Long-term trend of new particle formation events in the Yangtze River Delta, China and its influencing factors: 7-year dataset analysis

To evaluate the influence of anthropogenic emission reductions since 2013 in China, a long-term trend analysis of the particle number size distribution (PNSD) and new particle formation (NPF) events in the Yangtze River Delta (YRD) region was conducted based on the PNSD measurement (diameter ranging from 3 to 850 nm) at the Lin'an (LAN) regional background station from 2013 to 2019. A modified Mann-Kendall test and a Theil-Sen estimator were used to calculate the overall trend of particle number concentrations in different modes and the relevant influencing factors. We observed a significant decreasing trend in the Aitken and accumulation mode number concentrations, with annual decrease rates of approximately 5.6% and 8.2%, respectively, resulting in an approximately 6.0% decline in total particles annually. However, the nucleation mode particle number concentration showed no significant trend from 2013 to 2016, but an increasing trend from 2016 to 2019, which was related to the NPF events occurrence frequency. The regional NPF events of "banana shape" accounted for an increasing fraction of all NPF events. As a key parameter influencing the NPF event, the condensation sink decreased by approximately 63% from 2013 to 2019. Moreover, the estimated sulfuric acid concentration decreased by approximately 50%, with a higher reduction rate occurring during 2013-2016 as result of the effective SO2 reduction. Surface meteorological factors (including the air temperature, relative humidity, air pressure, and wind) and the air masses origin were found to played minor roles in the long-term trend of NPF events. As PNSD and NPF events are closely related to changes in the particle emissions and regional air pollution levels, studies concerning PNSD and NPF are necessary to provide important information regarding air qual-ity improvements and evaluating the efficacy of climate change mitigation strategies.