Sources and Distribution of Light NMHCs in the Marine Boundary Layer of the Northern Indian Ocean During Winter: Implications to Aerosol Formation

Non-methane hydrocarbons (NMHCs) are ubiquitous trace gases and profoundly affect the Earth's atmosphere and climate change. Mixing ratios of light NMHCs were measured over the northern Indian Ocean during winter-2018 as part of the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB-2018). Higher levels of NMHCs over the coastal regions were due to the efficient transport of anthropogenic and biogenic air masses and higher air-sea exchanges due to the higher biological productivity. Although oceanic emissions dominated the open ocean, the transport of aged continental air also influenced the levels of some NMHCs. The higher and lower propane/ethane ratios of 2.41 +/- 0.34 and 1.13 +/- 0.78 ppbv ppbv-1 over coastal and open oceans indicated the prevalence of fresh and aged air masses, respectively. Ethene and propene show a strong correlation, but the ethene/propene ratios over open ocean (2.2 +/- 0.25 ppbv ppbv-1) were slightly lower than the coastal region (2.5 +/- 0.34 ppbv ppbv-1). Principal component analysis reveals the major associated sources identified in this study are from oceanic and nearby anthropogenic sources, explaining nearly 51% and 21% of variance. Light alkenes accounted for similar to 70% of the total ozone and secondary organic aerosol formation potential. A higher alkene/alkane ratio, strong correlation of alkene with organic aerosol mass, and new particle formation events highlight the role of alkenes in secondary aerosol formation over the equatorial Indian Ocean. Overall, the levels of NMHCs were much higher than those measured nearly two decades ago during the Indian Ocean Experiment (INDOEX)-1999. Plain Language Summary Non-methane hydrocarbons (NMHCs) constitute a major fraction of volatile organic compounds (VOCs), which play a vital role in atmospheric chemistry and air quality. To characterize the impact of South Asian outflow and the role of oceanic sources, we performed in-situ measurements of key NMHCs over the Arabian Sea in winter of 2018. Various sources, such as anthropogenic emissions from the Indian subcontinent, oceanic emissions, biogenic emissions from the Western Ghats, and shipping lane emissions, contributed to the observed NMHCs concentrations. However, marine sources dominated the measurements in the remote regions. Light alkenes showed a higher potential for ozone and organic aerosol formations, accounting for similar to 70% of total formation potentials. The simultaneous measurements of organic aerosol mass concentrations provide the role of light alkenes in the new particle formation over the equatorial Indian Ocean. VOCs, with their diverse sources, influence the atmospheric chemistry and radiative balance over coastal as well as remote marine environments. Interestingly, the levels of NMHCs measured in this study are much higher than those measured about two decades ago during the INDOEX campaign. Therefore, it is essential to improve our understanding of the sources and effects of VOCs to develop effective strategies for mitigating their negative impacts.