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

Recent studies on fossil fuel CO2 simulation associated with Delta(CO2)-C-14 measurements is quite limited, particularly in China. In this study, the fossil fuel CO2 recently added to the atmosphere (delta CO(2)ff) over the Guanzhong basin, central China, during summer 2012 is simulated using a modified WRF-CHEM model constrained by measured CO2 mixing ratio and Delta(CO2)-C-14. The model well captures the temporal variation of observed CO2 mixing ratio and Delta(CO2)-C-14, and reasonably reproduces the distribution of observed Delta(CO2)-C-14. The simulation shows a significant variation of delta CO(2)ff during summertime, ranging from <5 ppmv to similar to 100 ppmv and no remarkable trend of delta CO(2)ff is found for June, July, and August. The delta CO(2)ff level is closely associated with atmospheric diffusion conditions. The diurnal cycle of delta CO(2)ff 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 delta CO(2)ff within the basin is clearly higher than the outside, reaching up to 40 ppmv in urban Xi'an and 15 ppmv 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 delta CO(2)ff by about 0.38 ppmv in the basin. (C) 2017 Elsevier B.V. All rights reserved

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 (R-2 = 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. (C) 2020 Published by Elsevier Ltd

Radiocarbon in the Atmosphere and Seawater in the South China Sea: Flux, Inventory and Air-Sea CO2 Exchange Rate Tracing

Delta C-14 values of the atmosphere and seawater dissolved inorganic carbon (DIC) were measured during a cruise in the South China Sea (SCS) in September 2015, in order to determine the C-14 flux and bomb C-14-based air-sea CO2 exchange rates for this region. The background atmospheric Delta C-14 value (13.8 +/- 5.0 parts per thousand) for the SCS during that period was lower than that (35.4 +/- 3.4 parts per thousand) of surface seawater (5 m) DIC, and a net transfer of C-14 from the sea to the atmosphere (7.4 +/- 5.0 x 10(11) atoms m(-2) yr(-1)) was determined at the wind speed of 5.2 +/- 1.7 m s(-1). Seawater DIC Delta C-14 profiles showed the highest value (37.9 +/- 3.7 parts per thousand) at a depth of 100 m, a rapid decrease below that depth to -220.3 +/- 3.2 parts per thousand at 1,500 m, and nearly constant values below 1,500 m. The average mean penetration depth of bomb C-14 was 585.5 +/- 99.2 m, and a value of 8.2 +/- 1.0 x 10(9) atoms cm(-2) was obtained for the bomb C-14 inventory in this region. Based on this inventory, a long-term (1954-2015) average air-sea CO2 exchange rate of 20.2 +/- 2.8 mol m(-2) yr(-1) was traced for the SCS. Combined with the pCO(2) measurements in this region, a net CO2 flux rate of 0.54 +/- 0.08 mol m(-2) yr(-1) was yielded for the SCS, which is comparable to the cruise measured flux (0.44 +/- 0.62 mol m(-2) yr(-1)) obtained from a synthesis study (Li et al., 2020, https://doi.org/10.1016/j.pocean.2020.102272). Our study highlights the importance of continued atmospheric and seawater C-14 observations on determining the air-sea flux in this region

LOW-FLOW PRESSURE GRADIENT PUMPING FOR ACTIVE ABSORPTION OF CO2 ON A MOLECULAR SIEVE

The authors have developed an active absorption system combining a molecular sieve with a pressure gradient as a way to overcome the shortcomings of the phosphoric acid solution displacement method. Taking advantage of the pressure gradient produced between the inside and outside of a bottle, as water moves through it, CO2 in the atmosphere can actively be absorbed onto a molecular sieve in its pathway. A comparative study showed that the technique was in agreement with the phosphoric acid displacement method, within error. We applied the new method to collect not only atmospheric CO2 samples, but also CO2 samples from soil respiration to verify its utility. Simple yet practical, our method is well suited to extended collection times in a variety of environments, and capable of providing relatively large amounts of carbon for high-precision accelerator mass spectrometry (AMS) C-14 analyses of atmospheric samples

High-resolution simulation of wintertime fossil fuel CO2 in Beijing, China: Characteristics, sources, and regional transport

In this study, a high-resolution simulation of fossil fuel CO2 in Beijing and surrounding areas, China, during January 2014 is performed to investigate the characteristics and sources of Beijing fossil fuel CO2 (FFCO2) mixing ratios and the impact of regional transport. The model reasonably reproduces the observed meteorological fields in the study domain, including temperature, relative humidity, wind speed, and wind direction. The simulated CO2 and CO mixing ratios in Beijing are in good agreement with the measurements. Elevated FFCO2 levels are produced by the model in megacities, such as Beijing, Tianjin, Shijiazhuang, and Baoding. The model result shows that FFCO2 mixing ratios are significantly correlated with observed total CO2, CO, and PM2.5 concentrations in Beijing. Sensitivity experiments show that Beijing FFCO2 is mainly from industry and residential emissions (35% for each), followed by power plant (21%) and transportation (9%) emissions in January 2014. Spatially, the largest contributor for Beijing FFCO2 is the local source, and regional transport also plays an important role in winter. The impact of regional transport is associated with wind direction, in which south/east (north/west) wind tends to accumulate (dilute) Beijing FFCO2. In addition, the roles of regional transport during haze and clean episodes are significantly distinct in winter, and more contribution is found in haze episodes

Measurement report: Source apportionment of carbonaceous aerosol using dual-carbon isotopes (C-13 and C-14) and levoglucosan in three northern Chinese cities during 2018-2019

To investigate the characteristics and changes in the sources of carbonaceous aerosols in northern Chinese cities after the implementation of the Action Plan for Air Pollution Prevention and Control in 2013, we collected PM2.5 samples from three representative inland cities, i.e., Beijing (BJ), Xi'an (XA), and Linfen (LF), from January 2018 to April 2019. Elemental carbon (EC), organic carbon (OC), levoglucosan, stable carbon isotope, and radiocarbon were measured in PM2.5 to quantify the sources of carbonaceous aerosol, combined with Latin hypercube sampling. The best estimate of source apportionment showed that the emissions from liquid fossil fuels contributed 29.3 +/- 12.7 %, 24.9 +/- 18.0 %, and 20.9 +/- 12.3% of the total carbon (TC) in BJ, XA, and LF, respectively, whereas coal combustion contributed 15.5 +/- 8.8 %, 20.9 +/- 18.0 %, and 42.9 +/- 19.4 %, respectively. Non-fossil sources accounted for 55 +/- 11 %, 54 +/- 10 %, and 36 +/- 14% of the TC in BJ, XA, and LF, respectively. In XA, 44.8 +/- 26.8% of non-fossil sources were attributed to biomass burning. The highest contributors to OC in LF and XA were fossil sources (74.2 +/- 9.6% and 43.2 +/- 10.8 %, respectively), whereas those in BJ were non-fossil sources (66.8 +/- 13.9 %). The main contributors to EC were fossil sources, accounting for 91.4 +/- 7.5 %, 66.8 +/- 23.8 %, and 88.4 +/- 10.8% in BJ, XA, and LF, respectively. The decline (6 %-16 %) in fossil source contributions in BJ since the implementation of the Action Plan indicates the effectiveness of air quality management. We suggest that specific measures targeted at coal combustion, biomass burning, and vehicle emissions in different cities should be strengthened in the future

Stable carbon isotopic characteristics of fossil fuels in China

Good knowledge on the stable carbon isotopic composition (delta C-13) of fossil fuels is critical for the estimation of atmospheric CO2 sources. Here, we complied a comprehensive delta C-13 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 delta C-13 signatures of Chinese fossil fuels. Results show that coal exhibits a narrow range and the most enriched in delta C-13 signature, oil displays intermediate variations both in the distribution and value of delta C-13. By contrast, natural gas is strongly depleted but became more enriched in delta C-13 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 delta C-13 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 delta C-13 of fossil fuels is used to estimate CO2 source contributions, and the measurement of delta(CO2)-C-13 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. (C) 2021 Elsevier B.V. All rights reserved

Emission characteristics of atmospheric carbon dioxide in Xi'an, China based on the measurements of CO2 concentration, Delta C-14 and delta C-13

Given that cities contributed most of China&#39;s CO2 emissions, understanding the emission characteristics of urban atmospheric CO2 is critical for regulating CO2 emissions. Regular observations of atmospheric CO2 concentration, Delta C-14 and delta C-13 values were performed at four different sites in Xi&#39;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 delta C-13 values, the peak to peak amplitude of which was 80.8 ppm for CO2 concentration and 4.0% for its delta C-13. With regard to the spatial variations, the urban CO2 &quot;dome&quot; effect was the most pronounced during the winter season. The use of Delta C-14 combines with delta C-13 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 delta C-13 value of fossil fuel (delta C-13(ff) = -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 N100% with the average contributions of 82.5 +/- 23.8% and 90.0 +/- 24.8%. Given the estimation of delta C-13 value of local sources (delta Cs-13) 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. (c) 2017 Elsevier B.V. All rights reserved

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 (813C) 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 CO2 excess (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 CO2 excess with the effect of weather removed. A Keeling plot shows that in contrast CO2 concentrations and 813C 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