Ratios of Peroxyacetyl Nitrate to Active Nitrogen Observed During Aircraft Flights Over the Eastern Pacific Oceans and

continental United States. The suite of observations included simultaneous measurements of peroxyacetyl nitrate (PAN) and active nitrogen (NO x = NO + NO2). At altitudes of 4.5-6.1 km in the middle free troposphere, PAN was usually 5-6 times NO x in maritime air masses and 2-4 times NO x in continental air masses. In air masses of tropical origin, or in the marine boundary layer, both PAN and NO x were typically less than 20-30 parts per trillion by volume, and the PAN to NO x ratio was less than one. The observations how that PAN can be a major component of the odd nitrogen budget in the middle free troposphere and strongly reinforce earlier views that the abundance is mainly governed by long-range transport processes including formation during transport and continental boundary layer to free tropospheric exchange of PAN and its precursors. Unlike reservoir HNO3, PAN can be transformed to active nitrogen and peroxy radicals by a variety of physical atmospheric processes that lead to air mass warming. Since NO x plays a critical role in determining photochemical 0 3 production, which in turn determines the oxidative power of the atmosphere, the observed large ratios of reservoir PAN to active NO x imply an important photochemical and dynamical role for PAN in the eastern Pacific remote free troposphere. 1. INTRODUCTION Th