The Reduction In C2H6 From 2015 To 2020 Over Hefei, Eastern China ...

Ethane (C2H6) is an important greenhouse gas and one of the most abundant volatile organic compounds (VOCs) in the atmosphere (Abad et al., 2011; Singh et al., 2001; Steinfeld, 1998). Although C2H6 is much less abundant than methane (CH4) and also less efficient relative to mass, it plays a significant role in tropospheric chemistry and climate change (Tzompa-Sosa et al., 2017). In the presence of nitrogen oxides (NOx = NO + NO2), C2H6 oxidation can enhance tropospheric ozone (O3) generation, which shows a positive radiative influence on climate (Sun et al., 2018a) and threatens crop yields (Sun et al., 2018a; Van Dingenen et al., 2009) and human health (Sun et al., 2018a; Tzompa-Sosa et al., 2017). In addition, as a major source of acetaldehyde (CH3CHO), C2H6 has a great impact on the production of peroxyacetyl nitrate (PAN) which is a key reservoir species of NOx (Fischer et al., 2014). The main sink of tropospheric C2H6 is predominantly destruction via reaction with the hydroxyl radical (OH) (Xiao et al., 2008), which determines the residence time of most tropospheric species (Steinfeld, 1998). As a result, tropospheric C2H6 can decrease the atmospheric oxidative capacity and indirectly impact the climate by extending the CH4 lifetime (Monks et al., 2018; Taylor et al., 2020). Atmospheric C2H6 has a relatively long residence time of a few months (Franco et al., 2016), allowing it to undergo intercontinental transport. As a result, observations of C2H6 can be assimilated into a chemical transport model to estimate nonlocal emissions and air quality, and provide valuable insights into model biases of C2H6 simulations (Tzompa-Sosa et al., 2017).

On a global scale, the main sources of C2H6 are leakage from production, processing, and transport of natural gas (62 %), and biofuel combustion (20 %) and biomass burning emission (18 %) largely occurred in the Northern Hemisphere (NH) (Franco et al., 2016; Xiao et al., 2008). Additional minor sources of C2H6 are from biogenic and oceanic sources. However, on a regional scale, the proportion of each C2H6 source may show large differences. The natural gas leakage contribution can reach 80 % of C2H6 emissions in regions with active oil and natural gas production (Gilman et al., 2013), where C2H6 emissions are highly correlated with CH4 emissions. In such regions, C2H6 can be applied as a tracer for the separation of fossil fuel CH4 emissions from multiple methane (CH4) sources (e.g., oil and gas, cows, wetlands, and rice yield) (McKain et al., 2015; Roscioli et al., 2015). The C2H6 abundance in the Southern Hemisphere (SH) is much lower than that in the NH, as the anthropogenic C2H6 sources are low in the SH and the residence time of C2H6 is shorter than the interhemispheric exchange rate. Many studies have concluded that C2H6 in the SH is primarily emitted from biomass burning and is closely correlated with CO and HCN emissions (Notholt et al., 2000; Rinsland et al., 2002; Vigouroux et al., 2012; Zeng et al., 2012).

C2H6 is one of the target gases of a global ground-based Fourier transform infrared spectroscopy (FTIR) network, namely the infrared working group (IRWG) of the Network for Detection of Atmospheric Composition Change (NDACC) (De Mazière et al., 2018). FTIR time series of C2H6 with different time periods have been reported at many stations for the validation of satellite data or chemical model simulation (Abad et al., 2011; Franco et al., 2015, 2016; Glatthor et al., 2009) or for the evaluation of local air quality and air pollutant transport caused by anthropogenic emission and biomass burning (Angelbratt et al., 2011; Lutsch et al., 2016, 2019; Nagahama and Suzuki, 2007; Rinsland et al., 2002; Simpson et al., 2012; Viatte et al., 2015, 2014; Vigouroux et al., 2012; Zeng et al., 2012; Zhao et al., 2002). Several FTIR sites have observed the decrease in C2H6 over the 1990–2010 period and have characterized consistent interannual trends in the −1 to −2.7 % yr−1 range (Franco et al., 2015, 2016; Simpson et al., 2012; Zeng et al., 2012). This declining trend has been largely attributed to the reduction in global fugitive emissions (Franco et al., 2015; Simpson et al., 2012). Recently, several studies concluded that the long-term decline in C2H6 in the NH reversed from 2009 onwards (Franco et al., 2015, 2016). Using ground-based FTIR C2H6 total columns derived at five selected NDACC sites, Franco et al. (2016) characterized the C2H6 evolution from 2009 to 2015 and determined growth rates of ∼ 3 % yr−1 at remote sites and of ∼ 5 % yr−1 at midlatitudes. This change is mainly attributed to the exploitation of shale gas and tight oil reservoirs in North America (Franco et al., 2016; Helmig et al., 2016).

The NDACC network has been operating for almost 3 decades around the globe (De Maziere et al., 2018; Sun et al., 2018a). However, most instruments are located in Europe and Northern America, the number of observation sites in the rest parts of world remains sparse, and there is only one qualified observations site in China, i.e., the Hefei site (32∘ N, 117∘ E; 30 ma.s.l.), located in a densely populated and highly industrialized area in eastern China (Sun et al., 2018a). The Hefei site is not yet affiliated with the NDACC network, but its observation routine has followed the NDACC standard convention since 2015 (Sun et al., 2018a). As the consequence of a series of actions for emission control, air pollution over China in recent years has been significantly decreased (Zhang et al., 2019; Zheng et al., 2018). However, the atmospheric pollution over densely populated and highly industrialized eastern China is still severe (Zhang et al., 2019; Zheng et al., 2018). The complexity, extension, and severity of the atmospheric pollution in eastern China are still unrivaled compared with the rest of world (Lu et al., 2018; Zheng et al., 2018). FTIR observations at Hefei have been used extensively for the evaluation of satellite data (Tian et al., 2018; Wang et al., 2017), chemical model simulation (Tian et al., 2018; Yin et al., 2020, 2019), local air quality (Shan et al., 2019; Sun et al., 2018a), and the transport of air pollutants caused by anthropogenic and biomass burning emissions (Sun et al., 2018a, 2000; Y. Sun et al., 2021).

In this study, we first present and then quantify the variability, sources, and transport of C2H6 over densely populated and highly industrialized eastern China using FTIR observation, GEOS-Chem model simulation, and the analysis of the meteorological fields. The seasonality and interannual variability of C2H6 over Hefei, eastern China, from 2015 to 2020 are investigated. The dependencies of C2H6 on meteorological and co-emitted gases (hereafter emission factors) are analyzed using generalized additive models (GAMs) (Wood and Simon, 2004). The ground-based FTIR C2H6 time series are, for the first time, applied to evaluate the GEOS-Chem model with respect to the simulation of C2H6 for specific polluted regions over eastern China. Furthermore, we run a series of GEOS-Chem sensitivity simulations to quantify the relative contributions of various source categories and regions to the observed C2H6 variability. The three-dimensional (3D) transport inflow and outflow pathways of C2H6 over the observation site are finally determined by the GEOS-Chem sensitivity simulations and the analysis of the meteorological fields. This study can not only enhance the understanding of regional emission, transport, and air clean actions over eastern China but can also contribute to form new reliable remote sensing data in this sparsely monitored region for climate change research.

The next section describes the retrieval of the FTIR tropospheric column-averaged dry-air mole fraction (troDMF) of C2H6, the configuration of GEOS-Chem model simulation, and the GAMs regression approach. Section 3 reports the variability of C2H6 troDMF and a comparison with the GEOS-Chem simulation. Section 4 reports the GAMs regression results and the interpretation. Section 5 reports the results for source attribution using a GEOS-Chem sensitivity simulation and the analysis of the meteorological fields. We conclude the study in Sect. 6.