The role of midlatitude cyclones in the emission, transport, production, and removal of aerosols in the Northern Hemisphere

Abstract

We examine the distribution of aerosol optical depth (AOD) across 27,707 northern hemisphere (NH) midlatitude cyclones for 2005-2018 using retrievals from the Moderate Resolution Spectroradiometer (MODIS) sensors and simulations from the Modern Era Retrospective analysis for Research and Applications, version 2 Global Modeling Initiative (M2GMI) chemical transport model. MODIS AOD and fine mode AOD (fAOD) cyclone-centered composites feature enhancements of 30-50% to the east of the cyclone center in the warm conveyor belt (WCB) airstream. Coarse mode AOD (cAOD) is more than doubled near the center of the cyclone, co-located with high surface wind speeds. Within the WCB, MODIS AOD (fAOD) maximizes at 0.2 (0.09) in spring and summer. M2GMI reproduces this magnitude and seasonality to within 16%. Annually, both MODIS and M2GMI show that fAOD accounts for 68% of AOD enhancement in WCBs while cAOD contributes the other 32%. AOD and fAOD are 60% larger in North Pacific WCBs and show a strong relationship with anthropogenic pollution. Composite profiles of M2GMI extinction show that sulfate, organic carbon, and dust enhancements maximize in the free troposphere while sea salt maximizes at the surface. Midlatitude cyclones account for 355 Tg yr-1 of sea salt aerosol emissions annually, or 60% of the 30-80N total. We find that large-scale precipitation in cyclone WCBs efficiently removes > 90% of sulfate but only 30-40 % of organic carbon and dust. Export of sulfate therefore occurs due to chemical production in the WCB while organic carbon and dust export occurs as a result of transport.