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%). Plain Language Summary: Non‐methane hydrocarbons (NMHCs) are abundant volatile organic compounds in the atmosphere and play an important role in atmospheric chemistry. The emissions of NMHCs from marine waters are a significant source in the atmosphere. We investigated the annual concentrations and fluxes of five NMHCs in Jiaozhou Bay, a typical anthropogenically impacted estuary on the east coast of North China. NMHCs concentrations and fluxes varied largely between seasons. Multiple processes including phytoplankton release, photochemical production, riverine input might contribute to this variability. The absence of correlations between NMHCs and environmental variables also suggested that the control of NMHCs could be more complex in estuarine systems. Based on our extrapolation, the contribution of coastal and estuarine environments to marine NMHCs emission budgets could be disproportionately high in comparison to their size, despite only accounting for a minor fraction of the global ocean. Given the strong variability of NMHCs emission to the atmosphere, it is necessary to consider the seasonality for better estimation of marine NMHCs budget. Key Points: NMHCs concentrations varied largely between seasons and higher abundance occurred in summerLack of proportionality between NMHCs and environmental variables suggested the complex control of NMHCs in estuarine systemsThe sea‐to‐air fluxes of NHMCs exhibited seasonal variations, and the emission of NMHCs was a source of atmospheric NMHCs [ABSTRACT FROM AUTHOR]