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

Abstract

Hourly measurements of O₃, NO, NO₂, PAN, HNO₃ and NO_y concentrations, and eddy-covariance fluxes of O₃ and NO_y 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 (R_c), and the Noah land surface model coupled with a photosynthesis-based Gas-exchange Evapotranspiration Model (Noah-GEM). Noah-GEM produced better V_d(O₃) variations due to its more realistically simulated stomatal resistance (R_s) than WDDM. V_d(O₃) is very sensitive to the minimum canopy stomatal resistance (R_i) 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(O₃) due to the leafless assumption in ‘autumn’ seasonal category for which an infinite R_i was assigned. Reducing R_i 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(O₃) agreed reasonably well. HNO₃ 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 R_b), both being sensitive to the surface roughness length (z₀). Using an appropriate value for z₀ (10% of canopy height), WDDM and Noah-GEM agreed well with the observed daytime V_d(NO_y). The differences in V_d(HNO₃) 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 NO₂ 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_x, PAN and HNO₃ accounted for 19%, 4%, and 70% of the measured NO_y fluxes, respectively.

Introduction

Global atmospheric emissions of nitrogen oxide have increased dramatically during the past 150 years, and the supply of reactive nitrogen to ecosystems has doubled due to anthropogenic activities such as nitrogen fertilization, biomass burning, and fossil fuel combustion (Galloway et al., 2008). Dry deposition is responsible for a significant portion of the total (wet and dry) nitrogen deposition (e.g. 34%, Munger et al., 1998; 58%, Sparks et al., 2008). Up to 43% of NO_x–N emissions over North America have been estimated to be removed from the atmosphere by dry deposition (Shannon and Sisterson, 1992). Reactive nitrogen oxides, called NO_y, is a class of oxidized nitrogen compounds including NO, NO₂, NO₃, N₂O₅, HNO₃, PAN (peroxyacetyl nitrate), other organic nitrates, and particle nitrate, which supply significant nutrient and acidic quantities to ecosystems. Augmented atmospheric deposition of NO_y associated with increased emissions of NO_x poses many environmental threats, including acidification of soil and surface water, eutrophication of lake, river and estuary, loss of biodiversity, damage to forests, and global climate change (Galloway et al., 2008). Increased anthropogenic emissions of NO_x combined with hydrocarbons have produced high levels of surface O₃ concentration. O₃ can penetrate the tissues of leaves easily through stomatal uptake, causing stomatal occlusion and leaf damage. The direct uptake by vegetation through the stomata is also a major sink of O₃ in the lower troposphere (Turnipseed et al., 2009).

Given the significant impacts of NO_y and O₃ deposition on atmospheric chemistry and ecosystem health, it is desirable to quantify the deposition amount and assess the effects. Measuring deposition fluxes for reactive nitrogen compounds and O₃ with the eddy-covariance technique (e.g. Munger et al., 1996, Turnipseed et al., 2006) or the gradient method (e.g. Meyers et al., 1989, Sievering et al., 2001) have formed the basis for deposition models aimed at predicting dry depositions of reactive nitrogen compounds and O₃.

Models have been developed (e.g. Wesely, 1989, Meyers et al., 1998, Zhang et al., 2002, Zhang et al., 2003, Niyogi et al., 2009, Wu et al., 2003) to estimate the dry deposition velocity (V_d) by commonly utilizing the resistance approach analogous to Ohm’s law in electrical circuits. Accurately parameterizing the complex surface-atmosphere exchange process remains challenging for V_d modeling due to large variability in surface conditions (e.g., vegetation types, and soil contents) at model sub-grid scales. It is difficult to fully describe the physiological processes concerning the vegetation stomatal responses to various environmental conditions, leaf age, injury, and so on. The rapid within-canopy chemical reactions are not often considered in simple single-layer models, neither for the role of horizontal flow to receptor surfaces over non-uniform surfaces and terrains (Wesely and Hicks, 2000). Therefore, large uncertainties still exist in modeling V_d. A recent study (Flechard et al., 2010) modeled the V_d of inorganic reactive nitrogen species (i.e. NH₃, NO₂, HNO₃, and HONO and aerosol NH₄⁺ and NO₃⁻) over 55 monitoring sites throughout Europe, using four existing dry deposition models. Their result revealed that differences between models can reach a factor 2–3 and are even greater than differences between monitoring sites. Hence, there is a continuous need to evaluate modeled V_d over different land-cover types and for different chemical compounds.

Observational deposition fluxes of SO₂ and O₃ are often used to evaluate models (Zhang et al., 2002, Wu et al., 2003). However, few studies have evaluated modeled V_d for nitrogen species primarily because accurate quantifications of dry deposition fluxes and speciation of the reactive nitrogen species are difficult and expensive to obtain (Horii et al., 2005). Munger et al. (1996) demonstrated that the dry deposition fluxes of NO_y can be measured reliably using the eddy-covariance technique and year-round observations have been conducted at the Harvard Forest Environmental Measurement Site (HFEMS) since 1990. In a campaign attempting to estimate NO_y concentration and deposition budget, concentrations of individual NO_y species (i.e. NO, NO₂, PAN and HNO₃) have been measured at HFEMS. The reactive nitrogen dataset along with the O₃ fluxes/concentrations available at HFEMS are used to evaluate two community dry deposition models here.

One model is the Weather Research and Forecasting-Chemistry model (WRF-Chem) dry deposition module (hereafter WDDM). WRF-Chem is a state-of-the-art, regional atmospheric chemistry model (Grell et al., 2005) and has been successfully applied for regional air quality studies (Wang et al., 2009). Due to lack of observational data, few studies have evaluated the ability of the WDDM for calculating nitrogen V_d, even though dry deposition is one of the most important sinks for pollutants. The other model is the Noah land surface model (LSM) (Chen and Dudhia, 2001) coupled with a photosynthesis-based Gas-exchange Evapotranspiration Model (Niyogi et al., 2009) (hereafter Noah-GEM). The Noah LSM has been used to provide surface heat fluxes as boundary conditions for WRF. It is of broad interest to develop capacities of computing V_d in Noah LSM (Charusombat et al., 2010). This evaluation effort is part of a broader effort to eventually integrate the balance of hydrosphere, biosphere, and atmosphere with environmental modeling such as atmospheric nitrogen input for the ecosystems in Noah. There are also plans to couple surface deposition and emission information more closely in Noah by linking with biogenic emission models such as MEGAN (Model of Emissions of Gases and Aerosols from Nature; Guenther et al., 2006). So one main purpose of this paper is to document current deficiencies in WDDM and raise the awareness of such problems. Also, because an investigation of nitrogen deposition calculation has not been done for these models, this study takes advantage of recently available nitrogen flux data to investigate nitrogen-deposition algorithms, which can serve well in the deposition models. The objectives are to: 1) assess the performances of WDDM and Noah-GEM in calculating V_d(NO_y) and V_d(O₃) over a temperate deciduous forest, 2) understand the sensitivity of modeled V_d(NO_y) and V_d(O₃) to the key variables/parameters, and 3) improve the models by comparing with the field observations.

We will first describe the measurements used in this study (Section 2) and the modeling framework and formulations of WDDM and Noah-GEM (Section 3). Next, the observation data and model results and discussions are presented in Section 4, which is followed by the conclusions in Section 5.