Treatment of non-ideality in the SPACCIM multiphase model – Part 2: Impacts on the multiphase chemical processing in deliquesced aerosol particles

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.

Basicity and acidity promote hydrolysis of methyl nitrate in aqueous aerosols

<p>The chemistry of organic nitrates (ONs), also known as alkyl nitrates (RONO<sub>2</sub>), controls the lifetime of nitrogen oxides in continental areas, which in turn affects air quality and varies ozone concentration throughout the troposphere. ONs can be emitted to the troposphere from marine sources. Also, they can be produced in the atmosphere through addition of NO to peroxy radicals or through the reaction of NO<sub>3</sub> radicals with volatile organic compounds. Atmospheric ONs may subsequently undergo oxidation or photolysis, in both gas and aerosol phases, or hydrolysis in aqueous aerosols. Though some recent studies have believed acid-catalysis promotes hydrolysis of ONs, earlier studies have claimed that acids have no effect on ON hydrolysis, and that it is the hydroxyl ion that can improve the hydrolysis process. The limited number of experimental studies performed so far have left this conflict with no appropriate answer, as mechanistic insight and full kinetics details have been partially or completely missing for the studied ONs. We report the detailed mechanism of methyl nitrate hydrolysis in acidic, neutral and basic conditions, in addition to analyzing the degradation of methyl nitrate into formaldehyde and nitrous acid in the presence of water and hydronium ions. According to the potential energy surfaces obtained at the CCSD(T)/cc-pVDZ//ωB97X-D/def2-TZVP level of theory (including the SMD solvent model) along with the rate coefficients estimated using asymmetric Eckart tunneling-corrected transition state theory (TST), mediation of water molecules and hydronium ions hinders degradation of methyl nitrate into formaldehyde and nitrous acid and, in general, this decomposition reaction is kinetically unfavorable. Furthermore, neutral hydrolysis of methyl nitrate is extremely slow with pseudo-first order rate coefficients (k; 298 K and 1 atm) falling below 10<sup>-27</sup> s<sup>-1</sup>. Similarly, hydrolysis of methyl nitrate by hydronium ions is observed to be extremely slow (k < 10<sup>-27</sup> s<sup>-1</sup>). However, under acidic conditions, protonation of methyl nitrate is quite feasible with the protonation Gibbs free energy of -429.1 kJ mol<sup>-1</sup>, at 298 K and 1 atm, and protonated methyl nitrate can hydrolyze into protonated methanol and nitric acid much faster relative to the hydronium ion-based and neutral hydrolysis (k = 3.83 s<sup>-1</sup>). On the other hand, the hydroxyl ions generated under basic conditions can hydrolyze methyl nitrate readily to give methanol and nitric acid (k = 6.63 × 10<sup>3</sup> s<sup>-1</sup>), or formaldehyde, nitrate and water (k = 9.40 × 10<sup>6</sup> s<sup>-1</sup>). In addition, regardless of the limitation on the rate of solvent-phase chemical reactions by the rate of diffusion, basic hydrolysis can produce methoxy ions and nitric acid quite fast (k = 8.95 × 10<sup>9</sup> s<sup>-1</sup>). In other words, methyl nitrate hydrolysis is faster in basic aerosols (i.e. some marine aerosols) and, to a less extent, in highly acidic aqueous aerosols (e.g. haze and urban aerosols).       </p>

Treatment of non-ideality in the multiphase model SPACCIM-Part2: Impacts on the multiphase chemical processing in deliquesced aerosol particles

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 in 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 fluxes 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 fluxes 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 fluxes reveals that the most important sink, the reaction with HSO3−, contributes with a 40 % higher flux in the non-ideal base case than in the ideal case, leading to more efficient sulfate formation. On the other hand, the chemical fluxes of the OH radical are about 50 % lower in the non-ideal base case than in the ideal case, including lower degradation fluxes 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 implicate 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.