Reply on CC1

Abstract

We report on airborne Limb and Nadir measurements of vertical profiles and total vertical column densities (VCDs) of glyoxal (C2H2O2) in the troposphere, which were performed from aboard the German research aircraft HALO (High Altitude and Long Range) in different regions and seasons around the globe between 2014 and 2019. The airborne Nadir and integrated Limb profiles excellently agree among each other. Our airborne observations are further compared to collocated glyoxal measurements of the TROPOspheric Monitoring Instrument (TROPOMI), with good agreement between both data sets for glyoxal observations in (1) pristine terrestrial, (2) pristine marine, (3) mixed polluted, and (4) biomass burning affected air masses with high glyoxal concentrations. Exceptions from the overall good agreement are observations of (1) faint and aged biomass burning plumes over the oceans and (2) of low lying biomass burning or anthropogenic plumes in the terrestrial or marine boundary layer, and (3) plumes detected under heavy aerosol loud, both of which contain elevated glyoxal that is mostly not captured by TROPOMI. These differences of airborne and satellite detected glyoxal are most likely caused by the overall small contribution of plumes of limited extent to the total atmospheric absorption by glyoxal and the difficulty to remotely detect weak absorbers located close to low reflective surfaces (e.g. the ocean in the visible wavelength range), or within dense aerosol layers. Observations of glyoxal in aged biomass burning plumes (e.g. observed over the Tropical Atlantic off the coast of West Africa in summer 2018, off the coast of Brazil by the end of the dry season 2019, and the East China Sea in spring 2018) could be traced back to related wildfires, such as a plume crossing over the Drake Passage that originated from the Australian bushfires in late 2019. Our observations of glyoxal in these over days aged biomass burning plumes thus confirm recent findings of enhanced glyoxal and presumably secondary aerosol (SOA) formation in aged wildfire plumes from yet to be identified longer-lived organic precursor molecules (e.g. aromatics, acetylene, or aliphatic compounds) co-emitted in the fires. Further, elevated glyoxal (median 44 ppt) as compared to other marine regions (median 10–19 ppt) is observed in the boundary layer over the tropical oceans, well in agreement with previous reports. The airborne data sets are further compared to glyoxal simulations performed with the global atmosphere-chemistry model EMAC (ECHAM/MESSy Atmospheric Chemistry). When using an EMAC setup that resembles recent EMAC studies focusing on complex chemistry, reasonable agreement is found for pristine air masses (e.g. the unperturbed free and upper troposphere), but notable differences exist for regions with high emissions of glyoxal and glyoxal producing volatile organic compounds (VOC) from the biosphere (e.g. the Amazon), mixed emissions from anthropogenic activities (e.g. continental Europe, the Mediterranean and East China Sea), and potentially from the sea (e.g. the tropical oceans). Also, the model tends to largely under-predict glyoxal in city plumes and aged biomass burning plumes. The potential causes for these differences are likely to be multifaceted, but they all point to missing glyoxal sources from the degradation of the cocktail of (potentially longer-chained) organic compounds emitted from anthropogenic activities, biomass burning, and from the organic micro-layer of the sea.