Lower stratosphere trace gas detection using aircraft-borne active chemical ionization mass spectrometry

Abstract

Acetaldehyde and acetone are expected to be formed as trace gases in the free troposphere from photochemical conversion of nonmethane hydrocarbons and to represent key precursors of peroxyacetyl nitrate (PAN)^1–5. PAN is important as a potential organic reservoir^1–5 for odd N and thus as a vehicle for odd N transport in the troposphere or even from the troposphere to the stratosphere. As PAN suffers from thermal decomposition in the lower troposphere, its transport into the stratosphere relies on its formation in the upper troposphere where temperatures are sufficiently low to prevent thermal decomposition. Neither PAN nor its key precursors, however, have been observed in the free troposphere and it is difficult, therefore, to assess the potential impact of non-methane hydrocarbons on upper tropospheric and stratospheric ozone^1–5. An indication of the presence of acetone around the tropopause level was first obtained recently from ambient positive-ion composition measurements⁶. Here, we report on a search for these trace gases and other species with large proton affinities in the tropopause using aircraft-borne active chemical ionization mass spectrometry (ACIMS), a novel method for atmospheric trace gas detection.