Reaction of N₂O₅ on tropospheric aerosols: Impact on the global distributions of NO_<i>x</i>, O₃, and OH

TLDR

Heterogeneous NOx removal is dominated by aerosol reactions in the northern hemisphere, while in the tropics and southern hemisphere liquid water clouds also act as a sink for N₂O₅ and NO₃. Modeling indicates that heterogeneous NO₃/N₂O₅ reactions on aerosols cut global NOₓ by 50 % (80 % in winter, 20 % in summer), lower O₃ by up to 25 % in spring subtropics, reduce OH by up to 30 % in winter/spring subtropics, and decrease global O₃ and OH burdens by ~9 % each, thereby improving agreement with observed nitrate deposition.

Abstract

Using a three‐dimensional global model of the troposphere, we show that the heterogeneous reactions of NO 3 and N 2 O 5 on aerosol particles have a substantial influence on the concentrations of NO x , O 3 , and OH. Due to these reactions, the modeled yearly average global NO x burden decreases by 50% (80% in winter and 20% in summer). The heterogeneous removal of NO x in the northern hemisphere (NH) is dominated by reactions on aerosols; in the tropics and southern hemisphere (SH), with substantial smaller aerosol concentrations, liquid water clouds can provide an additional sink for N 2 O 5 and NO 3 . During spring in the NH subtropics and at mid‐latitudes, O 3 ‐concentrations are lowered by 25%. In winter and spring in the subtropics of the NH calculated OH concentrations decreased by up to 30%. Global tropospheric average O 3 and OH burden (the latter weighted with the amount of methane reacting with OH) can drop by about 9% each. By including reactions on aerosols, we are better able to simulate observed nitrate wet deposition patterns in North America and Europe. O 3 concentrations in springtime smog situations are shown to be affected by heterogeneous reactions, indicating the great importance of chemical interactions resulting from NO x and SO 2 emissions. However, a preliminary analysis shows that under present conditions a change in aerosol concentrations due to limited SO 2 emission control strategies (e.g., reductions by a factor of 2 in industrial areas) will have only a relatively minor influence on O 3 concentrations. Much larger reductions in SO 2 emissions may cause larger increases in surface O 3 concentrations, up to a maximum of 15%, if they are not accompanied by a reduction in NO x or hydrocarbon emission.