Abstract:
The combustion of fossil fuels (coal and petroleum products) constitutes a source of
continuous release of anthropogenic SO₂ into the atmosphere. Furthermore, natural
sources such as volcanoes can inject large amounts of SO₂ directly into the troposphere
and sometimes even into the stratosphere. These event-based volcanic eruptions provide
solitary opportunities to study the transport and transformation of atmospheric con-
stituents. In this study, we present an episode of high SO₂ concentration over northern
India as a result of long-range transport from Africa using multiple satellite observa-
tions. Monthly averaged column SO₂ values over the Indo-Gangetic Plain (IGP) were
observed in the range of 0.6–0.9 Dobson units (DU) during November 2008 using
observations from the Ozone Monitoring Instrument (OMI). These concentrations were
conspicuously higher than the background concentrations (<0.3 DU) observed during
2005–2010 over this region. The columnar SO₂ loadings were highest on 6 November
over most of the IGP region and even exceeded 6 DU, a factor of 10–20 higher than
background levels in some places. These enhanced SO₂ levels were not reciprocated
in satellite-derived NO₂ or CO columns, indicating transport from a non-anthropogenic
SO₂ source. As most of the local aerosols over the IGP region occur below 3 km, a
well-separated layer at 4–5 km was observed from the Cloud-Aerosol Lidar and Infrared
Pathfinder Satellite Observations (CALIPSO) satellite. Wind fields and back-trajec-
tory analysis revealed a strong flow originating from the Dalaffilla volcanic eruption in
Ethiopia during 4–6 November 2008. Although volcanic SO₂ plumes have been exten-
sively studied over many parts of Asia, Europe, and the USA, analysis of such events
for the IGP region is being reported for the first time in this study.
