Annual Variations of Isoprene and Other Non-Methane Hydrocarbons in the Jiaozhou Bay on the East Coast of North China

Non-methane hydrocarbons (NMHCs) play an important role in atmospheric chemistry and the ocean is a source of NMHCs in the atmosphere. Here, we investigated the annual variations of NMHCs concentrations and fluxes (i.e., ethane, ethylene, propane, propylene, and isoprene) in the semi-enclosed shallow waters of Jiaozhou Bay from September 2016 to August 2017. Ethylene is the most abundant NMHCs, with an annual average concentration of 487.7 +/- 239.3 pmol L-1. The concentrations of other NMHCs were lower; the average concentrations for ethane, propane, propylene, and isoprene were 87.0 +/- 49.0, 65.7 +/- 22.9, 127.4 +/- 77.8, and 60.5 +/- 47.1 pmol L-1, respectively. NMHCs concentrations varied largely between seasons and higher abundance were observed in summer. The correlations between NMHCs and environmental variables such as chlorophyll a (Chl-a), dissolved organic carbon (DOC) were ambiguous, and principal component analysis (PCA) suggested nutrients contributed significantly to total variation. These results indicated that the control of NMHCs could be more complex in dynamic and anthropogenically impacted estuarine systems, which could be a result of multiple processes involving phytoplankton release, photochemical production, riverine input, and so forth. The average annual sea-to-air fluxes of ethane, ethylene, propane, propylene, and isoprene were 105.9-298.3, 583.5-1,337.0, 36.7-95.2, 85.2-389.6, and 25.2-182.4 nmol m(-2) d(-1), respectively. Although coastal and estuarine environments only account for a minor fraction of the global ocean (0.26%-0.39%), NMHC emissions from these areas could influence regional atmospheric chemistry and may be relatively important for global budgets (ethane, 1.70%; ethylene, 1.54%; propane, 1.29%; propylene, 0.87%; isoprene, 3.17%).