Tropospheric NO₂ column densities deduced from zenith-sky DOAS measurements in Shanghai, China, and their application to satellite validation

Abstract

Abstract. Zenith-sky scattered sunlight observations using differential optical absorption spectroscopy (DOAS) technique were carried out in Shanghai, China (31.3° N, 121.5° E) since December 2006. At this polluted urban site, the measurement provided NO2 total columns in the daytime. Here, we present a new method to extract time series of tropospheric vertical column densities (VCD) of NO2 from these observations. The derived tropospheric NO2 VCD is an important quantity for the estimation of emissions and for the validation of satellite observations. Our method makes use of assumptions on the relative NO2 height profiles and on the diurnal variation of the stratospheric NO2 VCD. The influence of these parameters on the retrieved tropospheric NO2 VCD is discussed; for a polluted site like Shanghai, the accuracy of our method is estimated to be <20% for solar zenith angle (SZA) lower than 85°. From simultaneously performed long-path DOAS measurement, the NO2 surface concentration at the same site was observed and the corresponding tropospheric NO2 VCD was estimated using the assumed seasonal NO2 profiles in the planetary boundary layer (PBL). By making a comparison between the tropospheric NO2 VCD from zenith-sky and long-path DOAS measurements, it was found that the former provided more realistic information about total tropospheric pollution than the latter, so it's more suitable for satellite data validation than the in situ measurement. A comparison between the tropospheric NO2 VCD from ground-based zenith-sky measurement and SCIAMACHY was also made. Satellite validation for a strongly polluted area is highly needed, but exhibits also a great challenge. Our comparison showed good agreement, considering in particular the different spatial resolutions between the two measurements.