NO and NO2 were simultaneously measured by photolytic conversion/chemiluminescence techniques during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX). The field site, located at an elevation of 3.4 km on the north side of the Mauna Loa Volcano, was subject to two airflow regimes which typically corresponded to upslope (marine boundary layer plus island sources) conditions during the day and downslope (middle free tropospheric) conditions at night to mid-morning. Median values of NOx (NOx=NO+NO2) were 37 and 31 pptv during upslope and downslope conditions, respectively, with the downslope measurements consistent with previous measurements made from aircraft in the middle free troposphere over the North Pacific. Although the difference in median NOx mixing ratios in the upslope and downslope regimes is small, the influence of island sources of NOx is apparent. Indeed, the median upslope values were approximately 2.5 times greater than measurements made previously in the remote marine boundary layer. The data have been examined according to downslope/free troposheric and upslope air flow regimes for relationships between NOx and the various species that were measured simultaneously (e.g., peroxyacetyl nitrate (PAN), HNO3, NOy, O3, CO, and hydrocarbons). While positive correlations between NOx and O3 and PAN were typically observed in free tropospheric air, these correlations were considerably weaker than those observed during previous campaigns.

This is likely primarily due to the lower sampling altitude during the MLOPEX study. NOx and dew point temperature were weakly anticorrelated in free tropospheric air masses. Linear correlations between NOx and the peroxides, formaldehyde, alkyl nitrates, and hydrocarbons were also weak in the free tropospheric air masses at the MLO. NOx/NOy was typically on the order of 0.1--0.2 in free tropospheric flow. Considerably higher values of NOx/NOy and HNO3/NOx values obtained under downslope conditions were similar to those previously obtained during aircraft measurements in the middle free troposphere over the northeast Pacific. On the whole, the downslope air masses sampled appear to becharacteristic of well-aged, marine free tropospheric air, and this conclusion is supported by 10-day trajectory analyses. ¿ American Geophysical Union 1992