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 measurements provided NO2 total columns in the daytime. Here, we present a new method to extract time series of tropospheric vertical column densities (VCDs) of NO2 from these observations. The derived tropospheric NO2 VCDs are important quantities 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 the diurnal variation of stratospheric NO2 VCDs. The main error sources arise from the uncertainties in the estimated stratospheric slant column densities (SCDs) and the determination of tropospheric NO2 air mass factor (AMF). For a polluted site like Shanghai, the accuracy of our method is conservatively estimated to be <25% for solar zenith angle (SZA) lower than 70°. From simultaneously performed long-path DOAS measurements, the NO2 surface concentrations at the same site were observed and the corresponding tropospheric NO2 VCDs were estimated using the assumed seasonal NO2 profiles in the planetary boundary layer (PBL). By making a comparison between the tropospheric NO2 VCDs from zenith-sky and long-path DOAS measurements, it is found that the former provides more realistic information about total tropospheric pollution than the latter, so it's more suitable for satellite data validation. A comparison between the tropospheric NO2 VCDs from ground-based zenith-sky measurements and SCIAMACHY was also made. Satellite validation for a strongly polluted area is highly needed, but exhibits also a great challenge. Our comparison shows good agreement, considering in particular the different spatial resolutions between the two measurements. Remaining systematic deviations are most probably related to the uncertainties of satellite data caused by the assumptions on aerosol properties as well as the layer heights of aerosols and NO2.
Highlights
Nitrogen dioxide (NO2) is one of the most important trace gases in tropospheric chemistry
The resulting tropospheric NO2 vertical column densities (VCDs) derived from zenith-sky measurements are firstly compared with the VCDs converted from the surface concentrations
By using a three-step strategy, the tropospheric NO2 VCD was derived (VCDtropo zenith), which is an important quantity for the estimation of emissions and for the validation of satellite observations
Summary
Nitrogen dioxide (NO2) is one of the most important trace gases in tropospheric chemistry. It directly participates in the photochemical formation of tropospheric ozone and contributes locally to radiative forcing (Solomon et al, 1999). Considering the importance of NO2 to human health and atmospheric chemistry, there have been many ground-based, air-borne and space-borne instruments carrying out NO2 observations. With the development of remote sensing techniques, especially the differential optical absorption spectroscopy (DOAS), the total amount of NO2 in the atmosphere can be acquired either from space or ground. After the launch of ERS-2 in 1995, the global distribution of total and tropospheric NO2 is mapped by the Global Ozone Monitoring Experiment (GOME) (Burrows et al, 1999b) which helps to improve the knowledge of atmospheric pollution and its transportation. Additional satellite instruments were launched since continuing the GOME time series: in 2002
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