Marine and Non-Local Sources of Sulfur Species and Organic Material in the Equatorial Pacific Marine Boundary Layer

Abstract

Marine aerosol and gases in the remote marine boundary layer (MBL) form a critical link between the ocean and atmosphere by regulating aerosol optical properties, cloud formation and properties, photochemistry, and biogeochemical cycles. The sources of marine aerosol were once thought to be relatively simple: large, primary sea salt aerosol (SSA) from wave-breaking and small, secondary non-sea-salt-SO2–4 (NSS) from condensed oxidation products of the organo-sulfur gas dimethylsulfide (DMS) in the atmosphere. With the advent of faster, more sensitive measurements, contributors to the marine organic and sulfurous aerosol population are coming into sharper focus. Of course, new complications inevitably appear. This dissertation explores components of this increasingly complex chemical story using a rich dataset collected during an airborne experiment in the remote equatorial Pacific. Rapid sulfur gas measurements coupled with size-resolved aerosol chemistry enabled the construction of balanced DMS, SO2, and NSS budgets that include entrainment and divergence. These two dynamic terms turn out to be critical mechanisms for adding oxidized sulfur to (entrainment) and removing it from (divergence) the remote MBL. Even in cleaner conditions, entrainment of continental material from 10 000 km away can match DMS as a source of NSS to the MBL. During episodes of enhanced long-range transport, distant sources such as biomass burning can eclipse the low-background marine atmosphere. In addition to some entrainment of submicrometer organic species, we were able to detect submicrometer organic aerosol concentrations that varied according to satellite chlorophyll a multiplied by wind speed. This connection strongly suggests emission from the marine biology. The natural marine signal near equatorial upwelling was up to 4x stronger than other organic aerosol signals. Methanesulfonic acid (MSA), a natural organo-sulfur gas, is often measured but poorly understood. Through our exploration of a unique vertical gradient in highresolution MSA measurements in the MBL, we highlight large unknowns in DMS oxidation chemistry as well as the emerging importance of gas-particle partitioning. We find strong evidence for MSA vapor degassing from small particles in the mixed layer near the warmer, drier ocean surface. There remains much debate and study and concerning the sources of natural aerosol and cloud condensation nuclei.