Effects of black carbon and boundary layer interaction on surface ozone in Nanjing, China
Atmospheric Chemistry and Physics Discussions
As an important solar-radiation absorbing aerosol, the effect of black carbon (BC) on surface ozone, by influencing photolysis rate, has been widely discussed by <q>offline</q> model studies. However, BC-boundary layer (BL) interactions also influence surface ozone. Using the <q>online</q> model simulations and processes analysis, we demonstrate the significant impact of BC-BL interaction on surface ozone. The absorbing effect of BC heats the air above the BL and suppresses BL development,
... eventually leads to changes in the contributions of ozone through chemical and physical processes (photochemistry, vertical mixing, and advection). Different from previous <q>offline</q> model studies, BL suppression leads large amounts of ozone precursors being confined below the BL which offsetting the influence from the reduction of photolysis rate, thus enhancing ozone photochemical formation before noon. Furthermore, the changes in physical process show a more significant influence on surface ozone. The weakened turbulence entrains much less ozone from the overlying ozone-rich air down to surface. As a result, the net contribution of ozone from physical and chemical processes leads to surface ozone reduction before noon. The maximum reduction reaches to 16.4&thinsp;ppb at 12:00. In the afternoon, the changes in chemical process are small which influence inconspicuously to surface ozone. However, physical process still influences the surface ozone significantly. Due to the delayed development of the BL, less vertically mixed BL continues to show an obvious ozone gradient near the top of the BL. Therefore, more ozone aloft can be entrained down to the surface, offsetting the surface ozone reduction. Comparing all the changes in the contributions of processes, the change in the contribution of vertical mixing plays a more important role in impacting surface ozone. Our results show the great impacts of BC-BL interactions on surface ozone. And more attention should be paid on the mechanism of aerosol-BL interactions when we deal with the ozone pollution control in China.