Reactive nitrogen transport and photochemistry in urban plumes over the North Atlantic Ocean

[1] Photochemical and transport processes involving reactive nitrogen compounds were studied in plumes of urban pollutants using measurements obtained from the NOAA WP-3 aircraft during the ICARTT study in July and August 2004. Observations close to Boston and New York City were used to characterize urban emissions, and plume transport and transformation processes were studied in aged plumes located up to 1000 km downwind from the east coast of North America. Pollution was observed primarily below 1.5 km altitude in well-defined layers that were decoupled from the marine boundary layer. In aged plumes located over the North Atlantic Ocean, the nitric acid (HNO₃) mixing ratios were large (up to 50 ppbv), and HNO₃ accounted for the majority of reactive nitrogen. Plume CO and reactive nitrogen enhancement ratios were nearly equivalent in fresh and aged plumes. Efficient transport of HNO₃ explained the observed trace gas ratios and abundances. Without substantial HNO₃ loss, the ratio of HNO₃ to NOx was between 13 and 42 in most highly aged plumes and sometimes exceeded calculated photochemical steady state values. Box model calculations that include nighttime reactions that convert NOx to HNO₃ reproduce the observations. Photolysis and OH oxidation of over 10 ppbv of HNO₃ that was in the troposphere for days resulted in reformation of hundreds of pptv of NOx, which is sufficient to maintain photochemical ozone production. Efficient transport of HNO₃ over the North Atlantic Ocean for days carried both HNO₃ and NOx far from their continental sources and increased their photochemical influence.