Implementation of HONO into the chemistry–climate model CHASER (V4.0): roles in tropospheric chemistry

[摘要] Nitrous acid (HONO) is an important atmospheric gas givenits contribution to the cycles of NO x and HO x , but its role inglobal atmospheric photochemistry is not fully understood. This studyimplemented three pathways of HONO formation in the chemistry–climate modelCHASER (MIROC-ESM) to explore three physical phenomena: gas-phase kineticreactions (GRs), direct emission (EM), and heterogeneous reactions oncloud and aerosol particles (HRs). We evaluated the simulations by theatmospheric aircraft-based measurements from EMeRGe-Asia-2018 (Effect ofMegacities on the Transport and Transformation of Pollutants on the Regionalto Global Scales), ATom-1 (atmospheric tomography), observations from theship R/V Mirai , EANET (Acid Deposition Monitoring Network in eastern Asia)/EMEP(European Monitoring and Evaluation Programme) ground-based stationaryobservations, and the OMI (Ozone Monitoring Instrument). We showed that theinclusion of the HONO chemistry in the modelling process reduced the modelbias against the measurements for PM 2.5 , NO 3 - /HNO 3 ,NO 2 , OH, HO 2 , O 3 , and CO, especially in the lower troposphereand 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 reactionson the aerosol surfaces contributed larger amounts of HONO (51 %) thanthose on the cloud surfaces (12 %). The model exhibited significantnegative biases for daytime HONO in the Asian off-the-coast region, comparedwith the airborne measurements by EMeRGe-Asia-2018, indicating the existenceof unknown daytime HONO sources. Strengthening of aerosol uptake of NO 2 near the surface and in the middle troposphere, cloud uptake, and direct HONOemission were all potential yet-unknown HONO sources. The most promisingdaytime source for HONO found in this study was the combination of enhancedaerosol uptake of NO 2 and surface-catalysed HNO 3 photolysis(maxST + JANO3-B case), which could also remedy the model bias for NO 2 and O 3 during EMeRGe. We also found that the simulated HONO abundanceand its impact on NO x –O 3 chemistry were sensitive to the yield ofthe heterogeneous conversion of NO 2 to HONO (vs. HNO 3 ). Inclusion of HONO reduced global tropospheric NO x (NO + NO 2 )levels by 20.4 %, thereby weakening the tropospheric oxidizing capacity(OH, O 3 ) occurring for NO x -deficit environments (remote regionsand upper altitudes), which in turn increased CH 4 lifetime (13 %)and tropospheric CO abundance (8 %). The calculated reduction effect onthe global ozone level reduced the model overestimates for tropospheric columnozone against OMI spaceborne observations for a large portion of the North Hemisphere. HRson the surfaces of cloud particles, which have been neglected in previousmodelling studies, were the main drivers of these impacts. This effect wasparticularly salient for the substantial reductions of levels of OH(40 %–67 %) and O 3 (30 %–45 %) in the NP region during summer, giventhe significant reduction of the NO x level (50 %–95 %). In contrast, HRson aerosol surfaces in China (Beijing) enhanced OH and O 3 winter meanlevels by 600 %–1700 % and 10 %–33 %, respectively, with regards to theirminima in winter. Furthermore, sensitivity simulations revealed that theheterogeneous formation of HONO from NO 2 and heterogenous photolysis ofHNO 3 coincided in the real atmosphere. Nevertheless, the global effectscalculated in the combined case (enhancing aerosol uptakes of NO 2 andimplementing heterogeneous photolysis of HNO 3 ), which most captured themeasured daytime HONO level, still reduced the global tropospheric oxidizingcapacity. Overall, our findings suggest that a global model that does notconsider HONO heterogeneous mechanisms (especially photochemicalheterogeneous formations) may erroneously predict the effect of HONO inremote areas and polluted regions.

[学科分类] 土木及结构工程学