Size-resolved Aerosol Composition in a Coastal Region Influenced by Ship Traffic and Wildfires

Size-resolved aerosol measurements were conducted at a coastal site in central California during the Nucleation in California Experiment (NiCE) between July and August of 2013. Two micro-orifice uniform deposit impactors (MOUDI) were used, one for daytime sampling and one for nighttime sampling; water-soluble and elemental composition were measured. This coastal area is just east of ship and marine emissions and is also influenced by continental pollution and wildfires near the California-Oregon border that occurred towards the end of NiCE. The measurements show that the five most abundant water-soluble species are chloride, sodium, sulfate, ammonium, and nitrate. Sulfate, ammonium, iron, vanadium, and several organic acids exhibit mass concentration peaks at an aerodynamic diameter of 0.32 µm; this size is thus associated with several secondarily produced species and ship emissions. Chloride, sodium, and silicon exhibit their mass peaks between 1.8 and 3.2 µm, which is the characteristic size for primarily emitted crustal species. Potassium has a bimodal distribution with the slightly larger peak at 0.32 µm and the other at 3.2 µm, suggestive of its association with both ship emissions and crustal emissions. An unexpected finding was that oxalate, malonate, and nitrate peak between those aerodynamic diameters at 1.0 µm, which is between the peaks for secondarily produced and primarily emitted species. For the samples influenced by biomass burning, oxalate and malonate exhibit an additional peak at 0.32 µm, coinciding with mass concentration enhancements of biomass burning tracers at this size. The relationship between oxalate and iron, a known sink for oxalate, is examined to determine if iron-oxalato complexation could potentially serve to reduce oxalate levels at the same modal diameter (0.32 µm) as other aqueous-phase tracer species such as sulfate and glyoxylate. Their interrelationship varies as a function of aerodynamic diameter and time of day.