Total reactive oxidized nitrogen (NOy) in the remote Pacific troposphere and its correlation with O₃ and CO: Mauna Loa Observatory Photochemistry Experiment 1988

As part of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX) total reactive oxidized nitrogen (NO y ) was measured during May and early June of 1988 at the Mauna Loa Observatory, the NOAA‐Geophysical Monitoring for Climatic Change Baseline Monitoring Station, located at 3.4‐km elevation on the island of Hawaii. Gold catalytic surface conversion of individual reactive oxidized nitrogen species to NO and subsequent quantification of the NO by NO/O 3 chemiluminescence was used to measure the NO y mixing ratio. The NO y abundance at the site was governed by the local downslope/upslope wind systems as well as synoptic‐scale transport. With some exceptions, downslope wind brought air representative of the free troposphere, while upslope winds transported air from below the trade wind inversion to the site. The upslope air masses could be a mix of marine boundary layer air and free tropospheric air modified by anthropogenic and natural emissions from island sources. It was possible to identify free tropospheric air in the downslope flow through meteorological and chemical tracers. Reflecting the remote location, low NO y mixing ratios with median values of 262 and 239 pptv were found in free tropospheric and upslope air masses, respectively. The median NO y levels in free tropospheric air are consistent with airborne NO y measurements made during NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (CITE 2) program over the northeastern Pacific Ocean at corresponding altitudes. The median NO y values in upslope flow are significantly higher than those measured in the remote marine boundary layer during CITE 2, reflecting probably the influence of island source and/or mixing of free tropospheric air with boundary layer air. The low correlation found between NO y and tracers of anthropogenic sources, such as carbon monoxide, tetrachloroethylene, and n‐propyl nitrate, in free tropospheric air samples is consistent with a stratospheric or upper tropospheric source for NO y . Simultaneous particulate nitrate (NO 3 − ) measurements suggest that at times not all aerosol NO 3 − was quantitatively converted to NO by the Au‐surf ace converter technique. These episodes were usually found during upslope flow and were characterized by high sodium concentrations, suggesting that possibly the sodium nitrate contained in these aerosols was not converted efficiently by the Au converter.