Abstract
We investigate the effects of aerosol optical depth (AOD), single scattering albedo (SSA), aerosol peak height (APH), measurement geometry (solar zenith angle (SZA) and viewing zenith angle (VZA)), relative azimuth angle, and surface reflectance on the accuracy of NO2 slant column density using synthetic radiance. High AOD and APH are found to decrease NO2 SCD retrieval accuracy. In moderately polluted (5 × 1015 molecules cm−2 < NO2 vertical column density (VCD) < 2 × 1016 molecules cm−2) and clean regions (NO2 VCD < 5 × 1015 molecules cm−2), the correlation coefficient (R) between true NO2 SCDs and those retrieved is 0.88 and 0.79, respectively, and AOD and APH are about 0.1 and is 0 km, respectively. However, when AOD and APH are about 1.0 and 4 km, respectively, the R decreases to 0.84 and 0.53 in moderately polluted and clean regions, respectively. On the other hand, in heavily polluted regions (NO2 VCD > 2 × 1016 molecules cm−2), even high AOD and APH values are found to have a negligible effect on NO2 SCD precision. In high AOD and APH conditions in clean NO2 regions, the R between true NO2 SCDs and those retrieved increases from 0.53 to 0.58 via co-adding four pixels spatially, showing the improvement in accuracy of NO2 SCD retrieval. In addition, the high SZA and VZA are also found to decrease the accuracy of the NO2 SCD retrieval.
Highlights
Nitrogen dioxide (NO2 ) is an important atmospheric trace gas because it adversely affects human health and plays a key role in the photochemistry of ozone [1,2]
This study investigates the effects of aerosol properties (aerosol optical depth (AOD), single scattering albedo (SSA), aerosol peak height (APH)), measurement geometry (solar zenith angle (SZA), and relative azimuth angle (RAA)), and surface reflectance (SFR) on the
True NO2 slant column density (SCD) are calculated from the true NO2 vertical column density (VCD) multiplied by the air mass factor (AMF) calculated from VLIDORT with the same input data used to generate synthetic radiances under low-AOD conditions (Figure 1)
Summary
Nitrogen dioxide (NO2 ) is an important atmospheric trace gas because it adversely affects human health and plays a key role in the photochemistry of ozone [1,2]. Valks et al [7], uncertainties in the tropospheric NO2 AMF in polluted regions due to uncertainties in cloud fraction, cloud top pressure, and a priori NO2 profile are 25%, 30% and 10%, respectively These spectral fitting errors occur due to uncertainties in the NO2 cross-section, spectral calibration, and instrument noise, such as dark current and stray light. Many previous studies have investigated uncertainties in DOAS NO2 retrieval in terms of atmospheric and surface condition, AMF and spectroscopy of the fitting method (fitting uncertainty). Their cross effects on the other factors (i.e., aerosol effects on NO2 SCD fitting precision) still remain uncertain. Changes in NO2 SCD precision are investigated using a spatial co-adding technique
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