Diurnal and seasonal changes of d13C and d18O in carbon dioxide measured by eddy covariance over an urban surface

Carbon dioxide (CO₂) is the major long-lived greenhouse gas emitted directly from urban areas worldwide - primarily by combustion of fossil fuels, including natural gas (space heating, cooking etc.), diesel and gasoline (transportation). CO₂ is also emitted in urban areas by human, plant and soil respiration and a minor fraction is taken up by photosynthesis of urban vegetation. Disentangling and attributing measured fluxes of carbon dioxide (CO₂) in urban environments to different fuel and biological sources remains a challenge. The stable isotope composition of CO₂ can provide valuable information about its sources. For example, linear mixing models have been used to identify the cause of enhanced CO₂ in the urban atmosphere using urban and background measurements of atmospheric mixing ratios in combination with known source signatures. Here we propose an alternative approach by continuously measuring fluxes of the stable isotopes ¹²CO₂, ¹³CO₂ and CO¹⁸O by means of eddy covariance (EC) over an urban surface. We aim to explore, characterize and explain the changing isotopic signature of urban CO₂ emissions without the need for background measurements or source sampling. The goal is (1) to independently determine the characteristic isotopic composition of the urban emission mix of a typical urban surface; (2) to determine whether the isotopic composition changes with the strength of fuel and respiratory emissions (and uptake by photosynthesis); and (3) compare EC-isofluxes to independent emission estimates and known source signatures.

EC fluxes of ¹²CO₂, ¹³CO₂ and CO¹⁸O were continuously measured on a 30-m-tall tower located in Vancouver, BC, Canada (Fluxnet ID "Ca-VSu) between March 15 and June 30, 2016. The tower is located near an intersection of two arterial roads in a residential area characterized by building emissions from natural gas furnaces and a moderate vegetation canopy (canopy cover: 12%, 17.2 trees / ha). EC-fluxes were measured using a 3-axis sonic anemometer (CSAT-3, Campbell Scientific Inc., (CSI)) and a closed-path tuneable diode laser absorption spectroscopy (TDLAS) system scanning absorption lines at 2308.171 (¹³CO₂), 2308.225 (¹³CO₂) and 2308.416 cm^-1 (CO¹⁸O) at 10 Hz (TGA200, CSI). Every 10 minutes, the TDLAS was calibrated using three tanks referenced against NOAA-ESRL/INSTAAR, UoC standards. Half-hourly isoflux ratios F¹³C and F¹⁸C were calculated as F¹³C = 1000 [(w'¹³CO₂' / w'¹²CO₂') / R_PDB-CO2) – 1] and F¹⁸O = 1000 [(w'CO¹⁸O' / w'¹²CO₂') / R_PDB-CO2) – 1]. F¹³C and F¹⁸C were analyzed for different wind sectors in combination with total CO₂ flux and source area analysis. Monthly average fluxes were determined by equally weighting hours of day (mean diurnal course approach) and equally weighting the four wind sectors in order to remove biases due to changes in the seasonal wind direction distribution.

From independent sample analysis it is known that δ¹³C in CO₂ changes depending on fuel type (from exhaust samples taken in Metro Vancouver: δ¹³C of gasoline -27.2‰; diesel -28.8‰; natural gas -41.6‰, respiration: ~-25‰, with respect to PDB-CO₂). These values are expected to change depending on the origin of fuels. δ¹⁸O is fractionated in catalytic converters (δ¹⁸O with converter: -12.5‰; no converter -18.6‰; natural gas -22.7‰, with respect to PDB-CO₂). Biogenic emissions exhibit a larger seasonal variability due to changes in plant water origin and fractionation during the dry season (measured δ¹⁸O of samples range between ~-15‰ and -5‰). During the study period, background atmospheric CO₂ consistently increased from δ¹³C = -8.9‰ (March) to -8.5‰ (June), and from δ¹⁸O = 0.0‰ (March) to 0.8‰ (June).

The average EC-measured F¹³C during the study period was -32.7‰. The average F¹³C matches well with the estimates from a top-down fuel inventory for the City of Vancouver (36% natural gas and 64% gasoline and diesel; leads to a calculated F¹³C = -32.1‰ using the independent sample analysis results). The EC-measured F¹³C steadily increased from -33.1‰ (March) to -31.7‰ (June), which is explained by a proportional decrease of emissions from home heating by natural gas. F¹³C was significantly correlated with traffic counts in the turbulent source area, with higher fluxes and higher F¹³C during daytime (proportionally more gasoline and diesel). Over the study area F¹³C was higher during daytime with -30.8‰ (proportionally more gasoline emissions) and lower at nighttime with -33.6‰, when traffic emissions were proportionally lower and natural gas use higher – this explanation assumes respiratory fluxes were minor and steady.

The average EC-measured F¹⁸O was -13.9‰, and monthly averages ranged between ‑14.8‰ (April) and -13.0‰ (June). F¹⁸O was lowest during the afternoon (‑13.9‰) and highest during the early morning (-13.0‰). The highest values were measured when wind was from the NW sector which is characterized by significant tree cover and negligible fuel sources, representing the biological processes (F¹⁸O = ‑11.9‰ over the study period, -6.9‰ in June). Lowest values were measured when the wind was from the SE sector that included the intersection of arterial roads (F¹⁸O = ‑15.3‰ over the study period, matching well the traffic signal).

To our knowledge, these are the first EC-isofluxes measured over an urban area. In summary, with the current experimental arrangement of using a TDLAS and strong emissions, EC-measured isofluxes can be resolved, resulting in F¹³C and F¹⁸O values consistent with independent source samples and the expected source distribution. EC isofluxes characterize the integral mix of CO₂ from urban areas, which could be useful in (1) partitioning total EC fluxes of CO₂ on urban EC-towers and hence validate fine-scale emission inventories at greater detail, and (2) providing valuable information about the characteristic isotopic composition of the emission mix from cities for use in regional inverse models using stable isotope tracers. Inverse models can estimate metropolitan emissions based on concentration and isotope measurements on aircraft or ground-based networks on coarser scales.