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

• Published in: Journal of geophysical research. Atmospheres 2024-02, Vol.129 (3), p.n/a
• Main Authors: Tripathi, Nidhi, Girach, Imran A., Kompalli, Sobhan Kumar, Murari, Vishnu, Nair, Prabha R., Babu, S. Suresh, Sahu, Lokesh Kumar
• Format: Article
• Language: English
• Subjects: Aerosol concentrations; Aerosol formation; Aerosols; Air masses; Air pollution; Air quality; Alkanes; Alkenes; Anthropogenic factors; Arabian Sea; Atmospheric chemistry; Biogenic emissions; Boundary layers; Climate change; Coastal zone; Correlation; Emissions; Ethane; Ethene; Ethylene; Gases; Human influences; Hydrocarbons; Indian Ocean; Light; Marine environment; Methane; Mixing ratio; NMHCs; oceanic emission; Oceans; OFP; Organic compounds; Outflow; Ozone; Particle formation; Principal components analysis; Radiation budget; Ratios; Remote regions; Secondary aerosols; Shipping; Shipping lanes; South Asia; Trace gases; VOCs; Volatile hydrocarbons; Volatile organic compounds; Water outflow; Winter
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2023JD039433

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 ∼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 ∼70% of total formation potentials. The simultaneous measurements of organic aerosol mass concentra