Modeling global impacts of heterogeneous loss of HO₂ on cloud droplets, ice particles and aerosols

Abstract. The abundance and spatial variability of the hydroperoxyl radical (HO₂) in the troposphere strongly affects atmospheric composition through tropospheric ozone production and associated HO_x chemistry. One of the largest uncertainties in the chemical HO₂ budget is its heterogeneous loss on the surface of cloud droplets, ice particles and aerosols. We quantify the importance of the heterogeneous HO₂ loss at global scale using the latest recommendations on the scavenging efficiency on various surfaces. For this we included the simultaneous loss on cloud droplets and ice particles as well as aerosol in the Composition-Integrated Forecast System (C-IFS). We show that cloud surface area density (SAD) is typically an order of magnitude larger than aerosol SAD, using assimilated satellite retrievals to constrain both meteorology and global aerosol distributions. Depending on the assumed uptake coefficients, loss on liquid water droplets and ice particles accounts for ∼53–70% of the total heterogeneous loss of HO₂, due to the ubiquitous presence of cloud droplets. This indicates that HO₂ uptake on cloud should be included in chemistry transport models that already include uptake on aerosol. Our simulations suggest that the zonal mean mixing ratios of HO₂ are reduced by ∼25% in the tropics and up to ∼50% elsewhere. The subsequent decrease in oxidative capacity leads to a global increase of the tropospheric carbon monoxide (CO) burden of up to 7%, and an increase in the ozone tropospheric lifetime of ∼6%. This increase results in an improvement in the global distribution when compared against CO surface observations over the Northern Hemisphere, although it does not fully resolve the wintertime bias in the C-IFS. There is a simultaneous increase in the high bias in C-IFS for tropospheric CO over the Southern Hemisphere, which constrains on the assumptions regarding HO₂ uptake on a global scale. We show that enhanced HO₂ uptake on aerosol types associated with anthropogenic sources could contribute to reductions in the low bias for CO simulated over the extra-tropical Northern Hemisphere.