Author(s)

Timmy Francis^1,2*, Shyam Sundar Kundu³, Ramabadran Rengarajan¹, Arup Borgohain³

¹Physical Research Laboratory, Ahmedabad, India.
²National Centre for Medium Range Weather Forecasting, Noida, India.
³North Eastern Space Application Centre, Umiam, India.

Systematic monitoring of the fluctuations in atmospheric SO₂ oxidation efficiency—measured as a molar ratio of SO₄^2- to total SOx (SOx=SO2+SO₄^2-), referred as S-ratio—have been performed during a major long range plume transport to northeast India (Shillong: 25.67°N, 91.91°E, 1064 m ASL) in March 2009. Anomalously low S-ratios (median, 0.03) were observed during the episode—associated with a cyclonic circulation—and the SO₄^2- and SO₂ exhibited unusual features in the ‘relative phase’ of their peaks. During initial days, when SO₂ levels were dictated by the long range influx, the SO₄^2- and SO₂ variabilities were in anti-phase—for the differing mobility/loss mechanisms. When SO₂ levels were governed by the boundary layer diurnality in the latter days, the anti-phase is explained by a ‘depleted OH level’—major portion being consumed in the initial period by the elevated SO₂ and other pollutants. Simulations with a global 3D chemical transport model, GEOS-Chem (v8-03-01), also indicated ‘suppressed oxidation conditions’—with characteristic low S-ratios and poor phase agreements. The modelled OH decreased steadily from the initial days, and OH normalized to SO₂—referred as OH_specific—was consistently low during the ‘suppressed S-ratio period’. Further, the geographical distribution of modelled OH showed a pronounced minimum over the region surrounding (20°N, 95°E) spanning parts of northeast India and the adjacent regions to the southeast of it—prevalent throughout the year, though the magnitude and the area of influence have a seasonality to it—with significant implications for reducing the oxidizing power of the regional atmosphere. A second set of measurements during January 2010—when prominent long range transports were absent—exhibited no anomalies, and the S-ratios were well within the acceptable limits (median, 0.32). This work highlights the GEOS-Chem model skill in simulating/detecting the ‘transient fluctuations’ in the oxidation efficiency, down to a regional scale.

Sulphur Dioxide, Sulphate, Atmospheric Oxidation, GEOS-Chem, OH Radical, Plume Transport

Francis, T. , Kundu, S. , Rengarajan, R. and Borgohain, A. (2017) SO₂ Oxidation Efficiency Patterns during an Episode of Plume Transport over Northeast India: Implications to an OH Minimum. Journal of Environmental Protection, 8, 1119-1143. doi: 10.4236/jep.2017.810071.