Dimethyl sulfide: Less important than long‐range transport as a source of sulfate to the remote tropical Pacific marine boundary layer

Abstract

Abstract During the Pacific Atmospheric Sulfur Experiment (PASE), dimethyl sulfide (DMS) was not the principal source of non–sea salt sulfate (NSS) mass in the remote marine boundary layer (MBL), according to an Eulerian sulfur budget based on observations of chemical concentrations from the NCAR C‐130 in relatively dry, subsiding regions of the tropical Pacific. Our three (DMS, SO 2 , and NSS) monthly‐average budgets are mutually consistent. The PASE‐average DMS emission was 3.0 ± 0.5μmol m −2 d −1 (our budget “units”). SO 2 sources include DMS + OH (1.4 ± 0.4 units, assuming 75% of reacted DMS forms SO 2 ) and entrainment from the free troposphere (FT) (0.8 ± 0.2 units). Clouds were the most important chemical reactors for SO 2 (−1.0 ± 0.5 units). SO 2 loss terms also include divergence (−0.9 ± 0.3 units), dry deposition (−0.5 ± 0.2 units), and OH + SO 2 (−0.22 ± 0.05 units). The total SO 2 loss balanced the SO 2 source. We assume that no SO 2 was lost to ozone oxidation on sea salt particles; we found negligible NSS on particles from 2.6 μm (the sea salt mass peak) to 10 μm diameter. Fine‐particle NSS sources include in‐cloud oxidation of SO 2 by H 2 O 2 (1.0 ± 0.5 units), OH + SO 2 (0.19 ± 0.05 units), and entrainment (1.1 ± 0.3 units in clean conditions; twice that when continental pollution is present). NSS sources balance NSS loss to divergence. Only about one fourth of emitted DMS becomes NSS. FT entrainment supplied two thirds and DMS oxidation produced one third of MBL NSS, rather similar source terms.