Validation of Urban \(NO_2\) 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 \(NO_2\) concentrations measured in Israeli cities to coinciding retrievals of tropospheric \(NO_2\) columns from satellite sensors (SCIAMACHY aboard ENVISAT and OMI aboard Aura). This provides a large statistical data set for validation of \(NO_2\) 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 \(NO_2\) is well-mixed throughout the boundary layer (BL), and using observed average seasonal boundary layer heights, near-surface \(NO_2\) concentrations are converted into BL \(NO_2\) columns. The agreement between OMI and (13:45) BL \(NO_2\) 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 \(NO_2\). OMI and BL \(NO_2\) columns show consistent seasonal cycles (winter \(NO_2\) 1.6–2.7× higher than summer). BL and coinciding OMI columns both show a strong weekly cycle with 45–50% smaller \(NO_2\) 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) \(NO_2\) 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 \(NO_2\) in summer than in winter.