Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> In this study, we present simultaneous airborne measurements of peroxyacetyl nitrate (PAN), ethane (C<span class="inline-formula">₂</span>H<span class="inline-formula">₆</span>), formic acid (HCOOH), methanol (CH<span class="inline-formula">₃</span>OH), and ethylene (C<span class="inline-formula">₂</span>H<span class="inline-formula">₄</span>) above the South Atlantic in September and October 2019. Observations were obtained from the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA), as two-dimensional altitude cross sections along the flight path. The flights were part of the SouthTRAC (Transport and Composition in the Southern Hemisphere Upper Troposphere/Lower Stratosphere) campaign with the German High Altitude and Long Range Research Aircraft (HALO). On two flights (8 September 2019 and 7 October 2019), large enhancements of all these substances were found between 7 and 14 km altitude with maximum volume mixing ratios (VMRs) of 1000 pptv for PAN, 1400 pptv for C<span class="inline-formula">₂</span>H<span class="inline-formula">₆</span>, 800 pptv for HCOOH, 4500 pptv for CH<span class="inline-formula">₃</span>OH, and 200 pptv for C<span class="inline-formula">₂</span>H<span class="inline-formula">₄</span>. One flight showed a common filamentary structure in the trace gas distributions, while the second flight is characterized by one large plume. Using backward trajectories, we show that measured pollutants likely reached upper troposphere and lower stratosphere (UTLS) altitudes above South America and central Africa, where elevated PAN VMRs are visible at the surface layer of the Copernicus Atmosphere Monitoring Service (CAMS) model during the weeks before both measurements. In comparison to results of the CAMS reanalysis interpolated onto the GLORIA measurement geolocations, we show that the model is able to reproduce the overall structure of the measured pollution trace gas distributions. For PAN, the absolute VMRs are in agreement with the GLORIA measurements. However, C<span class="inline-formula">₂</span>H<span class="inline-formula">₆</span> and HCOOH are generally underestimated by the model, while CH<span class="inline-formula">₃</span>OH and C<span class="inline-formula">₂</span>H<span class="inline-formula">₄</span>, the species with the shortest atmospheric lifetimes of the pollution trace gases discussed, are overestimated by CAMS. The good agreement between model and observations for PAN suggests that the general transport pathways and emissions locations are well captured by the model. The poorer agreement for other species is therefore most likely linked to model deficiencies in the representation of loss processes and emission strength.