Reactive nitrogen and its correlation with O3 and CO over the Pacific in winter and early spring

Abstract

Measurements of NO, NO_y, O₃, and CO were made during NASA's Global Tropospheric Experiment/Pacific Exploratory Mission‐West B (GTE/PEM‐West B) carried out over the western Pacific in February and March 1994. NO_x was calculated from NO using a photostationary state model ((NO_x)_mc). Correlations between these species are presented, and some insights into the sources of NO_x and NO_y are described. The boundaries between the lower, middle, and upper troposphere have been defined at potential temperatures of 311 K and 328 K, which correspond to the geometric altitudes of about 5 and 9 km at 30°N. Enhancements in the mixing ratios of NO_y and CO were observed in the lower and middle troposphere. A positive correlation was found between these two species suggesting that the high NO_y values were due to anthropogenic emissions over the continental surface. On the other hand, O₃ increased little with increase in CO. As a result, NO_y/O₃ ratios were higher in air more influenced by pollution. NO_y values in 55 and 28% of the air masses sampled in the lower and middle troposphere, respectively, were higher than the clean free tropospheric NO_y‐O₃ range when O₃ values simultaneously observed were used. High (NO_x)_mc/NO_y ratios between 0.15 and 0.3 were found in the boundary layer with relatively low mixing ratios of CO and NO_y during the three flights. These air masses were transported from a higher altitude (∼5 km) and a higher latitude (∼50°N) within a few days. The peroxyacetyl nitrate (PAN)/NO_y ratios were generally high (∼0.4) in these air masses, and the thermal decomposition of PAN was a probable source of NO_x. In the middle troposphere the (NO_x)_mc mixing ratio did not generally increase with NO_y or CO, suggesting that the transport of air masses affected by anthropogenic emissions did not increase the NO_x level significantly. In the upper troposphere, very minor effects from the continental surface sources were seen in the CO mixing ratio. By contrast, NO_y values in 33% of the air masses were higher than those expected when stratospheric air intrusion is assumed to be a single source of NO_y based on NO_y‐O₃ correlation analyses. This result suggests significant free tropospheric NO_y sources, namely exhaust from the aircraft and NO production by lightning activity. In fact, spikes in the (NO_x)_mc mixing ratios were observed near the aircraft corridor south of Tokyo at an altitude of 10 km. These two free tropospheric NO_x sources were considered to be important in determining the levels of the upper tropospheric NO_x and NO_y during PEM‐West B.