Summertime response of ozone and fine particulate matter to mixing layer meteorology over the North China Plain

Measurements of surface ozone (O-3 ), PM2.5 and its major secondary components (SO42- , NO3- , NH4+ , and organic carbon (OC)), mixing layer height (MLH), and other meteorological parameters were made in the North China Plain (NCP) during the warm season (June-July) in 2021. The observation results showed that the summertime regional maximum daily 8 h average ozone (MDA8 O-3 ) initially increased and reached the maximum value (195.88 mu g m (-3) ) when the MLH ranged from approximately 900 to 1800 m, after which the concentration of O 3 decreased with further increase in MLH. Interestingly, synchronous increases in PM2.5 concentration along with the development of the mixing layer (MLH < 1200 m) were observed, and the positive response of PM2.5 to MLH was significantly associated with the increase in SO42- and OC. It was found that this increasing trend of PM2.5 with elevated MLH was driven not only by the wet deposition process but also by the enhanced secondary chemical formation, which was related to appropriate meteorological conditions (50 % < RH < 70 %) and increased availability of atmospheric oxidants. Air temperature played a minor role in the change characteristics of PM2.5 concentration, but it greatly controlled the different change characteristics of SO42- and NO3- . The concentrations of PM2.5 , its major secondary components, and the oxidation ratios of sulfate (SOR) and nitrate (NOR) increased synchronously with elevated MDA8 O 3 concentrations, and the initial increase in PM(2.5 )along with increased MLH corresponded well with that of MDA8 O-3 . We highlight that the correlation between MLH and secondary air pollutants should be treated with care in hot weather, and the superposition-composite effects of PM2.5 and O-3 along with the evolution of mixing layer should be considered when developing PM2.5 -O-3 coordinated control strategies.