Assessment of surface ozone production in Qinghai, China with satellite-constrained VOCs and NOx emissions

Abstract

The implementation of the western development strategy of China and the migration of air pollutants from eastern China might lead to a rapid increase in volatile organic compounds (VOCs) concentrations in the Qinghai-Tibet Plateau (QTP) and an amplified role of Nitrogen Oxides (NOx) in ozone (O3) pollution. Qinghai province, situated on the northeast of the QTP, had fewer human activities compared to eastern China, while ozone pollution worsened over the years. To better capture recent emission trends and improve the accuracy of O3 simulation in Qinghai, this study proposed a top-down method, which combined the air quality modeling system RAMS-CMAQ, with formaldehyde (HCHO) and nitrogen dioxide (NO2) columns derived from TROPOMI as the constraints to improve the emission estimates of VOCs and NOx in July 2020, respectively. Through a series of sensitivity experiments, better quantified emission estimates of VOCs and NOx were obtained to be 1.33 and 0.34 Tg/yr, 2.5 and 2.1 times larger than the bottom-up ones. The results demonstrated the effectiveness of the top-down method and satellite observations constraints in improving VOCs and NOx emission estimates, resulting in a reduction in the differences between the observed and modeled HCHO and NO2 columns to 0.7 and 0.2 × 1015 molec/cm2, respectively. As a result, the simulated maximum daily 8-h average (MDA8) O3 concentrations increased from 58.9 to 81.6 μg/m3, which were closer to observations (85.4 μg/m3), the normalized mean bias (NMB) and normalized mean error (NME) values of hourly O3 concentrations changed from −24.7 % to −2.9 % and from 29.9 % to 22.3 %, respectively. This study showed the potential of top-down estimates to aide in the development of emission scenarios, which were critical for accurately simulating the O3 pollution and pollution control policy studies.