Impact of acidity and surface modulated acid dissociation on cloud response to organic aerosol
Abstract. Dissociation of organic acids is currently not included in most atmospheric aerosol models. Organic dissociation in aqueous aerosols could alter the H+ concentrations and affect the cloud activating properties. We implemented a simple representation of organic dissociation in a box model version of the aerosol–chemistry–climate model ECHAM-HAMMOZ and investigated the impact on cloud droplet number concentrations and short-wave radiative effect through changes in kinetically driven sulfate concentrations in an aerosol population. Organic dissociation has been observed in X-ray photo-electron spectroscopy measurements to be significantly suppressed in the aqueous surface. We therefore additionally introduced an empirical account of this mechanism to explore the potential further impact on aerosol effects. Malonic acid and decanoic acid were used as proxies for atmospheric organic acid aerosols. Both acids were found to yield sufficient hydrogen ion concentrations from dissociation in an aqueous droplet population to strongly influence the sulfur chemistry, leading to enhanced cloud droplet number concentrations and a cooling short-wave radiative effect. Further considering surface modulated suppressed dissociation, the impact on cloud microphysics was smaller than according to the well-known bulk solution organic dissociation, but still significant. Our results show that organic aerosol acidity can significantly influence predictions of aerosol formation and aerosol-cloud-climate effects. Furthermore, it may be important to also consider the specific influence of surface effects, also in relation to bulk solution phenomena such as organic acid dissociation.
Gargi Sengupta et al.
Status: open (extended)
- RC1: 'Comment on egusphere-2023-438', Anonymous Referee #1, 24 May 2023 reply
Gargi Sengupta et al.
Gargi Sengupta et al.
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Sengupta et al. investigated the impact of surface modulated organic acid dissociation (and related acidity change) in organic aerosol on aqueous-phase SO2 oxidation, cloud responses, and radiative effects using a box model. This is an interesting topic and the authors clearly showed that the impacts could be significant. However, how this result translates into a 3D aerosol-chemistry-climate model (not a box-model version) is unclear. I think that the authors should be clearer about the goal of this paper. Is it just to demonstrate that these impacts can be significant, or to improve aerosol-chemistry-climate models by providing information about this previously largely overlooked aspect? If it is closer to the latter, how to implement the results of this paper in an aerosol-chemistry-climate model is key information needed (e.g., how to set up similar calculations in aerosol-chemistry-climate models or what are the recommended values for relevant key parameters in aerosol-climate simulations). Otherwise, the authors should make it very clear that the results of this paper are not ensured to be transferable to real-world aerosol-chemistry-climate simulations. Even so, more could be done to show the implications of these box-model-based results for aerosol-chemistry-climate simulations.
Section 2.3: does inorganic ion surface propensity play a role in SO2 oxidation here? What about ionic strength, as Liu et al. (2020) showed to substantially affect this chemistry.
Line 289: the sentence containing “pKabulk+1” is confusing. pKabulk+1 does not need an extrapolation to be obtained from pKabulk, but properties at these pHs do.
Figures 1-6: all these results are only for pKabulk, pKabulk+1, and pKabulk+2. The pKas of malonic and decanoic acids are different (2.8 and 4.9, respectively). At a certain pH, these 2 acids in an aerosol can have behaviors quite different than described in this paper. In an aerosol with a more realistic composition, more acids have even more pKas. The authors should explore the behavior of such acid mixture, or at least, that of individual acids in a wide range of pH, not just near their pKa.
Line 215: “F-“ and “HF” should be “A-“ and “HA”, respectively.
Many numbers in the manuscript have too many significant digits, e.g., “14.70%”, “83.14%”, “21.73%”, and “64.72%” in Line 403.