Effects of heterogeneous reactions on tropospheric chemistry: a global simulation with the chemistry–climate model CHASER V4.0

Abstract. This study uses a chemistry–climate model CHASER (MIROC) to explore the roles of heterogeneous reactions (HRs) in global tropospheric chemistry. Three distinct HRs of N 2 O 5 , HO 2 , and RO 2 are considered for surfaces of aerosols and cloud particles. The model simulation is verified with EANET and EMEP stationary observations; R/V Mirai ship-based data; ATom1 aircraft measurements; satellite observations by OMI, ISCCP, and CALIPSO-GOCCP; and reanalysis data JRA55. The heterogeneous chemistry facilitates improvement of model performance with respect to observations for NO 2 , OH, CO, and O 3 , especially in the lower troposphere. The calculated effects of heterogeneous reactions cause marked changes in global abundances of O 3 ( −2.96  %), NO x ( −2.19  %), CO ( +3.28  %), and global mean CH 4 lifetime ( +5.91  %). These global effects were contributed mostly by N 2 O 5 uptake onto aerosols in the middle troposphere. At the surface, HO 2 uptake gives the largest contributions, with a particularly significant effect in the North Pacific region ( −24  % O 3 , +68  % NO x , +8  % CO, and −70  % OH), mainly attributable to its uptake onto clouds. The RO 2 reaction has a small contribution, but its global mean negative effects on O 3 and CO are not negligible. In general, the uptakes onto ice crystals and cloud droplets that occur mainly by HO 2 and RO 2 radicals cause smaller global effects than the aerosol-uptake effects by N 2 O 5 radicals ( +1.34  % CH 4 lifetime, +1.71  % NO x , −0.56  % O 3 , +0.63  % CO abundances). Nonlinear responses of tropospheric O 3 , NO x , and OH to the N 2 O 5 and HO 2 uptakes are found in the same modeling framework of this study ( R>0.93 ). Although all HRs showed negative tendencies for OH and O 3 levels, the effects of HR(HO 2 ) on the tropospheric abundance of O 3 showed a small increment with an increasing loss rate. However, this positive tendency turns to reduction at higher rates ( >5 times). Our results demonstrate that the HRs affect not only polluted areas but also remote areas such as the mid-latitude sea boundary layer and upper troposphere. Furthermore, HR(HO 2 ) can bring challenges to pollution reduction efforts because it causes opposite effects between NO x (increase) and surface O 3 (decrease).