Aerosol-heat flux interactions in the boundary layer during the SIJAQ campaign

The Satellite Integrated Joint Monitoring of Air Quality (SIJAQ) field campaign, carried out from October 10 to November 25, 2021, included the measurement of detailed heat flux and air pollutants. In this study, we interpreted interactions between particulate matter (PM2.5) and sensible heat flux (SHF) in the planetary boundary layer (PBL) over the Seoul metropolitan area during the SIJAQ campaign based on measurements and on- and offline WRF-Chem model simulations. Heat flux data obtained during the SIJAQ campaign was varied, depending on meteorological variables, including cloud cover, and revealed meaningful interactions between PM2.5 and SHF. Throughout the campaign, the measured PM2.5-SHF during the daytime (10:00-16:00 LST) showed overall negative correlation with the correlation coefficient (R) of -0.38. During the campaign, the measured SHF on average was 68 +/- 38 W/m(2) on high PM2.5 days (daily mean PM2.5 >= 50 mu g/m(3)), which is significantly lower (35% decrease) compared to the SHF of 105 +/- 57 W/m(2) on low PM2.5 days (daily mean PM2.5 < 50 mu g/m(3)). Both on- and offline WRF-Chem simulation also showed that SHF and PM2.5 were negatively correlated throughout the campaign with R of -0.49, in good agreement with the observations. As the comparative case study, we selected two extreme cases of 3-day periods with the lowest and highest PM2.5 concentrations and aerosol effects were estimated based on both on- and offline WRF-Chem simulation and the case-specific aerosol-cloud-SHF-PBL relationship was explored. The results showed that the simulated aerosol-induced SHF was similar to 14% (15 W/m(2)) lower on the highest PM2.5 case during the campaign, whereas there was little or no significant relationship on the lowest PM2.5 concentration case. Our modeling study also revealed the underlying impact of PM2.5 concentrations on reducing SHF (with a slope = -0.41), and this reduction follows a specific sequence of processes: elevated PM2.5 level led to a decrease in SHF, weakening the PBL evolution and reducing vertical mixing, ultimately resulting in high PM2.5 concentrations accumulated in the PBL over the Seoul metropolitan area.