C1ONO2, HOC1, and HC1. A number of research groups using complementary experimental techniques have measured these reaction probabilities on sulfuric acid solutions and aero-

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Abstract. A droplet train apparatus has been used to measure the heterogeneous reactive uptake of gaseous N20 s and C1ONO2 by concentrated sulfuric acid solutions. H2SO 4 concentrations in the range of 39 to 69 wt % were investigated between 229 and 260 K. Uptake rates normalized to the gas-liquid collision frequency, 3/0, for N2Os ranged from 0.086 to 0.16, decreasing moderately with increasing temperature and decreasing H2SO 4 concentration. Uptake rates for C1ONO2, measured over a slightly narrower concentration range of 39-59 wt % H2SO4, ranged between 0.0037 and 0.056, decreasing moderately with H2SO 4 temperature but significantly with increasing concentration. Results are compared with measurements from other laboratories using different experimental techniques. In general, the data from the different groups agree well. A phenomenological model is presented which addresses the solubility, diffusion, and chemical reactivity of XNO3 (X = C1, NO2) in sulfuric acid solutions and accounts for the dependence of the observed uptake rates on H2SO 4 concentration and temperature. Two XNO3 hydrolysis pathways are proposed, one involving direct reaction with H20 and the other involving participation of H + ions to promote bond dissociation. Differences between the concentration dependencies of 3/0 for C1ONO2 and N2Os can be ascribed largely to different rates of acid-catalyzed hydrolysis. The implications of these results for the effects of lower stratospheric sulfuric acid aerosols on ozone depletion chemistry are discussed.

The uptake of gas-phase ammonia by sulfuric acid surfaces was measured as a function of temperature (248−288 K), gas−liquid interaction time (2−15 ms), and acid concentration (20−70 wt % H2SO4) using a droplet train apparatus. The uptake coefficient increases as a function of acid concentration and reaches unity at about 55 wt % H2SO4. The increased NH3 uptake in acid solution is apparently due to reaction between NH3 and H+ at the gas−liquid interface. The results yielded parameters required to model the reaction of NH3 with H+ at the gas−liquid interface. These uptake experiments were expanded to include a detailed study of gas transport to a moving train of droplets. An analysis of previous sulfuric acid aerosol neutralization experiments shows that the uptake of ammonia by ternary NH3−H2SO4−H2O solutions is significantly lower than that by fresh binary H2SO4−H2O solutions. At typical tropospheric water and ammonia vapor concentrations, NH3 uptake coefficients need to be included in detailed microphysical models of sulfuric acid aerosols.

Isotope exchange for deuterated gas-phase acetic acid and ethanol in contact with water (H2O) droplets was studied using a droplet train apparatus. In these experiments, the gas-phase species interacts with liquid droplets and the loss of the species is monitored. The loss of the species may be due to the entry of the molecules into the bulk or to a reaction of the species at the gas−liquid interface, in this case isotope exchange. Studies were conducted as a function of pH in the range 0−14, droplet temperature in the range 291−263 K and gas−liquid interaction time in the range 2−15 ms. For deuterated acetic acid the isotope exchange probability with water molecules at the interface is near unity. On the other hand, isotope exchange probability for ethanol with surface water molecules at pH 7 is much smaller, ranging from 0.033 at 263 K to 0.051 at 291 K. Ethanol isotope exchange is both acid and base catalyzed. The exchange probability therefore increases both toward low and high pH and levels off to a plateau at pH 2 and 12, respectively. The maximum value of the isotope exchange probability at the plateau is significantly less than 1. It ranges between 0.14 and 0.18 with no clear trend in temperature. Results are explained in terms of a kinetic model in which it is assumed that the surface-adsorbed ethanol molecules are distributed between two distinct forms:  a weakly adsorbed state and a partially solvated state. Only the partially solvated molecules can interact with the near-surface ions in the interior of the liquid. A finite rate of entering the partially solvated state is responsible for the observed plateaus in isotope exchange at high and low pH. Parameters describing the gas uptake and isotope exchange processes are examined using two models to describe the surface species:  surface nucleation and Gibbs surface excess.