NO2 inter-comparison between Pandora spectrometer and in-situ measurements during MAPS campaign in 2015

To improve the accuracy of air quality information from satellite retrieval algorithm and model simulations, National Institute of Environmental Research (NIER) in Korea and National Aeronautics and Space Administration (NASA) in the United States performed the Megacity Air Pollution Studies (MAPS) campaign from May to June, 2015 as a preliminary work for the KORUS-AQ campaign in 2016. In this study we investigate and evaluate the nitrogen dioxide (NO2) data at six sites obtained from the ground-based remote sensing using Pandora spectrometers and Air Korea surface in-situ measurements during MAPS campaign. Basically measured NO2 shows spatially large differences among sites and also temporally larger diurnal variations, particularly in urban areas. To conduct inter-comparisons between Pandora and in-situ NO2, we need to estimate the tropospheric portion of NO2 from Pandora total column measurements. First we simply use the ratio between tropospheric and total column NO2 density obtained from Ozone Monitoring Instrument (OMI) measurements to calculate the tropospheric NO2 density from Pandora total column density. As a result, correlation coefficients between Pandora tropospheric NO2 and in-situ surface NO2 are in the range from 0.13 to 0.61. To enhance a correlation, next we use NO2 vertical profiles obtained from chemical transport model (GEOS-Chem) in the process of Pandora tropospheric NO2 estimation, and also compare with in-situ NO2 data. Since planetary boundary layer (PBL) height is known as another key factor to capture the near-surface signal from the total column information, we will calculate a ground-level NO2 using PBL height information from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset.