NO2 lidar profile measurements for satellite interpretation and validation

Satellite instruments are efficient detectors of air pollutants such as NO2. However, the interpretation of satellite retrievals is not a trivial matter. We describe a novel instrument, the RIVM NO2 mobile lidar, to measure tropospheric NO2 profiles for the interpretation and validation of satellite data. During the DANDELIONS campaign in 2006 we obtained an extensive collection of lidar NO2 profiles, coinciding with OMI and SCIAMACHY overpasses. On clear days and early mornings a comparison between lidar and in situ measurements showed excellent agreement. At other times the in situ monitors with molybdenum converters suffered from NOy interference. The lidar NO2 profiles indicated a well-mixed boundary layer, with high NO2 concentrations in the boundary layer and concentrations above not differing significantly from zero. The boundary layer concentrations spanned a wide range, which likely depends on the wind directions and on the intensity of local (rush hour) traffic which varies with the day of the week. Large diurnal differences were mainly driven by the height of the boundary layer, although direct photolysis or photochemical processes also contribute. Small-scale temporal and spatial variations in the NO2 concentrations of the order of 20-50% were measured, probably indicative of small-scale eddies. A preliminary comparison between satellite and lidar data shows that the satellite data tend to overestimate the amount of NO2 in the troposphere compared to the lidar data.