Shipping and algae emissions have a major impact on ambient air mixing ratios of NMHCs and methanethiol on Utö island in the Baltic Sea

Abstract. Mixing ratios of highly volatile organic compounds were studied on Utö Island in the Baltic Sea. Measurements of non-methane hydrocarbons (NMHCs) and methanethiol were conducted using an in situ thermal desorption-gas chromatograph-flame ionization detector/mass spectrometer (TD-GC-FID/MS) from March 2018 until March 2019. The mean mixing ratios of NMHCs (alkanes, alkenes, alkynes, aromatic hydrocarbons) were at the typical levels for rural/remote sites in Europe and as expected the highest mixing ratios were measured in winter while in summertime, the mixing ratios remained close to or below detection limits for most of the studied compounds. Sources of NMHCs during wintertime were studied using positive matrix factorization (PMF) together with wind direction analyses and source area estimates. Shipping was found to be a major local anthropogenic source of NMHCs with a 21 % contribution. It contributed especially on ethene, propene and ethyne mixing ratios. Other identified sources were gasoline fuel (15 %), traffic exhaust (14 %), local solvents (6 %), and long-range transported background (42 %). Contrary to NMHCs, high mixing ratios of methanethiol were detected in summertime (July mean 1000 pptv). The mixing ratios followed the variations of seawater temperatures and sea level height and were highest during the daytime. Biogenic phytoplankton or macroalgae emissions were expected to be the main source for methanethiol.