Diurnal variation of columnar and surface in-situ HCHO over Seoul in SIJAQ 2021

The GEMS capability to monitor daily changes in key air pollutants was examined with ground-based remote sensing in the SIJAQ 2021 campaign (October 16 - November 11, 2021). In particular, HCHO, a type of GEMS primary product, plays an important role in urban photochemistry as a unique radical source. Diurnal cycles of both columnar and in situ HCHOs were measured in Seoul during SIJAQ 2021. Surface in situ HCHO was higher than toluene known to be abundant in Seoul during the KORUS-AQ period. We analyzed the sensitivities for the main factors controlling HCHO behavior using the land surface-meteorology-chemistry coupled 0-D box-model. Primary emissions, along with background HCHO, contributed significantly to the diurnal variation of HCHO in Seoul. Assuming that HCHO is well mixed within PBL, the vertical integration of modeled HCHO was well matched with  the columnar HCHO.