Isotopic Interpretation of Particulate Nitrate in the Metropolitan City of Karachi, Pakistan: Insight into the Oceanic Contribution to NO x

Nitrogen oxide (NO x ) abatement has become the focus of air quality management strategies. In this study, we examined NO x sources and the atmospheric conversion of NO x in Karachi, Pakistan, a megacity in South Asia with serious particulate pollution problems. Oceanic contributions to NO x were quantified for the first time based on a novel approach using nitrogen/oxygen isotopic analysis in nitrate (δ15N–NO3 –; δ18O–NO3 –) and a Bayesian model. Our results showed that δ15N–NO3 – in Karachi varied between −10.2‰ and +12.4‰. As indicated by the δ18O–NO3 – findings (+66.2 ± 7.8‰), the •OH pathway dominated NO x conversion throughout the nearly two-year observation, but high NO3 – events were attributed to the O3 pathway. Coal combustion was the most significant source (32.0 ± 9.8%) of NO x in Karachi, with higher contributions in the autumn and winter; a similar situation occurred for biomass burning + lightning (30.3 ± 6.5%). However, mobile sources (25.2 ± 6.4%) and microbial processes (12.5 ± 7.5%) exhibited opposite seasonal trends. The oceanic contributions to NO x in Karachi were estimated to be 16.8%, of which lightning, shipping emissions, and microbial processes accounted for 20.3%, 46.3%, and 33.4%, respectively, emphasizing the dominance of shipping emissions as an oceanic NO x source.