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<p> Nitrous acid (HONO) is a photochemical source of hydroxyl radical that plays an important role in initiating radical reactions leading to photochemical air pollution and aerosol formation in the lower atmosphere. Field studies suggest nocturnal and daytime HONO sources that stem from heterogeneous reactions on environmental surfaces. However, the mechanisms are not well-understood and atmospheric models under-predict daytime HONO levels by 30&ndash;50%. The main aim of this research is to examine proposed nocturnal and daytime sources with a focus on elucidating the mechanisms of HONO formation and understanding the variables controlling this important process. Results from four projects are presented. The first project focuses on aqueous nitrate (₃^&ndash;) photolysis and the role that nitrogen dioxide (NO₂) hydrolysis plays in the formation process. The second involves linking the flux of HONO from agricultural and urban soil samples to ammonia oxidizing bacteria and archaea. The third project elucidates the mechanism of NO₂ reduction on surfaces containing humic acids (HA), specifically focusing on quinone and hydroquinone redox chemistry. The fourth project seeks to explain how nitrate photolysis on HA or iron surfaces can be an important daytime source of HONO. We suggest that NO₃^&ndash; photolysis in the presence of chromophoric compounds and OH radical scavengers, which are naturally present on the environment surfaces, help explain the higher than expected daytime HONO levels observed during recent field studies. </p>

Brine and disordered interfacial layers (DILs) on ice are known to significantly alter the interaction between trace gases and ice surfaces. In order to better predict the impact of a warming climate on atmospheric composition, a quantitative physical understanding of the interaction between ice surfaces and key atmospheric trace gases is essential. In the series of complementary studies presented in this dissertation, theoretical and experimental techniques were employed to gain insight into the phenomenon of interfacial layers. A brine layer model developed based on equilibrium thermodynamics is the first model to have the unique capability of taking into account the non-ideality of the brine, as well as incorporate equilibrium partitioning among the gas, brine, and ice phases. The model was applied to study ice systems containing NaCl, HNO_3, and HCl. We found that for all three systems, accounting for the non-ideal behavior of the brine was important, especially at low temperatures. Our findings also suggest that gas phase HNO_3 and HCl can induce the formation of brine layers on ice at temperatures close to the bulk melting point. In addition, we identified environmentally relevant regimes where brine is not predicted to exist, but the presence of DILs can still significantly impact air-ice chemical reactions. Next we examined the interaction of HNO_3 with water ice for partial pressures 2×10^-8 Torr to 1×10^-5 Torr and at temperatures from 216 to 256 K using (i) the surface-specific technique ellipsometry and (ii) a coated wall flow tube (CWFT) reactor, both coupled with chemical ionization mass spectrometry (CIMS) detection of HNO_3 in the gas phase. Our ellipsometry results show that exposure to HNO_3 induces surface disordering on ice at a range of environmentally relevant temperatures and HNO_3 partial pressures, particularly in the vicinity of the boundary between the ice and the HNO_3·3H_2O phases. The coated wall flow tube studies indicate that the nature of HNO_3 uptake changes from reversible adsorption to a continuous flux of HNO_3 into the bulk in the presence of a disordered interfacial layer. These results have implications for atmospheric chemistry in the upper troposphere and in Polar regions. Ellipsometry and CWFT-CIMS techniques were also used to study HCHO and CH_3CHO in partial pressure and temperature conditions akin to polar snowpack interstitial space. Although these light aldehydes do not interact as strongly with the ice surface as does HNO_3, they were also found to induce surface disorder, especially at low temperatures. HCHO and CH_3CHO were also observed to cause the formation of opaque domains. For HCHO, the opaque domains were associated with high solute loading on the surface combined with temperature cycling. CWFT-CIMS studies of HCHO uptake by ice, show a hint of hydrate formation signature, though the result is not clear enough to be conclusive. Acetaldehyde ellipsometry studies revealed a hitherto unknown DIL surface transition at around 223±2 K, in agreement with earlier findings that partitioning behavior of CH_3CHO is different above and below this transition temperature. Our findings regarding ice surface modification by HCHO and CH_3CHO alter the typical view of simple Langmuir adsorption and suggest that more complex interactions are occurring at the air-ice interface that can potentially impact exchange of trace gases between snow and the polar boundary layer.

The chemistry and dynamics of our atmosphere are complex and diverse. A plethora of different chemical reactions are thought to be important, of which only some are known, and even less are well understood. Many of these atmospheric reactions involve highly reactive or unstable trace species, which can be difficult to study experimentally. However, the advancement of computational methods and hardware now make it possible to investigate these chemical species theoretically to an accuracy that is useful for atmospheric chemistry. In this thesis, we have applied modern electronic structure methods to some prototypical hydrogen bonded complexes and sulfonic acid derivatives to better understand the roles of these trace species in the atmosphere. We have calculated fundamental and overtone vibrational spectra, electronic absorption spectra, and reaction energetics with high level ab initio methods. Where possible, we compare our calculated results to experiment and in the absence of experimental data we suggest that our theoretical findings may be of use to atmospheric modelers.

Thesis (M.S.)--Oregon Graduate Center, 1984.

M.S. / Environmental Science / A continuous real-time in-situ ammonia monitor has been developed. It utilizes the reaction between ammonia gas and sulfuric acid aerosol to form ammonium sulfate for ammonia measurement. A sensitivity-improved Flame Photometric Detector (FPD) is used with the instrument. The detection limit of the monitor for measuring ammonia is 0.4 ppb NH3 with a time resolution of five minutes. The detection limit of the FPD for measuring sulfur has been improved by a factor greater than ten. The present detection limit of the FPD is 0.07 ppb S, which is an improvement over the previous detection limit of 1 ppb S.

Kinetics of free-radical reactions with monoterpenes in the aqueous phase mimicing atmospheric aerosol chemistry

Thesis (Ph. D.)--York University, 1998. Graduate Programme in Chemistry. / Typescript. Includes bibliographical references (leaves 245-251). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pNQ39305.

<p> A sectional aerosol model (CARMA) has been developed and coupled with the Community Earth System Model (CESM1). Aerosol microphysics, radiative properties and interactions with clouds are simulated. The model described here uses 20 particle size bins for each aerosol component including freshly nucleated sulfate particles, as well as mixed particles containing sulfate, primary organics, black carbon, dust and sea salt. In this thesis, CESM1/CARMA is firstly constrained by a variety of observations, and then utilized to investigate several scientific topics including aerosol layers in the upper troposphere and lower stratosphere as well as forest fire smoke in the lower troposphere.</p><p> Recent studies reveal layers of enhanced aerosol scattering in the upper troposphere and lower stratosphere over Asia (ATAL) and North America (NATAL). The observed aerosol extinction enhancement is reproduced by CESM1/CARMA. Both model and observations indicate a strong gradient of the sulfur-to-carbon ratio from Europe to the Asia on constant pressure surfaces. We found that the ATAL is mostly composed of sulfates, surface-emitted organics and secondary organics; the NATAL is mostly composed of sulfates and secondary organics. The model also suggests emission increases in Asia between 2000 and 2010 led to an increase of aerosol optical depth of the ATAL by 0.002 on average, which is consistent with observations.</p><p> The Rim Fire of 2013, the third largest area burned by fire recorded in California history, is simulated by CESM1/CARMA. Modeled aerosol mass, number, effective radius, and extinction coefficient are within variability of data obtained from multiple airborne measurements. Simulations suggest Rim Fire smoke may block 4-6% of sunlight reaching the surface, with a cooling efficiency around 120-150 W m^-2 per unit aerosol optical depth. This study shows that exceptional events like the 2013 Rim Fire can be simulated by a climate model with one-degree resolution, though that resolution is still not sufficient to resolve the smoke peak near the source region.</p>

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