Computational Simulation of Secondary Organic Aerosol Formation in Laboratory Chambers

Species-specific effect of particle viscosity and particle-phase reactions on the formation of secondary organic aerosol

Secondary organic aerosol (SOA) is a major component of atmospheric fine particulate matter. Both particle viscosity and particle-phase chemistry play a crucial role in the formation and evolution of SOA; however, our understanding on how these two factors together with gas-phase chemistry collectively determine the formation of SOA is still limited. Here we developed a kinetic aerosol multilayer model coupled with gas-phase and particle-phase chemistry to simulate SOA formation. We take the atmospherically important α-pinene + OH oxidation system as an example application of the model. The simulations show that although the particle viscosity has negligible to small influences on the total SOA mass concentration, it strongly changes the concentration and distribution of individual compounds within the particle. This complicated effect of particle viscosity on SOA formation is a combined result of inhibited condensation or evaporation of specific organics due to slowed particle-phase diffusion. Furthermore, the particle-phase reactions alter the volatility and abundance of specific compounds and exacerbate their non-uniform distribution in highly viscous particles. Our results highlight an important species-specific effect of particle viscosity and particle-phase chemistry on SOA formation and demonstrate the capability of our model for quantifying such complicated effects on SOA formation and evolution.

Understanding the active role of water in laboratory chamber studies of reactions of the OH radical with alcohols of atmospheric relevance

In this work, we studied the reactions of three cyclic aliphatic alcohols with OH at room temperature, atmospheric pressure and different humidities in a Teflon reaction chamber. It was determined that the lower the solubility of the alcohol in water, the larger the effect of the humidity on the acceleration of the reaction. This experimental evidence allows suggesting that the acceleration is due to the reaction of the co-adsorbed reactants at the air-water interface of a thin water film deposited on the Teflon walls of the reaction chamber, instead of between co-reactants dissolved in the water film or due to gas phase catalysis as previously suggested. Therefore, formation of thin water films on different surfaces could have some implications on the tropospheric chemistry of these alcohols in the tropical regions of the planet with high humidity.

Vapors Are Lost to Walls, Not to Particles on the Wall: Artifact-Corrected Parameters from Chamber Experiments and Implications for Global Secondary Organic Aerosol

Atmospheric models of secondary organic aerosol (OA) (SOA) typically rely on parameters derived from environmental chambers. Chambers are subject to experimental artifacts, including losses of (1) particles to the walls (PWL), (2) vapors to the particles on the wall (V2PWL), and (3) vapors to the wall directly (VWL). We present a method for deriving artifact-corrected SOA parameters and translating these to volatility basis set (VBS) parameters for use in chemical transport models (CTMs). Our process involves combining a box model that accounts for chamber artifacts (Statistical Oxidation Model with a TwO-Moment Aerosol Sectional model (SOM-TOMAS)) with a pseudo-atmospheric simulation to develop VBS parameters that are fit across a range of OA mass concentrations. We found that VWL led to the highest percentage change in chamber SOA mass yields (high NO_x: 36-680%; low NO_x: 55-250%), followed by PWL (high NO_x: 8-39%; low NO_x: 10-37%), while the effects of V2PWL are negligible. In contrast to earlier work that assumed that V2PWL was a meaningful loss pathway, we show that V2PWL is an unimportant SOA loss pathway and can be ignored when analyzing chamber data. Using our updated VBS parameters, we found that not accounting for VWL may lead surface-level OA to be underestimated by 24% (0.25 μg m^-3) as a global average or up to 130% (9.0 μg m^-3) in regions of high biogenic or anthropogenic activity. Finally, we found that accurately accounting for PWL and VWL improves model-measurement agreement for fine mode aerosol mass concentrations (PM_2.5) in the GEOS-Chem model.

Effects of NO2 and RH on secondary organic aerosol formation and light absorption from OH oxidation of ο-xylene

Ο-xylene is an important aromatic volatile organic compound (VOC) in the atmosphere over urban areas. In this work, the effect of nitrogen dioxide (NO₂) concentration and relative humidity (RH) on the mass concentration of secondary organic aerosols (SOA) formed from ο-xylene OH oxidization was investigated in a photooxidation chamber. The ο-xylene SOA mass concentration increased from 54.2 μg m^-3 to 127.2 μg m^-3 during dry conditions, but decreased from 177.7 μg m^-3 to 146.5 μg m^-3 during high RH conditions when the initial NO₂ concentration increased form 0 ppbv to about 900 ppbv. An increase in the ratio of [NO₃^-]/[Org] and a decrease in the oxidation state of carbon (OS_C) of SOA suggested that acid-catalyzed heterogeneous reaction was responsible for enhancing SOA formation with increasing NO₂ concentrations in dry conditions. In contrast, in humid conditions, the high molecular diffusion capacity of SOA could promote the reactivity of OH towards the interior of SOA, and the enhancement of nitrous acid (HONO) formation under high NO₂ conditions could promote the SOA aging processes and be responsible for the decreasing trend of SOA formation with NO₂. Light absorption by SOA was also measured, and both NO₂ and RH enhanced the mass absorption coefficient (MAC_{λ = 365 nm}) value for the optical properties of ο-xylene SOA. The highest MAC_{λ = 365 nm} value of ο-xylene SOA was 0.89 m² g^-1, observed during humid conditions with an initial NO₂ concentration of 862 ppbv, which was 3.9 times higher than in the experiment conducted in the absence of NO₂ under dry conditions. The formation of nitrogen-containing organic compounds (NOCs) and humic-like substances (HULIS) were responsible for the increased MAC_{λ = 365 nm} values of ο-xylene derived SOA. This study provides new insight into the effect of NO₂ on SOA formation through the change in ο-xylene photooxidation under different RH conditions, and the complex effect of multiple environmental factors on SOA formation was also important and should not be ignored.

Cu-VWT Catalysts for Synergistic Elimination of NOx and Volatile Organic Compounds from Coal-Fired Flue Gas

A dual-function catalyst, designated as Cu5-VWT, has been constructed for the synergistic removal of NO_x and volatile organic compounds under complex coal-fired flue gas conditions. The removal of toluene, propylene, dichloromethane, and naphthalene all exceeded 99% (350 °C), and the catalyst could effectively block the generation of polycyclic aromatic hydrocarbons. Mechanistic studies have shown that Cu sites on the Cu5-VWT catalyst facilitate catalytic oxidation, while V sites facilitate NO_x reduction. Thus, toluene oxidation and NO_x reduction can proceed simultaneously. The removal of total hydrocarbons and nonmethane total hydrocarbons from 1200 m³·h^-1 real coal-fired flue gas by a monolithic catalyst were determined as 92 and 96%, respectively, much higher than those of 54 and 72% over a commercial VWT catalyst, indicating great promise for industrial application.

A review of secondary organic aerosols formation focusing on organosulfates and organic nitrates

Secondary organic aerosols (SOA) are crucial constitution of fine particulate matter (PM), which are mainly derived from photochemical oxidation products of primary organic matter and volatile organic compounds (VOCs), and can induce terrible impacts to human health, air quality and climate change. As we know, organosulfates (OSs) and organic nitrates (ON) are important contributors for SOA formation, which could be possibly produced through various pathways, resulting in extremely complex formation mechanism of SOA. Although plenty of research has been focused on the origins, spatial distribution and formation mechanisms of SOA, a comprehensive and systematic understanding of SOA formation in the atmosphere remains to be detailed explored, especially the most important OSs and ON dedications. Thus, in this review, we systematically summarize the recent research about origins and formation mechanisms of OSs and ON, and especially focus on their contribution to SOA, so as to have a clearer understanding of the origin, spatial distribution and formation principle of SOA. Importantly, we interpret the complex interaction with coexistence effect of SO_x and NO_x on SOA formation, and emphasize the future insights for SOA research to expect a more comprehensive theory and practice to alleviate SOA burden.

Reactions of C12-C14 n-Alkylcyclohexanes with Cl Atoms: Kinetics and Secondary Organic Aerosol Formation

Long-chain alkanes are a type of intermediate volatility organic compound (IVOC) in the atmosphere and a potential source of secondary organic aerosols (SOAs). C₁₂-C₁₄ n-alkylcyclohexanes are important compositions of IVOCs, with considerable concentrations and emission rates. The reaction rate constants and SOA formation of the reactions of C₁₂-C₁₄ n-alkylcyclohexanes with Cl atoms were investigated in the present study. The reaction rate constants of the long-chain alkanes obtained via the relative-rate method at 298 ± 0.2 K (in units of ×10^-10 cm³ molecule^-1 s^-1) were as follows: k_{hexylcyclohexane} = 5.11 ± 0.28, k_{heptylcyclohexane} = 5.56 ± 0.30, and k_{octylcyclohexane} = 5.74 ± 0.31. The gas-phase products of the reactions were identified as mainly small molecules of aldehydes, ketones, and acids. The particle-phase products were mostly monomers and oligomers, but there were still trimers even under high-NO_x conditions. Moreover, under high-NO_x conditions (urban atmosphere), the SOA yields of hexylcyclohexane are higher than that under low-NO_x conditions (remote atmosphere), indicating that more attention should be given to the SOA formation of Cl-initiated n-alkylcyclohexane oxidations in polluted regions. This research can further clarify the oxidation processes and SOA formation of n-alkylcyclohexanes in the atmosphere.

Effect of relative humidity on SOA formation from aromatic hydrocarbons: Implications from the evolution of gas- and particle-phase species

Relative humidity (RH) plays a significant role in secondary organic aerosol (SOA) formation, but the mechanisms remain uncertain. Using a 30 m³ indoor smog chamber, the influences of RH on SOA formation from two conventional anthropogenic aromatics (toluene and m-xylene) were investigated from the perspective of both the gas- and particle- phases based on the analysis of multi-generation gas-phase products and the chemical composition of SOA, which clearly distinguishes from many previous works mainly focused on the particle-phase. Compared to experiments with RH of 2.0%, SOA yields increased by 11.1%-133.4% and 4.0%-64.5% with higher RH (30.0%-90.0%) for toluene and m-xylene, respectively. The maximum SOA concentration always appeared at 50.0% RH, which is consistent with the change trend of SOA concentration with RH in the summertime field observation. The most plausible reason is that the highest gas-phase OH concentration was observed at 50.0% RH, when the increases in gas-phase OH formation and OH uptake to aerosols and chamber walls with increasing RH reached a balance. The maximum OH concentration was accompanied by a notable decay of second-generation products and formation of third-generation products at 50.0% RH. With further increasing RH, more second-generation products with insufficient oxidation degree will be partitioned into the aerosol phase, and the aqueous-phase oxidation process will also be promoted due to the enhanced uptake of OH. These processes concurrently caused the O/C and oxidation state of carbon (OSc) to first increase and then slightly decrease. This work revealed the complex influence of RH on SOA formation from aromatic VOCs through affecting the OH concentration, partitioning of advanced gas-phase oxidation products as well as aqueous-phase oxidation processes. Quantitative studies to elucidate the role of RH in the partitioning of oxidation products should be conducted to further clarify the mechanism of the influence of RH on SOA formation.

The oxidation mechanism of gas-phase ozonolysis of limonene in the atmosphere

Limonene with endo- and exo-double bonds is a significant monoterpene in the atmosphere and has high reactivity towards O3. We investigated the atmospheric oxidation mechanism of limonene ozonolysis using a high level quantum chemistry calculation coupled with RRKM-ME kinetic simulation. The additions of O3 can take place at both the endo- and exo-double bonds with a branching ratio of 0.87 : 0.13, forming four major highly energized CIs* (named Syn-2a*, Syn-2b*, Anti-2b* and Anti-2c*) with the relative higher fractions of 0.21 : 0.35 : 0.27 : 0.11. A yield of 4% for Limona-ketone was obtained as well. For the unimolecular isomerization pathways of limonene + O3 → POZs → CIs* → SOZ, VHP, or dioxirane, five, one, or none of the internal rotations are treated as hindered internal rotors for CIs*. We obtained percentages of 0.59 : 0.18 : 0.14 in total for separate isomerization routes in the formation of VHPs, dioxirane and SOZs from CIs* using the fourth-order Runge-Kutta method. Additionally, a yield of ∼5% was acquired for stabilized CIs compiling the fractions of different addition routes. About ∼10% of stabilized Anti-2b would isomerize to VHP and 90% would isomerize to SOZs. Isomerization to VHPs dominates the fate of stabilized Syn-2a, Syn-2b and Anti-2c. The overall yield of OH radicals was 0.61. Our study suggested a yield of 0.17 for stabilized SOZs and 0.18 for dioxirane, although both their fates are ambiguous.