LAUSR

The strictest ever Clean Air Action has been implemented since 2013 at nationwide scale in China. Nevertheless, PM2.5 (particulate matter with aerodynamic diameter smaller than 2.5 μm) mass and number of ultrafine particles (UFPs, the particles with diameters smaller than 100 nm) exhibited contrasting response to the emission mitigation policies. Unlike reduction of PM2.5 and number of accumulation mode particles, UFPs number decreased from 2012 to 2015, and then has begun to increase sharply since 2016, and annual change of number in nucleation mode particles can reach 40% at noontime during 2017–2019. We examined inter‐annual trends of mode diameter, frequency of new particle formation (NPF) events, formation and growth rates, and particles were found to be much finer and NPF events have been more frequent since 2017. The much higher temperature and stronger solar radiation induced by PM2.5 reduction could promote the formation of secondary aerosol particles at noon.