Evaluation and Interpretation of NO2 Measurements during the DISCOVER-AQ and KORUS-AQ Field Campaigns

Nitrogen dioxide (NO₂) is an important trace species in the troposphere; it has adverse human health effects and also contributes to the formation of tropospheric ozone, a criteria pollutant, and climate agent. Various types of NO₂ measurements were made during DISCOVER-AQ and KORUS-AQ field campaigns, including ground-based surface and airborne in-situ NO₂ mixing ratio, ground-based Pandora total column, and Ozone Monitoring Instrument (OMI) tropospheric column. We conduct a comprehensive analysis of these measurements. NO2 vertical profiles and columns are obtained from in-situ/ground-based measurements. OMI tropospheric column NO2 is adjusted by two methods for a more accurate retrieval of tropospheric NO2 abundances specifically for the campaigns. First, using high-resolution Community Multi-scale Air Quality (CMAQ) model simulations, the spatially coarse OMI NO2 pixels are downscaled into a finer spatial grid for comparisons with in-situ/ground-based measurements capturing small-scale features. Second, we utilize the profiles from the in-situ measurements during the campaigns as a priori profiles in the tropospheric air mass factor (AMF) calculation. We analyze the tropospheric NO2 columns from OMI and in-situ/ground-based measurements to evaluate OMI NO2 column measurement and to investigate the effects of accuracy of a priori profiles in the OMI NO2 column retrieval.