Role of atmospheric ammonia in the formation of inorganic secondary particulate matter: A study at Kanpur, India

Levels of fine Particulate Matter (PMfine), SO2 and NOx are interlinked through atmospheric reactions to a large extent. NOx, NH3, SO2, temperature and humidity are the important atmospheric constituents/conditions governing formation of fine particulate sulfates and nitrates. To understand the formation of inorganic secondary particles (nitrates and sulfates) in the atmosphere, a study was undertaken in Kanpur, India. Specifically, the study was designed to measure the atmospheric levels of $$ {\text{NH}}^{{\text{ + }}}_{{\text{4}}} ,\;{\text{Ca}}^{{{\text{2 + }}}} ,\;{\text{Mg}}^{{2 + }} ,\;{\text{Na}}^{{\text{ + }}} ,\;{\text{K}}^{{\text{ + }}} ,\;{\text{NO}}^{{\text{ - }}}_{{\text{3}}} ,\;{\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}} ,\;{\text{CI}}^{{\text{ - }}} ,\;{\text{NH}}_{{\text{3}}} \;{\left( {{\text{gas}}} \right)},\;{\text{HNO}}_{{\text{3}}} \;{\left( {{\text{gas}}} \right)},\;{\text{NO}}_{2} \;{\text{and}}\;{\text{PM}}_{{{\text{10}}}} \;{\left( {{{\text{PM}}_{{2.5}} } \mathord{\left/ {\vphantom {{{\text{PM}}_{{2.5}} } {{\text{PM}}_{{{\text{10}}}} \;{\text{ratio}}}}} \right. \kern-\nulldelimiterspace} {{\text{PM}}_{{{\text{10}}}} \;{\text{ratio}}} = 0.74} \right)} $$ covering winter and summer seasons and day and night samplings to capture the diurnal variations. Results showed $$ {\text{NO}}^{{\text{ - }}}_{{\text{3}}} ,\;{\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}} ,\;{\text{NH}}^{{\text{ + }}}_{{\text{4}}} ,\;{\text{K}}^{ + } $$ are found to be significantly high in winter season compared to the summer season. In winter, the molar ratio of $$ {\text{NH}}^{{\text{ + }}}_{{\text{4}}} $$ to $$ {\text{SO}}^{{{\text{2 - }}}}_{{\text{4}}} $$ was found to be greater than 2:1. This higher molar ratio suggests that in addition to (NH4)2SO4, NH4NO3 will be formed because of excess quantity of $$ {\text{NH}}^{{\text{ + }}}_{{\text{4}}} $$ present. In summer, the molar ratio was less than 2:1 indicating deficit of $$ {\text{NH}}^{{\text{ + }}}_{{\text{4}}} $$ to produce NH4NO3. The nitrogen conversion ratio (NO2 to NO3) was found to be nearly 50% in the study area that suggested quick conversion of NO2 into nitric acid. As an overall conclusion, this study finds that NH3 plays a vital role in the formation of fine inorganic secondary particles particularly so in winter months and there is a need to identify and assess sources of ammonia emissions in India.