Comment on gmd-2021-385

Abstract

Nitrous acid (HONO) is an important atmospheric gas given its contribution to the cycles of NOx and HOx, but its role in global atmospheric photochemistry is not fully understood. This study, for the first time, implemented three pathways of HONO formation in the chemistry-climate model CHASER (MIROC-ESM) to explore three physical phenomena: gas-phase kinetic reactions (GRs), direct emission (EM), and heterogeneous reactions on cloud/aerosol particles (HRs). We evaluated the simulations by the atmospheric measurements from the OMI (Ozone Monitoring Instrument), EANET (Acid Deposition Monitoring Network in eastern Asia) / EMEP (European Monitoring and Evaluation Programme) ground-based stationary observations, observations from the ship R/V Mirai, and aircraft-based measurements by ATom1 (atmospheric tomography) and EMeRGe-Asia-2018 (Effect of Megacities on the Transport and Transformation of Pollutants on the Regional to Global scales). We showed that the inclusion of the HONO chemistry in the modeling process reduces the model bias against the measurements for PM2.5, NO3−/HNO3, NO2, OH, O3, and CO, especially in the lower troposphere and the North Pacific (NP) region. We found that the retrieved global abundance of tropospheric HONO was 1.4 TgN. Of the three source pathways, HRs and EM contributed 63 % and 26 % to the net HONO production, respectively. We also observed that, reactions on the aerosol surfaces contributed larger amounts of HONO (51 %) than those on the cloud surfaces (12 %). The model exhibited significant negative biases for daytime HONO in the Asian off-coast region, compared with the airborne measurements by EMeRGe-Asia-2018, indicating the existence of unknown daytime HONO sources. Strengthening of aerosol uptake of NO2 near-surface and in the middle troposphere, cloud uptake, and direct HONO emission are all potential yet-unknown HONO sources. We also found that the simulated HONO abundance and its impact on NOx-O3 chemistry are sensitive to the yield of the heterogeneous conversion of NO2 to HONO (vs. HNO3). Inclusion of HONO reduces global tropospheric NOx (NO + NO2) levels by 20.4 %, thereby weakening the tropospheric oxidizing capacity, which in turn, increases CH4 lifetime (13 %) and CO abundance (8 %). HRs on the surfaces of cloud particles, which have been neglected in previous modeling studies, are the main drivers of these impacts. This effect is particularly salient for the substantial reductions of levels of OH (40–67 %) and O3 (30–45 %) in the NP region during summer given the significant reduction of NOx level (50–95 %). In contrast, HRs on aerosol surfaces in China (Beijing) enhance OH and O3 winter mean levels by 600–1700 % and 10–33 %, respectively, with regards to their minima in winter. Overall, our findings suggest that a global model that does not consider HONO heterogeneous mechanisms (especially HRs on cloud particle surfaces) may erroneously predict the effect of HONO in remote areas and polluted regions.