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.