Tropospheric NO<sub>2</sub> 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 measurements provided NO 2 total columns in the daytime. Here, we present a new method to extract time series of tropospheric vertical column densities (VCDs) of NO 2 from these observations. The derived tropospheric NO 2 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 NO 2 height profiles and the diurnal variation of stratospheric NO 2 VCDs. The main error sources arise from the uncertainties in the estimated stratospheric slant column densities (SCDs) and the determination of tropospheric NO 2 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 longpath DOAS measurements, the NO 2 surface concentrations at the same site were observed and the corresponding tropospheric NO 2 VCDs were estimated using the assumed seasonal NO 2 profiles in the planetary boundary layer (PBL). By making a comparison between the tropospheric NO 2 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 NO 2 VCDs from ground-based zenith-sky measurements and SCIAMACHY was also made. Satellite validation for a strongly polluted area is highly needed, butCorrespondence to: B. Zhou (binzhou@fudan.edu.cn) 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 NO 2 .

Ship-based MAX-DOAS measurements of tropospheric NO&lt;sub&gt;2&lt;/sub&gt;, SO&lt;sub&gt;2&lt;/sub&gt;, and HCHO distribution along the Yangtze River

“…In this study, all radiative transfer calculations were performed by using the radiative transfer model SCIATRAN 2.2 (Rozanov et al, 2005). Previous studies show that the uncertainties caused by aerosol SSA, AP, and surface albedo assumptions are less than 10 % (Chen et al, 2009;Wang et al, 2012b). Uncertainty of lidar measurement of aerosol extinction profiles also contributes to the uncertainty in the AMF calculations.…”

“…A fixed set of single scattering albedo (SSA) of 0.95, asymmetry parameter (AP) of 0.68, and surface albedo of 0.06 is assumed in the radiative transfer calculations (Chen et al, 2009;Pinker et al, 1995). In this study, all radiative transfer calculations were performed by using the radiative transfer model SCIATRAN 2.2 (Rozanov et al, 2005).…”

Validation of OMI, GOME-2A and GOME-2B tropospheric NO&lt;sub&gt;2&lt;/sub&gt;, SO&lt;sub&gt;2&lt;/sub&gt; and HCHO products using MAX-DOAS observations from 2011 to 2014 in Wuxi, China: investigation of the effects of priori profiles and aerosols on the satellite products

“…However, validation studies for the satellite products in this region are still sparse. Chen et al (2009), Irie et al (2012, Kanaya et al (2014) and Chan et al (2015) validated the satellite NO 2 tropospheric VCD products using MAX-DOAS (or zenith-sky DOAS) measurements in Rudong, Hefei and Shanghai. So far there are no validation reports for SO 2 and HCHO products in the YRD region.…”

Tropospheric nitrogen dioxide column retrieval from ground-based zenith–sky DOAS observations

“…It is recommended, however, to use daily observations (or to take a weekly mean) of the stratospheric NO 2 amount in the absence of frequent tropospheric pollution events in order to reduce the uncertainties introduced by the temporal variance and/or seasonality of the stratospheric NO 2 content. In Chen et al (2009) a strategy was applied to determine the stratospheric contribution in case of a heavy polluted site. Due to severe tropospheric contamination at the measurement site in Shanghai (China), no clean reference day could be identified in the data set.…”

“…They affect the retrievals in a systematic way. In Chen et al (2009) and Wang et al (2012), a thorough sensitivity study is applied with varying input parameters in the radiative transfer simulations. The influence of parameters such as aerosol and NO 2 layer height, AOD and NO 2 profile, surface albedo, etc.…”