Comparison of GOME tropospheric NO2 columns withNO2 profiles deduced from ground-based in situ measurements

[摘要] Nitrogen dioxide (NO₂) vertical tropospheric column densities (VTCs)retrieved from the Global Ozone Monitoring Experiment (GOME) are compared tocoincident ground-based tropospheric NO₂ columns. The ground-basedcolumns are deduced from in situ measurements at different altitudes in theAlps for 1997 to June 2003, yielding a unique long-term comparison of GOMENO₂ VTC data retrieved by a collaboration of KNMI (Royal NetherlandsMeteorological Institute) and BIRA/IASB (Belgian Institute for Space Aeronomy)with independently derived tropospheric NO₂ profiles. A first comparisonrelates the GOME retrieved tropospheric columns to the tropospheric columnsobtained by integrating the ground-based NO₂ measurements. For a secondcomparison, the tropospheric profiles constructed from the ground-basedmeasurements are first multiplied with the averaging kernel (AK) of the GOMEretrieval. The second approach makes the comparison independent from the apriori NO₂ profile used in the GOME retrieval. This allows splitting thetotal difference between the column data sets into two contributions: one thatis due to differences between the a priori and the ground-based NO₂ profileshapes, and another that can be attributed to uncertainties in both the remainingretrieval parameters (such as, e.g., surface albedo or aerosol concentration)and the ground-based in situ NO₂ profiles. For anticyclonic clear sky conditionsthe comparison indicates a good agreement between the columns (n=157, R=0.70/0.74 forthe first/second comparison approach, respectively). The mean relative difference(with respect to the ground-based columns) is −7% with a standard deviation of40% and GOME on average slightly underestimating the ground-based columns.Both data sets show a similar seasonal behaviour with a distinct maximum of springNO₂ VTCs. Further analysis indicates small GOME columns being systematicallysmaller than the ground-based ones. The influence of different shapes in the apriori and the ground-based NO₂ profile is analysed by considering AKinformation. It is moderate and indicates similar shapes of the profiles forclear sky conditions. Only for large GOME columns, differences between theprofile shapes explain the larger part of the relative difference. Incontrast, the other error sources give rise to the larger relativedifferences found towards smaller columns. Further, for the clear sky cases,errors from different sources are found to compensate each other partially.The comparison for cloudy cases indicates a poorer agreement between thecolumns (n=60, R=0.61). The mean relative difference between the columns is60% with a standard deviation of 118% and GOME on averageoverestimating the ground-based columns. The clear improvement afterinclusion of AK information (n=60, R=0.87) suggests larger errors in the apriori NO₂ profiles under cloudy conditions and demonstrates theimportance of using accurate profile information for (partially) clouded scenes.

[学科分类] 大气科学