Measurements of NOx and PAN and estimates of O3 production over the seasons during Mauna Loa Observatory Photochemistry Experiment 2

Abstract

Measurements of peroxyacetyl nitrate (PAN) and NOx and a variety of other constituents were made over approximately 1‐month‐long intensives in the autumn of 1991 and the winter, spring, and summer of 1992 during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2). PAN and NOx in the free troposphere had maximum abundances in spring in concert with the well‐known maximum in O3. The ratio of the spring to summer averages was a factor of 4.1 for PAN, a factor of 1.6 for O3, and only a factor of 1.4 for NOx. During most intensives, variations over periods of a few days to a week were often larger than the average seasonal amplitude. In free tropospheric air masses local to Hawaii, average PAN/NOx ratios were a maximum in winter through spring but in the range of 0.25–0.86 in all intensives. PAN decomposition is unlikely to be the major net source of NOx in local air masses in summer and fall. The low HNO3/NOx ratios determined during MLOPEX 1 were confirmed during MLOPEX 2. Intensive average ratios of 1.6–3.8 over the year are lower than some model predictions. Both the low ratio and the magnitude of NOx imply a shortcoming in our understanding of the transformations and sources of NOy constituents in the central Pacific, The 3‐ to 4‐km altitude region near Hawaii was a net importer of O3, on average, over the year. The average net rate of production of O3 in free tropospheric air was near zero in winter, −0.4 to −0.8 ppbv/d in spring, −1.4 ppbv/d in summer, and −0.6 ppbv/d in autumn. Thus the spring maximum in O3 is not due to local photochemistry. We believe, as has been concluded from the long‐term measurements of long‐lived constituents by the Climate Monitoring and Diagnostics Laboratory, that the variation of ozone precursors over the year and on shorter timescales of a few days to a week is controlled predominantly by changes in long‐range transport: more frequent sampling of higher‐latitude and higher‐altitude air masses in winter and spring versus more frequent sampling of well‐aged air from lower altitudes and latitudes in summer and autumn.