Isotopic Interpretation of Particulate Nitrate in the Metropolitan City of Karachi, Pakistan: Insight into the Oceanic Contribution to NOx.

Nitrogen oxide (NOx) abatement has become the focus of air quality management strategies. In this study, we examined NOx sources and the atmospheric conversion of NOx in Karachi, Pakistan, a megacity in South Asia with serious particulate pollution problems. Oceanic contributions to NOx 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 NOx 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 NOx 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 NOx 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 NOx source.