Evaluating the calculated dry deposition velocities of reactive nitrogen oxides and ozone from two community models over a temperate deciduous forest
Highlights
► The first study on evaluating Vd(NOy) in the dry deposition models. ► Compared the performance of two models with different canopy treatments. ► Assessed the sensitivity of key parameters to simulate Vd(O3) and Vd(NOy). ► Improved the models by comparing with the field observations. ► Further developed the GEM model.
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 NOx–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 NOy, 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 NOy associated with increased emissions of NOx 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 NOx 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 NOy 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 (Vd) 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 Vd 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 Vd. A recent study (Flechard et al., 2010) modeled the Vd 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 Vd 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 Vd 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 NOy 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 NOy concentration and deposition budget, concentrations of individual NOy 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 Vd, 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 Vd 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 Vd(NOy) and Vd(O3) over a temperate deciduous forest, 2) understand the sensitivity of modeled Vd(NOy) and Vd(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.
Section snippets
Site description
The HFEMS is located in a temperate 80–100 year-old mixed deciduous forest in central Massachusetts (42.54 N, 72.18 W; elevation, 340 m), which consists of red oak (Quercus rubra), red maple (Acer rubrum) with scattered hemlock (Tsuga canadensis), red pine (Pinus resinosa), and white pine (Pinus strobus). The canopy height near the observation tower is approximately 20 m with a peak leaf area index (LAI) of 3.4 m2 m−2 during summer. The nearest sources of significant pollution are a secondary road
Modeling framework
The resistance method determines Vd as the reciprocal of a total resistance (Rt) which consists of a series of resistances to perform gas transport from the atmosphere down to the surface.Table A.1 describes each resistance component, and Table A.2 compares the formulations between WDDM and Noah-GEM.
Further developments of GEM
The GEM model (Niyogi et al., 2009) was further developed here (see Appendix B), but the parameters were kept the same and not specifically tuned for this study. Rs is the
The observations of O3 deposition and its environmental drivers
Fig. 2 shows the time series of hourly-averaged [O3] and F(O3) from June–October 2000. There was a distinct seasonal cycle of [O3] showing maxima in summer, associated with the high solar radiation and temperature. The peak values ranged from 40 to 80 ppbv, slightly lower than the observations in 1991–1994 (Munger et al., 1996). F(O3) followed the same seasonal trend with maxima during summer, closely coinciding with high concentrations and canopy growth (Munger et al., 1996). As shown in Fig. 3
Summary and conclusions
We evaluated the ability of two models (WDDM and Noah-GEM) to calculate Vd(O3) and Vd(NOy) against direct observations at HFEMS, and identified key variables/parameters and uncertainties in the two models. WDDM employs Wesely (1989) parameterization for Rc, which uses a simple Rs scheme based on the Ri parameter prescribed for each season and land-cover category. The uncertainty in prescribed Ri dominates the errors in estimating Vd for O3 and other gases that are controlled by the stomatal
Acknowledgements
This work was supported by the Natural Science Foundation of China (grant Nos. 40875076, U0833001), the National Program on Key Basic Research Project of China (973) (grant No. 2010CB428504) and the Fundamental Research Funds for the Central Universities. We also gratefully acknowledge the NCAR Advanced Study Program (ASP), BEACHON and Water System Programs. This research also benefited through the NOAA/JCSDA grant (NA06NES4400013), NASA-Terrestrial Hydrology Program and DOE ARM program. The
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