Hydrogen cyanide in the upper troposphere: GEM-AQ simulation and comparison with ACE-FTS observations

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. Our model results show 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. 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.

What drives the observed variability of HCN in the troposphere and lower stratosphere?

“…Li et al: Observed variability of HCN in the troposphere and lower stratosphere main sink. Recently Lupu et al (2009) reproduced similar seasonal variations of the HCN tropospheric column using the GEM-AQ tropospheric global 3-D model.…”

What causes the irregular cycle of the atmospheric tape recorder signal in HCN?

“…Global HCN emission inventories are subject to large uncertainties. However, there are no indications of substantial interannual variability of HCN emissions outside of Indonesia [Schultz et al, 2008;Lupu et al, 2009], so they are neglected here.…”

“…Over land the uptake of HCN is neglected. Lupu et al [2009] performed a model study of HCN in the upper troposphere and were able to reproduce the observed variability of the volume mixing ratio of HCN in the troposphere and a residence time of ≈5 month.…”

Large-scale upper tropospheric pollution observed by MIPAS HCN and C<sub>2</sub>H<sub>6</sub> global distributions

“…Both the "HCN-plumes" and "C 2 H 6 -plumes" agree fairly well in location and in magnitude of the mixing-ratios. A more detailed comparison can be made with the ACE HCN data in Lupu et al (2009) Li et al, 2003). These authors report mean values of 260 pptv at 9 km and 240 pptv at 11 km altitude over the Northern Pacific (10 • N-40 • N), whereas the respective MIPAS values are 220-240 pptv (Fig.…”

“…The lifetime of stratospheric HCN, where the main sink is reaction with OH, is still assumed to be several years. Recent spaceborne observations of HCN are performed by the Atmospheric Chemistry Instrument (ACE) on SCISAT (Rinsland et al, 2005;Lupu et al, 2009) and by the Microwave Limb Sounder (MLS) on EOS (Pumphrey et al, 2006). C 2 H 6 is the most important non-methane hydrocarbon (NMHC) in the troposphere (Singh et al, 2001).…”