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

Abstract

[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.

Seasonal and interannual variations in HCN amounts in the upper troposphere and lower stratosphere observed by MIPAS

“…A third distinct plume was observed above northern Africa in boreal spring. Possibly due to ocean uptake (Li et al, 2000Singh et al, 2003), the tropospheric HCN amounts exhibit minima above the tropical and subtropical oceans, which are most pronounced during boreal winter and spring. There is generally good agreement with the HCN climatology obtained from spaceborne ACE-FTS measurements and with airborne in situ data from the INTEX-B campaign.…”

“…This minimum is probably caused by ocean uptake (cf. Li et al, 2000Li et al, , 2003Singh et al, 2003) during a long period without southern hemispheric biomass burning. Because of its long middle atmospheric lifetime of 2.5 years (Cicerone and Zellner, 1983), stratospheric HCN is able to map seasonal cycles.…”

“…In recent years ocean uptake has been assumed to be the additional, major sink of HCN. Inclusion of this process in model calculations constrained by aircraft observations leads to a tropospheric lifetime of 5-6 months (Li et al, 2000Singh et al, 2003).…”

“…These observations led to the conclusion that biomass burning is a major source of atmospheric HCN and that there must be an additional sink of tropospheric HCN. Today, HCN is considered as an almost unambiguous tracer of biomass burning Singh et al, 2003;Yokelson et al, 2007;Lupu et al, 2009). HCN has been used as tracer of biomass burning by, for example, Glatthor et al (2009) and Tereszchuk et al (2013).…”

Measurements of volatile organic compounds over West Africa

“…Benzene is predominantly emitted during fossil fuel combustion and biomass burning and it has a lifetime of 10 days against oxidation by OH. It is believed that biomass burning is the main source of acetonitrile to the atmosphere (de Gouw et al, 2003c), while estimates of its atmospheric lifetime range from ∼1 year against oxidation by OH, to months if ocean uptake is assumed to be significant (de Gouw et al, 2003c;Sanhueza et al, 2004;Singh et al, 2003). The global atmospheric budget of acetone, reviewed in Jacob et al (2002), includes primary sources from anthropogenic and biogenic emissions and a photochemical ocean source, in addition to secondary production from oxidation of anthropogenic and biogenic VOC.…”