Insights into seasonal variation of wet deposition over southeast Asia via precipitation adjustment from the findings of MICS-Asia III
Authors: Itahashi Syuichi , Ge Baozhu , Sato Keiichi , Wang Zhe , Kurokawa Junichi , Tan Jiani , Huang Kan , Fu Joshua S. , Wang Xuemei , Yamaji Kazuyo , Nagashima Tatsuya , Li Jie , Kajino Mizuo , Carmichael Gregory R. , Wang Zifa 
Published: June 2021

Abstract. Asia has attracted research attention because it has the highest
anthropogenic emissions in the world, and the Model Inter-Comparison Study
for Asia (MICS-Asia) phase III was carried out to foster our understanding
of the status of air quality over Asia. This study analyzed wet deposition
in southeast Asian countries (Myanmar, Thailand, Lao People's Democratic
Republic (PDR), Cambodia, Vietnam, the Philippines, Malaysia, and Indonesia)
with the aim of providing insights into the seasonal variation of wet
deposition. Southeast Asia was not fully considered in MICS-Asia phase II
due to a lack of observational data; however, the analysis period of
MICS-Asia III, namely the year 2010, is covered by ground observations of
the Acid Deposition Monitoring Network in East Asia (EANET), and the
coordinated simulation domain was extended to cover these observation sites.
The analyzed species are wet depositions of S (sulfate aerosol, sulfur
dioxide (SO2), and sulfuric acid (H2SO4)), N (nitrate
aerosol, nitrogen monoxide (NO), nitrogen dioxide (NO2), and nitric
acid (HNO3)), and A (ammonium aerosol and ammonia (NH3)). The wet
deposition simulated with seven models driven by a unified meteorological
model in MICS-Asia III was used with the ensemble approach, which
effectively modulates the differences in performance among models. By
comparison with EANET observations, although the seven models generally
captured the wet depositions of S, N, and A, there were difficulties
capturing these in some cases. Considering the model performance for ambient
aerosol concentrations over southeast Asia, this failure of models is
considered to be related to the difficulty in capturing the precipitation in
southeast Asia, especially during the dry and wet seasons. Generally,
meteorological fields overestimate the precipitation during the dry season,
which leads to the overestimation of wet deposition during this season. To
overcome this, a precipitation-adjusted approach that scaled the modeled
precipitation to the observed value was applied, and it was demonstrated
that the model performance was improved. Satellite measurements were also
used to adjust for precipitation data, which adequately accounted for the
spatiotemporal precipitation patterns, especially in the dry season. As the
statistical scores were mostly improved by this adjustment, the estimation
of wet deposition with precipitation adjustment was considered to be
superior. To utilize satellite measurements, the spatial distribution of wet
deposition was revised. Based on this revision, it was found that Vietnam,
Malaysia, and Indonesia were upward corrected, and Myanmar, Thailand, Lao
PDR, Cambodia, and the Philippines were downward-corrected; these
corrections were up to ±40 %. The improved accuracy of
precipitation amount was key to estimating wet deposition in this study.
These results suggest that the precipitation-adjusted approach has the
potential to obtain accurate estimates of wet deposition through the fusion
of models and observations.