The diurnal variability of sulfate and nitrate aerosols during wintertime in the Indo-Gangetic Plain: implications for heterogeneous phase chemistry

Abstract

We have conducted this study (November 09–February 10) during the daytime (average PM₁: 113 μg m⁻³; n = 51) and nighttime (average PM₁: 159 μg m⁻³; n = 49) in the Indo-Gangetic Plain (IGP). Air-mass back trajectories suggest the impact of local emission and long-range transport (predominantly from the north–west direction). Mass fractions of SO₄²⁻ and NO₃⁻ in PM₁ are significantly (p < 0.05) different during both the daytime and nighttime, whereas NH₄⁺/PM₁ were similar during the day and night. The relatively high concentration of SO₄²⁻ observed during the daytime was explained based on heterogeneous-phase reactivity due to the positive response of Fe and Mn species (inferred from correlation and multi-linear regression analysis: MLRA). Likewise, the lower concentration of NO₃⁻ was explained based on the negative response of Fe during heterogeneous phase formation. The role of wet-bulb temperature and solar flux has also been studied. From the field-based measurements our study shows that heterogeneous formation of SO₄²⁻ (involving Fe and Mn) and NO₃⁻ occur via selective endothermic pathways. The proposed mechanism for sulfate and nitrate formation via heterogeneous phase reactivity is in good agreement with the field-based measurements obtained during this study (IGP). The impact of the heterogeneous-phase reactivity via endothermic pathways relates to the uptake of various reactive species during the winter in IGP. This, in turn, has implications for fog-formation and tropospheric oxidative cleansing. Furthermore, the uptake of various species will lead to altered size, morphology and optical properties of aerosols. This would have impact on regional scale radiative forcing estimates and future climate projections.