Impacts of haze and nitrogen oxide alleviation on summertime ozone formation: A modeling study over the Yangtze River Delta, China.

The strict emission control measures have profoundly changed the air pollution in the Yangtze River Delta (YRD) region, China. However, the impacts of decreasing fine particulates (PM) and nitrogen oxide (NOx) on summer ozone (O) formation still remain disputable. We perform simulations in the 2018 summer over the YRD using the WRF-Chem model that considers the aerosol radiative forcing (ARF) and HO heterogeneous loss on aerosol surface. The model reasonably reproduces the measured spatiotemporal surface O and PM concentrations and aerosol compositions. Model sensitivity experiments show that the NOx mitigation during recent years changes daytime O formation in summer from the transition regime to the NOx-sensitive regime in the YRD. The decreasing NOx emission generally weakens O formation and lowers ambient O levels in summer during recent years, except for some urban centers of megacities. While, the haze alleviation characterized by a decline in ambient PM concentration in the past years largely counteracts the daytime O decrease caused by NOx mitigation, largely contributing to the persistently high levels of summertime O. The counteracting effect is dominantly attributed to the attenuated ARF and minorly contributed by the suppressed HO uptake and heterogeneous loss on aerosol surface. These results highlight that the repeated O pollution in the YRD is closely associated with NOx and haze alleviation and more efforts must be taken to achieve lower O levels.