A modified Vegetation Photosynthesis and Respiration Model (VPRM) for
the eastern USA and Canada, evaluated with comparison to atmospheric
observations and other biospheric models
Abstract
Increasing atmospheric CO2 measurements in North
America, especially in urban areas, may help enable the development of
an operational CO2 emission monitoring system. However,
isolating the fossil fuel emission signal in the atmosphere requires
factoring out CO2 fluctuations due to the biosphere,
especially during the growing season. To help improve simulations of the
biosphere, here we customize the Vegetation Photosynthesis and
Respiration Model (VPRM) at high-resolution for an eastern North
American domain, upwind of coastal cities from Washington D.C. to
Boston, MA, optimizing parameters using domain-specific flux tower data
from 2001 to the present. We run three versions of VPRM from November
2016 to October 2017 using i) annual (VPRMann) and ii)
seasonal parameters (VPRMseas), and then iii) modifying
the respiration equation to include the Enhanced Vegetation Index (EVI),
a squared temperature term and interactions between temperature and
water stress (VPRMnew). VPRM flux estimates are
evaluated by comparison with other models (the Carnegie-Ames-Stanford
Approach model, or CASA, and the Simple Biosphere Model v4), and with
comparison to atmospheric CO2 mole fraction data at 21
surface towers. Results show that VPRMnew is relatively
unbiased and outperforms all other models in explaining
CO2 variability from April to October, while
VPRMann overestimates growing season sinks by
underestimating summertime respiration. Despite unknown remaining errors
in VPRMnew, and uncertainties associated with other
components of the atmospheric CO2 comparisons,
VPRMnew appears to hold promise for more effectively
separating anthropogenic and biospheric signals in atmospheric inversion
systems in eastern North America.