10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Characteristics and Formation Mechanism of Nitrate during Haze Events in Beijing
QINGCHENG XU, Shuxiao Wang, Yang Hua, Jiming Hao, Tsinghua University
Abstract Number: 695 Working Group: Air Quality in Megacities: from Sources to Control
Abstract Water-soluble inorganic ions (WSI) is major components of PM2.5 in Beijing in recent years. Since year 2015, sulfate concentrations in PM2.5 has gradually decreased owing to the effective control of coal combustion. However, the contribution of nitrate to PM2.5 displayed a significant increase during haze events in Beijing. In this study, a highly time-resolved online analyzer (Monitoring for AeRosols and Gases, MARGA) was employed to measure the WSI in PM2.5 in Beijing from 5 February to 15 November, 2017. Three typical haze events during this period were selected and investigated. During our campaign, the mass ratio of nitrate to sulfate changed from 2.4 to 2.67 when the air pollution aggravated from slightly polluted (150>AQI>100,115>PM2.5>75) to moderately polluted (AQI>150, PM2.5>115). This study used mass concentration of nitrate/sulfate to CO(gas) to exclude the effects of planetary boundary layer in four selected haze event. The results show that time-series of PM2.5 is significantly correlated with nitrate (with a correlation coefficient of 0.53), while it is not correlated with sulfate. During heavy pollution episodes in winter, nitrate concentrations increased from 7.5 μg/m3 to 45.6μg/m3, while sulfate increased from 4.2μg/m3 to 20.1μg/m3, indicating that nitrate is a more important driver for the growth of PM2.5. Nitrate also dominates the increase of WSI in the pollution episodes in autumn, with a concentration of 52.5μg/m3, and contributes up to 67% of WSI. The average ratio of NH4+ to SO42- was 1.62 in autumn, much higher than that in summer (0.86). One of the reason is that the emission control of coal combustion in Beijing and surrounding areas results in an NH3-rich and SO2-lean atmosphere, which promoted the formation of ammonium nitrate. Even in summer, when the meteorological conditions are much more suitable for the photochemical reaction of ammonium sulfate than ammonium nitrate, there are still significant amount of ammonium nitrate formation in the night with variations in temperature and relative humidity. This study indicates that nitrate is becoming the most important component of WSI in PM2.5 and is driving the rapid growth of PM2.5 concentrations during heavy pollution episodes. Therefore, more efforts shall be made to reduce the nitrogen oxide and ammonia emissions in North China.