Abstract. The Indo-Gangetic Plain (IGP) is home to 9 % of the global population and is responsible for a
large fraction of agricultural crop production in Pakistan, India, and Bangladesh. Levels of fine particulate matter (mean diameter <2.5 µm, PM2.5)
across the IGP often exceed human health recommendations, making
cities across the IGP among the most polluted in the world. Seasonal
changes in the physical environment over the IGP are dominated by the
large-scale south Asian monsoon system that dictates the timing of
agricultural planting and harvesting. We use the WRF-Chem model to study the seasonal anthropogenic,
pyrogenic, and biogenic influences on fine particulate matter and its
constituent organic aerosol (OA) over the IGP
that straddles Pakistan, India, and Bangladesh during 2017–2018. We find that surface air quality
during pre-monsoon (March–May) and monsoon (June–September) seasons is
better than during post-monsoon (October–December) and winter
(January–February) seasons, but all seasonal mean values of PM2.5
still exceed the recommended levels, so that air pollution is a year-round problem. Anthropogenic
emissions influence the magnitude and distribution of PM2.5 and
OA throughout the year, especially over urban sites, while pyrogenic
emissions result in localised contributions over the central and upper
parts of IGP in all non-monsoonal seasons, with the highest impact during
post-monsoon seasons that correspond to the post-harvest season in the
agricultural calendar. Biogenic emissions play an important role in
the magnitude and distribution of PM2.5 and OA during the monsoon
season, and they show a substantial contribution to secondary OA (SOA),
particularly over the lower IGP. We find that the OA contribution to
PM2.5 is significant in all four seasons (17 %–30 %), with primary
OA generally representing the larger fractional contribution. We find
that the volatility distribution of SOA is driven mainly by the mean
total OA loading and the washout of aerosols and gas-phase aerosol
precursors that result in SOA being less volatile during the
pre-monsoon and monsoon season than during the post-monsoon and winter
seasons.
Source apportionment of carbonaceous fine particulate matter (PM 2.5 ) in two contrasting cities across the Indo–Gangetic Plain
