Measurement Report: Diurnal Variability of NO₂ and HCHO Lower Tropospheric Vertical Profiles in Southeastern Los Angeles

Abstract. Ground level ozone in excess of United States ambient air quality standards remains a prevalent issue across Southern California, particularly in the summer months. To improve our understanding of the vertical distribution of ozone precursors in southern California, we used ground-based MAX-DOAS measurements in Whittier CA to simultaneously retrieve both near-surface mole fractions and vertical column densities of both NO₂ and HCHO. While vertical column densities of NO₂ are well correlated with TROPOMI observations over the study period (R=0.77), HCHO VCDs and FNRs derived from MAX-DOAS observations are less well correlated (R=0.44 and 0.35, respectively). These observations also show differing diurnal cycles between near surface mixing ratios and vertical column densities due to variability in the vertical profile that will be increasingly critical to understand given the ongoing shift from sun synchronous to geostationary satellite observations. Ratios of HCHO to NO₂, commonly referred to as FNR, derived from satellite-based measurements are used to diagnose ozone production chemistry over regions without consistent surface based measurements. Using ground-based measurements, we determine FNRs using both surface mole fractions and vertical column densities, finding FNRs derived from surface mole fractions are generally lower than those derived from column based measurements. Evaluating ozone exceedance probability as a function of FNR for both quantities suggests the transition between a VOC limited and NOx limited regimes may begin at lower FNR values than those derived from satellite based measurements in East LA. We find these differences in FNR derived form ground based and satellite based measurements are driven by variability in the vertical distribution of HCHO. These impacts are most pronounced in late afternoon, when ozone exceedances are most prevalent.