Exploring the impact of new particle formation events on PM2.5 pollution during winter in the Yangtze River Delta, China

New particle formation(NPF) events are an increasingly interesting topic in air quality and climate science. In this study, the particle number size distributions, and the frequency of NPF events over Hefei were investigated from November 2018 to February 2019. The proportions of the nucleation mode, Aitken mode, and accumulation mode were 24.59%,53.10%, and 22.30%, respectively, which indicates the presence of abundant ultrafine particles in Hefei. Forty-six NPF events occurred during the observation days, accounting for 41.82% of the entire observation period. Moreover, the favorable meteorological conditions,potential precursor gases, and PM2.5range of the NPF events were analyzed. Compared to non-NPF days, the NPF events preferentially occurred on days with lower relative humidity, higher wind speeds, and higher temperatures. When the PM2.5was 15–20, 70–80, and105–115 μg/m~3, the frequency of the NPF events was higher. Nucleation mode particles were positively related to atmospheric oxidation indicated by ozone when PM2.5ranged from 15 to 20 μg/m~3, and related to gaseous precursors like SO2and NO2when PM2.5was located at 70-80 and 105–115 μg/m~3. On pollution days, NPF events did not directly contribute to the increase in the PM2.5in the daytime, however, NPF events would occur during the night and the growth of particulate matter contributes to the nighttime PM2.5contents. This could lead to pollution that lasted into the next day. These findings are significant to the improvement of our understanding of the effects of aerosols on air quality.