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Tropospheric deliquesced particles are characterised by concentrated non-ideal solutions ("aerosol liquid water" or ALW) that can affect the occurring multiphase chemistry. However, such non-ideal solution effects have generally not yet been considered in and investigated by current complex multiphase chemistry models in an adequate way. Therefore, the present study aims at accessing the impact of non-ideality on multiphase chemical processing in concentrated aqueous aerosols. Simulations with the multiphase chemistry model (SPACCIM-SpactMod) are performed under different environmental and microphysical conditions with and without a treatment of non-ideal solutions in order to assess its impact on aqueous-phase chemical processing. The present study shows that activity coefficients of inorganic ions are often below unity under 90 % RH-deliquesced aerosol conditions and that most uncharged organic compounds exhibit activity coefficient values of around or even above unity. Due to this behaviour, model studies have revealed that the inclusion of non-ideality considerably affects the multiphase chemical processing of transition metal ions (TMIs), oxidants, and related chemical subsystems such as organic chemistry. In detail, both the chemical formation and oxidation rates of Fe(II) are substantially lowered by a factor of 2.8 in the non-ideal base case compared to the ideal case. The reduced Fe(II) processing in the non-ideal base case, including lowered chemical rates of the Fenton reaction (- 70 %), leads to a reduced processing of HOx/HOy under deliquesced aerosol conditions. Consequently, higher multiphase H2O2 concentrations (larger by a factor of 3.1) and lower aqueous-phase OH concentrations (lower by a factor of ≈4) are modelled during non-cloud periods. For H2O2 , a comparison of the chemical reaction rates reveals that the most important sink, the reaction with HSO3- , contributes with a 40 % higher rate in the non-ideal base case than in the ideal case, leading to more efficient sulfate formation. On the other hand, the chemical formation rates of the OH radical are about 50 % lower in the non-ideal base case than in the ideal case, leading to lower degradation rates of organic aerosol components. Thus, considering non-ideality influences the chemical processing and the concentrations of organic compounds under deliquesced particle conditions in a compound-specific manner. For example, the reduced oxidation budget under deliquesced particle conditions leads to both increased and decreased concentration levels, e.g. of important C2/C3 carboxylic acids. For oxalic acid, the present study demonstrates that the non-ideality treatment enables more realistic predictions of high oxalate concentrations than observed under ambient highly polluted conditions. Furthermore, the simulations imply that lower humidity conditions, i.e. more concentrated solutions, might promote higher oxalic acid concentration levels in aqueous aerosols due to differently affected formation and degradation processes. [ABSTRACT FROM AUTHOR]

Despite the high abundance of secondary aerosols in the atmosphere, their formation mechanisms remain poorly understood. In this study, the Master Chemical Mechanism (MCM) and the Chemical Aqueous-Phase Radical Mechanism (CAPRAM) are used to investigate the multiphase formation and processing of secondary aerosol constituents during the advection of air masses towards the measurement site of Mt. Tai in northern China. Trajectories with and without chemical–cloud interaction are modeled. Modeled radical and non-radical concentrations demonstrate that the summit of Mt. Tai, with an altitude of ∼1.5 km a.m.s.l., is characterized by a suburban oxidants budget. The modeled maximum gas-phase concentrations of the OH radical are 3.2×106 and 3.5×106 molec. cm -3 in simulations with and without cloud passages in the air parcel, respectively. In contrast with previous studies at Mt. Tai, this study has modeled chemical formation processes of secondary aerosol constituents under day vs. night and cloud vs. non-cloud cases along the trajectories towards Mt. Tai in detail. The model studies show that sulfate is mainly produced in simulations where the air parcel is influenced by cloud chemistry. Under the simulated conditions, the aqueous reaction of HSO3- with H2O2 is the major contributor to sulfate formation, contributing 67 % and 60 % in the simulations with cloud and non-cloud passages, respectively. The modeled nitrate formation is higher at nighttime than during daytime. The major pathway is aqueous-phase N2O5 hydrolysis, with a contribution of 72 % when cloud passages are considered and 70 % when they are not. Secondary organic aerosol (SOA) compounds, e.g., glyoxylic, oxalic, pyruvic and malonic acid, are found to be mostly produced from the aqueous oxidations of hydrated glyoxal, hydrated glyoxylic acid, nitro-2-oxopropanoate and hydrated 3-oxopropanoic acid, respectively. Sensitivity studies reveal that gaseous volatile organic compound (VOC) emissions have a huge impact on the concentrations of modeled secondary aerosol compounds. Increasing the VOC emissions by a factor of 2 leads to linearly increased concentrations of the corresponding SOA compounds. Studies using the relative incremental reactivity (RIR) method have identified isoprene, 1,3-butadiene and toluene as the key precursors for glyoxylic and oxalic acid, but only isoprene is found to be a key precursor for pyruvic acid. Additionally, the model investigations demonstrate that an increased aerosol partitioning of glyoxal can play an important role in the aqueous-phase formation of glyoxylic and oxalic acid. Overall, the present study is the first that provides more detailed insights in the formation pathways of secondary aerosol constituents at Mt. Tai and clearly emphasizes the importance of aqueous-phase chemical processes on the production of multifunctional carboxylic acids. [ABSTRACT FROM AUTHOR]

Mechanism reductions of the detailed aqueous phase chemistry mechanism CAPRAM 3.0i are performed. Manual methods and automatic techniques are both applied in order to provide a less computationally intensive mechanism which is operational in regional chemistry transport models (CTMs). The finally reduced mechanism contains less than 200 reactions (4 times smaller than the detailed CAPRAM 3.0i) and describes the main characteristics of inorganic and organic aqueous phase processes occurring in tropospheric warm clouds. Most of the chemical reduction potential is realized in the CAPRAM 3.0i organic chemistry. The number of aqueous phase species decreases from 380 in the full mechanism to 130 in the final reduced version. The calculated percentage deviations between the full and reduced mechanism are on average below 5% for the most important organic and inorganic target compounds such as oxidants, inorganic and organic acids, carbonyls and alcohols. Comparisons of the required CPU times between the full and reduced mechanisms show reductions of approximately 40%. 2-D test simulations with the CTM MUSCAT were performed using prescribed meteorological conditions in order to examine the applicability of the reduced mechanism at regional scale. Simulations with the reduced CAPRAM 3.0i mechanism and a much less complex mechanism with only limited inorganic chemistry (INORG) were compared to evaluate the effects of more detailed chemistry. The model results show large differences in the level of oxidants and the inorganic and organic mass processing. Prospectively, the reduced mechanism represents the basis for studying aerosol cloud processing effects at regional scale with future CTMs and will allow more adequate interpretation of field data. [ABSTRACT FROM AUTHOR]

Coastal regions cover nearly 22% of Earth’s continental surface area and give living space formore than 39% of world’s population with an urbanization rate of 46% (Kummu et al., 2016).Overall, coastal regions are characterised by strong interactions between the marine andcontinental environment. Thus, in polluted coastal areas, where clean marine air masses mixwith anthropogenic air masses, halogen atoms can have a significant impact on air quality(Muniz-Unamunzaga et al., 2018; von Glasow et al., 2013). The marine atmosphereis highly influenced by halogen chemistry, whereas the continental atmosphereis highly influenced by NOx chemistry. Hence, the formation of halogen nitratesand their subsequent multiphase chemistry can be a crucial tropospheric chemistryaspect. In this study, detailed halogen multiphase chemistry simulations are carried outinvestigating the importance of the interaction of halogen radicals with NOx forming halogennitrates on the activation of chlorine atoms. For this purpose, two meteorological scenariosimulations are performed, one with cloud occurrence and one without. The simulationsimply that especially the hydrolysis of iodine nitrate within aerosols leads to an enhancedchlorine atom activation by ICl photolysis. Whereas ClNO2 photolysis has largercontributions to chlorine activation in the morning, the ICl photolysis dominates chlorineactivation at afternoon. Overall, the average contributions to chlorine atom activation in thecloud and cloud-free scenarios by ClNO2photolysis are 42 % and 62 % and byICl photolysis 35 % and 28 %, respectively. Therefore, the simulations implythe hydrolysis of iodine nitrates affects the atmospheric oxidation capacity, VOCoxidation, and ozone formation potential and has to be considered in chemical transportmodels. References: Kummu, M., de Moel, H., Salvucci, G., Viviroli, D., Ward, P. J., and Varis, O.: Over thehills and further away from coast: global geospatial patterns of human and environment overthe 20th–21st centuries, Environ. Res. Lett., 11, 10.1088/1748-9326/11/3/034010,2016. Muniz-Unamunzaga, M., Borge, R., Sarwar, G., Gantt, B., de la Paz, D., Cuevas, C. A.,and Saiz-Lopez, A.: The influence of ocean halogen and sulfur emissions in the air quality ofa coastal megacity: The case of Los Angeles, Sci. Total Environ., 610-611, 1536-1545,10.1016/j.scitotenv.2017.06.098, 2018. von Glasow, R., Jickells, T. D., Baklanov, A., Carmichael, G. R., Church, T. M., Gallardo,L., Hughes, C., Kanakidou, M., Liss, P. S., Mee, L., Raine, R., Ramachandran, P., Ramesh,R., Sundseth, K., Tsunogai, U., Uematsu, M., and Zhu, T.: Megacities and largeurban agglomerations in the coastal zone: interactions between atmosphere, land,and marine ecosystems, Ambio, 42, 13-28, 10.1007/s13280-012-0343-9, 2013. [ABSTRACT FROM AUTHOR]

The article focuses on the bulk tropospheric aqueous-phase chemistry along with its kinetics and mechanisms. Topics discussed include introduction to aqueous-phase chemistry, aqueous aerosol chamber studies, use of electrospray ionization mass spectrometry (ESI-MS) as analytical technique for tropospheric aqueous-phase chemistry studies, comparison among aqueous aerosol, cloud and fog chemistry and kinetics of dehydration reactions in alkyl radical reformation.