Global distribution of peroxyacetyl nitrate

Abstract

Nitrogen oxides (NO_x) have a central role in the chemistry of the atmosphere, especially in key processes relating to ozone, hydroxyl-radical (OH) and acid formation^1–7. High reactivity of NO_x (lifetime of 0.5–2 days) precludes hemispheric-scale transport and it has been proposed that non-methane hydrocarbons present in the troposphere can transform NO_x into its organic forms principally as peroxyacetyl nitrate (PAN)^8,9. PAN is highly stable in the colder regions of the middle and upper troposphere and can provide a mechanism for NO_x storage and transport. Once transported, PAN and its homologues can easily release free NO_x in warmer atmospheric conditions. PAN is probably ubiquitous and its concentrations could exceed those of NO_x in clean tropospheric conditions^10–12. Here we present the first view of the global distribution of PAN based on extensive shipboard and aircraft measurements. PAN is more abundant in the Northern than in the Southern Hemisphere and in the continental than in the marine troposphere. In contrast to its behaviour in polluted atmospheres, PAN mixing ratios in winter greatly exceed those in summer. These measurements provide a basis for assessing the significance of PAN as a reservoir of NO_x and for extending and validating reactive nitrogen chemistry theory in the troposphere.