Characterization of Gaseous Pollutants and Water-Soluble Inorganic Ions in ${\mathrm{PM}}_{2.5}$ during Summer Time at an Urban Site of North India

Gaseous toxic air pollutants are normally associated with acute short-term and chronic long-term health effects, and hence measurement of these pollutants is essential on a daily basis to understand the science of their occurrences. In this study, gaseous ${\mathrm{O}}_3$, ${\mathrm{SO}}_2$, ${\mathrm{NO}}_2$, ${\mathrm{NH}}_3$, ${\mathrm{HNO}}_2$, ${\mathrm{HNO}}_3$, and HCl and water-soluble inorganic ions (${\mathrm{Na}}^{+}$, ${\mathrm{K}}^{+}$, ${\mathrm{Ca}}^{2+}$, ${\mathrm{Mg}}^{2+}$, ${\mathrm{NH}}_4^{+}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{SO}}_4^{2-}$, and ${\mathrm{Cl}}^{-}$) in ${\mathrm{PM}}_{2.5}$ were measured during a summer in Kanpur, India, using an annular denuder technique on a time resolution of 12 h. The denuder technique revealed that evaporation losses of ${\mathrm{NH}}_4^{+}$, ${\mathrm{Cl}}^{-}$, and ${\mathrm{NO}}_3^{-}$ from a Teflon filter and trapped in nylon filters were temperature sensitive. The evaporation losses were $17.8\pm 8.7\%$, $29.5\pm 19.3\%$, and $37.1\pm 25.9\%$ during daytime, and $10.2\pm 6.4\%$, $17.7\pm 10.1\%$, and $19.3\pm 12.2\%$ during nighttime for ${\mathrm{NH}}_4^{+}$, ${\mathrm{Cl}}^{-}$, and ${\mathrm{NO}}_3^{-}$, respectively. Overall, 24 h average concentrations of ${\mathrm{O}}_3$, ${\mathrm{SO}}_2$, ${\mathrm{NO}}_2$, ${\mathrm{NH}}_3$, ${\mathrm{HNO}}_2$, ${\mathrm{HNO}}_3$, and HCl were 49.6, 8.6, 34.2, 21.5, 3.6, 9.8, and $1.4{\mathrm{\mu gm}}^{-3}$, respectively; and 2.1, 1.5, 2.9, 1.1, 13.1, 12.9, 19.7, and $1.3{\mathrm{\mu gm}}^{-3}$ were observed for ${\mathrm{Na}}^{+}$, ${\mathrm{K}}^{+}$, ${\mathrm{Ca}}^{2+}$, ${\mathrm{Mg}}^{2+}$, ${\mathrm{NH}}_4^{+}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{SO}}_4^{2-}$, and ${\mathrm{Cl}}^{-}$, respectively, in ${\mathrm{PM}}_{2.5}$. Levels of ${\mathrm{O}}_3$, ${\mathrm{NH}}_3$, ${\mathrm{HNO}}_2$, ${\mathrm{HNO}}_3$, ${\mathrm{PM}}_{2.5}$, ${\mathrm{K}}^{+}$, ${\mathrm{Ca}}^{2+}$, ${\mathrm{Mg}}^{2+}$, ${\mathrm{NO}}_3^{-}$, and ${\mathrm{SO}}_4^{2-}$ showed significant diurnal variations at 95% confidence level. However, ${\mathrm{SO}}_2$, ${\mathrm{NO}}_2$, HCl, ${\mathrm{Na}}^{+}$, ${\mathrm{NH}}_4^{+}$, and ${\mathrm{Cl}}^{-}$ did not show any significant diurnal variations. Ion balance showed that an abundant quantity of ${\mathrm{NH}}_3$ was present in the study area to neutralize ${\mathrm{H}}_2{\mathrm{SO}}_4$, ${\mathrm{HNO}}_3$, and HCl to form ${({\mathrm{NH}}_4)}_2{\mathrm{SO}}_4$, ${\mathrm{NH}}_4{\mathrm{NO}}_3$, and ${\mathrm{NH}}_4\mathrm{Cl}$, and thermodynamics for the formation of ${\mathrm{NH}}_4{\mathrm{NO}}_3$ was favored under more humid conditions.