Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic

A large number of oxygenated organic chemicals (peroxyacyl nitrates, alkyl nitrates, acetone, formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airborne field campaign over th e Atlantic. In this paper, we present a first picture of the distribution o f these oxygenated organic chemicals (Ox-organic) in the troposphere and th e lower stratosphere, and assess their source and sink relationships. In bo th the troposphere and the lower stratosphere, the total atmospheric abunda nce of these oxygenated species (Sigma Ox-organic) nearly equals that of to tal nonmethane hydrocarbons (Sigma NMHC), which have been traditionally mea sured. A sizable fraction of the reactive nitrogen (10-30%) is present in i ts oxygenated organic form. The organic reactive nitrogen fraction is domin ated by peroxyacetyl nitrate (PAN), with alkyl nitrates and peroxypropionyl nitrate (PPN) accounting for <5% of total NOy. Comparison of observations with the predictions of the Harvard three-dimensional global model suggests that in many key areas (e.g., formaldehyde and peroxides) substantial diff erences between measurements and theory are present and must be resolved. I n the case of CH3OH, there appears to be a large mismatch between atmospher ic concentrations and estimated sources, indicating the presence of major u nknown removal processes. Instrument intercomparisons as well as disagreeme nts between observations and model predictions are used to identify needed improvements in key areas. The atmospheric chemistry and sources of this gr oup of chemicals is poorly understood even though their fate is intricately linked with upper tropospheric NOx and HOx cycles.