Abstract
Abstract. The atmospheric carbon dioxide (CO2) mixing ratio and its carbon isotope (δ13C-CO2) composition contain important CO2 sink and source information spanning from ecosystem to global scales. The observation and simulation for both CO2 and δ13C-CO2 can be used to constrain regional emissions and better understand the anthropogenic and natural mechanisms that control δ13C-CO2 variations. Such work remains rare for urban environments, especially megacities. Here, we used near-continuous CO2 and δ13C-CO2 measurements, from September 2013 to August 2015, and inverse modeling to constrain the CO2 budget and investigate the main factors that dominated δ13C-CO2 variations for the Yangtze River delta (YRD) region, one of the largest anthropogenic CO2 hotspots and densely populated regions in China. We used the WRF-STILT model framework with category-specified EDGAR v4.3.2 CO2 inventories to simulate hourly CO2 mixing ratios and δ13C-CO2, evaluated these simulations with observations, and constrained the total anthropogenic CO2 emission. We show that (1) top-down and bottom-up estimates of anthropogenic CO2 emissions agreed well (bias < 6 %) on an annual basis, (2) the WRF-STILT model can generally reproduce the observed diel and seasonal atmospheric δ13C-CO2 variations, and (3) anthropogenic CO2 emissions played a much larger role than ecosystems in controlling the δ13C-CO2 seasonality. When excluding ecosystem respiration and photosynthetic discrimination in the YRD area, δ13C-CO2 seasonality increased from 1.53 ‰ to 1.66 ‰. (4) Atmospheric transport processes in summer amplified the cement CO2 enhancement proportions in the YRD area, which dominated monthly δs (the mixture of δ13C-CO2 from all regional end-members) variations. These findings show that the combination of long-term atmospheric carbon isotope observations and inverse modeling can provide a powerful constraint on the carbon cycle of these complex megacities.
Highlights
Urban landscapes account for 70 % of global CO2 emissions and represent less than 3 % of Earth’s land area (Seto et al, 2014)
When excluding ecosystem respiration and photosynthetic discrimination in the Yangtze River delta (YRD) area, δ13C-CO2 seasonality increased from 1.53 ‰ to 1.66 ‰. (4) Atmospheric transport processes in summer amplified the cement CO2 enhancement proportions in the YRD area, which dominated monthly δs variations. These findings show that the combination of longterm atmospheric carbon isotope observations and inverse modeling can provide a powerful constraint on the carbon cycle of these complex megacities
Anthropogenic CO2 emission is produced from fossil fuel burning and cement production
Summary
Urban landscapes account for 70 % of global CO2 emissions and represent less than 3 % of Earth’s land area (Seto et al, 2014) Such CO2 hotspots play a dominant role in controlling the rise in atmospheric CO2 concentrations, which exceeded 412 ppm in December 2019 for global monthly average observations (https://www.esrl.noaa.gov/gmd/ccgg/trends/, last access: 1 August 2020). As the urban population is expected to increase by 2.5 to 6 billion people in 2050, anthropogenic CO2 emissions are projected to increase dramatically, especially in developing regions and countries (Sargent et al, 2018; Ribeiro et al, 2019) Under such a scenario, the observations of atmospheric CO2 and δ13C-CO2 in urban landscapes are of great importance to monitoring these potential CO2 emissions hotspots (Lauvaux et al, 2016; Nathan et al, 2018; Graven et al, 2018; Pillai et al, 2016; Staufer et al, 2016)
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