Comparing sources of carbonaceous aerosols during haze and nonhaze periods in two northern Chinese cities.

Radiocarbon (14C), stable carbon isotope (13C), and levoglucosan in PM2.5 were measured in two northern Chinese cities during haze events and nonhaze periods in January 2019, to ascertain the sources and their differences in carbonaceous aerosols between the two periods. The contribution of primary vehicle emissions (17.8 ± 3.7%) to total carbon in Beijing during that haze event was higher than that of primary coal combustion (7.3 ± 4.2%), and it increased significantly (7.1%) compared to the nonhaze period. The contribution of primary vehicle emissions (4.1 ± 2.8%) was close to that of primary coal combustion (4.3 ± 3.3%) during the haze event in Xi'an, and the contribution of primary vehicle emissions decreased by 5.8% compared to the nonhaze period. Primary biomass burning contributed 21.1 ± 10.5% during the haze event in Beijing and 40.9 ± 6.6% in Xi'an (with an increase of 3.3% compared with the nonhaze period). The contribution of secondary fossil fuel sources to total secondary organic carbon increased by 29.2% during the haze event in Beijing and by 18.4% in Xi'an compared to the nonhaze period. These results indicate that specific management measures for air pollution need to be strengthened in different Chinese cities in the future, that is, controlling vehicle emissions in Beijing and restricting the use of coal and biomass fuels in winter in Xi'an.

Tracing fossil fuel CO2 by 14C in maize leaves in Guanzhong Basin of China.

Quantifying fossil fuel CO2 (CO2ff) in the atmosphere provides a benchmark method to monitor anthropogenic carbon emissions. Radiocarbon (14C) in atmospheric CO2ff has been widely studied using the 14C in plants to document regional CO2ff patterns. However, annual CO2ff variations, reflecting spatial distributions based on plant samples, are still scarce. In this paper, the spatial distribution and temporal CO2ff changes in the Guanzhong Basin is established using Δ14C measurements from maize leaves collected in 2011 and 2012. With regard to spatial distribution, samples collected around Xi'an City showed lower Δ14C values (more CO2ff), while sites located near the perimeter of the basin showed higher Δ14C values (less CO2ff). This is likely due to the concentration of anthropogenic activities in the center of the Guanzhong Basin. The observed CO2ff mole fractions generally matched with PKU CO2 inventory and the ODIAC CO2 inventory data in the spatial distribution trend. However, it seems that thermal power plants were not well captured by the PKU CO2 inventory. Our results provide useful information for the improvement of the inventory and verification of regional carbon cycle models.

Characteristics and source apportionment of particulate carbon in precipitation based on dual-carbon isotopes (13C and 14C) in Xi'an, China.

Wet deposition is a dominant removal pathway of carbonaceous particles from the atmosphere, but few studies have assessed the particulate carbon in precipitation in Chinese cities. To assess the characteristics and sources of particulate carbon, we measured the concentrations, fluxes, stable carbon isotopes, and radiocarbon of particulate carbon, and some cations concentrations in precipitation in Xi'an, China, in 2019. In contrast to rainfall samples, particulate carbon in snowfall samples in Xi'an showed extremely high concentrations and wet deposition fluxes. The concentrations as well as wet deposition fluxes showed no significant (p > 0.05) differences between urban and suburban sites, and they also exhibited low seasonality in rainfall samples. Water-insoluble organic carbon (WIOC) accounted for the majority (∼90%) of the concentrations and wet deposition fluxes of water-insoluble total carbon (WITC) in precipitation. The best estimates of source apportionment of WITC in precipitation showed that biological sources were the main contributor (80.0% ± 10.5%) in summer, and their contributions decreased to 47.3% ± 12.8% in winter. The contribution of vehicle exhaust emissions accounted for 11.7% ± 3.5% in summer and 39.0% ± 4.3% in winter, while the contributions of coal combustion were relatively small in summer (8.3% ± 7.0%) and winter (13.8% ± 8.5%). Biomass burning accounted for 25.7% ± 9.3% and 89.9% ± 0.7% of the biological sources in summer and winter, respectively, with the remainder comprising other sources of contemporary carbon. These results highlight the nonnegligible contributions of biogenic emissions and biomass burning to particulate carbon in precipitation in this city in summer and winter, respectively.

Stable carbon isotopic characteristics of fossil fuels in China.

Good knowledge on the stable carbon isotopic composition (δ13C) of fossil fuels is critical for the estimation of atmospheric CO2 sources. Here, we complied a comprehensive δ13C database including 336 coal, 580 oil, and 1160 natural gas data based on the extensive literature search, and conducted field measurements in two megacities, to characterize the δ13C signatures of Chinese fossil fuels. Results show that coal exhibits a narrow range and the most enriched in δ13C signature, oil displays intermediate variations both in the distribution and value of δ13C. By contrast, natural gas is strongly depleted but became more enriched in δ13C signature due to the shift in production from isotopically light oil-type gas to isotopically heavy coal-type gas. We found an obvious overlap between the δ13C distributions of oil and natural gas, and the carbon isotopic difference between oil and natural gas is minimized in Ordos Basin. Therefore, we suggested that the geographic origin is a certain factor that must be considered when δ13C of fossil fuels is used to estimate CO2 source contributions, and the measurement of δ13CO2 signatures of local end members is a better alternative in the absence of detailed information about the geographical origins of fossil fuels. This work is helpful in improving the ability to quantify CO2 sources of fossil fuel emissions in China, and also make a contribute to the global carbon isotope database.

The impact of COVID-19 lockdown on atmospheric CO2 in Xi'an, China.

Lockdown measures to control the spread of the novel coronavirus disease (COVID-19) sharply limited energy consumption and carbon emissions. The lockdown effect on carbon emissions has been studied by many researchers using statistical approaches. However, the lockdown effect on atmospheric carbon dioxide (CO2) on an urban scale remains unclear. Here we present CO2 concentration and carbon isotopic (δ13C) measurements to assess the impact of COVID-19 control measures on atmospheric CO2 in Xi'an, China. We find that CO2 concentrations during the lockdown period were 7.5% lower than during the normal period (prior to the Spring Festival, Jan 25 to Feb 4, 2020). The observed CO2excess (total CO2 minus background CO2) during the lockdown period was 52.3% lower than that during the normal period, and 35.7% lower than the estimated CO2excess with the effect of weather removed. A Keeling plot shows that in contrast CO2 concentrations and δ13C were weakly correlated (R2 = 0.18) during the lockdown period, reflecting a change in CO2 sources imposed by the curtailment of traffic and industrial emissions. Our study also show that the sharp reduction in atmospheric CO2 during lockdown were short-lived, and returned to normal levels within months after lockdown measures were lifted.

Time series of atmospheric Δ14CO2 recorded in tree rings from Northwest China (1957-2015).

Radiocarbon (14C) is a unique and important tool for understanding carbon cycle in the nature, and its use can be significantly enhanced where reliable historical atmospheric Δ14CO2 records can be established. In China, continuous atmospheric Δ14CO2 records since the 1950s are scarce, a period when dramatic variations of Δ14CO2 occurred caused by intensive human activities. In this research, Δ14C of Qinghai spruce tree rings collected from Huangzhong (HZ) (36.27°N, 101.67°E, 2982 m amsl) were measured by Accelerator Mass Spectrometry, and a Δ14CO2 time series from 1957 to 2015 was reconstructed. The results show that HZ Δ14C was generally higher than the contemporaneous average level in the mid-high latitudes of the Northern Hemisphere. The peak value of HZ Δ14C occurred in 1964 (as bomb peak) was higher than that of other tree ring records in East Asia at a similar latitude, likely due to the impact of the atmosphere nuclear tests at Semipalatinsk (Kazakhstan). The record shows no obvious disturbance of Lop Nor nuclear weapons tests (in Northwest China) during 1964-1980, except for 1971. A local Suess effect began to appear since 2001, and the estimated atmospheric fossil fuel-derived CO2 (CO2ff) concentration increased from 3.5 ppm to 8.8 ppm from 2006 to 2015. This is associated with the implementation of the "Western Development" strategy in China. HZ Δ14C records document background Δ14C data, useful for regional carbon cycle research and atmospheric CO2ff quantification in the region. These data also provide baseline values for assessment environmental safety connected with nuclear power plants in China.

Fossil fuel CO2 traced by radiocarbon in fifteen Chinese cities.

China is an important fossil fuel CO2 (CO2ff) emitter and the international community is thus concerned with quantifying reductions in Chinese carbon emissions in the recent past. Compared to traditional statistical method, radiocarbon (14C) offers a different approach to quantify atmospheric CO2 derived from fossil fuel emissions. Here, we carry out a multi-year (2011-2016) CO2ff tracing by 14C in Xi'an, and a three-year (2014-2016) CO2ff tracing in 15 Chinese cities. The Xi'an results show that average CO2ff concentrations fell 35.9 ± 6.6% from 2014- 2016, compared to 2011-2013, and the timing of this decrease coincides with the implementation of nationwide carbon reduction measures in China, known as the Action Plan on Prevention and Control of Air Pollution. A WRF-Chem forward modeling simulation reveals that the CO2ff in Xi'an is mainly derived from local sources, and a source apportionment combined stable-carbon isotope showed that the CO2ff in this city is dominated by coal combustion (72.6 ± 10.4%). Strong CO2ff differences are found between January and July in most Chinese cities. High CO2ff concentrations often correspond to severe haze episodes and there are generally positive correlations between CO2ff and fine particulate (PM2.5) concentrations. Our study provides scientific data to understand the effects of CO2ff reduction strategies in China that can be applied to other countries as well.

Determining diurnal fossil fuel CO2 and biological CO2 by Δ14CO2 observation on certain summer and winter days at Chinese background sites.

Diurnal atmospheric Δ14CO2 was measured on two consecutive days in summer and winter, 2016 at Shangdianzi, Lin'an and Luhuitou regional background sites, and at Waliguan global background site in China. The objectives of this study were to determine diurnal fossil fuel CO2 (CO2ff) and biological CO2 (CO2bio) concentrations and to ascertain the factors influencing them. Evident CO2ff inputs (0-33.0 ± 1.4 ppm) were found, with some small morning and afternoon rush hour signals. Particularly, the long-range transport of air masses influenced the seasonal differences and rapid diurnal variations in CO2ff. Diurnal CO2bio showed violent variations (-20.9-113.3 ppm), with high values at night and low or negative values during the daytime. Diurnal CO2bio variations resulted from the tradeoffs between photosynthetic CO2 uptake and biological respiration CO2 emission as well as atmospheric boundary layer heights variations. These results might help to understand the roles of fossil fuel sources and biological sources on atmospheric CO2 diurnal variations at Chinese background sites.

Simulations of summertime fossil fuel CO2 in the Guanzhong basin, China.

Recent studies on fossil fuel CO2 simulation associated with Δ14CO2 measurements is quite limited, particularly in China. In this study, the fossil fuel CO2 recently added to the atmosphere (δCO2ff) over the Guanzhong basin, central China, during summer 2012 is simulated using a modified WRF-CHEM model constrained by measured CO2 mixing ratio and Δ14CO2. The model well captures the temporal variation of observed CO2 mixing ratio and Δ14CO2, and reasonably reproduces the distribution of observed Δ14CO2. The simulation shows a significant variation of δCO2ff during summertime, ranging from <5ppmv to ~100ppmv and no remarkable trend of δCO2ff is found for June, July, and August. The δCO2ff level is closely associated with atmospheric diffusion conditions. The diurnal cycle of δCO2ff presents a double-peak pattern, a nocturnal one and a rush-hour one, related to the development of planetary boundary layer and CO2 emission from vehicles. The spatial distributions of summertime δCO2ff within the basin is clearly higher than the outside, reaching up to 40ppmv in urban Xi'an and 15ppmv in its surrounding areas, indicative of large local fossil fuel emissions. Furthermore, we find that neglecting the influence of summer heterotrophic respiration in terrestrial biosphere would slightly underestimate the calculated δCO2ff by about 0.38ppmv in the basin.

Emission characteristics of atmospheric carbon dioxide in Xi'an, China based on the measurements of CO2 concentration, △14C and δ13C.

Given that cities contributed most of China's CO2 emissions, understanding the emission characteristics of urban atmospheric CO2 is critical for regulating CO2 emissions. Regular observations of atmospheric CO2 concentration, △14C and δ13C values were performed at four different sites in Xi'an, China in 2016 to illustrate the temporal and spatial variations of CO2 emissions and recognize their sources and sinks in urban carbon cycles. We found seasonal variations in CO2 concentration and δ13C values, the peak to peak amplitude of which was 80.8ppm for CO2 concentration and 4.0‰ for its δ13C. With regard to the spatial variations, the urban CO2 "dome" effect was the most pronounced during the winter season. The use of △14C combines with δ13C measurements aid in understanding the emission patterns. The results show that in the winter season, emissions from fossil fuel derived CO2 (CO2ff) contributed 61.8±10.6% and 57.4±9.7% of the excess CO2 (CO2ex) in urban and suburban areas respectively. Combining with the result of estimated δ13C value of fossil fuel (δ13Cff=-24‰), which suggest coal burning was the dominant source of fossil fuel emissions. In contrast, the proportions of CO2ff in CO2ex varied more in the summer season than that in the winter season, ranging from 42.3% to >100% with the average contributions of 82.5±23.8% and 90.0±24.8%. Given the estimation of δ13C value of local sources (δ13Cs) was -21.9‰ indicates that the intensively biogenic activities, such as soil respiration and corn growth have significantly impacted urban carbon cycles, and occasionally played a role of carbon sink.

Atmospheric fossil fuel CO2 traced by 14CO2 and air quality index pollutant observations in Beijing and Xiamen, China.

Radiocarbon (14C) is the most accurate tracer available for quantifying atmospheric CO2 derived from fossil fuel (CO2ff), but it is expensive and time-consuming to measure. Here, we used common hourly Air Quality Index (AQI) pollutants (AQI, PM2.5, PM10, and CO) to indirectly trace diurnal CO2ff variations during certain days at the urban sites in Beijing and Xiamen, China, based on linear relationships between AQI pollutants and CO2ff traced by 14C ([Formula: see text]) for semimonthly samples obtained in 2014. We validated these indirectly traced CO2ff (CO2ff-in) concentrations against [Formula: see text] concentrations traced by simultaneous diurnal 14CO2 observations. Significant (p < 0.05) strong correlations were observed between each of the separate AQI pollutants and [Formula: see text] for the semimonthly samples. Diurnal variations in CO2ff traced by each of the AQI pollutants generally showed similar trends to those of [Formula: see text], with high agreement at the sampling site in Beijing and relatively poor agreement at the sampling site in Xiamen. AQI pollutant tracers showed high normalized root-mean-square (NRMS) errors for the summer diurnal samples due to low [Formula: see text] concentrations. After the removal of these summer samples, the NRMS errors for AQI pollutant tracers were in the range of 31.6-64.2%. CO generally showed a high agreement and low NRMS errors among these indirect tracers. Based on these linear relationships, monthly CO2ff averages at the sampling sites in Beijing and Xiamen were traced using CO concentration as a tracer. The monthly CO2ff averages at the Beijing site showed a shallow U-type variation. These results indicate that CO can be used to trace CO2ff variations in Chinese cities with CO2ff concentrations above 5 ppm.

Δ14CO2 from dark respiration in plants and its impact on the estimation of atmospheric fossil fuel CO2.

Radiocarbon (14C) has been widely used for quantification of fossil fuel CO2 (CO2ff) in the atmosphere and for ecosystem source partitioning studies. The strength of the technique lies in the intrinsic differences between the 14C signature of fossil fuels and other sources. In past studies, the 14C content of CO2 derived from plants has been equated with the 14C content of the atmosphere. Carbon isotopic fractionation mechanisms vary among plants however, and experimental study on fractionation associated with dark respiration is lacking. Here we present accelerator mass spectrometry (AMS) radiocarbon results of CO2 respired from 21 plants using a lab-incubation method and associated bulk organic matter. From the respired CO2 we determine Δ14Cres values, and from the bulk organic matter we determine Δ14Cbom values. A significant difference between Δ14Cres and Δ14Cbom (P < 0.01) was observed for all investigated plants, ranging from -42.3‰ to 10.1‰. The results show that Δ14Cres values are in agreement with mean atmospheric Δ14CO2 for several days leading up to the sampling date, but are significantly different from corresponding bulk organic Δ14C values. We find that although dark respiration is unlikely to significantly influence the estimation of CO2ff, an additional bias associated with the respiration rate during a plant's growth period should be considered when using Δ14C in plants to quantify atmospheric CO2ff.

Variation of atmospheric 14CO2 and its spatial distribution.

The atmospheric 14CO2 is usually presented in the Δ14C notation, which cannot reflect its absolute quantity change. This article presents the atmospheric radiocarbon activity concentrations (aacn, reported in mBq/m3) in recent years at nine observation stations. The aacn at Schauinsland decrease from 1977 to 1993 but between 1993 and 2003 keeps at a relative steady state. Atmospheric aacns were higher in the northern hemisphere than that in the southern hemisphere. The aacns in the northern hemisphere show clear seasonal cycle with higher value in winter and lower value in summer, while this seasonality is not obvious in the southern hemisphere. Vegetation plays as a role of sink in summer and a role of source in winter, and atmosphere-biosphere radiocarbon exchange might be the main driver of the aacns seasonality. The annual mean aacns in both hemispheres show slightly increasing trends since 2002, which may be mainly caused by decreasing air-sea 14C flux as the air-sea 14C gradient decline.

Authentication of Chinese vintage liquors using bomb-pulse 14C.

The older a bottle of Chinese vintage liquor is, the higher the price it commands. Driven by the potential for higher profits, some newly-founded distilleries openly sell liquor whose storage ages are exaggerated. In China, the market for vintage liquor has become fraught with uncertainty and a pressing need has arisen to establish an effective method to authenticate the age of vintage liquors. A radiocarbon (14C) dating method is described here that can verify cellar-stored years of Chinese liquors distilled within the last fifty years. Two different flavored Chinese liquors produced in "the golden triangular region" in the Upper Yangtze River region in southwest China, with known cellar-stored years, were analyzed to benchmark the technique. Strong flavored liquors are found to be consistent with local atmospheric Δ14C values. A small offset of 2-3 years between predicted vintage years of soy-sauce flavored liquors and strong flavored liquors is found to be associated with the fermentation cycle of certain varieties. The technique can measure cellar-stored years of a wide range of liquors including those with fundamentally different aromas. This demonstrates the strength of our method in identifying suspect Chinese vintage liquors.

Observations of Atmospheric Δ14CO2 at the Global and Regional Background Sites in China: Implication for Fossil Fuel CO2 Inputs.

Six months to more than one year of atmospheric Δ14CO2 were measured in 2014-2015 at one global background site in Waliguan (WLG) and four regional background sites at Shangdianzi (SDZ), Lin'an (LAN), Longfengshan (LFS) and Luhuitou (LHT), China. The objectives of the study are to document the Δ14CO2 levels at each site and to trace the variations in fossil fuel CO2 (CO2ff) inputs at regional background sites. Δ14CO2 at WLG varied from 7.1 ± 2.9‰ to 32.0 ± 3.2‰ (average 17.1 ± 6.8‰) in 2015, with high values generally in autumn/summer and low values in winter/spring. During the same period, Δ14CO2 values at the regional background sites were found to be significantly (p < 0.05) lower than those at WLG, indicating different levels of CO2ff inputs at those sites. CO2ff concentrations at LAN (12.7 ± 9.6 ppm) and SDZ (11.5 ± 8.2 ppm) were significantly (p < 0.05) higher than those at LHT (4.6 ± 4.3 ppm) in 2015. There were no significant (p > 0.05) seasonal differences in CO2ff concentrations for the regional sites. Regional sources contributed in part to the CO2ff inputs at LAN and SDZ, while local sources dominated the trend observed at LHT. These data provide a preliminary understanding of atmospheric Δ14CO2 and CO2ff inputs for a range of Chinese background sites.

Atmospheric Fossil Fuel CO2 Traced by Δ(14)C in Beijing and Xiamen, China: Temporal Variations, Inland/Coastal Differences and Influencing Factors.

One year of atmospheric Δ(14)CO2 were observed in 2014 in the inland city of Beijing and coastal city of Xiamen, China, to trace temporal CO2ff variations and to determine the factors influencing them. The average CO2ff concentrations at the sampling sites in Beijing and Xiamen were 39.7 ± 36.1 ppm and 13.6 ± 12.3 ppm, respectively. These contributed 75.2 ± 14.6% and 59.1 ± 26.8% to their respective annual ΔCO2 offsets over background CO2 concentrations. Significantly (p < 0.05) high CO2ff values were observed in winter in Beijing. We did not find any significant differences in CO2ff values between weekdays and weekends. Diurnal CO2ff variations were plainly evident, with high values between midnight and 4:00, and during morning and afternoon rush hours. The sampling site in the inland city of Beijing displayed much higher CO2ff inputs and overall temporal variations than the site in the coastal city of Xiamen. The variations of CO2ff at both sites were controlled by a combination of emission sources, topography, and atmospheric dispersion. In particular, diurnal observations at the urban site in Beijing showed that CO2ff was easily accumulated under the southeast wind conditions.

The spatial distribution of fossil fuel CO2 traced by Δ(14)C in the leaves of gingko (Ginkgo biloba L.) in Beijing City, China.

Atmospheric fossil fuel CO2 (CO2ff ) information is an important reference for local government to formulate energy-saving and emission reduction in China. The CO2ff spatial distribution in Beijing City was traced by Δ(14)C in the leaves of gingko (Ginkgo biloba L.) from late March to September in 2009. The Δ(14)C values were in the range of -35.2 ± 2.8∼15.5 ± 3.2 ‰ (average 3.4 ± 11.8 ‰), with high values found at suburban sites (average 12.8 ± 3.1 ‰) and low values at road sites (average -8.4 ± 18.1 ‰). The CO2ff concentrations varied from 11.6 ± 3.7 to 32.5 ± 9.0 ppm, with an average of 16.4 ± 4.9 ppm. The CO2ff distribution in Beijing City showed spatial heterogeneity. CO2ff hotspots were found at road sites resulted from the emission from vehicles, while low CO2ff concentrations were found at suburban sites because of the less usage of fossil fuels. Additionally, CO2ff concentrations in the northwest area were generally higher than those in the southeast area due to the disadvantageous topography.