Implementation of HONO into the chemistry–climate model CHASER (V4.0): roles in tropospheric chemistry
Authors: Ha Phuc Thi Minh , Kanaya Yugo , Taketani Fumikazu , Andrés Hernández Maria Dolores , Schreiner Benjamin , Pfeilsticker Klaus , Sudo Kengo 
Published: February 2023

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
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 and aerosol particles (HRs). We evaluated the simulations by the
atmospheric aircraft-based measurements from EMeRGe-Asia-2018 (Effect of
Megacities on the Transport and Transformation of Pollutants on the Regional
to Global Scales), ATom-1 (atmospheric tomography), observations from the
ship R/V Mirai, EANET (Acid Deposition Monitoring Network in eastern Asia)/EMEP
(European Monitoring and Evaluation Programme) ground-based stationary
observations, and the OMI (Ozone Monitoring Instrument). We showed that the
inclusion of the HONO chemistry in the modelling process reduced the model
bias against the measurements for PM2.5, NO3-/HNO3,
NO2, OH, HO2, 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-the-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 the surface and in the middle troposphere, cloud uptake, and direct HONO
emission were all potential yet-unknown HONO sources. The most promising
daytime source for HONO found in this study was the combination of enhanced
aerosol uptake of NO2 and surface-catalysed HNO3 photolysis
(maxST+JANO3-B case), which could also remedy the model bias for NO2
and O3 during EMeRGe. We also found that the simulated HONO abundance
and its impact on NOx–O3 chemistry were sensitive to the yield of
the heterogeneous conversion of NO2 to HONO (vs. HNO3). Inclusion of HONO reduced global tropospheric NOx (NO + NO2)
levels by 20.4 %, thereby weakening the tropospheric oxidizing capacity
(OH, O3) occurring for NOx-deficit environments (remote regions
and upper altitudes), which in turn increased CH4 lifetime (13 %)
and tropospheric CO abundance (8 %). The calculated reduction effect on
the global ozone level reduced the model overestimates for tropospheric column
ozone against OMI spaceborne observations for a large portion of the North Hemisphere. HRs
on the surfaces of cloud particles, which have been neglected in previous
modelling studies, were the main drivers of these impacts. This effect was
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 the NOx level (50 %–95 %). In contrast, HRs
on aerosol surfaces in China (Beijing) enhanced OH and O3 winter mean
levels by 600 %–1700 % and 10 %–33 %, respectively, with regards to their
minima in winter. Furthermore, sensitivity simulations revealed that the
heterogeneous formation of HONO from NO2 and heterogenous photolysis of
HNO3 coincided in the real atmosphere. Nevertheless, the global effects
calculated in the combined case (enhancing aerosol uptakes of NO2 and
implementing heterogeneous photolysis of HNO3), which most captured the
measured daytime HONO level, still reduced the global tropospheric oxidizing
capacity. Overall, our findings suggest that a global model that does not
consider HONO heterogeneous mechanisms (especially photochemical
heterogeneous formations) may erroneously predict the effect of HONO in
remote areas and polluted regions.