Characterization of OMI tropospheric NO2 measurements in East Asia based on a robust validation comparison

Abstract

In the period from June 2006 to December 2008, we measured the tropospheric nitrogen dioxide (NO2) column by ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) at an urban site in China (Tai'an) and three sites in Japan, covering urban (Yokosuka), suburban (Tsukuba), and remote areas (Hedo). This robust dataset is used to characterize Ozone Monitoring Instrument (OMI) tropospheric NO2 column data (the standard product, version 3). Correlations between MAX-DOAS and OMI data, both of which show very low NO2 at Hedo and moderate/high levels at the other sites, have correlation coefficients (R2) as high as 0.64, indicating that relative changes in OMI NO2 data are reliable. However, OMI data have a negative bias of 31% on average. Assuming that these results are valid for OMI data taken over China, we find an increasing trend in tropospheric column NO2 at about 5% per year on average in the industrial areas of China (30°-40°N and 110°-123°E) over 2005-2008, but its spatial distribution is highly inhomogeneous.