Contribution of particulate nitrate to airborne measurements of total reactive nitrogen

Simultaneous measurements of speciated, total reactive nitrogen (NO y ) and particulate NO 3 − (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 NH 4 NO 3 , KNO 3 , NaNO 3 , and Ca(NO 3 ) 2 were measured in the laboratory. Only NH 4 NO 3 showed quantitative conversion, and its conversion efficiency was as high as that for HNO 3 . 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 (∑(NO y ) i ) at 0–4 km. Particulate NO 3 − concentrations measured by a particle‐into‐liquid sampler (PILS) were nearly equal to NO y − ∑(NO y ) i under low‐dust‐loading conditions. The PILS data showed that the majority of the particulate NO 3 − was in the form of NH 4 NO 3 under these conditions, suggesting that NH 4 NO 3 particles were quantitatively converted to detectable NO by the NO y converter, consistent with the laboratory experiments. The contribution of particulate NO 3 − to NO y was most important at 0–2 km, where NO 3 − constituted 10–30% of NO y during TRACE‐P. On average, the amounts of particulate NO 3 − and gas‐phase HNO 3 were comparable in this region.