Abstract
This study focuses on the estimation of top-down NOx emissions over East Asia, integrating information on the levels of NO2 and NO, wind vector, and geolocation from Ozone Monitoring Instrument (OMI) observations and Weather Research and Forecasting (WRF)-Community Multiscale Air Quality (CMAQ) model simulations. An algorithm was developed based on mass conservation to estimate the 30 km × 30 km resolved top-down NOx emissions over East Asia. In particular, the algorithm developed in this study considered two main atmospheric factors—(i) NOx transport from/to adjacent cells and (ii) calculations of the lifetimes of column NOx (τ). In the sensitivity test, the analysis showed the improvements in the top-down NOx estimation via filtering the data (τ ≤ 2 h). The best top-down NOx emissions were inferred after the sixth iterations. Those emissions were 11.76 Tg N yr−1 over China, 0.13 Tg N yr−1 over North Korea, 0.46 Tg N yr−1 over South Korea, and 0.68 Tg N yr−1 over Japan. These values are 34%, 62%, 60%, and 47% larger than the current bottom-up NOx emissions over these countries, respectively. A comparison between the CMAQ-estimated and OMI-retrieved NO2 columns was made to confirm the accuracy of the newly estimated NOx emission. The comparison confirmed that the estimated top-down NOx emissions showed better agreements with observations (R2 = 0.88 for January and 0.81 for July).
Highlights
Smog events in East Asia have been recognized as severe air pollution problems, resulting in deteriorated air quality in the atmosphere [1,2,3] and harmful effects on human health and the ecological system [4,5,6]
The Weather Research and Forecasting (WRF) simulation was configured with the following atmospheric physical schemes: the Yonsei University (YSU) scheme for planetary boundary layer (PBL) physics [53], the five-layer thermal diffusion Land Surface Model (LSM) scheme for land surface, the Dudhia scheme for the shortwave radiation [54], the rapid radiative transfer model (RRTM) scheme for the longwave radiation [55]; and Kain–Fritsch scheme for the cumulus parametrization [56]
The NO2 columns (ΩNO2,Community Multiscale Air Quality (CMAQ)) calculated from the initial CMAQ simulation with the consideration of the bottom-up NOx emission were compared with the Ozone Monitoring Instrument (OMI)-retrieved NO2 columns
Summary
Smog events in East Asia have been recognized as severe air pollution problems, resulting in deteriorated air quality in the atmosphere [1,2,3] and harmful effects on human health and the ecological system [4,5,6]. There has been a lack of capability in more accurately forecasting the levels of pollutants due to many uncertainties related to the meteorological fields, anthropogenic and biogenic emissions, chemical and physical parameterizations, boundary, and initial conditions, and land uses and land covers [7,8,9]. Among these uncertainties, the accuracy of emissions is one of the most important for improving the performance of air quality forecasting. The NOx emissions in East Asia have still been highly uncertain [10,11,12,13,14]
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