We analyzed seasonality and interannual variability of tropospheric hydrogen cyanide (HCN) columns in densely populated eastern China for the first time. The results were derived from solar absorption spectra recorded with a ground-based high-spectral-resolution Fourier transform infrared (FTIR) spectrometer in Hefei (31°54′N, 117°10′E) between 2015 and 2018. The tropospheric HCN columns over Hefei, China, showed significant seasonal variations with three monthly mean peaks throughout the year. The magnitude of the tropospheric HCN column peaked in May, September, and December. The tropospheric HCN column reached a maximum monthly mean of (9.8±0.78)×1015moleculescm?2 in May and a minimum monthly mean of (7.16±0.75)×1015moleculescm?2 in November. In most cases, the tropospheric HCN columns in Hefei (32°N) are higher than the FTIR observations in Ny-?lesund (79°N), Kiruna (68°N), Bremen (53°N), Jungfraujoch (47°N), Toronto (44°N), Rikubetsu (43°N), Izana (28°N), Mauna Loa (20°N), La Reunion Maido (21°S), Lauder (45°S), and Arrival Heights (78°S) that are affiliated with the Network for Detection of Atmospheric Composition Change (NDACC). Enhancements of tropospheric HCN column were observed between September?2015 and July?2016 compared to the same period of measurements in other years. The magnitude of the enhancement ranges from 5% to 46% with an average of 22%. Enhancement of tropospheric HCN (ΔHCN) is correlated with the concurrent enhancement of tropospheric CO (ΔCO), indicating that enhancements of tropospheric CO and HCN were due to the same sources. The GEOS-Chem tagged CO simulation, the global fire maps, and the potential source contribution function (PSCF) values calculated using back trajectories revealed that the seasonal maxima in May are largely due to the influence of biomass burning in Southeast Asia (SEAS) (41±13.1%), Europe and boreal Asia (EUBA) (21±9.3%), and Africa (AF) (22±4.7%). The seasonal maxima in September are largely due to the influence of biomass burnings in EUBA (38±11.3%), AF (26±6.7%), SEAS (14±3.3%), and North America (NA) (13.8±8.4%). For the seasonal maxima in December, dominant contributions are from AF (36±7.1%), EUBA (21±5.2%), and NA (18.7±5.2%). The tropospheric HCN enhancement between September?2015 and July 2016 at Hefei (32°N) was attributed to an elevated influence of biomass burnings in SEAS, EUBA, and Oceania (OCE) in this period. In particular, an elevated number of fires in OCE in the second half of 2015 dominated the tropospheric HCN enhancement between September and December?2015. An elevated number of fires in SEAS in the first half of 2016 dominated the tropospheric HCN enhancement between January and July?2016.
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