The occurrence of peroxyacetyl nitrate over the Atlantic Ocean east of North America during WATOX‐86

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Abstract
Measurements of PAN (peroxyacetyl nitrate) over the western Atlantic Ocean, collected during four aircraft missions in January 1986, are reported. The observed concentrations at circa 150‐200 km off the coast while the general airflow was directly from the coast to the sampling location were surprisingly low, ranging from 20‐510 ppt in the planetary boundary layer (PBL) and 25‐230 ppt in the free troposphere (FT). Most PAN was found in pollution plumes in the PBL. With the exception of one small plume, the level in the FT appeared to be indicative of a continental background air mass. It is speculated that the low average PAN level in the PBL was due to efficient dilution of the air, transported over the open ocean, thereby preventing extensive oxidation of NOx into PAN and inorganic nitrates, or alternatively to the preferential production of other not identified organic nitrates. Averaged over the total data set the PAN mixing ratio was determined at 184 ± 115 ppt and 94 ± 48 ppt for the PBL and FT, respectively. Comparison with NOx data suggests that for flux calculations most of the N flux in the PBL can be assumed to be in the form of NOx (≡ NO + NO2), but a similar assumption for FT flow would lead to an underestimation of total N flux.