[1] We report the first in situ measurements of hydrogen cyanide (HCN) and methyl cyanide (CH 3 CN, acetonitrile) from the Pacific troposphere (0-12 km) obtained during the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) airborne mission (February-April 2001). Mean HCN and CH 3 CN mixing ratios of 243 ± 118 (median 218) ppt and 149 ± 56 (median 138) ppt, respectively, were measured. These in situ observations correspond to a mean tropospheric HCN column of 4.2 Â 10 15 molecules cm À2 and a CH 3 CN column of 2.5 Â 10 15 molecules cm À2 . This is in good agreement with the 0-12 km HCN column of 4.4 (±0.6) Â 10 15 molecules cm À2 derived from infrared solar spectroscopic observations over Japan. Mixing ratios of HCN and CH 3 CN were greatly enhanced in pollution outflow from Asia and were well correlated with each other as well as with known tracers of biomass combustion (e.g., CH 3 Cl, CO). Volumetric enhancement (or emission) ratios (ERs) relative to CO in free tropospheric plumes, likely originating from fires, were 0.34% for HCN and 0.17% for CH 3 CN. ERs with respect to CH 3 Cl and CO in selected biomass burning (BB) plumes in the free troposphere and in boundary layer pollution episodes are used to estimate a global BB source of 0.8 ± 0.4 Tg (N) yr À1 for HCN and 0.4 ± 0.1 Tg (N) yr À1 for CH 3 CN. In comparison, emissions from industry and fossil fuel combustion are quite small (<0.05 Tg (N) yr À1 ). The vertical structure of HCN and CH 3 CN indicated reduced mixing ratios in the marine boundary layer (MBL). Using a simple box model, the observed gradients across the top of the MBL are used to derive an oceanic loss rate of 8.8 Â 10 À15 g (N) cm À2 s À1 for HCN and 3.4 Â 10 À15 g (N) cm À2 s À1 for CH 3 CN. An air-sea exchange model is used to conclude that this flux can be maintained if the oceans are undersaturated in HCN and CH 3 CN by 27% and 6%, respectively. These observations also correspond to an open ocean mean deposition velocity (v d ) of 0.12 cm s À1 for HCN and 0.06 cm s À1 for CH 3 CN. It is inferred that oceanic loss is a dominant sink for these cyanides and that they deposit some 1.4 Tg (N) of nitrogen annually to the oceans. Assuming loss to the oceans and reaction with OH radicals as the major removal processes, a mean atmospheric residence time of 5.0 months for HCN and 6.6 months for CH 3 CN is calculated. A global budget analysis shows that the sources and sinks of HCN and CH 3 CN are roughly in balance but large uncertainties remain in part due to a lack of observational data from the atmosphere and the oceans. Pathways leading to the oceanic (and soil) degradation of these cyanides are poorly known but are expected to be biological in nature.

Nitrogen oxides (NOx) have a central role in the chemistry of the atmosphere, especially in key processes relating to ozone, hydroxyl-radical (OH) and acid formation. High reactivity of NOx (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 NOx into its organic forms principally as peroxyacetyl nitrate (PAN). PAN is highly stable in the colder regions of the middle and upper troposphere and can provide a mechanism for NOx storage and transport. Once transported, PAN and its homologues can easily release free NOx in warmer atmospheric conditions. PAN is probably ubiquitous and its concentrations could exceed those of NOx in clean tropospheric conditions. 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 NOx and for extending and validating reactive nitrogen chemistry theory in the troposphere.

No abstract