Identification of Urban Carbon Emission Peaks through Tree-Ring 14C.

Independent identification of carbon emission peaks determined from fuel inventories is a challenging goal. Because of the complete depletion of radiocarbon (14C) in fossil fuel sources, the measurement of atmospheric 14CO2 has proven to offer a means of achieving this goal. Here, we present a study identifying peak carbon emissions from two Chinese cities using urban tree-ring Δ14C time series during 2000-2019. After subtracting background atmospheric Δ14C from urban tree-ring Δ14C to isolate local Δ14C (Δ14Clocal), we find a minimum in 2010 (-51.1 ± 4.5‰) in Beijing and in 2013 in Xi'an (-52.5 ± 0.5‰). These levels correspond to an urban carbon emission peak in 2010 and in 2013 in the two respective cities. The urban carbon emission peaks are further identified by the declines of the mean absolute interannual rate of decrease of tree-ring Δ14C during a period, with the respective values of 3.6 and 6.4 ‰/yr after and before a turning point in Beijing and 3.0 and 6.0 ‰/yr after and before a turning point in Xi'an. This study provides an observation method to identify carbon emission peaks in basin cities.

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.

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.

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.

Measurement of the vertical distributions of atmospheric pollutants using an uncrewed aerial vehicle platform in Xi'an, China.

Vertical observations of atmospheric pollutants play crucial roles in a comprehensive understanding of the distribution characteristics and transport of atmospheric pollutants. A hexacopter uncrewed aerial vehicle equipped with miniature monitors was employed to measure the vertical distribution of atmospheric pollutants within a height of 1000 m at a rural site in Xi'an, China, in 2021. The concentrations of carbon monoxide (CO) and particulate matter (PM) showed generally decreasing trends with increasing height. The ozone (O3) concentration showed a general increasing trend with height followed by a gradual decreasing trend. Vertical decrements of PM2.5 and CO from 0 to 1000 m were significantly (p < 0.05) lower on observation days during summer (14.0 ± 8.1 µg m-3 and 8.7 ± 6.6 ppb, respectively), compared with those in winter (78.3 ± 14.1 µg m-3 and 34.8 ± 17.3 ppb, respectively). The horizontal transport of PM and CO mostly occurred in the morning and at night during winter observations at an altitude of 400-500 m. During the winter haze, the PM and CO profile concentrations below 500 m increased substantially with the decrease in the height of the thermal inversion layer. Vertical O3 transportation was observed in the afternoon and evening during summer, and a ∼37.7% (11.6 ppb) increase in ground-level O3 was observed in relation to vertical transport from the upper atmosphere. The results provide insights into the vertical distribution and transport of atmospheric pollutants in rural areas near cities.

Vertical measurements of atmospheric CO2 and 14CO2 at the northern foot of the Qinling Mountains in China.

The CO2 and 14CO2 levels in air samples from the northern foot of the Qinling Mountains (Xi'an, China) were determined. In 2021, a hexacopter unmanned aerial vehicle sampled air at different heights, from near-ground to 2000 m. The objectives of this study were to determine vertical characteristics of CO2 and 14CO2, the sources of different-height CO2, and the influence of air mass transport. The CO2 concentrations mainly exhibited a slight decreasing trend with increasing height during summer observations, which was in contrast to the increasing trend that was followed by a subsequent gradual decreasing trend during early winter observations, with peak CO2 levels (443.4 ± 0.4-475.7 ± 0.5 ppm) at 100-500 m. The variation in vertical concentrations from 20 to 1000 m in early winter observations (21.6 ± 19.3 ppm) was greater than that in summer observations (14.6 ± 14.3 ppm), and the maximum vertical variation from 20 to ∼2000 m reached 61.1 ppm. Combining Δ14C and δ13C vertical measurements, the results showed that fossil fuel CO2 (CO2ff, 56.1 ± 15.2 %), which mainly come from coal combustion (81.2 ± 3.4 %), was the main contributor to CO2 levels in excess of the background level (CO2ex) during early winter observations. In contrast, biological CO2 (CO2bio) dominated CO2ex in summer observations. The vertical distributions of CO2ff in early winter observations and CO2bio in summer observations were consistent with those of CO2 during early winter and summer observations, respectively. The strong correlation between winter CO2bio and ΔCO (r = 0.81, p < 0.01) indicated that biomass burning was the main contributor to CO2bio during early winter observations. Approximately half of the air masses originated from the Guanzhong Basin during observations. The results provide insights into the vertical distribution of different-sources of atmospheric CO2 in scientific support of formulating carbon emission-reduction strategies.

Atmospheric CO2 and 14CO2 observations at the northern foot of the Qinling Mountains in China: Temporal characteristics and source quantification.

A two-year (March 2021 to February 2023) continuous atmospheric CO2 and a one-year regular atmospheric 14CO2 measurement records were measured at the northern foot of the Qinling Mountains in Xi'an, China, aiming to study the temporal characteristics of atmospheric CO2 and the contributions from the sources of fossil fuel CO2 (CO2ff) and biological CO2 (CO2bio) fluxes. The two-year mean CO2 mole fraction was 442.2 ± 16.3 ppm, with a yearly increase of 4.7 ppm (i.e., 1.1 %) during the two-year observations. Seasonal CO2 mole fractions were the highest in winter (452.1 ± 17.7 ppm) and the lowest in summer (433.5 ± 13.3 ppm), with the monthly CO2 levels peaking in January and troughing in June. Diurnal CO2 levels peaked at dawn (05:00-07:00) in spring, summer and autumn, and at 10:00 in winter. 14C analysis revealed that the excess CO2 (CO2ex, atmospheric CO2 minus background CO2) at this site was mainly from CO2ff emissions (67.0 ± 26.8 %), and CO2ff mole fractions were the highest in winter (20.6 ± 17.7 ppm). Local CO enhancement above the background mole fraction (ΔCO) was significantly (r = 0.74, p < 0.05) positively correlated with CO2ff in a one-year measurement, and ΔCO:CO2ff showed a ratio of 23 ± 6 ppb/ppm during summer and winter sampling days, much lower than previous measurements and suggesting an improvement in combustion efficiency over the last decade. CO2bio mole fractions also peaked in winter (14.2 ± 9.6 ppm), apparently due to biomass combustion and the lower and more stable wintertime atmospheric boundary layer. The negative CO2bio values in summer indicated that terrestrial vegetation of the Qinling Mountains had the potential to uptake atmospheric CO2 during the corresponding sampling days. This site is most sensitive to local emissions from Xi'an and to short distance transportation from the southern Qinling Mountains through the valleys.

[Main Chemical Components in Atmospheric Precipitation and Their Sources in Xi'an].

In order to understand the current status of main chemical components of atmospheric precipitation in Xi'an, the pH, electrical conductivity, mass concentration of water-soluble ions and heavy metals, wet deposition fluxes, and their sources in precipitation samples in urban and suburban areas of Xi'an in 2019 were studied. The results showed that the pH, conductivity, water-soluble ions, and heavy metals in precipitation in Xi'an in winter were higher than those in other seasons. The main water-soluble ions in precipitation were Ca2+, NH+4, SO2-4, and NO-3, and the sum of these ions accounted for (88.5%±2.8)% of the total ion concentration in urban and suburban areas. The main heavy metals were Zn, Fe, and Zn and Mn; their sum accounted for (54.0%±3)% and (47.0%±8)% of the total metal concentration. The wet deposition fluxes of water-soluble ions in precipitation in urban and suburban areas were (253.2±58.4) mg·(m2·month)-1 and (241.9±61.1) mg·(m2·month)-1, respectively. They showed higher values in winter than those in other seasons. The wet deposition fluxes of heavy metals were (86.2±37.5) mg·(m2·month)-1 and (88.1±37.4) mg·(m2·month)-1, respectively, with little seasonal difference. The source analysis using PMF showed that the water-soluble ions in urban and suburban precipitation mainly came from combustion sources (57.5% and 32.32%), followed by motor vehicles (24.4% and 17.2%) and dust sources (18.1% and 27.0%). The ions in suburban precipitation were also affected by local agriculture (11.1%). Heavy metals in precipitation in urban and suburban areas mainly came from industrial sources (51.8% and 46.7%), and the contribution rate of coal and motor vehicle mixed sources in winter was 10.7% and 6.1% higher than that in summer, respectively.

Pollution characteristics of volatile organic compounds in the atmosphere of Haicang District in Xiamen City, Southeast China.

The compositions, spatial distributions, seasonal variations and ozone formation potential (OFP) of volatile organic compounds (VOCs) were investigated in the atmosphere of Haicang District, Xiamen City, Southeast China. Twenty-four types of VOCs were measured in this study, and ethanol, methylene chloride, toluene, ethyl acetate and isopropyl alcohol were the abundant species based on concentration rank. The concentrations of total VOCs (TVOCs) in industrial areas were higher than those in residential and administrative areas and background site. For industrial areas, the TVOCs concentrations in summer were higher than those in winter, which might result from higher emissions from industrial activities because of stronger evaporation in summer. In contrast, non-industrial areas showed higher concentrations in winter due to the unfavorable meteorological conditions. The spatial distribution of BTEX (benzene, toluene, ethylbenzene and xylene) followed the order of industrial areas > residential and administrative areas > background site, and the concentrations in summer were lower than those in winter for most sites. The high ratios (8.9-14.0) of T/B in this study indicated that industrial emissions were the main sources in this district. X/B ratios were used to assess the ages of air parcels and provided evidence of the transport of air parcels among these sites. Total OFP (TOFP) showed the trend of increase with the increase of TVOCs, and toluene was found as the major contributor to TOFP.

One-year measurement of organic and elemental carbon in size-segregated atmospheric aerosol at a coastal and suburban site in Southeast China.

To understand the influence of the urbanization process on the air quality in the urban neighbourhood area, the size distribution and seasonal variations of elemental and organic carbon in aerosols were studied at a coastal and suburban site in Xiamen City, China. A total of 87 samples were obtained during the one-year measurement campaign from June 2009 to May 2010. The results indicated that 79.3 ± 3.2% of the organic carbon (OC) and 88.3 ± 1.7% of the elemental carbon (EC) were associated with fine particles (PM(2.5)), which consist of 32.0 ± 8.3% of the total carbonaceous aerosol (TCA). The concentrations of the OC and EC in PM(2.5) were 17.8 ± 11.2 and 3.8 ± 1.9 µg m(-3), respectively, and high concentrations were usually observed when the wind direction was northeast (NE). High OC/EC ratios (average 5.1) in PM(2.5) indicated the formation of secondary organic carbon (SOC), which contributed 60.0% to the OC and 11.0% to the particulate matter. At this site, SOC had a significant negative correlation with the temperature (R(2) = 0.42), and a favorable meteorological condition for SOC formation was found in the wintertime. The OC/EC ratios increased with particle size, while the fractions of the carbonaceous aerosols to particulate matter decreased. OC, EC and SOC concentrations and OC/EC ratios followed the same seasonal pattern of winter > spring > autumn > summer, which mainly resulted from the various origins of the air masses in different seasons. This study indicates the requirement for mitigating the pollution of carbonaceous aerosol at this coastal and suburban area in Xiamen City.

Chemical compositions of PM2.5 aerosol during haze periods in the mountainous city of Yong'an, China.

Haze phenomena were found to have an increasing tendency in recent years in Yong'an, a mountainous industrial city located in the center part of Fujian Province, China. Atmospheric fine particles (PM2.5) in the urban area during haze periods in three seasons (spring, autumn and winter) from 2007 to 2008 were collected, and the mass concentrations and chemical compositions (seventeen elements, water soluble inorganic ions (WSIIs) and carbonaceous species) of PM2.5 were determined. PM2.5 mass concentrations did not show a distinct difference among the three seasons. The carbonaceous species organic carbon (OC) and elemental carbon (EC) constituted up to 19.2%-30.4% of the PM2.5 mass during sampling periods, while WSIIs made up 25.3%-52.5% of the PM2.5 mass. The major ions in PM2.5 were SO4(2-), NO3(-) and NH4(+), while the major elements were Si, K, Pb, Zn, Ca and Al. The experimental results (from data based on three haze periods with a 10-day sampling length for each period) showed that the crustal element species was the most abundant component of PM2.5 in spring, and the secondary ions species (5O4(2-), NO3(-), NH4(+), etc.) was the most abundant component in PM2.5 in autumn and winter. This indicated that dust was the primary pollution source for PM2.5 in spring and combustion and traffic emissions could be the main pollution sources for PM2.5 in autumn and winter. Generally, coal combustion and traffic emissions were considered to be the most prominent pollution sources for this city on haze days.

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.

Mercury in leaf litter in typical suburban and urban broadleaf forests in China.

To study the role of leaf litter in the mercury (Hg) cycle in suburban broadleaf forests and the distribution of Hg in urban forests, we collected leaf litter and soil from suburban evergreen and deciduous broadleaf forests and from urban forests in Beijing. The Hg concentrations in leaf litter from the suburban forests varied from 8.3 to 205.0 ng/g, with an average (avg) of (49.7 +/- 36.9) ng/g. The average Hg concentration in evergreen broadleaf forest leaf litter (50.8 + 39.4) ng/g was higher than that in deciduous broadleaf forest leaf litter (25.8 +/- 10.1) ng/g. The estimated Hg fluxes of leaf litter in suburban evergreen and deciduous broadleaf forests were 179.0 and 83.7 mg/(ha x yr), respectively. The Hg concentration in organic horizons (O horizons) ((263.1 +/- 237.2) ng/g) was higher than that in eluvial horizons (A horizons) ((83.9 +/- 52.0) ng/g). These results indicated that leaf litterfall plays an important role in transporting atmospheric mercury to soil in suburban forests. For urban forests in Beijing, the Hg concentrations in leaf litter ranged from 8.8-119.0 (avg 28.1 +/- 16.6) ng/g, with higher concentrations at urban sites than at suburban sites for each tree. The Hg concentrations in surface soil in Beijing were 32.0-25300.0 ng/g and increased from suburban sites to urban sites, with the highest value from Jingshan (JS) Park at the centre of Beijing. Therefore, the distribution of Hg in Beijing urban forests appeared to be strongly influenced by anthropogenic activities.

Influences of high-level atmospheric gaseous elemental mercury on methylmercury accumulation in maize (Zea mays L.).

Maize (Zea mays L.) leaves play an important role in stomatal uptake and surface adsorption of atmospheric mercury (Hg). However, the influence of atmospheric gaseous elemental mercury (GEM) on methylmercury (MeHg) accumulation in maize plants is poorly understood. In this study, we conducted a field open-top chambers (OTCs) experiment and a soil Hg-enriched experiment to investigate the response of MeHg accumulation in maize tissues to different GEM levels in the air. Maize upper leaves had a higher average MeHg concentration (0.21 ± 0.08 ng g-1) than bottom leaves (0.15 ± 0.05 ng g-1) in the OTCs experiment, which was inconsistent with that in the soil Hg-enriched experiment (maize upper leaves: 0.41 ± 0.07 ng g-1, maize bottom leaves: 0.60 ± 0.05 ng g-1). Additionally, significantly positive correlations were found between MeHg concentrations in maize leaves and air Hg levels, suggesting that elevated air Hg levels enhanced MeHg accumulation in maize leaves, which was possibly attributed to methylation of Hg on leaf surfaces. Mature maize grains from the OTCs experiment had low MeHg concentrations (0.12-0.23 ng g-1), suggesting a low accumulation capability of MeHg by maize grains. Approximately 93-96% of MeHg and 51-73% of total Hg in maize grains were lost from the grain-filling stage to the grain-ripening stage at all GEM level treatments, implying that self-detoxification in maize grains occurred. MeHg concentrations in maize roots showed a significant linear relationship (R2 = 0.98, p < 0.01) with soil Hg levels, confirming that MeHg in maize roots is primarily from soil. This study provides a new finding that elevated air GEM levels could enhance MeHg accumulation in maize leaves, and self-detoxification may occur in maize grains. Further studies are needed to clarify these mechanisms of Hg methylation on maize leaf surfaces and self-detoxification of Hg by maize grains.

14C-AMS measurements in modern tree rings to trace local fossil fuel-derived CO2 in the greater Xi'an area, China.

Fossil fuel-derived CO2 (CO2ff) time series are critical to understanding urban carbon emissions, and to devise strategies to mitigate emission reduction. Using tree ring 14C archives, we reconstruct an historical CO2ff time series from 1991 to 2015 in the greater Xi'an region, China. CO2ff concentrations from the urban sites reached 22.5 ppm, with an average of 14.0 ppm, while average values from rural and mountain sites averaged about 6.0 ppm. These values provide a good measure of the distribution of anthropogenic CO2 emissions in the region. We also observed CO2ff concentration increases from both urban and rural sites during the study period, with more significant increases among urban sites. The persistent rise in CO2ff was attributed to increasing energy consumption caused by regional socio-economic development, which are corroborated by strong correlations between CO2ff and socioeconomic parameters.

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.