Assessing biotic contributions to CO₂ fluxes in northern China using the Vegetation, Photosynthesis and Respiration Model (VPRM-CHINA) and observations from 2005 to 2009

• Main Authors: A. Dayalu, J. W. Munger, S. C. Wofsy, Y. Wang, T. Nehrkorn, Y. Zhao, M. B. McElroy, C. P. Nielsen, K. Luus
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-11-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/15/6713/2018/bg-15-6713-2018.pdf
• ISSN: 1726-4170; 1726-4189

<p>Accurately quantifying the spatiotemporal distribution of the biological component of CO₂ surface–atmosphere exchange is necessary to improve top-down constraints on China's anthropogenic CO₂ emissions. We provide hourly fluxes of CO₂ as net ecosystem exchange (NEE; µmol&thinsp;CO₂&thinsp;m⁻²&thinsp;s⁻¹) on a 0.25° × 0.25° grid by adapting the Vegetation, Photosynthesis, and Respiration Model (VPRM) to the eastern half of China for the time period from 2005 to 2009; the minimal empirical parameterization of the VPRM-CHINA makes it well suited for inverse modeling approaches. This study diverges from previous VPRM applications in that it is applied at a large scale to China's ecosystems for the first time, incorporating a novel processing framework not previously applied to existing VPRM versions. In addition, the VPRM-CHINA model prescribes methods for addressing dual-cropping regions that have two separate growing-season modes applied to the same model grid cell. We evaluate the VPRM-CHINA performance during the growing season and compare to other biospheric models. We calibrate the VPRM-CHINA with ChinaFlux and FluxNet data and scale up regionally using Weather Research and Forecasting (WRF) Model v3.6.1 meteorology and MODIS surface reflectances. When combined with an anthropogenic emissions model in a Lagrangian particle transport framework, we compare the ability of VPRM-CHINA relative to an ensemble mean of global hourly flux models (NASA CMS – Carbon Monitoring System) to reproduce observations made at a site in northern China. The measurements are heavily influenced by the northern China administrative region. Modeled hourly time series using vegetation fluxes prescribed by VPRM-CHINA exhibit low bias relative to measurements during the May–September growing season. Compared to NASA CMS subset over the study region, VPRM-CHINA agrees significantly better with measurements. NASA CMS consistently underestimates regional uptake in the growing season. We find that during the peak growing season, when the heavily cropped North China Plain significantly influences measurements, VPRM-CHINA models a CO₂ uptake signal comparable in magnitude to the modeled anthropogenic signal. In addition to demonstrating efficacy as a low-bias prior for top-down CO₂ inventory optimization studies using ground-based measurements, high spatiotemporal resolution models such as the VPRM are critical for interpreting retrievals from global CO₂ remote-sensing platforms such as OCO-2 and OCO-3 (planned). Depending on the satellite time of day and season of crossover, efforts to interpret the relative contribution of the vegetation and anthropogenic components to the measured signal are critical in key emitting regions such as northern China – where the magnitude of the vegetation CO₂ signal is shown to be equivalent to the anthropogenic signal.</p>