Modeling heterogeneous sulphate production in maritime stratiform clouds

Abstract

Abstract. A size-resolved droplet physico chemical model linked to a Lagrangian parcel framework was developed and used to study the heterogeneous sulphate production process in marine stratiform clouds. This model study provides a rigorous approach to cloud chemistry processing by treating the effects of non ideal chemistry on both aqueous phase equilibration and droplet growth using the Pitzer method to overcome many of the shortcomings of alternative thermodynamic methods. In addition, the current study uses a multi modal representation of seasalt aerosol, rather than the single mode used in most previous work. The results of the model simulations showed a distribution of sulphate production across the aerosol size range that was non linear and that a significant fraction (=75-90%) of the sulphate was produced in droplets formed on sea-salt aerosol particles. The number of sea-salt particles affected both the total amount of sulphate produced and the fraction produced on cloud droplets formed around non-sea-salt sulphate aerosol. Dissolved ozone and hydrogen peroxide were both important as oxidant species, although the ozone-driven oxidation pathway was dominant in droplets formed on sea-salt aerosol particles. The rate of sulphate production was found to be a non linear function of the gaseous sulphur dioxide concentration at cloud base and only very weakly dependent on sulphate nuclei concentration. A comparison of sulphate production in cloud and below cloud indicates that sulphate production in sea-salt nuclei is not limited by the carbonate buffer and that in-cloud production can exceed, many times, that of cloud-free production due to the "virtual" buffering capacity of the cloud. The use of non ideal solution effects appear important, particularly for clean-to-moderately polluted SO2 concentrations (< 500 parts per trillion) and when sea-salt nuclei are present, resulting in at least 40% lower sulphate production when compared to ideal solution simulations.

Closure evaluation of size‐resolved aerosol pH in the New England coastal atmosphere during summer

“…For the sea‐salt‐dominated size fractions (≥2.4 μm), deviations between concentrations and activities at RHs greater than 80% (23 of 32 samples) are generally small [e.g., Chameides and Stelson , 1992] and have only minor influences on estimated pHs (typically <0.2 pH unit [ Keene et al , 2002]). For sea salt at lower RHs and for smaller aerosol size fractions under all conditions, activities and corresponding concentrations for some species (H + and SO 4 2− in particular) diverge substantially [e.g., Kim et al , 1993; Sander and Crutzen , 1996; O'Dowd et al , 2000]. Consequently, our estimates of the ambient pHs for these size fractions and conditions are considered qualitative.…”

Ocean–Atmosphere Interactions of Particles

“…Detailed cloud parcel modelling studies of the heterogeneous oxidation of SO 2 to aerosol sulphate were conducted by O'Dowd et al (2000) both for activated droplets and un-activated haze particles. The simulations showed that dissolved ozone and hydrogen peroxide were the dominant oxidants, with the ozone oxidation pathway dominant on sea salt Gantt et al 2011) activated droplets.…”

Statistical evaluation of aerosol data from Ben Macdhui mountain, South Africa

“…Two intensive sampling campaigns were designed for the summer (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) March) and winter (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). The aim of these field campaigns was to sample atmospheric particulate matter at high flow rates and extended sampling intervals periods (12 to 24 hours) and to subject the samples to an array of analytical techniques to obtain a comprehensive suite of chemical components to better understand of the nature of the impacting sources and transport mechanisms.…”

“…The importance of sulphuric acid in aerosol formation in a humid environment has been emphasized in a number of studies. [7,8] It is clear that the understanding of the transformation of sulphur dioxide in the atmosphere is not completely understood. The formation of the dominant atmospheric sulphate compound, ammonium sulphate, poses some interesting questions about the main pathway of SO 2 oxidation in the atmosphere.…”