Characterizing tropospheric ozone and CO around Frankfurt between 1994–2012 based on MOZAIC-IAGOS aircraft measurements

Abstract

Abstract. In the framework of the MOZAIC-IAGOS program, ozone and carbon monoxide vertical profiles are available since 1994 and 2002, respectively. This study investigates the variability and trends of both species at several tropospheric layers above the Frankfurt and Munich airports where about 21 600 flights have been performed over the 1994–2012 period, which represents the densest dataset in the world (about 75 flights per month on average). Over that period, most mean ozone trends are positive but insignificant at a 95 % confidence level, except in winter where significant upward trends (around +0.38 ± 0.24 ppb yr−1) are found. Conversely, a significant increase of annual background ozone is highlighted, mostly during winter and autumn. Mean annual ozone trends increase with altitude, the largest increase being found in summer due to a noticeable decrease of highest ozone concentrations observed in the lower troposphere during the second half period. Over the 2002–2012 period, most mean CO trends are significantly negative, the decrease being higher in the lower troposphere compared to the mid- and upper troposphere with again, major differences in summer. Trends in the ozone seasonal cycle are also investigated, with a focus on the phase. Ozone maxima occur earlier and earlier with a shift around −10.6 ± 2.9 days decade−1 in the lower troposphere, in agreement with previous studies. The analysis of other ozone datasets in Europe (including surface stations and ozone soundings) confirms this trend, but highlights strong heterogeneities in the phase change. Interestingly, this shift is shown to decrease with altitude, with trends of −4.3 ± 2.4 and −2.0 ± 1.7 days decade−1 in the mid- and upper troposphere, respectively. The geographic origin of the air masses sampled by aircraft is analysed with FLEXPART backward simulations and suggests, together with trends and phase changes results, that an increase of the Asian contribution to ozone in the upper troposphere may compensate during summer the decrease of European and North American contributions associated to emission control over these two regions.