Ratios of reactive nitrogen species over the Pacific during PEM‐West A

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Abstract
Measurements of total reactive odd nitrogen (NOy) and known individual odd nitrogen species were made over the Pacific Ocean in September and October 1991 during the NASA Global Tropospheric Experiment, Pacific Exploratory Mission‐West A (GTE/PEM‐West A). The ratios between NOx (NO + NO2) calculated from observed NO assuming photochemical equilibrium ((NOx)mc), peroxyacetyl nitrate (PAN), and NOy have been investigated for these data. The mixing ratios of (NOx)mc, PAN, and NOy in the lower and middle troposphere generally decreased with the air mass age, which has been defined by the propane/ethane ratio (C3H8/C2H6). The (NOx)mc/NOy ratios in the continental air showed a C‐shaped vertical profile and median values were 0.27, 0.11, and 0.20 in the lower, middle, and upper troposphere, respectively. In the maritime and tropical air the (NOx)mc/NOy ratios generally increased with altitude and were about 0.1 and 0.15 in the lower and upper troposphere, respectively. In the middle troposphere the (NOx)mc/NOy ratios were similar in continental, maritime, and tropical air. These results are consistent with the fact that the (NOx)mc/NOy ratio did not decrease with the air mass age in the middle troposphere, suggesting the existence of NOx sources in the free troposphere. The PAN/NOy ratios in the continental air were about 0.3 at all altitudes. In maritime and tropical air the PAN/NOy ratio increased sharply at altitudes between 3 and 5 km with values of about 0.03 below 3 km and between 0.1 and 0.2 in the middle and upper troposphere. The PAN/NOy ratio generally decreased with the air mass age in the middle troposphere. In spite of a longer photochemical lifetime of PAN than NOx the PAN/(NOx)mc ratio also generally decreased with air mass age. These observations indicate that the reactive nitrogen ratios in the continental air mass are influenced by anthropogenic emissions at all altitudes, although relative contributions from various NOx sources in the upper troposphere are still poorly understood. Some influence from the continental air can be seen in the anthropogenic origin species in the maritime air mass when compared with the tropical air mass (Kondo et al., this issue). Similar (NOx)mc/NOy ratios, however, were generally found between the maritime and tropical air masses and a difference was only seen in the slightly higher PAN/NOy ratios in the maritime middle tropospheric air. The high‐latitude air is characterized by low (NOx)mc/NOy ratios and high PAN/(NOx)mc ratios, probably due to the low atmospheric temperatures.