Particle number size distributions and new particle formation events over the northern Indian Ocean during continental outflow

Abstract

The particle number concentrations, size distributions, and new particle formation (NPF) events were investigated in the South Asian outflow over South-eastern Arabian Sea (SEAS), northern (NIO) and the equatorial Indian Ocean (EIO) during the winter of 2018 based on the extensive measurements of particle number size distributions in size range 10–414 nm in the marine atmospheric boundary layer (MABL) as part of the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB - 2018) experiment. The total particle concentrations were higher (as high as 10⁶ cm⁻³; mean ∼8857 ± 13,425 cm⁻³) over the SEAS in the proximity to the continental outflow, which gradually decreased to reach the lowest values over the remote EIO (∼3206 ± 4406 cm⁻³). Several NPF events with conspicuous bimodal size distributions and 3–10 times enhancement in nucleation particle (<25 nm) concentrations were noticed. NPF occurred more frequently over the remote oceanic EIO, but more intensely over the regions in the continental proximity demonstrating the varying precursor vapor source strengths. The vertical winds and dew point temperatures revealed the entrainment of free tropospheric air masses to the MABL enabling the downward transport of the nucleation mode particles, possibly formed in the free tropospheric altitudes, to the MABL which advected southward along with the primary outflow aerosols due to north-easterly surface winds. Collocated submicron aerosol chemical composition measurements revealed that most of the events were associated with enhanced particulate organics mass concentrations. Further investigations suggested that volatile organic compounds were possibly involved in the NPF pathways whose sources were advected polluted plumes in the outflow regions and marine emissions over the remote ocean. The newly formed particles grew to larger sizes with a median growth rate of 12 nm h−1, which suggest that these NPF events can contribute considerably to the regional aerosol loading, and potentially affect regional climate.