Enhanced sulfur in the upper troposphere and lower stratosphere in spring 2020

Abstract

Abstract. Sulfur compounds in the upper troposphere and lower stratosphere (UTLS) impact the atmosphere radiation budget, either directly as particles or indirectly as precursor gas for new particle formation. In situ measurements in the UTLS are rare but are important to better understand the impact of the sulfur budget on climate. The BLUESKY mission in May and June 2020 explored an unprecedented situation. (1) The UTLS experienced extraordinary dry conditions in spring 2020 over Europe, in comparison to previous years, and (2) the first lockdown of the COVID-19 pandemic caused major emission reductions from industry, ground, and airborne transportation. With the two research aircraft HALO and Falcon, 20 flights were conducted over central Europe and the North Atlantic to investigate the atmospheric composition with respect to trace gases, aerosol, and clouds. Here, we focus on measurements of sulfur dioxide (SO2) and particulate sulfate (SO42-) in the altitude range of 8 to 14.5 km which show unexpectedly enhanced mixing ratios of SO2 in the upper troposphere and of SO42- in the lowermost stratosphere. In the UT, we find SO2 mixing ratios of (0.07±0.01) ppb, caused by the remaining air traffic, and reduced SO2 sinks due to low OH and low cloud fractions and to a minor extent by uplift from boundary layer sources. Particulate sulfate showed elevated mixing ratios of up to 0.33 ppb in the LS. We suggest that the eruption of the volcano Raikoke in June 2019, which emitted about 1 Tg SO2 into the stratosphere in northern midlatitudes, caused these enhancements, in addition to Siberian and Canadian wildfires and other minor volcanic eruptions. Our measurements can help to test models and lead to new insights in the distribution of sulfur compounds in the UTLS, their sources, and sinks. Moreover, these results can contribute to improving simulations of the radiation budget in the UTLS with respect to sulfur effects.

The influence of extratropical cross-tropopause mixing on the correlation between ozone and sulfate aerosol in the lowermost stratosphere

“…Figure 3 shows that direct mixing of sulfate aerosol was not observed during the campaign, due to the low sulfate mixing ratios at the chemical tropopause, so some other processes need to be taken into account. Previous studies with focus on the Raikoke eruption in 2019 determined no significant contribution from this volcanic eruption (Tomsche et al, 2022;Reifenberg et al, 2022). The following section will show the influence of cross-tropopause mixing, especially of the precursor gas SO 2 and the potential influence of a more current volcanic eruption.…”

“…The one sigma detection limit is 0.0006 to 0.0017 ppbv and 0.005 to 0.009 ppbv for SO 2 and HNO 3 , respectively. The total uncertainty for SO 2 is 22.7 % (Tomsche et al, 2022) and 16 % for HNO 3 (Marsing, 2021).…”

“…Krüger et al (2022) found substantial reduction in aerosol particles in the lower troposphere in this period and Reifenberg et al (2022) could explain the observed reduction in some tracer concentrations with the reduced emissions of pollutants. Tomsche et al (2022) investigated the SO2 concentrations in the UTLS region above Europe, which was influenced by changes in sulfur sources such as aviation as well as sinks. Here we focus on the transport processes in the extratropical transition layer, which has been probed with a set of instrumentations onboard HALO and DLR-Falcon.…”

“…An additional in-situ data set is provided by the atmospheric chemical ionization mass spectrometer (AIMS) deployed on DLR-Falcon and includes information on gaseous SO 2 and HNO 3 . For the detection of upper tropospheric and lower stratospheric SO 2 and HNO 3 mixing ratios the AIMS uses SF 5 − reagent ions (Voigt et al, 2014;Jurkat et al, 2016;Marsing et al, 2019;Tomsche et al, 2022). The one sigma detection limit is 0.0006 to 0.0017 ppbv and 0.005 to 0.009 ppbv for SO 2 and HNO 3 , respectively.…”

“…While degassing volcanoes contribute to the tropospheric SO 2 budget (Voigt et al, 2014), explosive eruptions can directly inject SO 2 into the lower stratosphere (Kremser et al, 2016). Another direct sulfate aerosol source are aircraft which emit soot and volatile sulfate containing particles at cruise altitudes into the upper troposphere/lower stratosphere (Voigt et al, 2010;Williamson et al, 2021;Tomsche et al, 2022).…”