Abstract
Abstract. We investigate the spatial and temporal distribution of hydrogen cyanide (HCN) in the upper troposphere through numerical simulations and comparison with observations from a space-based instrument. To perform the simulations, we used the Global Environmental Multiscale Air Quality model (GEM-AQ), which is based on the three-dimensional global multiscale model developed by the Meteorological Service of Canada for operational weather forecasting. The model was run for the period 2004–2006 on a 1.5°×1.5° global grid with 28 hybrid vertical levels from the surface up to 10 hPa. Objective analysis data from the Canadian Meteorological Centre were used to update the meteorological fields every 24 h. Fire emission fluxes of gas species were generated by using year-specific inventories of carbon emissions with 8-day temporal resolution from the Global Fire Emission Database (GFED) version 2. The model output is compared with HCN profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument onboard the Canadian SCISAT-1 satellite. High values of up to a few ppbv are observed in the tropics in the Southern Hemisphere; the enhancement in HCN volume mixing ratios in the upper troposphere is most prominent in October. Low upper-tropospheric mixing ratios of less than 100 pptv are mostly recorded at middle and high latitudes in the Southern Hemisphere in May–July. Mixing ratios in Northern Hemisphere peak in the boreal summer. The amplitude of the seasonal variation is less pronounced than in the Southern Hemisphere. The comparison with the satellite data shows that in the upper troposphere GEM-AQ performs well globally for all seasons, except at northern high and middle latitudes in summer, where the model has a large negative bias, and in the tropics in winter and spring, where it exhibits large positive bias. This may reflect inaccurate emissions or possible inaccuracies in the emission profile. The model is able to explain most of the observed variability in the upper troposphere HCN field, including the interannual variations in the observed mixing ratio. A complementary comparison with daily total columns of HCN from two middle latitude ground-based stations in Northern Japan for the same simulation period shows that the model captures the observed seasonal variation and also points to an underestimation of model emissions in the Northern Hemisphere in the summer. The estimated average global emission equals 1.3 Tg N yr−1. The average atmospheric burden is 0.53 Tg N, and the corresponding lifetime is 4.9 months.
Highlights
Hydrogen cyanide (HCN) is a minor constituent of the atmosphere emitted primarily from biomass burning
The model is able to explain most of the observed variability in the upper troposphere HCN field, including the interannual variations in the observed mixing ratio
The authors concluded that a global biomass burning source of 0.63 Tg N yr−1 combined with a residential coal burning source in Asia of 0.20 Tg N yr−1 provided a best fit to the TRACE-P observations
Summary
Hydrogen cyanide (HCN) is a minor constituent of the atmosphere emitted primarily from biomass burning. In their study, Li et al (2000) showed that seasonal variation of observed HCN total columns at 6 sites in the Northern Hemisphere can be reproduced in a model scenario where biomass burning is the main source and ocean uptake is the main sink They estimated a global biomass burning HCN source of 1.4–2.9 Tg N yr−1 and an atmospheric lifetime for HCN of 2.1–4.4 months. Li et al (2003) used a chemistry and transport model, GEOS-CHEM, to analyse HCN observations in the Asian outflow from the Transport and Chemical Evolution over the Pacific (TRACE-P) aircraft mission which was conducted over the Northwest Pacific between February and April 2001 (Singh et al, 2003) Their modelling study confirmed that the ocean is a dominant sink for HCN and estimated a deposition velocity of 0.13 cm s−1 from the observed vertical gradients in the remote marine air. The emissions used include yearly-averaged anthropogenic and monthly-averaged biogenic, ocean and soil fluxes, as well as year-specific 8-dayaveraged biomass burning fluxes
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