Elevated levels of biogenic nonmethane hydrocarbons in the marine boundary layer of the Arabian sea during the intermonsoon

Abstract

The Arabian Sea is one of the most biologically productive ocean regimes and hence possesses a perennially intense oxygen minimum zone. High productive oceans can potentially modify the production and sea?to?air exchange of trace gases including volatile organic compounds (VOCs). We measured nonmethane hydrocarbons (NMHCs) such as ethane, ethene, propane, propene, 1?butene, cis?2?butene, and 1?pentene in the marine air and characterized phytoplankton species in seawater of the Arabian Sea during the premonsoon season of the year 2017. The light alkenes namely ethene and propene were the dominant VOCs in the marine air with average mixing ratios of 8.92 � 3.50 and 3.38 � 1.30 ppbv, respectively. Daytime enhancements of ethene, propene, 1?butene, cis?2?butene, and 1?pentene were 20�40% higher than their nighttime values. The high levels of alkenes were associated with the higher abundances of Trichodesmium and Thalassiosira species. The ?ethene/?propene (2.63 ppb ppb?1) was comparable to those reported for other oceanic regions. Estimated emission fluxes of ethene (2.0�6.9 � 109 molecules cm?2 s?1) were higher than those reported for several other oceanic regions. Such high emission rates of NMHCs in remote regions can significantly affect the regional tropospheric oxidation chemistry leading to the formation of ozone and secondary organic aerosols. Our observations highlight the need to evaluate the biogeochemical processes controlling the oceanic emissions of NMHCs over the northern Indian Ocean.