Fine particulate matter (PM 2.5 , aerodynamic diameter ≤2.5 µm) impacts the climate, reduces visibility and severely influences human health. The Indo-Gangetic Plain (IGP), home to about one-seventh of the world's total population and a hotspot of aerosol loading, observes strong enhancements in the pM 2.5 concentrations towards winter. We performed high-resolution (12 km × 12 km) atmospheric chemical transport modeling (WRF-Chem) for the post-monsoon to winter transition to unravel the underlying dynamics and influences of regional emissions over the region. Model, capturing the observed variations to an extent, reveals that the spatial distribution of PM 2.5 having patches of enhanced concentrations (≥100 µgm −3) during post-monsoon, evolves dramatically into a widespread enhancement across the IGP region during winter. A sensitivity simulation, supported by satellite observations of fires, shows that biomass-burning emissions over the northwest IGP play a crucial role during post-monsoon. Whereas, in contrast, towards winter, a large-scale decline in the air temperature, significantly shallower atmospheric boundary layer, and weaker winds lead to stagnant conditions (ventilation coefficient lower by a factor of ~4) thereby confining the anthropogenic influences closer to the surface. Such changes in the controlling processes from post-monsoon to winter transition profoundly affect the composition of the fine aerosols over the IGP region. The study highlights the need to critically consider the distinct meteorological processes of west-to-east IGP and changes in dominant sources from post-monsoon to winter in the formulation of future pollution mitigation policies.

[1] Studies on the solar eclipse-induced changes in near-surface ozone and its precursors NO x and CO were carried out at two nearby tropical coastal locations, Thumba (very close to the sea) and the Centre for Earth Science Studies (CESS), which is 4.5 km off the Thumba coast and with varying topography, during the annular eclipse of 15 January 2010. The surface ozone decreased by 12 and 13 ppb (35% and 52%) over Thumba and CESS, with the time lag of 40 min and 25 min from the maximum phase of eclipse, respectively, and at CESS, post-eclipse recovery was faster compared to Thumba. No pronounced change was observed in NO x , but CO showed an enhancement toward the ending phase of the eclipse. The diurnal patterns of ozone and their differences at the two sites were strongly dependent on local meteorology, in particular, the mesoscale dynamics and topography. While the temperature decreased by 1.2°C at Thumba, the decrease was almost double ($2.1°C) at CESS. The early fall in temperature caused the early setting in of land-breeze (post-eclipse effect), which in turn triggered an early evening decrease in near-surface ozone compared to the control conditions. The present study points to the role of mesoscale meteorology/dynamics in controlling the evolution of solar eclipse-induced changes in ozone in a relatively clean environment. The chemical box model simulations reproduced these broad features: a percentage decrease and the time lag in surface ozone. The observation of total column ozone showed a decrease and fluctuations, after the eclipse maximum.

Abstract. The present study examines the role of tropical cyclones in the enhancement of tropospheric ozone. The most significant and new observation reported is the increase in the upper-tropospheric (10-16 km) ozone by 20-50 ppbv, which has extended down to the middle (6-10 km) and lower troposphere ( < 6 km). The descent rate of enhanced ozone layer during the passage of tropical cyclone is 0.8-1 km day −1 , which is three times that of a clear-sky day (non-convective). Enhancement of surface ozone concentration by ∼ 10 ppbv in the daytime and 10-15 ppbv in the night-time is observed during a cyclone. Potential vorticity, vertical velocity and potential temperature obtained from numerical simulation, reproduces the key feature of the observations. A simulation study indicates the downward transport of stratospheric air into the troposphere. Space-borne observations of relative humidity indicate the presence of sporadic dry air in the upper and middle troposphere over the cyclonic region. These observations quantitatively constitute experimental evidence of redistribution of stratospheric ozone during cyclonic storms.

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