In situ measurements of HCN and CH3CN over the Pacific Ocean: Sources, sinks, and budgets

We report the first in situ measurements of hydrogen cyanide (HCN) and methyl cyanide (CH3CN, 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 CH3CN 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 × 1015 molecules cm-2 and a CH3CN column of 2.5 × 1015 molecules cm-2. This is in good agreement with the 0-12 km HCN column of 4.4 (±0.6) × 1015 molecules cm-2 derived from infrared solar spectroscopic observations over Japan. Mixing ratios of HCN and CH3CN 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., CH3Cl, 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 CH3CN. ERs with respect to CH3Cl 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-1 for HCN and 0.4 ± 0.1 Tg (N) yr-1 for CH3CN. In comparison, emissions from industry and fossil fuel combustion are quite small (<0.05 Tg (N) yr-1). The vertical structure of HCN and CH3CN 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 × 10-15 g (N) cm-2 s-1 for HCN and 3.4 × 10-15 g (N) cm-2 s-1 for CH3CN. An air-sea exchange model is used to conclude that this flux can be maintained if the oceans are undersaturated in HCN and CH3CN by 27% and 6%, respectively. These observations also correspond to an open ocean mean deposition velocity (vd) of 0.12 cm s-1 for HCN and 0.06 cm s-1 f