Jonathan E. Pleim

A non-local closure model for vertical mixing in the convective boundary layer

Abstract A simple non-local closure model for vertical mixing in Convective Boundary Layers (CBL) has been developed specifically for application in regional or mesoscale atmospheric chemistry models. The model, named the Asymmetrical Convective Model (ACM), is based on the concept that vertical transport within the CBL is inherently asymmetrical. Upward transport by buoyant plumes originating in the surface layer is simulated by mixing from the lowest model layer directly to all other layers in the CBL. Downward transport, however, proceeds only to the next layer in order to emulate gradual compensatory subsidence. The ACM is similar to the model developed by Blackadar (1978, 4th Symp. on Atmospheric Turbulence, Diffusion and Air Quality, pp. 443–447, Reno, Am. Meteorol. Soc.) but differs in its treatment of downward transport. The realism of the ACM is tested through comparisons to large-eddy simulations of several idealized test cases. These tests show that while the ACM shares the Blackadar models ability to simulate rapid transport upward from the surface layer to all levels in the CBL, it is clearly superior in its treatment of material emitted from elevated sources either within or above the CBL. The ACM is also tested in the context of the Regional Acid Deposition Model (RADM) both to determine sensitivity to different CBL mixing schemes and to compare to vertically resolved aircraft measurements. These tests demonstrate quicker upward transport of ground-level emissions by the ACM as compared to the eddy diffusion scheme currently used in RADM. The ACM also affects ozone photochemistry in the boundary layer resulting in lower ozone concentrations in areas of high NOx emissions.

The electrical analogy does not apply to modeling dry deposition of particles

Abstract The most commonly used expresion for dry deposition of particles is based on the electrical analogy. Because the electrical analogy is not consistent with the mass conservation equation, this expression for dry deposition velocity cannot be justified. This paper presents the correct expression.

A detailed evaluation of the Eta‐CMAQ forecast model performance for O3, its related precursors, and meteorological parameters during the 2004 ICARTT study

[1]xa0The Eta-Community Multiscale Air Quality (CMAQ) models forecast performance for ozone (O3), its precursors, and meteorological parameters has been assessed over the eastern United States with the observations obtained by aircraft, ship, ozonesonde, and lidar and two surface networks (AIRNOW and AIRMAP) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results at the AIRNOW sites show that the model was able to reproduce the day-to-day variations of observed daily maximum 8-hour O3 and captured the majority (73%) of observed daily maximum 8-hour O3 within a factor of 1.5 with normalized mean bias of 22%. The model in general reproduced O3 vertical distributions on most of the days at low altitudes, but consistent overestimations above ∼6 km are evident because of a combination of effects related to the specifications of lateral boundary conditions from the Global Forecast System (GFS) as well as the models coarse vertical resolution in the upper free troposphere. The model captured the vertical variation patterns of the observed values for other parameters (HNO3, SO2, NO2, HCHO, and NOy_sum (NOy_sum = NO + NO2 + HNO3 + PAN)) with some exceptions, depending on the studied areas and air mass characteristics. The consistent underestimation of CO by ∼30% from surface to high altitudes is partly attributed to the inadequate representation of the transport of pollution associated with Alaska forest fires from outside the domain. The model exhibited good performance for marine or continental clear airflows from the east/north/northwest/south and southwest flows influenced only by Boston city plumes but overestimation for southeast flows influenced by the long-range transport of urban plumes from both New York City and Boston.

Evaluation of real‐time PM2.5 forecasts and process analysis for PM2.5 formation over the eastern United States using the Eta‐CMAQ forecast model during the 2004 ICARTT study

[1] The performance of the Eta-Community Multiscale Air Quality (CMAQ) modeling system in forecasting PM 2.5 and chemical species is assessed over the eastern United States with the observations obtained by aircraft (NOAA P-3 and NASA DC-8) and four surface monitoring networks (AIRNOW, IMPROVE, CASTNet and STN) during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) study. The results of the statistical analysis at the AIRNOW sites show that the model was able to reproduce the day-to-day and spatial variations of observed PM 2.5 and captured a majority (73%) of PM 2.5 observations within a factor of 2, with normalized mean bias of -21%. The consistent underestimations in regional PM 2.5 forecast at other networks (IMPROVE and STN) were mainly due to the underestimation of total carbonaceous aerosols at both urban and rural sites. The significant underestimation of the other category, which predominantly is composed of primary emitted trace elements in the current model configuration, is also one of the reasons leading to the underestimation of PM 2.5 at rural sites. The systematic overestimations of SO 2- 4 both at the surface sites and aloft, in part, suggest too much SO 2 cloud oxidation due to the overestimation of SO 2 and H 2 O 2 in the model. The underestimation of NH + 4 at the rural sites and aloft may be attributed to the exclusion of some sources of NH 3 in the emission inventory. The systematic underestimations of NO - 3 may result from the general overestimations of SO 2- 4 . Note that there are compensating errors among the underestimation of PM 2.5 species (such as total carbonaceous aerosols) and overestimation of PM 2.5 species (such as SO 2- 4 ), leading to generally better performance of PM 2.5 mass. The systematic underestimation of biogenic isoprene (by ∼30%) and terpene (by a factor of 4) suggests that their biogenic emissions may have been biased low, whereas the consistent overestimations of toluene by the model under the different conditions suggest that its anthropogenic emissions might be too high. The contributions of various physical and chemical processes governing the distribution of PM 2.5 during this period are investigated through detailed analysis of model process budgets using the integrated process rate (IPR) analysis along back trajectories at five selected locations in Pennsylvania and Georgia. The results show that the dominant processes for PM 2.5 formation and removal vary from the site to site, indicating significant spatial variability.

Simulation of Atmospheric Boundary Layer Processes Using Local- and Nonlocal-Closure Schemes

A soil‐vegetation‐atmospheric boundary layer model was developed to study the performance of two localclosure and two nonlocal-closure boundary layer mixing schemes for use in meteorological and air quality simulation models. Full interaction between the surface and atmosphere is achieved by representing surface characteristics and associated processes using a prognostic soil‐vegetation scheme and atmospheric boundary layer schemes. There are 30 layers in the lowest 3 km of the model with a high resolution near the surface. The four boundary layer schemes are tested by simulating atmospheric boundary layer structures over densely and sparsely vegetated regions using the observational data from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) and from Wangara. Simulation results indicate that the near-surface turbulent fluxes predicted by the four boundary layer schemes differ from each other, even though the formulation used to represent the surface-layer processes is the same. These differences arise from the differing ways of representing subgrid-scale vertical mixing processes. Results also indicate that the vertical profiles of predicted parameters (i.e., temperature, mixing ratio, and horizontal winds) from the four mixed-layer schemes differ from each other, particularly during the daytime growth of the mixed layer. During the evening hours, after the mixed layer has reached its maximum depth, the differences among these respective predicted variables are found to be insignificant. There were some general features that were associated with each of the schemes in all of the simulations. Compared with observations, in all of the cases the simulated maximum depths of the boundary layer for each scheme were consistently either lower or higher, superadiabatic lapse rates were consistently either stronger or weaker, and the intensity of the vertical mixing was either stronger or weaker. Also, throughout the simulation period in all case studies, most of the differences in the predicted parameters are present in the surface layer and near the top of the mixed layer.

Sensitivity of ozone to model grid resolution—I. Application of high-resolution regional acid deposition model

Abstract This paper examines the sensitivity of ozone (O3) predictions to grid resolution in Eulerian grid models. A high-resolution version of the regional acid deposition model (HR-RADM) was developed and applied to simulate O3 formation at different grid resolutions. Horizontal grid-cell sizes of 20, 40, and 80 km were selected for this sensitivity study. Individual meteorological and chemical processes that contribute to O3 and its precursors were further separated and analyzed to determine their importance to O3 formation and the effects of grid resolution on these regulating processes. We first examined the model predictions of O3 maxima and minima at different grid resolutions over several major source areas. The results showed that the coarser-grid model tended to underpredict O3 maxima and overpredict O3 minima over the major source areas, because emission strengths were not as well resolved. Process contribution analyses of O3 over these source areas revealed that grid resolution significantly influences the magnitude of O3 formation and loss processes, especially chemistry and vertical transport. We also compared the process contributions between two different grid resolutions over an equal source area with nearly equal emissions to examine the nonlinearities of processes and their interactions with respect to grid resolution. These comparisons showed that for nonreactive species, the average transport applied to a coarse-grid cell is the same as that applied to the same area at higher resolution. For reactive species, however, the average transport is no longer the same between two different grid resolutions because the transport process interacts closely with chemistry, which is nonlinearly related to grid resolution. As a result, over the same source area, the coarser grid tended to predict more O3 but less NO2 from chemistry and to export more O3 and NO but less NO2 by vertical transport than did the finer grid.

A COUPLED LAND-SURFACE AND DRY DEPOSITION MODEL AND COMPARISON TO FIELD MEASUREMENTS OF SURFACE HEAT, MOISTURE, AND OZONE FLUXES

We have developed a coupled land-surface and drydeposition model for realistic treatment of surface fluxes ofheat, moisture, and chemical dry deposition within acomprehensive air quality modelling system. A new land-surfacemodel (LSM) with explicit treatment of soil moisture andevapotranspiration and an indirect soil moisture nudging schemehas been added to a mesoscale meteorology model. The new drydeposition model uses the same aerodynamic and bulk stomatalresistances computed for evapotranspiration in the LSM. Thisprovides consistent land-surface and boundary layer propertiesacross the meteorological and chemical components of the system. The coupled dry deposition model also has the advantage of beingable to respond to changing soil moisture and vegetationconditions. Modelled surface fluxes of sensible and latent heatas well as ozone dry deposition velocities were compared to twofield experiments: a soybean field in Kentucky during summer 1995and a mixed forest in the Adirondacks of New York in July 1998.

Sub-grid-scale features of anthropogenic emissions of NOx and VOC in the context of regional eulerian models

Abstract Model uncertainty is a major issue concerning regional-scale air quality simulation. One major source of uncertainty in regional Eulerian models is due to sub-grid-scale (SGS) effects related to anthropogenic emissions. Regional models typically have horizontal grid resolution (Δ) of 20–80 km. Since NOx chemistry in plumes is nonlinear and often diffusion-limited, the sudden dilution of plumes over regional grid dimensions, as in current models, can lead to a fundamental distortion of their chemistry, resulting in over-production of ozone, peroxides, sulfates and nitrates in the source region, and a related over-depletion of NOx. The corresponding model uncertainty over the whole regional domain remains unquantified. In this paper, we use high-resolution information from urban and regional emission inventories and plume field studies to examine SGS features of the emissions of anthropogenic NOx and VOC (volatile organic compounds), and of their mesoscale dispersion and chemistry. Such examination provides useful insight into some of the main sources of SGS uncertainty, as well as guidance for reducing it. The mesoscale chemistry of power plant plumes is very diffusion-limited, being controlled by VOC entrainment from the background. The crosswind spread of large point-source plumes typically takes 4–6 h to reach 30 km in convective conditions, and at least a full diurnal cycle to reach 80 km. For Δ much larger than 20–30 km, regional models will not be able to capture the essence of the behavior of rural point-source plumes even with plume-in-grid treatment, or to resolve the NOx emissions from many large power plants in urban peripheries from the urban VOC emissions. Within urban areas, there is progressive improvement in the resolution of the important ratio VOC/NOx as Δ is decreased below 20 km. The nature of these emissions-related SGS features suggests that significant gain in regional model accuracy should result by limiting Δ to 20–30 km in the regional domain, by the use of finer nested gridding in metropolitan sub-domains, and by a reactive plume-in-grid treatment of major point-source emissions.

A multilayer biochemical dry deposition model. 1. Model formulation

[1]xa0A multilayer biochemical dry deposition model has been developed based on the NOAA Multilayer Model (MLM; Meyers et al. [1998]) to study gaseous exchanges between the soil, plants, and the atmosphere. Most of the parameterizations and submodels have been updated or replaced. The numerical integration was improved, and an aerodynamic resistance based on Monin-Obukhov theory was added. An appropriate parameterization for the leaf boundary layer resistance was chosen. A biochemical stomatal resistance model was chosen based on comparisons of four different existing stomatal resistance schemes. It describes photosynthesis and respiration and their coupling with stomatal resistance for sunlit and shaded leaves separately. Various aspects of the photosynthetic process in both C3 and C4 plants are considered in the model. To drive the photosynthesis model, the canopy radiation scheme has been updated. Leaf area index measurements are adjusted to account for stem area index. A normalized soil water stress factor was applied to potential photosynthesis to account for plant response to both drought and water-logging stresses. A new cuticle resistance model was derived based on membrane passive transport theory and Ficks first law. It accounts for the effects of diffusivity and solubility of specific gases in the cuticle membrane, as well as the thickness of the cuticle membrane. The model is designed for use in the nationwide dry deposition networks, for example, the Clean Air Status And Trends Network (CASTNet), and mesoscale models, for example, the Community Multiscale Air Quality model (CMAQ) and even the Weather Research and Forecasting model (WRF).

Multiscale Air Quality Simulation Platform (MAQSIP): Initial applications and performance for tropospheric ozone and particulate matter

[1]xa0The performance of the Multiscale Air Quality Simulation Platform (MAQSIP) in simulating the regional distributions of tropospheric ozone and particulate matter (PM) is evaluated through comparisons of model results from three-dimensional simulations against available surface and aircraft measurements. These applications indicate that the model captures the dynamic range of observations and the spatial trends represented in measurements. Some discrepancies also exist, however, and they are discussed in the context of model formulation, input data specification and assumptions, and variability and bias in measurements. The daily normalized bias (within ±20%) and normalized gross errors (<25%) for predicted surface level O3 over an entire summer season are within the suggested performance criteria for management evaluation studies and are comparable to, if not smaller than, those reported previously for other regional O3 models. Comparisons of modeled PM composition with speciated fine particle concentration measurements show that the model is able to capture the spatial variability in fine PM mass as well as in the inorganic component fractions. Both measurements and model results show that in the summertime in the eastern U.S., SO42− is a relatively large component of fine PM mass; in contrast, NO3− is a significant fraction in the western U.S. in the wintertime case studied. The ability of the model to simulate the observed visibility indices (extinction coefficient and deciview) are evaluated through comparisons of model estimates using both a detailed Mie theory-based calculation (based on predicted aerosol size and number distributions) and an empirical mass reconstruction algorithm. Both modeled and observed data show that among the various aerosol components, in the eastern U.S. SO42− contributes the largest fraction to the aerosol extinction (35–85%), while organic mass contributes up to 20–25%. In contrast, in the western U.S., SO42− and NO3− have comparable contributions (20–50%) to the observed aerosol extinction. Comparisons with limited observational aircraft data, however, show moderate to poor correlation with measurements in the free troposphere. While these discrepancies can be attributed in part to model initialization and lateral boundary conditions specification, there is a need for further evaluation of the representation of boundary layer-free troposphere exchange mechanisms as well as the chemical mechanisms currently used in the model for representing chemistry in the free troposphere.