First comparison between ground‐based and satellite‐borne measurements of tropospheric nitrogen dioxide in the Po basin

Abstract

In this paper we present in situ and tropospheric column measurements of NO2 in the Po river basin (northern Italy). The aim of the work is to provide a quantitative comparison between ground‐based and satellite measurements in order to assess the validity of spaceborne measurements for estimating NO2 emissions and evaluate possible climatic effects. The study is carried out using in situ chemiluminescent instrumentation installed in the Po valley, a UV/Vis spectrometer installed at Mount Cimone (44.2°N, 10.7°E, 2165 m asl), and tropospheric column measurements obtained from the Global Ozone Monitoring Experiment (GOME) spectrometer. Results show that the annual cycle in surface concentrations and also some specific pollution periods observed by the air quality network are well reproduced by the GOME measurements. However, tropospheric columns derived from the surface measurements assuming a well‐mixed planetary boundary layer (PBL) are much larger than the GOME columns and also have a different seasonal cycle. This is interpreted as indication of a smaller and less variable mixing height for NO2 in the boundary layer. Under particular meteorological conditions the agreement between UV/Vis tropospheric column observations and GOME measurements in the Mount Cimone area is good (R2 = 0.9) with the mixing properties of the atmosphere being the most important parameter for a valid comparison of the measurements. However, even when the atmospheric mixing properties are optimal for comparison, the ratio between GOME and ground‐based tropospheric column data may not be unity. It is demonstrated that the values obtained (less than 1) are related to the fraction of the satellite ground pixel occupied by the NO2 hot spot.