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Abstract: This paper presents a multi-pollutant sensitivity study of an air quality model over Europe with a focus on aerosols. Following the evaluation presented in the companion paper, the aim here is to study the sensitivity of the model to input data, mathematical parameterizations and numerical approximations. To that end, 30 configurations are derived from a reference configuration of the model by changing one input data set, one parameterization or one numerical approximation at a time. Each of these configurations is compared to the same reference simulation over two time periods of the year 2001, one in summer and one in winter. The sensitivity of the model to the different configurations is evaluated through a statistical comparison between the simulation results and through comparisons to available measurements. The species studied are ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), coarse and fine aerosol particles (PM c and PM2.5), sulfate, nitrate, ammonium, chloride and sodium. For all species, the modeled concentrations are very sensitive to the parameterization used for vertical turbulent diffusion and to the number of vertical levels. For the other configurations considered in this work, the sensitivity of the modeled concentration to configuration choice varies with the species and the period of the year. O3 is impacted by options related to boundary conditions. PM c is sensitive to sea-salt related options, to options influencing deposition and to options related to mass transfer between gas and particulate phases. PM2.5 is sensitive to a larger number of options than PM c : sea-salt, boundary conditions, heterogeneous reactions, aqueous chemistry and gas/particle mass transfer. NO2 is strongly influenced by heterogeneous reactions. Nitrate shows the highest variability of all species studied. As with NO2, nitrate is strongly sensitive to heterogeneous reactions but also to mass transfer, thermodynamic related options, aqueous chemistry and computation of the wet particle diameter. While SO2 is mostly sensitive to aqueous chemistry, sulfate is also sensitive to boundary conditions and, to a lesser extent, to heterogeneous reactions. As with nitrate, ammonium is largely impacted by the different configuration choices, although the sensitivity is slightly lower than for nitrate. NH3 is sensitive to aqueous chemistry, mass transfer and heterogeneous reactions. Chloride and sodium are impacted by sea-salt related options, by options influencing deposition and by options concerning the aqueous-phase module. [Copyright &y& Elsevier]

A fine-resolution, meso-urban meteorological model was updated and applied, using new data and techniques in this study, in driving fine-resolution photochemical simulations to evaluate the air-quality impacts of urban heat island mitigation. The drag-force-based formulation of the model improves the simulation of fine-resolution meteorological and air-pollutant concentration fields in the urban canopy layer. Compared to the meso scale, the meso-urban modeling of heat island mitigation produces larger localized impacts on meteorology and ozone air quality and captures phenomena of interest that are typically not detectable at the coarser scale. These include, for example, cool islands, heat islands, flow convergence associated with heat island circulation, flow divergence at the leading edge of urban areas, and vertical variation in turbulent kinetic energy budget components within the canopy layer in response to vertical changes in densities of buildings and vegetation. They also include fine-resolution features in the simulated ozone concentration field and its response to surface modifications. Model results show that heat island mitigation is effective in reducing local ozone concentrations. This paper presents results from Sacramento, California, as an example using increased urban albedo as the control mechanism. For the region, episodic conditions, and surface modification scenarios examined in this paper, air temperature is decreased by up to 3°C. Changes in ozone consist overwhelmingly of decreases but can also involve increases with the latter being confined to small areas and short time intervals. While larger reductions in ozone are detected, decreases of up to 5–10ppb are more representative and the daily maximum 8-h average can be decreased by up to 13%. [Copyright &y& Elsevier]

In the companion paper, we showed that MM5 simulation using a satellite-derived high resolution Texas Forest Service (TFS) land use and land cover (LULC) data set (M2), compared to the MM5 results with the default USGS-LULC (M1), improved representation of the complicated features of the atmospheric planetary boundary layer (PBL) in the Houston ship channel (HSC) area, where large industrial emission sources are concentrated. In the present paper, the study is extended to investigate these effects on air quality simulations. Two emission inputs, namely E1 and E2, are prepared with the M1 and M2 meteorology data, respectively, to reflect the differences in the point source plume rise estimates while keeping the biogenic and mobile emissions the same. Air quality simulations were performed with CMAQ using the M1E1 and M2E2 inputs. The simulation results demonstrate the importance of utilizing high resolution LULC data. In the default LULC data, the HSC area was classified as grass land cover, and MM5 predicted confined mixing, resulting in over-prediction of ozone (O3) precursors, such as NO x (NO plus NO2), and highly reactive volatile organic compounds (HRVOC) species, including ethylene and propylene, over the HSC area. In the TFS data, the area was classified as the impervious “urban” land use and MM5 predicted enhanced mixing of the precursor species, leading to better agreements with measurements. The high resolution LULC also resolves the location of water body near the HSC more accurately, predicting shallower PBL heights than the default LULC during daytime. With favorable wind conditions, the O3 precursors were transported from the HSC emission source towards the area, trapping the pollutants in a confined shallow mixing layer that occasionally led to a rapid photochemical production of O3. The above comparison includes the changes in both meteorological and plume-rise emissions inputs. We performed two additional CMAQ simulations using the same meteorological result (M2) but with different emission point sources E1 and E2 to determine the importance of emission changes on the air quality simulations. The sensitivity tests with the different plume-rise emission inputs due to two different meteorological inputs show little impact on the air quality simulations in this case. [Copyright &y& Elsevier]

Abstract: In this paper, we examine the extent to which an ensemble generated from a single air quality model correctly represents the global uncertainty of ozone simulations with a regional scale chemistry-transport model and whether it is suitable for ozone data assimilation. An ensemble of 30 members is constructed from a reference simulation in which the model parameters that are the most uncertain for determining ozone concentrations are randomly perturbed. Comparisons of the simulated ensemble using the model CHIMERE with observations are examined over the summer season both at the surface and higher in the troposphere. Although the ensemble overestimates vertical and horizontal correlation between errors, the average model error is well represented both at the surface and in the vertical dimension (after an adjustment for the latter). A variability of about 7–8ppb is found. The results found in this study with respect to tropospheric ozone model errors and error correlation lengths both from ensemble simulations and model to observation comparisons are of particular interest for data assimilation when constructing model error covariance matrices. [ABSTRACT FROM AUTHOR]

A gas-phase reaction mechanism for the atmospheric photooxidations of over 100 alkanes, alkenes, aromatic hydrocarbons, alcohols, ethers and other compounds representative of the range of reactive organics emitted into polluted atmospheres is described. Most of these organic species are represented using generalized reactions with variable rate constants and product yield coefficients for which individual assignments were made or estimated. This mechanism employs 19 species to represent the reactive oxygenated and organic nitrate products, and includes the gas-phase reactions of SO2, but does not include heterogeneous or liquid-phase reactions. The evaluation of this mechanism, by comparison of its predictions against the results of over 500 environmental chamber experiments, is described in a separate paper. This detailed mechanism can be used in assessments of relative atmospheric reactivities of organic compounds, and can provide the basis for the derivation of more condensed mechanisms for use in air quality simulation models. [Copyright &y& Elsevier]