Evaluating the calculated dry deposition velocities of reactive nitrogen oxides and ozone from two community models over a temperate deciduous forest

摘要

Hourly measurements of O_3, NO, NO_2, PAN, HNO_3 and NOy concentrations, and eddy-covariance fluxes of O_3 and NOj, over a temperate deciduous forest from June to November, 2000 were used to evaluate the dry deposition velocities (V_d) estimated by the WRF-Chem dry deposition module (WDDM), which adopted Wesely (1989) scheme for surface resistance (Rc), and the Noah land surface model coupled with a photosynthesis-based Gas-exchange Evapotranspiration Model (Noah-GEM). Noah-GEM produced better V_d(O_3) variations due to its more realistically simulated stomatal resistance (R_s) than WDDM. V_d(O_3) is very sensitive to the minimum canopy stomatal resistance (Ri) which is specified for each seasonal category assigned in WDDM. Treating Sep-Oct as autumn in WDDM for this deciduous forest site caused a large underprediction of V_d(O_3) due to the leafless assumption in 'autumn' seasonal category for which an infinite R_j was assigned. Reducing Rj to a value of 70 s m~1, the same as the default value for the summer season category, the modeled and measured V_d(O_3) agreed reasonably well. HNO_3 was found to dominate the NO_y, flux during the measurement period; thus the modeled V_d(NO_y) was mainly controlled by the aerodynamic and quasi-laminar sublayer resistances (R_a and Rb), both being sensitive to the surface roughness length (zo). Using an appropriate value for z_0 (10% of canopy height), WDDM and Noah-GEM agreed well with the observed daytime V_d(NO_y). The differences in V_d(HNO_3) between WDDM and Noah-GEM were small due to the small differences in the calculated R_a and R_b between the two models; however, the differences in R_c of NO_2 and PAN between the two models reached a factor of 1.1-1.5, which in turn caused a factor of 1.1-1.3 differences for V_d- Combining the measured concentrations and modeled V_d. NO,,, PAN and HNO_3 accounted for 19%, 4%, and 70% of the measured NO_y fluxes, respectively.