Effects of atmospheric oxidation processes on the latitudinal distribution differences in MSA and nss-SO42- in the Northwest Pacific

Abstract

Dimethyl sulfide (DMS) derived aerosols play an important role in the oceanic atmosphere. The methanesulfonic acid (MSA)/non-sea-salt sulfate (nss-SO42-) ratio is often used to determine the contribution of the DMS sulfate source. However, there is competition between the pathways of DMS to MSA and sulfate, and the importance of the differences in the reaction pathways for MSA/nss-SO42- remains unclear. Here, the MSA, nss-SO42-, and other environmental data from the Northwest Pacific Ocean in July and September were used to evaluate the impact of atmospheric oxidation processes on the ratio. In July, nss-SO42- was mostly contributed by anthropogenic sources, and in September, it was mostly produced mainly from DMS (biogenic sulfate [bio-SO42-]). Differences in sources explain most of the spatiotemporal distribution characterization of MSA and nss-SO42-. However, the atmospheric oxidation process was the main factor responsible for the difference in the spatial distribution of MSA and bio-SO42- in September. The air temperature, surface incoming shortwave flux, ozone and relative humidity affect atmospheric oxidation processes and thus the MSA/bio-SO42– ratio. Therefore, when nss-SO42- is mainly from the DMS source and the net primary production (NPP) is similar between sea areas, the MSA/nss-SO42- ratio needs to be used carefully to evaluate the sulfate contribution from the DMS source. In addition, it should be noted that the contribution of the DMS source aerosol to the Northwest Pacific is not negligible (up to 45%) with gentle wind and high NPP.