Peroxyacetyl nitrate (PAN) in the unpolluted atmosphere: An important reservoir for nitrogen oxides

Abstract

Based on the measured atmospheric distributions of ethane and propane (at midlatitudes in the northern hemisphere) and a simplified mechanism for their oxidation, it is predicted that acetaldehyde, acetone, and PAN [CH 3 C(O)OONO 2 ] are ubiquitous components of the troposphere and the lower stratosphere. Average acetaldehyde concentrations (from ethane oxidation) of 22 parts per trillion (ppt), 3 ppt, and 7 ppt; average acetone concentrations (from propane oxidation) of 111 ppt, 15 ppt, and 3 ppt and average PAN concentrations of 17 to 34 ppt, 90 to 360 ppt, and 40 to 85 ppt are estimated for the lower troposphere (∼ 2 km), upper troposphere (∼ 9 km), and the lower stratosphere (∼ 15 km), respectively. These calculations suggest that in the troposphere, nitrogen oxides (NO x ) contained in their organic form may be as much or more abundant as their inorganic form. This organic form of reactive nitrogen is in chemical equilibrium [CH 3 C(O)OONO 2 ⇄ CH 3 C(O)OO + NO s ] with inorganic NO 2 and acts as reservoir of inorganic NO x . Measurement methods for PAN are currently available to verify these predicted results.