Mapping gaseous dimethylamine, trimethylamine, ammonia, and their particulate counterparts in marine atmospheres of China's marginal seas – Part 1: Differentiating marine emission from continental transport

Abstract

To study sea-derived gaseous amines, ammonia, and primary particulate aminium ions in the marine atmosphere of China's marginal seas, an onboard URG-9000D Ambient Ion Monitor-Ion Chromatograph (AIM-IC, Thermo Fisher) was set up on the front deck of the R/V Dongfanghong-3 to semi-continuously measure the spatiotemporal variations in the concentrations of atmospheric trimethylamine (TMAgas), dimethylamine (DMAgas), and ammonia (NH3gas) along with their particulate matter (PM2.5) counterparts. In this study, we differentiated marine emissions of the gas species from continental transport using data obtained from 9 to 22 December 2019 during the cruise over the Yellow and Bohai seas, facilitated by additional short-term measurements collected at a coastal site near the Yellow Sea during the summer, fall, and winter of 2019. The data obtained from the cruise and coastal sites demonstrated that the observed TMAgas and protonated trimethylamine (TMAH+) in PM2.5 over the Yellow and Bohai seas overwhelmingly originated from marine sources. During the cruise, no significant correlation (P>0.05) was observed between the simultaneously measured TMAH+ and TMAgas concentrations. Additionally, the concentrations of TMAH+ in the marine atmosphere varied around 0.28±0.18 µg m−3 (average ± standard deviation), with several episodic hourly average values exceeding 1 µg m−3, which were approximately 1 order of magnitude larger than those of TMAgas (approximately 0.031±0.009 µg m−3). Moreover, there was a significant negative correlation (P<0.01) between the concentrations of TMAH+ and NH4+ in PM2.5. Therefore, the observed TMAH+ in PM2.5 was overwhelmingly derived from primary sea-spray aerosols. Using TMAgas and TMAH+ in PM2.5 as tracers for sea-derived basic gases and sea-spray particulate aminium ions, the values of non-sea-derived DMAgas, NH3gas, and non-sea-spray particulate DMAH+ in PM2.5 were estimated. The estimated average values of each species contributed 16 %, 34 %, and 65 % of the observed average concentrations for non-sea-derived DMAgas, NH3gas, and non-sea-spray particulate DMAH+ in PM2.5, respectively. Uncertainties remained in the estimations, as TMAH+ may decompose into smaller molecules in seawater to varying extents. The non-sea-derived gases and non-sea-spray particulate DMAH+ likely originated from long-range transport from the upwind continents based on the recorded offshore winds and increased concentrations of non-sea-salt SO42- (nss-SO42-) and NH4+ in PM2.5. The lack of a detectable increase in particulate DMAH+, NH4+, and nss-SO42- concentrations in several SO2 plumes did not support the secondary formation of particulate DMAH+ in the marine atmosphere.