Understanding the physical mechanisms of PM2.5 formation in Seoul, Korea: assessing the role of aerosol direct effects using the WRF-CMAQ model

This study evaluated the impact of the aerosol direct effect (ADE) on the meteorological conditions and air quality in Seoul, South Korea (Korea), using the WRF-CMAQ coupled model. Two experiments were conducted: NF simulation without the ADE feedback and YF simulation with the ADE feedback. The ADE resulted in a decrease in shortwave radiation at the surface and an increase in shortwave radiation in the atmosphere. Additionally, the 2-m temperature, 10-m wind speed, and planetary boundary layer (PBL) height decreased. The changes in meteorological conditions due to ADE-induced atmospheric stability and restrained vertical mixing resulted in deteriorated air quality. During the simulation, PM_2.5 concentrations increased due to ADE, with daily averages rising by 4.2 µg·m⁻³ on transport day and 3.2 µg·m⁻³ on accumulation day. Process analysis (PA) was employed to investigate contributions of physical/chemical processes affecting ADE. Local emissions, aerosol processes, and horizontal advection were identified as key factors in PM_2.5 increases in Seoul. Differences in the contributions of processes were noted between transport and accumulation day due to ADE considerations. On transport day, the YF simulation exhibited a higher influx of PM_2.5 through horizontal advection, attributed to transport from upwind regions. Conversely, on accumulation day, horizontal advection decreased due to enhanced atmospheric stability, while vertical transport increased from restrained vertical mixing induced by ADE. Thus, ADE emphasized the contribution of processes to PM_2.5. Emissions and horizontal advection were primary contributors on transport day, while the emissions process dominated on accumulation day.