Abstract
Abstract. An improved tropospheric nitrogen dioxide (NO2) retrieval algorithm from the Global Ozone Monitoring Experiment-2 (GOME-2) instrument based on air mass factor (AMF) calculations performed with more realistic model parameters is presented. The viewing angle dependency of surface albedo is taken into account by improving the GOME-2 Lambertian-equivalent reflectivity (LER) climatology with a directionally dependent LER (DLER) dataset over land and an ocean surface albedo parameterisation over water. A priori NO2 profiles with higher spatial and temporal resolutions are obtained from the IFS (CB05BASCOE) chemistry transport model based on recent emission inventories. A more realistic cloud treatment is provided by a clouds-as-layers (CAL) approach, which treats the clouds as uniform layers of water droplets, instead of the current clouds-as-reflecting-boundaries (CRB) model, which assumes that the clouds are Lambertian reflectors. On average, improvements in the AMF calculation affect the tropospheric NO2 columns by ±15 % in winter and ±5 % in summer over largely polluted regions. In addition, the impact of aerosols on our tropospheric NO2 retrieval is investigated by comparing the concurrent retrievals based on ground-based aerosol measurements (explicit aerosol correction) and the aerosol-induced cloud parameters (implicit aerosol correction). Compared with the implicit aerosol correction utilising the CRB cloud parameters, the use of the CAL approach reduces the AMF errors by more than 10 %. Finally, to evaluate the improved GOME-2 tropospheric NO2 columns, a validation is performed using ground-based multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements at different BIRA-IASB stations. At the suburban Xianghe station, the improved tropospheric NO2 dataset shows better agreement with coincident ground-based measurements with a correlation coefficient of 0.94.
Highlights
Tropospheric nitrogen dioxide (NO2) is an important air pollutant that harms the human respiratory system, even over short exposure periods (Gamble et al, 1987; Kampa and Castanas, 2008), and contributes to the formation of tropospheric ozone, urban haze, and acid rain (Charlson and Ahlquist, 1969; Crutzen, 1970; McCormick, 2013)
The quality of Global Ozone Monitoring Experiment-2 (GOME-2) NO2 measurements is strongly related to the calculation of the air mass factor (AMF), which is determined by a radiative transfer model, depending on a set of model parameters, such as viewing geometry, surface albedo, vertical distribution of NO2, cloud, and aerosol
To account for the surface bidirectional reflectance distribution function (BRDF) in our NO2 AMF calculation over land, the surface reflectivity is described by a Global Ozone Monitoring Experiment (GOME)-2 dependent LER (DLER) dataset (Tilstra et al, 2019) that captures the viewing zenith angle (VZA) dependency
Summary
Tropospheric nitrogen dioxide (NO2) is an important air pollutant that harms the human respiratory system, even over short exposure periods (Gamble et al, 1987; Kampa and Castanas, 2008), and contributes to the formation of tropospheric ozone, urban haze, and acid rain (Charlson and Ahlquist, 1969; Crutzen, 1970; McCormick, 2013). The quality of GOME-2 NO2 measurements is strongly related to the calculation of the AMF, which is determined by a radiative transfer model, depending on a set of model parameters, such as viewing geometry, surface albedo, vertical distribution of NO2, cloud, and aerosol. High sensitivity towards absorbers near the surface is obtained for the smallest elevation angles, whereas measurements at higher elevations provide information on the rest of the column This technique allows for the determination of vertically resolved abundances of atmospheric trace species in the lowermost troposphere (Hönninger et al, 2004; Wagner et al, 2004; Wittrock et al, 2004; Heckel et al, 2005). 3, we improve the AMF calculation in the reference retrieval algorithm by accounting for the dependency of surface albedo on direction over land and over water, applying the advanced IFS (CBA) a priori NO2 profiles with higher model resolution, and implementing the more realistic CAL cloud model.
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