Carbon monoxide (CO) is an important atmospheric trace gas, which affects air quality and the radiative balance of the Earth (IPCC, 2013). CO reacts with hydroxyl radicals (OH) and changes the atmospheric oxidizing capacity. CO is also a good trace gas to study long-distance transport of fire and biomass burning emissions (Duflot et al., 2010; Zhou et al., 2018), as it has a relatively long lifetime of about 2 months (Khalil and Rasmussen, 1990). Atmospheric CO is mainly emitted from biomass burning, fossil fuel combustion, and oxidation from methane (CH4) and other biogenic non-methane hydrocarbons (NMHCs) and is removed mainly by the reaction with OH and partly by uptake by soil microorganisms (Holloway et al., 2000). Ethane (C2H6) and acetylene (C2H2) are two major NMHCs. The sources of C2H2 and C2H6 are combustions from fossil fuel, biofuels, and biomass burning, and the sink of C2H2 and C2H6 is the reaction with OH (Xiao et al., 2007, 2008). C2H6 is a strong source of peroxyacetyl nitrate (PAN), a reservoir for nitrogen dioxide (NO2). NO2 normally has a short lifetime, but in the form of PAN it can be transported over long distances to a remote place, leading to an important impact on the tropospheric ozone formation. The C2H2 oxidation by OH can form secondary organic aerosols, affecting the atmospheric chemistry (Volkamer et al., 2009). C2H6 has a similar lifetime to CO of about 2 months (Rudolph, 1995). C2H2 has a relatively short lifetime of 2 to 4 weeks (Xiao et al., 2007). Hydrogen cyanide (HCN) is a colorless and highly poisonous gas that is less reactive than CO and has a lifetime of 2-4 months. Atmospheric HCN’s main sources are biomass burning, biogenic emissions, and biofuels combustion, and it is mainly removed by the reaction with OH and ocean uptake (Li et al., 2000, 2003). Formaldehyde (H2CO) is another important trace gas, mainly produced by methane and NMHCs oxidation in the atmosphere (Fortems-Cheiney et al., 2012). H2CO in the atmosphere reacts quickly with OH, NO3, Cl, and Br, leading to a typical lifetime of a few hours (Anderson et al., 2017). H2CO is a major intermediate product in the degradation of isoprene in the atmosphere, and it strongly affects the tropospheric ozone formation (Finlayson-Pitts and Pitts, 1993). The main sources and sinks of global CO, C2H2, C2H6, H2CO, and HCN are summarized in Table 1.
Holloway et al. (2000)Xiao et al. (2007)Xiao et al. (2008)Fortems-Cheiney et al. (2012)Li et al. (2003)
Because of the common sources and sinks (Table 1), a number of studies have used the ratio of C2H2 to CO as a tracer of the age of air to investigate the relative importance of dilution and chemistry (Xiao et al., 2007; Parker et al., 2011). In addition, the emission factors of biomass burning or forest fire were calculated and evaluated based on available aircraft observations (Goode et al., 2000; Xiao et al., 2007; Wetzel et al., 2021) and ground-based measurements (Zhao et al., 2002; Vigouroux et al., 2012; Lutsch et al., 2016). However, the variations and correlations of CO, C2H2, C2H6, H2CO, and HCN in polluted area in northern China are not well known due to limited measurements. It is expected that these species are strongly affected by the anthropogenic emission there, but can we also capture the fire emissions?
In June 2018, we started measuring mid-infrared high spectral resolution solar absorption spectra using a new ground-based Fourier transform infrared (FTIR) spectrometer (Bruker IFS 125HR) at Xianghe (39.75∘ N, 116.96∘ E; 36 m a.s.l.), China. Such FTIR measurements are compliant with the Network for Detection of Atmospheric Composition Change – InfraRed Working Group (NDACC-IRWG) (De Mazière et al., 2018) protocols. In contrast to most NDACC FTIR sites, which are located in remote areas, the Xianghe site is located in a highly populated area with strong anthropogenic emissions from fossil fuel and biofuels combustion among other sources (Yang et al., 2020; Zhou et al., 2021). In this study, we present the first time series of FTIR retrievals of CO, C2H2, C2H6, H2CO, and HCN at Xianghe, covering 3 years of measurements, and their use to investigate the variations in CO, C2H2, C2H6, H2CO, and HCN columns on both seasonal and synoptic scales, as well as their correlations. Moreover, the FTIR measurements, atmospheric model, and satellite measurements are used to understand the sources of CO, C2H2, C2H6, H2CO, and HCN columns in this region. Section 2 gives a brief introduction to the FTIR instrument and discusses the retrieval strategies, retrieval information, and uncertainties. The time series, variations, and correlations of CO, C2H2, C2H6, H2CO, and HCN total columns are analyzed in Sect. 3. the FLEXible PARTicle dispersion (FLEXPART) model is used to understand the sources of the observed air masses at Xianghe. The VIIRS satellite observations are applied to locate the fire emission in the boreal forest. Based on the CO anthropogenic emissions and the ratios of C2H2 and C2H6 to CO derived from the FTIR measurements, we derive the C2H2 and C2H6 emissions in northern China and compare them to the inventories. Finally, the conclusions are drawn in Sect. 4.