SO₂ Observations and Sources in the Western Pacific Tropical Tropopause Region

Abstract

Sulfur dioxide (SO 2 ) is one of the primary source gases for aerosols in the atmosphere. Even at low concentrations, its presence in the upper troposphere and lower stratosphere (UT/LS) provides an important source for aerosol nucleation and growth, and SO 2 has been postulated to be important for the stratospheric sulfur budget. To understand aerosol nucleation and global radiative effects, it is therefore important to quantify how much SO 2 emitted from biogenic and anthropogenic processes at the surface is transported into the UT/LS. Much of the potential transport is expected to occur in regions of significant deep convection such as the western Pacific warm pool. Here we use measurements from the National Aeronautics and Space Administration (NASA) WB-57F Pacific Oxidants Sulfur Ice and DehydratiON (POSIDON) mission that was based in Guam in 2016 to examine the distribution and sources of SO 2 in the western Pacific UT/LS region. During POSIDON, a few events related to volcanic emissions as well as transport by tropical cyclones were observed that brought up to 86 pptv of SO 2 above 14-km altitude. Overall however, the measurements corroborate our earlier findings that SO 2 near the tropical tropopause is typically quite low (~5 pptv) and globally the transport of SO 2 into the stratosphere is likely a minor source of stratospheric aerosols.

Improved tropospheric and stratospheric sulfur cycle in the aerosol–chemistry–climate model SOCOL-AERv2

“…One laboratory study has identified the SO 2 + H 2 O 2 reaction on the surface of ice as a possible sink for SO 2 , although complicating factors, like partial-pressure-dependent reaction prob- abilities and surface poisoning during the reaction, make it difficult to extrapolate the measurements to atmospheric conditions (Clegg and Abbatt, 2001). Furthermore, Rotstayn and Lohmann (2002) found improved model agreement with Arctic sulfate measurements when they included SO 2 oxidation also in ice water. In addition, physical uptake of SO 2 without conversion to S(VI) on ice has been observed in the laboratory (Huthwelker et al, 2001) and may lead to gravitational settling; uptake of SO 2 on ice is not considered in either SOCOL-AERv1 or v2.…”

Author response to both referees

“…One laboratory study has identified the SO 2 + H 2 O 2 reaction on the surface of ice as a possible sink for SO 2 , although complicating factors, like partial-pressure-dependent reaction prob- (Deshler et al, 2003;Deshler, 2008) abilities and surface poisoning during the reaction, make it difficult to extrapolate the measurements to atmospheric conditions (Clegg and Abbatt, 2001). Furthermore, Rotstayn and Lohmann (2002) found improved model agreement with Arctic sulfate measurements when they included SO 2 oxidation also in ice water. In addition, physical uptake of SO 2 without conversion to S(VI) on ice has been observed in the laboratory (Huthwelker et al, 2001) and may lead to gravitational settling; uptake of SO 2 on ice is not considered in either SOCOL-AERv1 or v2.…”

Improved tropospheric and stratospheric sulfur cycle in the aerosol-chemistry-climate model SOCOL-AERv2

“…There has been an ongoing debate in the literature regarding the magnitude of the cross-tropopause SO 2 flux and its relative importance in establishing the Junge layer (Rollins et al, 2018). The debate has been fueled by a lack of in situ measurements in the UTLS region, and the high temporal and spatial variability of UTLS SO 2 .…”

“…The debate has been fueled by a lack of in situ measurements in the UTLS region, and the high temporal and spatial variability of UTLS SO 2 . Figure 5 compares the tropical UTLS SO 2 measured by two aircraft campaigns (Rollins et al, 2017(Rollins et al, , 2018 with two annual mean satellite products, MIPAS (Höpfner et al, 2015) and ACE-FTS (Doeringer et al, 2012), averaged during volcanically quiescent periods. The two in situ measurements and the ACE-FTS satellite product all show SO 2 mixing ratios of 5 to 10 pptv around the tropical tropopause (∼17 km).…”