The presence of oxygenated organic compounds in the troposphere strongly influences key atmospheric processes. Such oxygenated species are, for example, carriers of reactive nitrogen and are easily photolysed, producing free radicals-and so influence the oxidizing capacity and the ozone-forming potential of the atmosphere-and may also contribute significantly to the organic component of aerosols. But knowledge of the distribution and sources of oxygenated organic compounds, especially in the Southern Hemisphere, is limited. Here we characterize the tropospheric composition of oxygenated organic species, using data from a recent airborne survey conducted over the tropical Pacific Ocean (30 degrees N to 30 degrees S). Measurements of a dozen oxygenated chemicals (carbonyls, alcohols, organic nitrates, organic pernitrates and peroxides), along with several C2-C8 hydrocarbons, reveal that abundances of oxygenated species are extremely high, and collectively, oxygenated species are nearly five times more abundant than non-methane hydrocarbons in the Southern Hemisphere. Current atmospheric models are unable to correctly simulate these findings, suggesting that large, diffuse, and hitherto-unknown sources of oxygenated organic compounds must therefore exist. Although the origin of these sources is still unclear, we suggest that oxygenated species could be formed via the oxidation of hydrocarbons in the atmosphere, the photochemical degradation of organic matter in the oceans, and direct emissions from terrestrial vegetation.

Abstract. 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 the Atlantic. In this paper, we present a first picture of the distribution of these oxygenated organic chemicals (Ox-organic) in the troposphere and the lower stratosphere, and assess their source and sink relationships. In both the troposphere and the lower stratosphere, the total atmospheric abundance of these oxygenated species (ZOx-organic) nearly equals that of total nonmethane hydrocarbons (ZNMHC), which have been traditionally measured.

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mixing ratios were found to increase in the stratosphere at a rate that was comparable to the decline in OCS levels. No evidence of particle formation could be observed. Ethane, propane, and acetylene mixing ratios declined rapidly in the LS with C1 atoms likely playing a key role in this process. A number of reactive hydrocarbons/halocarbons (e.g. C6H 6, CH3 I) were present at low but measurable concentrations.

Airborne measurements of aerosol (condensation nuclei, CN) and selected trace gases made over areas of the North Atlantic Ocean during Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) (October/November 1997), the south tropical Pacific Ocean during Pacific Exploratory Mission (PEM)‐Tropics A (September/October 1996), and PEM‐Tropics B (March/April 1999) have been analyzed. The emphasis is on interpreting variations in the number densities of fine (>17 nm) and ultrafine (>8 nm) aerosol in the upper troposphere (8–12 km). These data suggest that large number densities of highly volatile CN (104 − 105 cm−3) are present in the upper troposphere and particularly over the tropical/subtropical region. CN number densities in all regions are largest when the atmosphere is devoid of nonvolatile particles. Through marine convection and long‐distance horizontal transport, volatile CN originating from the tropical/subtropical regions can frequently impact the abundance of aerosol in the middle and upper troposphere at mid to high latitudes. Nonvolatile aerosols behave in a manner similar to tracers of combustion (CO) and photochemical pollution (peroxyacetylnitrate (PAN)), implying a continental pollution source from industrial emissions or biomass burning. In the upper troposphere we find that volatile and nonvolatile aerosol number densities are inversely correlated. Results from an aerosol microphysical model suggest that the coagulation of fine volatile particles with fewer but larger nonvolatile particles, of principally anthropogenic origin, is one possible explanation for this relationship. In some instances the larger nonvolatile particles may also directly remove precursors (e.g., H2SO4) and effectively stop nucleation.

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