Identification of Anthropogenic CO2 Using Triple Oxygen and

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Identification of anthropogenic CO2 using triple oxygen and clumped isotopes Amzad Hussain Laskar, Sasadhar Mahata, and Mao-Chang Liang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02989 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 4, 2016

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Identification of anthropogenic CO2 using triple oxygen and clumped isotopes

3 Amzad H. Laskar1, Sasadhar Mahata1, Mao-Chang Liang1,2,3*

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11529, Taiwan

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Graduate Institute of Astronomy, National Central University, Taoyuan City 32001, Taiwan

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Research Center for Environmental Changes, Academia Sinica, Taipei

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Department of Physics, University of Houston, Houston, TX 77204, USA

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*Correspondence author

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Address: Phone: +886 2653-9885 #852

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Fax: +886 2783-3584 E-mail: [email protected]

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Abstract

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Quantification of contribution from various sources of CO2 is important for understanding the

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atmospheric CO2 budget. Considering the number and diversity of sources and sinks, the

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widely used proxies such as concentration and conventional isotopic compositions (δ13C and

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δ18O) are not always sufficient to fully constrain the CO2 budget. Additional constraints may

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help in understanding the mechanisms of CO2 production and consumption. Anomaly in

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triple oxygen isotopes or

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isotopes, called clumped isotopes, are two recently developed tracers with potentials to

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independently constrain some important processes that regulate CO2 in the atmosphere. The

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clumped isotope for CO2, denoted by ∆47, is the excess of

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distribution of isotopes in a CO2 molecule. We measured the concentration, δ13C, δ18O, ∆17O

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and ∆47 in air CO2 samples collected from the Hsuehshan tunnel (length: 12.9 km), and

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applied linear and polynomial regressions to obtain the fossil fuel end-members for all these

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isotope proxies. The other end-members, the values of all these proxies for background air

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CO2, are either assumed or taken the values obtained over the tunnel and ocean. The fossil

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fuel (anthropogenic) CO2 end-member values for δ13C, δ18O, ∆17O, and ∆47 are estimated

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using the two component mixing approach: the derived values are -26.76±0.25‰,

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24.57±0.33‰, -0.219±0.021‰ and 0.267±0.019‰, respectively. These four major CO2

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isotope tracers along with the concentration were used to estimate the anthropogenic

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contribution in the atmospheric CO2 in urban and sub-urban locations. We demonstrate that

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∆17O and ∆47 have the potential to independently estimate anthropogenic contribution and

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advantages of these two over the conventional isotope proxies are discussed.

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O excess (denoted by ∆17O) and molecules containing two rare

13 16

C O18O over a random

48 49 50 51 52 53 54 55

Key words: Triple oxygen isotopes, clumped isotopes, tunnel, anthropogenic CO2



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Introduction

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The isotopic compositions of the sources of atmospheric CO2, for example, anthropogenic

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emission and respiration, are important to understand the CO2 budget. Traffic is a major

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anthropogenic source responsible for the increase of atmospheric CO2. CO2 emission from

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transportation has grown steadily since 1970 throughout the globe, accounting for about 21%

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of anthropogenic emissions. According to the International Energy Agency, such high level

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of emissions from transportation will remain through 2020.1 From 1990 to 2010, the increase

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of CO2 emission from the transport sector was ~92%, though the increase was not uniform

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everywhere; it was larger in the South and Southeast Asia (300-400%).1 Robust

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quantification of CO2, the main player for long-term climate change, emitted from major

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sources is important for assessing the global carbon budget and modeling future climate.

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The widely used proxies such as temporal and spatial records of concentration and

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conventional isotopic ratios (δ13C and δ18O) have limitations in identifying sources. δ13C

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values, for example, produced from high temperature combustion and low temperature

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respiration are essentially indistinguishable. δ18O in atmospheric CO2 is mainly controlled by

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isotopic exchange with various water reservoirs with significant modification on a local scale

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due to respiration and exchange with leaf and soil waters and vehicle and industrial emission

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in urban/industrial regions. Given that terrestrial waters are spatiotemporally and isotopically

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highly inhomogeneous2,3 and the associated isotopic fractionation between water and CO2

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exchange is temperature dependent4,5, the use of δ18O for assessing anthropogenic

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contribution is limited6, though widely used for estimating gross primary production.7-13

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Anomaly in

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δ18O plot, expressed as ∆17O and clumped isotopes in CO2 (excess of mass 47, mainly

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13 18

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independent constraints for improving the atmospheric CO2 budget and mechanisms for CO2

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production (for definition of ∆17O and ∆47, see method section).

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O, the deviation of δ17O from conventional ~0.5 terrestrial slope in δ17O vs.

C O16O over a random distribution of isotopes, denoted by ∆47) provide additional and

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The major processes that regulate atmospheric CO2 are photosynthesis and respiration, and

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they follow the terrestrial mass dependent fractionation line with a slope of ~0.5 in triple

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oxygen isotope plot. Some photochemical reactions in the stratosphere, however, re-partition

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the oxygen isotope distribution in oxygen-containing species with a deviation from ~0.5 3 ACS Paragon Plus Environment

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slope. Two representative molecules in the upper troposphere and stratosphere are O3 (ref. 14)

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and CO2 (ref. 15,16); oxygen anomaly in O3 is transferred to CO2 via the coupled

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photochemistry between O2, O3, and CO2.17-20 Thus, the ∆17O value of near surface

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atmospheric CO2 is controlled mainly by the influx of stratospheric CO2 with an elevated

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∆17O value and isotopic modification due to various biospheric and hydrospheric activities

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such as photosynthesis and respiration and isotopic exchange with various water reservoirs

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such as oceans, soil and plant leaves at the surface. Therefore, in principle, gross carbon

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exchange between the atmosphere and biosphere and stratospheric intrusion can be quantified

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using

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exchange or productivity using

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especially in urban and industrial areas, viz. anthropogenic CO2, must be accurately

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determined as ∆17O value of fossil fuel released CO2 is different from that generated by the

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above mentioned processes. During combustion, anomaly in air O2, or more precisely deficit

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in

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from the plant respired CO2 or that equilibrated with water.12,20,24 Thus ∆17O has strong

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relevance in quantitative estimation of the sources of CO2, especially in urban and industrial

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areas where fossil fuel burning rate is rapidly increasing.

17

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O anomaly in atmospheric CO2.21-24 But to precisely estimate the gross carbon 17

O, its value for another significant fraction of CO2

O is transferred to CO2, making ∆17O a potential proxy to distinguish combustion CO2

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Similar to ∆17O, ∆47 is another useful tracer for quantifying anthropogenic emission. The ∆47

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values of CO2 depend on the formation temperature of the CO2. For example, for fossil fuel

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released CO2, it gives the temperature of combustion and for carbonates, the temperature of

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precipitation. ∆47 values in CO2 increases with decrease in the temperature of its formation or

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exchange with water. At lower temperatures (~0 to 70 °C), ∆47 varies inversely with the

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square of temperature. There is a mismatch observed between the actual combustion

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temperatures of vehicles and that predicted by ∆47, probably due to post-combustion

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exchange at lower temperatures inside the catalytic converter and exhaust pipe. Even then ∆47

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values of car exhaust CO2 can be used as an end-member to estimate the anthropogenic

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fraction of CO2 as its value is reproducible.25,26 When CO2 interacts with water, ∆47 value in

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the CO2 gets modified depending on the temperature of the water. At thermodynamic

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equilibrium, CO2 is expected to reflect the temperature at which exchange takes place.27,28

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Thus the fluxes that involve CO2 equilibration with water are expected to have ∆47 at

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thermodynamic equilibrium. Therefore, ∆47 values in ambient air CO2 represent a dynamical

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balance of CO2 fluxes between biosphere-atmosphere exchange, ocean-atmosphere exchange, 4 ACS Paragon Plus Environment


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and production from combustion sources.26 Tunnel air CO2 is expected to be dominated by

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anthropogenic CO2 with reduced ∆47 values, marine air should reflect thermodynamic

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equilibrium ∆47 values at the sea surface temperature, and semi-urban and urban air CO2 may

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have a significant fraction of anthropogenic CO2, signature of which should be reflected in

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their ∆47 values.

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Unlike conventional isotopes, studies using these two rare isotopologues, viz., 13C16O18O and

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12 17

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their accurate and precise analysis.26-32 So far the main focus of clumped isotopes was

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paleoclimate studies. Triple oxygen isotopes in atmospheric CO2 are also less explored. The

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available data are not sufficient to use these as proxies to identify and quantify the sources of

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atmospheric CO2. Accurate estimations of the end-members of these proxies will largely help

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to constrain the sources of CO2 and their contribution to the atmosphere. A long tunnel is

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selected that provides an ideal environment for estimating the end-members using

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interpolation due to large gradient of CO2 concentrations from one end to the other and

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unbiased sampling of all types of car engines.

C O18O for atmospheric CO2 are limited mainly because of the challenges associated with

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Here we present high precision measurements of stable isotopes including ∆17O and ∆47 in air

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CO2 in traffic (tunnel and urban road) as well as relatively clean areas such as sub-urban,

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forest and marine environments. The aim is to constrain the end-members associated with

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high temperature combustion of fossil fuels (anthropogenic end-member) for ∆17O and ∆47

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using binary mixing models and use them to quantify the contribution of CO2 emitted from

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different sources in urban and sub-urban areas. The background end-members are either

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assumed or taken from the values observed over the tunnel and South China Sea. We also

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showed how ∆47 and ∆17O can be used to identify the sources of CO2 in a respiration

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dominated area where δ13C and δ18O fail.

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Materials and methods

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Sampling locations. Air samples were collected from the Hsuehshan tunnel, located in the

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Taipei-Yilan Expressway in Northern Taiwan, extending south-eastward from metropolitan 5 ACS Paragon Plus Environment

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Taipei through the Hsuehshan Mountain to Yilan County (Figure 1). 14 samples were

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analyzed which were collected during 12 - 15 December, 2013 from Yilan to Taipei bound

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bore at two points, S1 and S2, inside the tunnel, located at 1.7 and 2.3 km away from the

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entry and exit, respectively. Detailed description of the tunnel is given in the Supporting

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Information and in the literature33-35. In addition, we collected four air samples on 24th July,

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2014 and five on 30th December, 2014 from a busy urban street, Roosevelt road, Taipei

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(Figure 1), to estimate the anthropogenic contribution of CO2 in urban areas using the

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obtained end-members. We also collected nine marine air CO2 samples over the South China

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Sea in October, 2013 for background isotopic values assuming they were free of

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anthropogenic influences. The details of the sampling were discussed elsewhere.26 The

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concentration, δ13C and δ18O values above the tunnel were similar to the values observed

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over the South China Sea indicating insignificant contribution from the local anthropogenic

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sources.35 Therefore, the background end-members for concentration, δ13C, and δ18O were

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assumed to be the values observed above the tunnel and for ∆47, it was the thermodynamic

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equilibrium value at the ambient temperature. For ∆17O, it was assumed to be the average

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value observed over South China Sea, as no ∆17O values were available for air CO2 around

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the tunnel. Also, the ∆17O values in CO2 are independent of the exchange temperatures unlike

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δ18O and ∆47, and hence its values observed over the South China Sea can safely be used as

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the background.

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In regions where respiration significantly contributes to air CO2, ∆17O and ∆47 can be used to

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distinguish between the sources (e.g., anthropogenic and respiration) but not δ13C and δ18O.

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To demonstrate this we carried out analysis of five air CO2 samples collected from a small

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(~0.7 km2) and dense natural forest located near the west end of the Academia Sinica

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campus. The samples were collected during July-August, 2015 from ~100 m inside the forest

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on a small plateau at a height of ~30 m from the ground in the hill slope. Dense vegetation

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allowed little sunlight penetrating to the surface. The relative humidity was 80-90 % during

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the sampling days and wind speed was nearly zero due to presence of hills on three sides of

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the sampling spot.

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Sample collection and measurement. Air was compressed to ~2 bar in pressure in 1 L and 2

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L glass flasks by pumping through a column packed with magnesium perchlorate for

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moisture removal. Concentration of CO2 was measured using a LI-COR infrared gas 6 ACS Paragon Plus Environment


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analyzer. For isotopic analysis, CO2 from air was extracted by cryogenic techniques.26,36

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Details about the concentration measurement and sample extraction and purification can be

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found in the Supporting Information. Stable isotopic compositions including ∆17O and ∆47 of

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the purified CO2 were measured using a Finnigan MAT 253 gas source stable isotope ratio

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mass spectrometer at Research Center for Environmental Changes, Academia Sinica, Taiwan.

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∆17O in CO2 was measured as O2 (ref. 32). For ∆47 measurement, the mass spectrometer was

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configured to measure ion beams corresponding to M/Z 44 through 49.

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All the sample CO2 for δ13C, δ18O and ∆47 were analyzed against a working CO2 called AS-2

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with δ13C=-32.62‰ (VPDB) and δ18O=36.64‰ (VSMOW). δ13C values are expressed

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relative to VPDB and δ18O to VSMOW. ∆47 is calculated using the following relation:

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  R47 R46 R45 ∆ 47 =  − − + 1 × 1000 2 2  2 R13 R18 + 2 R17 R18 + R13 (R17 ) 2 R18 + 2 R13 R17 + (R17 ) R13 + 2 R17 

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where R13 and R18 (ratios

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masses 44, 45 and 46 in the same CO2 sample and R17 is calculated assuming a mass

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dependent relation with R18. Routine monitoring of masses 48 and 49 along with 44 to 47,

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was carried out check possible interference by sample impurities on the measurements of ∆47.

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The reproducibility (standard deviation) of measurement for air CO2 was 0.07, 0.08 and 0.01

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‰ for δ13C, δ18O and ∆47, respectively.26,37 The accuracy of measurements was better than

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0.05 ‰ for δ13C and δ18O and 0.01 ‰ for ∆47.26 All the ∆47 values are expressed in the

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absolute reference frame38 (see Supporting Information for details).

13

C/12C and

18

(1)

O/16O) are obtained by measuring the traditional

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The CO2-O2 oxygen isotope exchange method mediated by hot platinum was followed to

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measure the ∆17O of CO2 samples.24,32 The same CO2 used for ∆47 measurement was

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recovered from the sample reservoir of the mass spectrometer by freezing back into a sample

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bottle for ∆17O analysis. The recovered CO2 was exchanged with an equal amount of O2,

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taken from a large reservoir, for about 2 hours inside a quartz tube in presence of platinum at

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670 oC. The O2 used for exchange was measured before and after exchange for δ17O and δ18O

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relative to the same working O2. The working O2 was a high purity commercial O2 called AP-

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4.36 ∆17O is defined as

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∆17 O = ln 1 + δ 17 O − 0 .516 × ln 1 + δ 18 O .

(

)

(

)

(2)


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Note that slope λ=0.516 is used for reporting all of the ∆17O values. The accuracy and

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analytical precision for ∆17O of CO2 are better than 0.01‰ (1-σ standard deviation).24 More

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details about ∆17O measurements, selection of λ value, precision and associated corrections

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are discussed elsewhere.24,32,39,42

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Results and discussion

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Concentration of CO2 and stable isotopic compositions including ∆17O and ∆47 values for air

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CO2 collected from Hsuehshan Tunnel, South China Sea, Roosevelt road and a forest site

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near Academia Sinica campus are summarized in Table 1. Inside the tunnel, concentration

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varied between 792 to 989 ppmv with an average of 902 ppmv and 1918 to 2845 ppmv with

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an average of 2407 ppmv at the sampling locations S1 and S2, respectively. The

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corresponding δ13C values fall in the range of -17.3 to -19.6‰ and -23.0 to -24.4‰. This

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large variation in concentration as well as in the isotopic composition even at the same spot

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was due to the fact that the air inside the tunnel was not always homogeneously mixed, that

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car flow rate was not even, and that vehicle emission was not uniform, resulting in spatial

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heterogeneities. Also, the incorporation of fresh air through the chimneys in the tunnel

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(Figure 1) might be different at different times at a given spot. Variability in all other isotopic

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species including ∆17O and ∆47 was expected inside the tunnel due to the same reason as

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pointed above.

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Inside the tunnel the δ18O value lied in the range of 30.98 to 33.21‰ and 26.37 to 27.90‰, at

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S1 and S2, respectively. Oxygen isotopes in CO2 readily exchange with water, and therefore,

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the δ18O values inside tunnel may get modified if such exchange happens. However, there

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was no visible liquid water present inside the tunnel during sampling. Even if tiny amount

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was present, we do not expect significant changes during 2 to 8 hour time period, the

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residence time of air inside the tunnel (see S1 of the Supporting Information), due to long

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exchange time (>10 hours).5,40 This argument is also valid for ∆17O and ∆47 inside the tunnel

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as the exchange time scale is approximately equal for δ18O, ∆17O and ∆47.5,40

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The averaged CO2 concentration over the South China Sea between latitudes 18o03′ N and

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21o17 ′ N was 395±7 ppmv, and the values of δ13C and δ18O were -8.43±0.19‰ and

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40.20±0.20‰, respectively (Table 1). Both the concentration and δ13C values were similar to

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those observed at Mauna Loa during the sampling period, suggesting little contribution from

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local/regional anthropogenic sources and δ18O was close to isotopic equilibrium with the

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surface sea water.26 The averaged values of CO2 concentration, δ13C, and δ18O for air CO2

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near Roosevelt Road, a busy street in Taipei city on 24th July, 2014, were 515±36 ppmv, -

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12.22±0.97‰, and 37.63±0.96‰, and the values on 30th December, 2014 were 500±50

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ppmv, -11.05±0.90‰ and 39.31±0.94‰ respectively (Table 1). Both the concentration and

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isotopic compositions at Roosevelt road show signatures of a significant contribution from

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anthropogenic sources. A minor fraction could also be contributed by respiration, which

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cannot be identified using δ13C and δ18O as discussed later in this section. The lower

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concentration and higher δ13C and δ18O values on 30th Dec, 2014 compared to 24th July, 2014

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was probably due to presence of relatively less number of vehicles on the road. This is

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because of less attendance of government employees and workers on the eve of new year. No

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clear diurnal variability in concentration or in isotopic composition was observed during the

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sampling hours (~12 – 20 hour).

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The average concentration, δ13C and δ18O values in the dense forest near Academia Sinica

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campus were 438±16 ppmv, -9.99±0.50‰ and 40.38±0.60‰, respectively (Table 1). The

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relatively higher concentration and lower δ13C values in this site compared to the background

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values observed over the South China Sea were due to local respiration.26 The conventional

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isotopic proxies along with concentration can estimate the excess amount of CO2 but cannot

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identify its origin. This is due to similar δ13C value of anthropogenic and respired CO2. For

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δ18O, though respired (~25‰) and anthropogenic (~23.5‰) CO2 are slightly different, but

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several influencing factors make it difficult to estimate the origin of CO2 using δ18O. ∆17O

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and ∆47 along with concentration can easily distinguish between these sources in such cases

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as discussed below.

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∆17O values varied between +0.020 and -0.044‰ with an average of -0.008±0.024‰ at S1

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and -0.063 and -0.144‰ with an average of -0.109±0.031‰ at S2 (Table 1). In urban area


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near Roosevelt road, ∆17O varied between 0.182 and 0.310‰ with an average of

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0.248±0.047‰ on 24th July, 2014 and 0.214 and 0.301‰ with an average of 0.254±0.030‰

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on 30th December, 2014. Most of the values were significantly less than the ∆17O values of

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atmospheric CO2 observed over places where anthropogenic emissions were less or

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absent,21,24,41,42 indicating that a significant fraction of CO2 had anthropogenic origin. Over

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the South China Sea during 15-17 October, 2013, ∆17O varied between 0.294 to 0.400‰ with

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an average of 0.335‰ (Table 1). In the forest air CO2, the average ∆17O value was

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0.326±0.032‰, which was similar to the background value, observed over the South China

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Sea, indicating that the respiration was the major source of the elevated CO2 concentration

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here. This is due to the fact that anomaly in 17O in respired CO2 is little or absent.23 This is an

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advantage of ∆17O to identify the source which is not possible by δ13C and δ18O alone as

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discussed above.

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The ∆47 values lied in the range of 0.520 to 0.742‰ with an average of 0.617±0.071‰ and

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0.327 to 0.471‰ with an average of 0.405±0.041‰ at S1 and S2, respectively (Table 1). In

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the urban station near Roosevelt road the value of ∆47 varied between 0.754 and 0.833‰ with

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an average of 0.807±0.028‰ on 30th December, 2014. These values were significantly less

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than that expected at thermodynamic equilibrium (0.974±0.020‰) at the ambient

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temperatures of 16±5 oC in December28, indicating the presence of a significant fraction of

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CO2 emitted from combustion. For marine air CO2 over the South China Sea, ∆47 varied

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between 0.901 and 0.934‰ with an average of 0.918‰. These values were close the

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thermodynamic equilibrium value of 0.91 ‰ at the average sea surface temperature of of 28.2

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o

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the forest CO2 near Academia Sinica in June-August, 2016 varied between 0.887 and 0.920‰

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which were similar to that expected at the ambient temperatures of 31±2 oC during the

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sampling period. Since the respired CO2 is in thermodynamic equilibrium as demonstrated

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previously26, this higher concentration at the forest site was due to respiration and not due to

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anthropogenic emissions, as the letter is expected to lower the ∆47 values as observed at

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Roosevelt road. This is an advantage of ∆47, just as ∆17O, over δ13C and δ18O to distinguish

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between the respired and anthropogenic CO2 sources.

C, during the sampling time in the region as discussed in a previous work.26 ∆47 values of

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A two component mixing model is applied to all of the four isotope proxies and the results

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are shown in Figure 2, with the intercepts summarized in Table 2. The associated main

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sources of CO2, obtained from the intercepts are also given in Table 2. Figure 2A shows the

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two component mixing plots (also called Keeling plots) for δ13C and δ18O with the air CO2

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data from the tunnel and South China Sea. The reason for estimating the end-members using

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only ocean and tunnel CO2 data is that the marine air CO2 is close to the background without

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significant contribution from any other sources and tunnel CO2 is mainly mixtures of two

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components viz. background and anthropogenic (vehicle exhausts) CO2. On the other hand,

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the air CO2 in other locations such as sub-urban and urban areas may contain significant

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contributions from other sources such as biospheric respiration. The Keeling plot intercept for

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δ13C is -27.76±0.25‰, showing a fossil fuel signature.6 The Keeling plot for δ18O gives an

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intercept of 24.57±0.33‰, a value close to the air O2 (~23.5‰). This is expected as the

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source of O2 in fossil fuel combustion is the atmospheric O2. The slight enrichment in

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end-member compared to the

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interaction of CO2 with water inside the exhaust pipes of the vehicles. This enrichment in 18O

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compared to the atmospheric O2 is much less than that reported by some previous

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researchers.43-45 The significant enhancement in δ18O in those previous studies could be

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artificial as isotopic exchange post collection of the samples with water condensed in the

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inner surface of the exhaust pipe and sampling tubes is likely.

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Figure 2B shows the two component mixing analysis for ∆17O and ∆47. The fossil fuel end-

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member (intercept) for ∆17O is -0.219 ±0.021‰, a value similar to the atmospheric O2.45 This

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is expected due to the fact that the atmospheric O2 is used in combustion. The fossil fuel end-

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member for ∆47 is 0.264±0.036‰ (intercept), obtained using a polynomial fit to the data

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points. This end-member is similar to the ∆47 values of 0.273±0.021 for car exhausts reported

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by Laskar and Liang.26 Both ∆17O and ∆47 do not follow conservative mixing i.e., their values

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in a mixture of two CO2 components cannot be expressed as a linear combination of the

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individual values unlike δ13C, δ18O, etc. However, we showed that the deviation in ∆17O in

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the mixture obtained by assuming a linear mixing is not significantly different from the exact

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value when the difference in δ18O between the two components are in the range of current

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consideration; the maximal bias is ~0.008‰, less than the analytical uncertainty in the

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present case (see S6 of Supporting Information). Therefore, for simplicity we assumed linear

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mixing for ∆17O. The linear approximation for mixing ∆47 could be erroneous. For a mixture

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of car exhaust and air CO2, linear mixing can be deviated by as much as 0.08‰ from the

18

18

O

O value of atmospheric O2 is probably due to partial


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338

exact ∆47 value of the mixture (see S6 and Figure S3 in the Supporting Information). We

339

showed that a second order polynomial fits well for ∆47 for a mixture of two gases (Figure S3

340

in Supporting Information). The anthropogenic end-member obtained using the second order

341

polynomial is 0.267‰ for the ∆47, which is similar to the values observed for car exhausts26.

342

This end member corresponds to a temperature of 285 oC. This temperature is much less than

343

the temperature inside the combustion chamber (>800 oC). The higher ∆47 value is probably

344

due to exchange of exhaust CO2 with water vapor at lower temperature inside the catalytic

345

converter and condensed water droplets on the inner surface of the exhaust pipe. The catalytic

346

converters, used to reduce environmental polluting species such as hydrocarbons, carbon

347

monoxide and oxides of nitrogen, have temperatures in the range of 200 to 400 oC.46-48.

348

Normally water in gas phase does not exchange isotopes with CO2, but in the presence of hot

349

catalysts, exchange may take place on its surface. Affeck et al.25,49 measured the ∆47 value of

350

car exhausts and estimated the end-member to be 0.44‰ which is significantly different from

351

the present value. The difference could be due to different car models and the variations in

352

the temperatures of the catalytic converters from car to car.

353 354

Strong correlations were observed among δ13C, δ18O, ∆17O and ∆47, indicating that the same

355

process governs all the isotopic compositions inside the tunnel and over the sea (Figure S4 in

356

the Supporting Information). Inside the tunnel, air CO2 was mainly a mixture of background

357

and vehicle emissions and over the ocean it was mainly the background air without any

358

significant contribution from any third sources in both the places. Table 3 shows the

359

estimation of fraction of CO2 from local anthropogenic emissions/respiration at the Academia

360

Sinica campus, Roosevelt road, and forest site, calculated using the end-members described

361

above. We assumed the background air CO2 as the other end-member with concentration,

362

δ13C, δ18O and ∆17O values of 395 ppmv, -8.43, 41.3, and -0.237‰, respectively. The

363

background end-members for δ13C and δ18O were assumed to be the values observed over the

364

tunnel (-8.5‰ and 41.3‰)35; for ∆17O, it was the value obtained over the South China Sea

365

(Table 1); and for ∆47, it was the thermodynamic equilibrium value of 0.97±0.02‰ at the

366

mean temperature of 16±5 oC at Roosevelt road during December. The background ∆47

367

values for the forest site and Academia Sinica campus were 0.895±0.012‰ at 31±2 oC, and

368

0.95±0.02 at 21±5 oC which are the thermodynamic equilibrium values during the sampling

369

periods, respectively.

370 12 ACS Paragon Plus Environment


371

Applying

372

f anth = 1 −

a

two-component

mixing

model

to

the

Page 14 of 27

air

CO2

data

yields

δ obs − δ anth × 100 % , where δ’s are the δ13C, δ18O or ∆17O values and f is the δ bgd − δ anth

373

fraction of CO2 and subscripts obs, bgd, and anth indicate observed, background and

374

anthropogenic, respectively. Anthropogenic CO2 estimated by different proxies at different

375

sites are summarized in Table 3. Note that the estimation of anthropogenic CO2 fraction by

376

∆17O is carried out by assuming a linear mixing of end to end members since the error

377

introduced by this assumption is less than analytical uncertainty as mentioned above. For ∆47

378

the deviation is significant, and hence instead of linear mixing we used a second order

379

polynomial fit to estimate the anthropogenic fraction.

380 381

At urban Roosevelt or sub-urban Academia Sinica campus, all five tracers give a similar

382

estimate for fanth. The majority of the excess CO2 in these sites are of anthropogenic origin

383

and hence all the isotope proxies give similar estimates. However, in the forest site, the

384

source of excess CO2 could be respiratory and/or anthropogenic. Possibility of anthropogenic

385

sources cannot a priori be ruled out as the site is not very far from the Academia Sinica

386

Campus, where we observed significant anthropogenic contribution. δ13C and δ18O may be

387

biased and fail to identify the sources. From the analysis of the samples from the forest site,

388

both ∆17O and ∆47 show little anthropogenic CO2 (Table 3), implying the excess of CO2 is

389

largely respiratory. This demonstrates a major advantage of the two rare proxies over their

390

major counterparts. Anthropogenic fraction of CO2 estimated by ∆47 at Roosevelt road on 30th

391

December, 2014 is significantly higher than the estimates obtained by the other isotope

392

proxies. However, this estimate, within the uncertainty, is similar to that estimated using

393

concentration. The errors associated with the estimates specially those using ∆17O and ∆47

394

are large which can be improved with more measurements and hence precise estimates of the

395

end-members.

396 397

All the proxies discussed above have advantages and disadvantages, and therefore, a

398

combined study based on multiple proxies improves the identification and quantification of

399

CO2 from different sources and the associated dynamics. Radiocarbon (14C) is another useful

400

tracer for quantifying the anthropogenic CO2 fraction50; however, it cannot distinguish the

401

difference between CO2 from two sources with modern carbon. At present, applications of 13 ACS Paragon Plus Environment

Page 15 of 27


402

∆17O and ∆47 in atmospheric CO2 are limited due to difficulty in measurements, but with

403

advances in the analytical techniques, automatic high precision analysis of these species such

404

as in-situ measurements of ∆17O using laser spectroscopy51 are becoming possible. In such

405

cases, these proxies can be used to monitor the anthropogenic fraction of CO2 in urban,

406

industrial areas. Refinement of the end-members even for other CO2 emitting processes such

407

as plant and soil respiration and human breath can be made and used for better monitoring of

408

the sources and modeling atmospheric carbon cycling.

409 410

In summary, two new proxies have been implemented in atmospheric CO2 budget estimation.

411

The anthropogenic end-members estimated for fossil fuel combusted CO2 are -0.219±0.021‰

412

and 0.267±0.036‰ for ∆17O and ∆47, respectively, which can be used to independently

413

quantify the fraction of anthropogenic CO2 as demonstrated here for a semi-urban, a busy

414

urban location, and in a forest site. The two new tracers are demonstrated to have advantages

415

over δ13C and δ18O in identifying sources of CO2 in certain cases. Thus, ∆17O and ∆47 can be

416

used to complement the deficiencies in the most widely used proxies such as concentration,

417

δ13C, and δ18O.

418

Associated content

419

Supporting Information

420

Additional details of Hsuehshan tunnel, sample collection and associated treatments,

421

analytical techniques including corrections, precision and accuracy of measurements,

422

neoconservative nature of ∆17O and ∆47 for mixing of two gases. A table giving the data for

423

accounting nonlinearity in the source of the mass spectrometer and conversion of ∆47 values

424

to absolute reference frame. Figures showing the calibrations for nonlinearity correction in

425

the source of the mass spectrometer, deviation of ∆17O and ∆47 from exact values for simple

426

linear mixing and correlations among isotopic parameters. This information is available free

427

of charge via the Internet at http://pubs.acs.org/.

428 429

Notes The authors declare no competing financial interest

430 431

Acknowledgement

432

We thank Mr. Wei-Kang Ho and Mr. Kuei-Pin Chang for collecting samples and helping in

433

laboratory setups. Special thanks to Dr. Chung-Ho Wang and Institute of Earth Sciences,

434

Academia Sinica for providing laboratory space. This work was supported by a Ministry of 14 ACS Paragon Plus Environment


435

Science and Technology (MOST-Taiwan) grant 105-2111-M-001-006-MY3 to Academia

436

Sinica and MOST-105-2119-M-002-001 to National Taiwan University.

437 438 439 440 441 442

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587 588 589 590 591 592 593 594 595 596 597 598


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599 600 601



602

Figures

603 604 605 606

Figure 1. Locations of Hsuehshan Tunnel connecting Taipei City and Yilan County and Roosevelt road and Academia Sinica Campus in Taipei city. S1 and S2 are the sampling sites near the entry and exit points, respectively, of the Yilan to Taipei bound bore of the tunnel.

607 608 609


Page 22 of 27

Page 23 of 27


610 611 612 613 614 615

Figure 2. Two component mixing plots for (A) δ13C (circles) and δ18O (stars) and (B) ∆17O (circles) and ∆47 (stars). A second order polynomial is fitted with the ∆47 (see S6 in Supporting Information for details). The dotted lines show the expected ∆47 and ∆17O values calculated using mixing of the two estimated end members. Data include air CO2 collected over South China Sea (gray) and from tunnel (black). For detailed descriptions refer to text.

616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634



635 636 637

Page 24 of 27

Table 1. Stable isotopic compositions including ∆17O and ∆47 values of air CO2 from Hsuehshan Tunnel, Roosevelt road, South China Sea and forest near Academia Sinica Campus (for details about the sites, see text). Hsuehshan Tunnel (position S1: 1.7 km inside from the entry point) Conc. δ13C δ18O ∆47 (‰) (ARF) ∆48 Date, time (ppmv) (VPDB) (VSMOW) ±1σ§ (SE) (‰)† 14/12/2013, 11:55 793 -17.31 33.21 0.637±0.010 0.54 14/12/2013, 16:00 853 -18.31 32.03 0.520±0.012 1.46 14/12/2013, 19:55 860 -17.83 32.60 0.742±0.008 2.96 15/12/2013, 12:00 988 -18.58 31.69 0.576±0.012 2.87 15/12/2013, 16:40 989 -19.60 30.98 0.657±0.010 2.55 15/12/2013, 20:30 932 -18.89 32.03 0.572±0.010 0.79 Average 902 -18.42 32.09 0.617 Std dev (1σ) 72 0.73 0.69 0.071 Hsuehshan Tunnel (position S2: 2.3 km inside from the exit point)

∆17O (‰) 0.020 -0.016 NA -0.044 NA 0.007 -0.008 0.024

12/12/2013, 16:30 13/12/2013, 16:40 13/12/2013, 22:00 14/12/2013, 16:20 14/12/2013, 20:25 15/12/2013, 12:40 15/12/2013, 15:10 15/12/2013, 21:25 Average Std dev (1σ)

2152 2490 1918 2533 2230 2612 2845 2478 2407 273

24/7/2014, 14:15 24/7/2014, 17:47 24/7/2014, 20:15 25/7/2014, 08:27

506 525 464 564

-23.519 27.61 -24.045 26.73 -23.091 27.90 -24.185 26.69 -23.76 27.09 -24.191 26.53 -24.427 26.37 -24.055 27.17 -23.91 27.01 0.40 0.50 Roosevelt road -11.54 38.11 -12.43 37.44 -11.20 38.80 -13.72 36.19

Average Std dev (1σ) 30/12/2014, 12:30 30/12/2014, 15:00 30/12/2014, 17:00 30/12/2014, 18:00 30/12/2014, 20:00 Average Std dev (1σ)

515 36 510 478 461 594 457

-12.22 0.97 -10.41 -11.50 -9.69 -12.30 -11.34

15/10/2013, 8:15 15/10/2013, 13:15 15/10/2013, 18:00 16/10/2013, 7:00

500 50 403 400 406 391

37.63 0.96 40.00 38.49 40.70 38.14 39.24 -11.05 39.31 0.90 0.94 South China Sea* -8.42 40.85 -8.46 40.80 -8.75 40.54 -8.76 40.53

0.471±0.011 0.420±0.047 0.409±0.013 0.425±0.012 0.327±0.009 0.359±0.013 0.410±0.009 0.420±0.015 0.405 0.041

2.13 1.05 1.69 1.28 2.32 1.16 1.77 2.62

-0.063 NA -0.106 NA -0.144 -0.142 -0.090 NA -0.109 0.031

NA NA NA NA

NA NA NA NA

0.272 0.182 0.310 0.229

0.823±0.010 0.754±0.007 0.833±0.013 0.819±0.014 0.806±0.021 0.807 0.028

-0.11 0.08 -0.03 0.45 0.37

0.248 0.047 0.257 0.262 0.301 0.214 0.234

0.901±0.017 0.919±0.011 0.933±0.013 0.903±0.023

0.41 0.39 0.36 0.89


0.254 0.030 0.332 0.301 0.313 0.294

Page 25 of 27


16/10/2013, 12:05 16/10/2013, 14:00 16/10/2013, 17:20 16/10/2013, 20:20 17/10/2013, 8:40 Average Std dev (1σ)

638 639 640 641 642 643

397 391 395 388 383

-8.44 -8.30 -8.31 -8.19 -8.26 -8.43

40.86 40.96 41.02 40.52 40.41 40.73

0.910±0.015 0.934±0.021 0.908±0.016 0.930±0.018 0.925±0.018 0.918

0.30 1.15 0.31 0.40 0.39

0.329 0.400 0.346 NA 0.317 0.335 0.034

395 7 0.19 0.20 0.012 Forest air near Academia Sinica Campus* 411 -9.07 0.890±0.017 0.388 41.43 0.29 7/7/2015 10:30 458 -10.43 0.890±0.017 0.292 39.74 14/7/2015 10:30 441 -9.99 0.887±0.015 0.324 40.86 1.00 28/7/2015 10:40 448 -10.46 0.920±0.009 0.317 40.09 0.30 11/8/2015 10:40 433 -9.99 0.888±0.016 0.311 39.80 0.40 18/8/2015 10:30 438 -9.99 0.895 0.326 40.38 Average 16 0.50 0.60 0.012 0.032 Std dev (1σ) *∆47 values of air CO2 collected over South China Sea and in the forest near Academia Sinica Campus are taken from ref. 26. § the errors are 1 SE based on 10 acquisitions each of 10 cycles. † ∆48 is used to check potential contaminants present in the sample CO2 (for definition of ∆48 see Supporting Information). Samples with ∆48>5 are not considered.

644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661



662

Page 26 of 27

Table 2. Anthropogenic and background end-members for all the four isotope proxies.

663

Proxy name

Anthropogenic end-members*

Background members

With CO2 from ocean Source of CO2 and tunnel

664 665 666 667 668 669

δ13C (VPDB)

-26.76±0.25

Fossil fuel carbon

-8.43±0.19§

δ18O (VPDB)

24.57±0.33

Atmospheric O2

41.43±0.18§

∆17O

-0.219±0.021

Atmospheric O2

0.335± 0.035†

∆47 (ARF)

0.267±0.036

Fossil fuel combustion

0.974±0.023$

*obtained from the intercepts of the two component mixing plots in Figure 2 § average δ18O value observed above the tunnel † observed over the South China Sea $ assumed the equilibrium value at the mean temperature of 16±5 oC during sampling time

670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685


end

Page 27 of 27

686 687 688 689 690 691


Table 3. Estimation of the fraction of local anthropogenic CO2 at different locations using the background and anthropogenic end-members given in Table 2. The CO2 concentration above 395 ppmv (value observed over South China Sea, to represent regional background) is assumed to be due to anthropogenic emission or respiration. The errors are 1σ standard deviation obtained considering errors associated with all the parameters and propagating them.

692 Proxy

Roosevelt road (24th July, 2014)

Roosevelt road (30th Dec, 2014)

Forest site

value†

ACO2§

value

ACO2

value

ACO2

value

ACO2

Conc. (ppmv)

411±11

4±2

515±36

23±7

500±50

21±10

438±16

10±4

δ13C(‰)

-8.78±0.05

2±3

-12.22±0.97

21±5

-11.05±0.90

14±5

-9.99±0.50

11±3

δ O(‰)

40.87±0.46

3±3

37.63±0.96

22±5

39.31±0.94

13±5

40.38±0.60

6±4

∆17O(‰)

0.331±0.038

3±8

0.284±0.047

15±9

0.254±0.030

17±7

0.326±0.032

1±7

∆47(‰)

0.897±0.027

2±3

NA

NA

0.807±0.028

30±9

0.895±0.012

0±3

18

693 694 695 696

Academia Sinica*

*concentration as well as isotopic values for Academia Sinica Campus was taken from Laskar and Liang (2016)26 and Liang and Mahata (2015)24. † average values of the proxies § anthropogenic CO2 fraction in %

697


Identification of Anthropogenic CO2 Using Triple Oxygen and

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