The seasonal variation, characteristics and secondary generation of PM2.5 in Xi'an, China, especially during pollution events.

As an important central city in western China, Xi'an has the worst atmospheric pollution record in China and many measures have been taken to improve the air quality in the past few years. In this study, PM 2.5 samples were collected across four seasons from 2017 to 2018 in Xi'an. Organic carbon and elemental carbon, water soluble ions, and elements were monitored to assess the air quality. The average annual PM 2.5 concentration was (134.9 ± 48.1 μg/m3), with the highest concentration in winter (188.8 ± 93.2 μg/m3), and lowest concentration in summer (71.2 ± 12.1 μg/m3). The secondary generation of sulfate (SO 4 2−) and nitrate (NO 3 −) was strong in spring, and secondary organic carbon (SOC) was formed in all seasons. The compositions of PM 2.5 changed greatly during a sandstorm occurred and the Spring Festival. The sandstorm played a positive role in removing local pollutant NO 3 −, but also increased the concentration of SO 4 2−, however both the concentration of SO 4 2− and NO 3 − greatly increased by secondary generation during Spring Festival. Potential source analysis showed that during the sandstorm, pollutants were transported over a long distance from the northwest of China, whereas it was mainly from the local and surrounded emissions during the Spring Festival. Except Ca2+ and geological dust (GM), the other components in PM 2.5 increased significantly on the day of the Spring Festival. During sampling time in Xi'an, the positive matrix factorization (PMF) model analysis showed that PM 2.5 mainly came from vehicle emission, coal combustion, and biomass burning. • The average annual PM 2.5 concentration in Xi'an has increased from 2004 to 2007, and gradually decreased from 2012 to 2018. • It has higher exposure levels in wintertime in Xi'an, especially during the pollution events with seasonal variations. • NO 3 – decreased and SO 4 2– increased during the sandstorm, whereas both SO 4 2– and NO 3 – increased during the Spring Festival. • As one of the three main sources for PM 2.5 in Xi'an, the contribution of vehicle emission has elevated in the past years. [ABSTRACT FROM AUTHOR]