Contribution of particulate nitrate to airborne measurements of total reactive nitrogen

Simultaneous measurements of speciated, total reactive nitrogen (NO_y) and particulate NO₃ (particle diameter <1.3 μm) were made on board the NASA P-3B aircraft over the western Pacific in February-April 2001 during the Transport and Chemical Evolution over the Pacific (TRACE-P) experiment. Gas-phase and particulate NO_y was measured using a gold tube catalytic converter. For the interpretation of particulate NO_y, conversion efficiencies of particulate NH₄NO₃, KNO₃, NaNO₃, and Ca(NO₃)₂ were measured in the laboratory. Only NH₄NO₃ showed quantitative conversion, and its conversion efficiency was as high as that for HNO₃. NO_y measured on board the aircraft was found to be systematically higher by 10-30% than the sum of the individual NO_y gas components (Σ(NO_y)_i) at 0-4 km. Particulate NO₃^- concentrations measured by a particle-into-liquid sampler (PILS) were nearly equal to NO_y - Σ(NO_y)_i under low-dust-loading conditions. The PILS data showed that the majority of the particulate NO₃^- was in the form of NH₄NO₃ under these conditions, suggesting that NH₄NO₃ particles were quantitatively converted to detectable NO by the NO_y converter, consistent with the laboratory experiments. The contribution of particulate NO₃ ^- to NO_y was most important at 0-2 km, where NO₃^- constituted 10-30% of NO_y during TRACE-P. On average, the amounts of particulate NO₃ ^- and gas-phase HNO₃ were comparable in this region.