Retrieval of tropospheric NO 2 using the MAX-DOAS method combined with relative intensity measurements for aerosol correction

Multi-Axis Differential Optical Absorption Spec- troscopy (MAX-DOAS) is a technique to measure trace gas amounts in the lower troposphere from ground-based scat- tered sunlight observations. MAX-DOAS observations are especially suitable for validation of tropospheric trace gas observations from satellite, since they have a representative range of several kilometers, both in the horizontal and in the vertical dimension. A two-step retrieval scheme is presented here, to derive aerosol corrected tropospheric NO2 columns from MAX- DOAS observations. In a first step, boundary layer aerosols, characterized in terms of aerosol optical thickness (AOT), are estimated from relative intensity observations, which are de- fined as the ratio of the sky radiance at elevation and the sky radiance in the zenith. Relative intensity measurements have the advantage of a strong dependence on boundary layer AOT and almost no dependence on boundary layer height. In a second step, tropospheric NO2 columns are derived from differential slant columns, based on AOT-dependent air mass factors. This two-step retrieval scheme was applied to cloud free periods in a twelve month data set of observations in De Bilt, The Netherlands. In a comparison with AERONET (Cabauw site) a mean difference in AOT (AERONET mi- nus MAX-DOAS) of 0.01±0.08 was found, and a corre- lation of 0.85. Tropospheric-NO2 columns were compared with OMI-satellite tropospheric NO2. For ground-based ob- servations restricted to uncertainties below 10%, no signifi- cant difference was found, and a correlation of 0.88.