Nonlinear relationship between air pollution and precursor emissions in Qingdao, eastern China

Exploring the nonlinear relationship between air pollution and precursor emissions in Qingdao, eastern China is crucial for improving air quality. We simulated 32 emission reduction scenarios based on different volatile organic compound (VOC) and nitrogen oxide (NOx) emission reduction ratios using the Weather Research and Forecasting-Comprehensive Air Quality Model Extensions model. The emission reduction of VOCs was beneficial for reducing fine particulate matter (PM2.5) concentration in January and ozone (O-3) concentration in June. However, NOx must be reduced by at least 48% and 70% to decrease PM2.5 and O-3 concentrations, respectively, when VOCs are not reduced. The responses of PM2.5 and O-3 concentrations to emission reductions from different sources were also evaluated. The reduction in VOC emissions from different sources decreased the PM2.5 concentration in January, and O-3 concertation in June, while NOx reduction resulted in an increase. Controlling VOC emissions from industry has a positive effect on improving local PM2.5 and O-3, while the emission reductions of NOx from transportation and industry are not conducive to reducing PM2.5 and O-3 concentrations. The synergistic emission reduction pathways for NOx and VOCs during PM2.5 and O-3 combined pollution were also analyzed. The VOC and NOx emission reductions were beneficial for reducing the comprehensive Air Quality Index (sAQI) values. When the NOx emission reduction was large, the sAQI improvement gradually exceeded that of VOCs. A collaborative optimization path should be adopted that focuses on controlling VOCs first, and further control of combined pollution should depend on the deep reduction of NOx.