Evaluation of accuracy on bottom-up NH3 emissions and sensitivity of PM2.5 to emission control in South Korea

Abstract

While winter has traditionally been the focus of air quality research in South Korea, early summer also experiences high levels of fine particulate matter (PM2.5), largely driven by elevated ammonia (NH3) emissions from agricultural activities. The South Korean government decided to reduce agricultural-based NH3 emissions by 30 % in the Special Act on Fine Aerosol to reduce the levels of PM2.5 due to the significant contributions of NH3 to the formation of secondary inorganic aerosols. In this study, we utilized the CMAQ v5.2.1 model to simulate air quality over East Asia during the KORUS-AQ campaign (1 May - June 12, 2016) and evaluate the accuracy of the NH3 emission, their impacts on PM2.5, and the effectiveness of various emission control strategies, particularly in South Korea. The accuracy of NH3 emissions was evaluated by comparing model-predicted NH3 concentrations with the EANET and AMoN-China networks and with the MARGA equipment in Seoul. The comparison at 60 monitoring stations revealed negative mean biases in China (often exceeding 10 mu g m- 3), indicating a possible underestimation of NH3 emissions in the KORUSv5.0 inventory in most parts of China except for some overestimations in Henan and Sichuan provinces. On the other hand, biases were smaller in South Korea and Japan, ranging from -3.80 to 2.99 ppb (or -2.68 to 2.11 mu g m- 3 at the standard ambient temperature and pressure). We found from the sensitivity simulations by incrementally reducing NH3 emissions in South Korea from 0 % to 100 % that NH3 reductions lead to lower aerosol pH and decreased nitrate partitioning (e.g., from 1.31 to -0.54 and from 0.40 to 0.05 for SK, respectively). In addition, the South Korean government's reduction policy of NH3 emissions can improve the mean concentration of PM2.5 between 0.59 and 1.31 mu g m-3 in South Korea. Chemical regime analysis concludes that South Korea exhibits NH3-rich conditions, where NOx plays a more dominant role in nitrate formation. Furthermore, the sensitivity simulations of the multi-pollutant reductions indicate that i) a 40 % of NH3 and SO2 & NOx, ii) a 60 % of NH3, or iii) a 60 % reduction of SO2 & NOx emissions are required to achieve a 10 % decrease in PM2.5 in South Korea.