Global impacts of organic aerosol acidity on sulfate and cloud formation

TL;DR

This work delivers the first global, fully coupled assessment of organic aerosol acidity, including surface-specific dissociation, on sulfate formation and aerosol–cloud–climate interactions. By embedding concentration-dependent OA acid dissociation into ECHAM-HAMMOZ and testing four scenarios (Ka = 0, bulk pK_a^B, and two surface-shifted pK_a^{S1,S2}), the study shows OA acidity enhances aqueous SO_4^{2-} production via acid-catalyzed oxidation of SO_2, increases cloud droplet numbers, and strengthens shortwave cloud radiative cooling, with global mean SWCRF reaching up to −0.97 W m^{-2} for the strongest surface-specific case. The results reveal substantial spatial heterogeneity, with land and mid-latitude regions often more affected and surface-specific effects sometimes exceeding bulk acidity, highlighting the importance of capturing OA acidity and surface-specific phenomena in climate models. Overall, the findings indicate OA acidity is a non-negligible driver of sulfate burden, cloud formation, and climate forcing, providing a compelling case for integrating pH-dependent OA chemistry into global assessments and informing observational strategies of aerosol acidity.

Abstract

Organic aerosols (OA) comprise a major fraction of atmospheric particulate matter and frequently contain acidic species, yet their contribution to overall aerosol acidity has not been explicitly considered in global climate models. We implement concentration-dependent OA acid dissociation, including recently demonstrated surface-specific effects, into the ECHAM-HAMMOZ global climate model and assess the impacts on aqueous aerosol sulfate chemistry and aerosol--cloud--climate interactions. We show that enhanced aerosol acidity from OA acid dissociation drives increased sulfate formation from aqueous-phase oxidation of $\mathrm{SO_2}$. The microphysics of additional secondary sulfate aerosol changes global cloud droplet number concentrations (CDNC), with enhancements up to $13.9\%$. Increased cloud formation leads to a significant global mean cooling effect with a shortwave cloud radiative forcing (SWCRF) up to $-0.97~\mathrm{W\,m^{-2}}$. We also find that surface-specific acid dissociation effects can further modify both aerosol chemistry and resulting aerosol--cloud--climate responses, in some cases with even stronger impact than bulk acidity conditions. Our results demonstrate significant effects of considering OA acidity, as well as surface-specific phenomena, in global climate models.

Figures (4)

• Figure 1: Sulfate aerosol (SU) burden shown as (a) the total column burden for the no OA acid dissociation condition ($\rm K_a=0$) as 5-year ($1999-2003$) medians in absolute units ($\rm kg \, m^{-2}$), and the column burden differences (b) with respect to $\rm K_a=0$ ($\Delta \mathrm{SU} |_{\mathrm{K_{a}=0}}$, %) for bulk OA acid dissociation ($\mathrm{pK^{B}_a}$), and with respect to $\mathrm{pK^{B}_a}$ ($\Delta \mathrm{SU} |_{\mathrm{pK_a^B}}$, %) for the surface-specific OA acid dissociation (c) $\mathrm{pK^{S1}_a}$ and (d) $\mathrm{pK^{S2}_a}$. Statistical significances of differences given as $p < 0.01$ (strong), $p < 0.05$ (medium), or $p < 0.1$ (weak) are represented by large, medium, and small circles, respectively.
• Figure 2: CDNC burden shown as the total column CDNC for the no OA acid dissociation condition ($\rm K_a=0$, panel a) as 5-year ($1999-2003$) medians in absolute units ($\rm m^{-2}$), and the column burden differences with respect to $\rm K_a=0$ ($\Delta \mathrm{CDNC} |_{\mathrm{K_{a}=0}}$, %) for bulk OA acid dissociation ($\mathrm{pK^{B}_a}$, panel b), and with respect to $\mathrm{pK^{B}_a}$ ($\Delta \mathrm{CDNC} |_{\mathrm{pK_a^B}}$, %) for the surface-specific OA acid dissociation $\mathrm{pK^{S1}_a}$ (panel c) and $\mathrm{pK^{S2}_a}$ (panel d). Statistical significance (white circles) is indicated as large ($p < 0.01$), medium ($p < 0.05$), and small ($p < 0.1$).
• Figure 3: Cloud liquid water content (LWC) burden shown as (a) the total column burden for the no OA acid dissociation condition ($\rm K_a=0$) as 5-year ($1999-2003$) medians in absolute units ($\rm kg \, m^{-2}$), and the column burden differences (b) with respect to $\rm K_a=0$ ($\Delta \mathrm{LWC} |_{\mathrm{K_{a}=0}}$, %) for bulk OA acid dissociation ($\mathrm{pK^{B}_a}$), and with respect to $\mathrm{pK^{B}_a}$ ($\Delta \mathrm{LWC} |_{\mathrm{pK_a^B}}$, %) for the surface-specific OA acid dissociation (c) $\mathrm{pK^{S1}_a}$ and (d) $\mathrm{pK^{S2}_a}$. Statistical significances of differences given as $p < 0.01$ (strong), $p < 0.05$ (medium), or $p < 0.1$ (weak) are represented by large, medium, and small circles, respectively.
• Figure 4: CDNC column burden differences with respect to $\rm K_a=0$ ($\Delta \mathrm{CDNC} |_{\mathrm{K_{a}=0}}$, %) for the surface-specific OA acid dissociation $\mathrm{pK^{S1}_a}$ (panel a) and $\mathrm{pK^{S2}_a}$ (panel b). Statistical significance (white circles) is indicated as large ($p < 0.01$), medium ($p < 0.05$), and small ($p < 0.1$).