Validation of urban NO₂ concentrations and their diurnal and seasonal variations observed from the SCIAMACHY and OMI sensors using in situ surface measurements in Israeli cities

Abstract. We compare a full-year (2006) record of surface air NO₂ concentrations measured in Israeli cities to coinciding retrievals of tropospheric NO₂ columns from satellite sensors (SCIAMACHY aboard ENVISAT and OMI aboard Aura). This provides a large statistical data set for validation of NO₂ satellite measurements in urban air, where validation is difficult yet crucial for using these measurements to infer NO_x emissions by inverse modeling. Assuming that NO₂ is well-mixed throughout the boundary layer (BL), and using observed average seasonal boundary layer heights, near-surface NO₂ concentrations are converted into BL NO₂ columns. The agreement between OMI and (13:45) BL NO₂ columns (slope=0.93, n=542), and the comparable results at 10:00 h for SCIAMACHY, allow a validation of the seasonal, weekly, and diurnal cycles in satellite-derived NO₂. OMI and BL NO₂ columns show consistent seasonal cycles (winter NO₂ 1.6–2.7× higher than summer). BL and coinciding OMI columns both show a strong weekly cycle with 45–50% smaller NO₂ columns on Saturday relative to the weekday mean, reflecting the reduced weekend activity, and validating the weekly cycle observed from space. The diurnal difference between SCIAMACHY (10:00) and OMI (13:45) NO₂ is maximum in summer when SCIAMACHY is up to 40% higher than OMI, and minimum in winter when OMI slightly exceeds SCIAMACHY. A similar seasonal variation in the diurnal difference is found in the source region of Cairo. The surface measurements in Israel cities confirm this seasonal variation in the diurnal cycle. Using simulations from a global 3-D chemical transport model (GEOS-Chem), we show that this seasonal cycle can be explained by a much stronger photochemical loss of NO₂ in summer than in winter.