Derivation of tropospheric NO3profiles using off-axis differential optical absorption spectroscopy measurements during sunrise and comparison with simulations

Abstract

[1] Early morning vertical profiles of tropospheric NO3 were derived by spectroscopy of scattered sunlight in off-axis geometry during sunrise. The measurements were carried out in the urban area of Heidelberg, Germany (April, July, and August 1999). The retrieval algorithm is based on the nitrate radical's rapid photolysis during sunrise, radiative transfer, as well photochemical modeling. We derived NO3 near-ground concentrations of typically (0.2–18) × 107 cm−3, concentrations above 3 km of (5–50) × 107 cm−3, and a NO3 maximum at a height of ∼350 m with concentrations of (100–900) × 107 cm−3. Assuming the case of very high terpene levels even at higher altitudes, which is very unlikely for the atmospheric conditions during the measurements, we obtain a different mathematical solution of our inversion problem yielding the NO3 maximum at altitudes of ∼2.5 km. The enriched layer was found to hold the bulk of the tropospheric nighttime NO3. The retrieved profile confirms earlier suggestions that ground-level measurements may be significantly underestimating the oxidative capacity of the boundary layer under stable nocturnal conditions. The NO3 layer is probably formed as a result of the vertical profiles of the NO3 educts (NO2 and O3), with NO2 concentrations falling off more slowly with height than the NO3 scavengers, e.g., anthropogenic NO as well as volatile organic compounds emitted at ground level. Independently from these measurements model simulations with a comprehensive three-dimensional model system were performed for an area, which includes the measurement site. A pronounced maximum (3.3 × 109 cm−3) of the nocturnal NO3 concentration was found at ∼250 m above the surface, i.e., at the top of the nocturnal stable boundary layer. The average NO3 concentration close to the surface was 5 × 107 cm−3. The only significant difference between the observations and the model results was a steeper decrease of the NO3 concentration above the maximum of the observed profiles.