UC San Diego

The marine boundary layer (MBL) serves as host to great exchanges of mass and energy across the air-sea interface that drive multi-scale dynamics, biogeochemical cycles and global climate change. Reactive trace gases in the MBL can set the atmosphere's oxidative capacity, aerosol production rates and greenhouse gas warming potential. The ocean surface is a broad source and sink for important reactive trace gases, but direct observations of trace gas air-sea exchange have been limited to a handful of species, to date. This doctoral dissertation addresses this deficit by developing methods for the measurement of reactive trace gas air-sea exchange, as well as novel observations of air-sea exchange rates of ozone and secondary organic aerosol (SOA) precursors. Terrestrial biogenic volatile organic compounds (BVOCs) determine global SOA production rates, but estimates of their marine source span several orders of magnitude. A chemical-ionization method for the sensitive detection of BVOC was developed and deployed to the remote MBL aboard a research vessel during the High Wind Gas Exchange Study. Direct observations of BVOC (dimethyl sulfide, isoprene and monoterpene) mixing ratios and air-sea exchange were taken via eddy covariance. Dimethyl sulfide and monoterpene air-sea exchange rates were positive (i.e. emitted) in both remote and coastal waters. In coastal areas, monoterpene air-sea exchange rates rivaled dimethyl sulfide. Reactive nitrogen species (NOy) including alkyl nitrates (RONO₂), dinitrogen pentoxide (N₂O₅) and nitryl chloride (ClNO₂) are the main source of nitrogen oxides (NOx) to the remote MBL and set ozone production rates. During a realistic mesocosm study and detailed laboratory monoculture experiments, alkyl nitrates were found to be driven by heterotrophic bacteria abundance suggesting a dark production mechanism for short -chained RONO₂. In an ambient coastal polluted atmosphere, simultaneous eddy covariance measurements of N₂O₅ and ClNO₂ air-sea exchange were taken from a pier. Contrary to what was predicted by heterogeneous chemistry, measurements demonstrated that the air-sea interface is a sink for both species, thus a terminal sink for NOx. Depending on aerosol surface area, this demonstrates the air-sea interface can play a controlling role in NOx processing in polluted coastal environments