Regional Air Quality Modeling System Research Articles

The influence of gas-particle partitioning and surface-atmosphere exchange on ammonia during BAQS-Met

Abstract. The Border Air Quality and Meteorology study (BAQS-Met) was an intensive field campaign conducted in Southwestern Ontario during the summer of 2007. The focus of BAQS-Met was determining the causes of the formation of ozone and fine particulate matter (PM2.5), and of the regional significance of trans-boundary transport and lake breeze circulations on that formation. Fast (1 Hz) measurements of ammonia were acquired using a Quantum Cascade Laser Tunable Infrared Differential Absorption Spectrometer (QC-TILDAS) at the Harrow supersite. Measurements of PM2.5 ammonium, sulfate and nitrate were made using an Ambient Ion Monitor Ion Chromatograph (AIM-IC) with hourly time resolution. The median mixing ratio of ammonia was 2.5 ppb, with occasional high spikes at night resulting from local emissions. Measurements were used to assess major local emissions of NH3, diurnal profiles and gas-particle partitioning. The measurements were compared with results from A Unified Regional Air-quality Modelling System (AURAMS). While the fraction of total ammonia (NHx≡NH3 + NH4+) observed in the gas phase peaks between 0.1 and 0.8, AURAMS tended to predict fractions of either less than 0.05 or greater than 0.8. The model frequently predicted acidic aerosol, in contrast with observations wherein NHx almost always exceeded the observed equivalents of sulfate. One explanation for our observations is that the net flux of ammonia from the land surface to the atmosphere increases when aerosol sulfate is present, effectively buffering the mixing ratio of gas phase ammonia, a process not included in the model. A simple representation of an offline bi-directional flux parameterization using the ISORROPIA thermodynamic model was successful at reducing the population of zero gas fraction points, but not the higher gas fraction points.

Relative impact of windblown dust versus anthropogenic fugitive dust in PM2.5 on air quality in North America

A new windblown dust emissions module was recently implemented into A Unified Regional Air Quality Modeling System (AURAMS), a Canadian regional air quality model, to investigate the relative impact of windblown dust versus anthropogenic fugitive dust on air quality in North America. In order to apply the windblown dust emissions module to the entire North American continent, a soil grain size distribution map was developed using the outputs of 4 monthly runs of AURAMS for 2002 and available PM2.5 dust content observations. The simulation results using the new soil grain size distribution map showed that inclusion of windblown dust emissions is essential to predict the impact of dust aerosols on air quality in North America, especially in the western United States. The windblown dust emissions varied widely by season, whereas the anthropogenic fugitive dust emissions did not change significantly. In the spring (April), the continental monthly average emissions rate of windblown dust (4.1 × 107 kg/d) was much higher than that of anthropogenic fugitive dust (1.5 × 107 kg/d). The total amount of windblown dust emissions in North America predicted by the model for 2002 was comparable to that of anthropogenic fugitive dust emissions. Even with the inclusion of windblown dust emissions, however, the model still had difficulty simulating dust concentrations. Further improvements are needed, in terms of both limitations of the windblown dust emission module and uncertainties in the anthropogenic fugitive dust emissions inventories, for improved dust modeling.

A modeling study of the impact of heterogeneous reactions on mineral aerosol surfaces on tropospheric chemistry over East Asia

Nine heterogeneous reactions (uptake of H 2O 2, HNO 3, HO 2, N 2O 5, NO 2, NO 3, O 3, OH and SO 2 on mineral aerosol surfaces) are incorporated into a Regional Air Quality Model System (RAQMS) to investigate their impacts on tropospheric chemistry in East Asia during the dust storm period in March 2006. Comparison with observations shows the model system well represents the behaviors of the gaseous and aerosol species. Most of the reaction probability γ values used for this study are the best estimation specifically for dust samples from deserts of China, derived from analysis of a number of recent laboratory studies. There are large variations in gas and aerosol concentrations while taking heterogeneous reactions on mineral aerosol surface into account, especially during dust storm events. The domain-averaged monthly mean percentage changes in SO 2, NO 2, O 3, HNO 3, NH 3, total sulfate, total nitrate and total ammonium concentrations are −4.4%, −3.8%, −2.1%, −22.0%, 12.7%, 6.6%, 26.1%, and −9.5%, respectively below 3 km. These changes indicate the considerable perturbation of heterogeneous reactions on mineral aerosol surface to tropospheric chemical system and components. The strength of heterogeneous reactions is determined by both reaction probability and gas precursor concentration. Among the nine reactions, dust uptakes of HNO 3, SO 2, and N 2O 5 exert relatively large influences on the other chemical components, whereas the reactions regarding H 2O 2, HO 2, and OH have little impacts.

The Meteorology-Chemistry Interface Processor (MCIP) for the CMAQ modeling system: updates through MCIPv3.4.1

Abstract. The Community Multiscale Air Quality (CMAQ) modeling system, a state-of-the-science regional air quality modeling system developed by the US Environmental Protection Agency, is being used for a variety of environmental modeling problems including regulatory applications, air quality forecasting, evaluation of emissions control strategies, process-level research, and interactions of global climate change and regional air quality. The Meteorology-Chemistry Interface Processor (MCIP) is a vital piece of software within the CMAQ modeling system that serves to, as best as possible, maintain dynamic consistency between the meteorological model and the chemical transport model (CTM). MCIP acts as both a post-processor to the meteorological model and a pre-processor to the emissions and the CTM in the CMAQ modeling system. MCIP's functions are to ingest the meteorological model output fields in their native formats, perform horizontal and vertical coordinate transformations, diagnose additional atmospheric fields, define gridding parameters, and prepare the meteorological fields in a form required by the CMAQ modeling system. This paper provides an updated overview of MCIP, documenting the scientific changes that have been made since it was first released as part of the CMAQ modeling system in 1998.

Simulation on SO2 and NOX Emission from Coal–Fired Power Plants in North-Eastern North America

MM5-SMOKE-CMAQ regional air quality modeling system was used to simulate pollutants emission from coal–fired power plants in North-Eastern North America. The effects of SO2 and NOX on air quality producing from coal-fired power plants in the summer of 2001 were analyzed. Simulations show the contributions of SO2 and NOX emission from coal-fired power plants using different scenarios, coal-fired power plants from US and Canada contribute 67.2% and 32.8% for total SO2 concentration, 17.6% and 6.0% for total NOX concentration in researched domain. Some control measures for coal-fired power plants were discussed. Further controls for the emissions of SO2 and NOX from coal-fired power plants are necessary to reduce the adverse environmental effects.

Modelling the impacts of ammonia emissions reductions on North American air quality

Abstract. A unified regional air-quality modelling system (AURAMS) was used to investigate the effects of reductions in ammonia emissions on regional air quality, with a focus on particulate-matter formation. Three simulations of one-year duration were performed for a North American domain: (1) a base-case simulation using 2002 Canadian and US national emissions inventories augmented by a more detailed Canadian emissions inventory for agricultural ammonia; (2) a 30% North-American-wide reduction in agricultural ammonia emissions; and (3) a 50% reduction in Canadian beef-cattle ammonia emissions. The simulations show that a 30% continent-wide reduction in agricultural ammonia emissions lead to reductions in median hourly PM2.5 mass of <1 μg m−3 on an annual basis. The atmospheric response to these emission reductions displays marked seasonal variations, and on even shorter time scales, the impacts of the emissions reductions are highly episodic: 95th-percentile hourly PM2.5 mass decreases can be up to a factor of six larger than the median values. A key finding of the modelling work is the linkage between gas and aqueous chemistry and transport; reductions in ammonia emissions affect gaseous ammonia concentrations close to the emissions site, but substantial impacts on particulate matter and atmospheric deposition often occur at considerable distances downwind, with particle nitrate being the main vector of ammonia/um transport. Ammonia emissions reductions therefore have trans-boundary consequences downwind. Calculations of critical-load exceedances for sensitive ecosystems in Canada suggest that ammonia emission reductions will have a minimal impact on current ecosystem acidification within Canada, but may have a substantial impact on future ecosystem acidification. The 50% Canadian beef-cattle ammonia emissions reduction scenario was used to examine model sensitivity to uncertainties in the new Canadian agricultural ammonia emissions inventory, and the simulation results suggest that further work is needed to improve the emissions inventory for this particular sector. It should be noted that the model in its current form neglects coarse mode base cation chemistry, so the predicted effects of ammonia emissions reductions shown here should be considered upper limits.

Evaluation of a unified regional air-quality modeling system (AURAMS) using PrAIRie2005 field study data: The effects of emissions data accuracy on particle sulphate predictions

The effects of the accuracy of major-point source emissions input data on the predictions of a regional air-quality model (AURAMS) were investigated through a series of scenario simulations. The model domain and time period were chosen to correspond to that of PrAIRie2005, an air-quality field study with airborne and ground-based mobile measurement platforms that took place between August 12th and September 7th, 2005, over the city of Edmonton, Alberta, Canada. The emissions data from standard sources for three coal-fired power-plants located west (typically upwind) of the city were compared to the continuous emissions monitoring system (CEMS) taking place at the time of the study – the latter showed that the original emissions inventory data considerably overestimated NO x , SO 2, and primary particulate emissions during the study period. Further field investigation (stack sampling) in the fall of 2006 showed that the measured primary particle size distribution and chemical speciation for the emissions were strikingly different from the distribution and speciation originally used in the model. The measured emissions were used to scale existing emissions data in accord with the CEMS and in-stack measurements. The effects of these improvements to the emissions data were examined sequentially in nested AURAMS simulations (finest horizontal resolution 3 km), and were compared to airborne aerosol mass spectrometer (Aerodyne AMS) measurements of particle sulphate, and particle distributions from an airborne passive cavity aerosol spectrometer probe (PCASP). The emissions of SO 2 had the greatest impact on predicted PM 1 sulphate, while the primary particle size distribution and chemical speciation had a smaller role. The revised emissions data greatly improved the comparisons between observations and model values, though over-predictions of fine-mode sulphate still occur near the power-plants, with the use of the revised emissions data. The modified emissions also had a significant impact on the larger particles of the particulate matter, with more primary PM in sizes greater than 1 μm diameter than had previously been estimated, and higher large particle concentrations close to the power-plants.

A comparative performance evaluation of the AURAMS and CMAQ air-quality modelling systems

A harmonized comparative performance evaluation of A Unified Regional Air-quality Modelling System (AURAMS) v1.3.1b and Community Multiscale Air Quality (CMAQ) v4.6 air-quality modelling systems was conducted on the same North American grid for July 2002 using the same emission inventories, emissions processor, and input meteorology. Comparison of AURAMS- and CMAQ-predicted O 3 concentrations against hourly surface measurement data showed a lower normalized mean bias (NMB) of 20.7% for AURAMS versus 46.4% for CMAQ. However, AURAMS and CMAQ had more similar normalized mean errors (NMEs) of 46.9% and 54.2%, respectively. Both models did similarly well in predicting daily 1-h O 3 maximums; however, AURAMS performed better in calculating daily minimums. CMAQ's poorer performance for O 3 is partly due to its inability to correctly predict nighttime lows. Total PM 2.5 hourly surface concentration was under-predicted by both AURAMS and CMAQ with NMBs of −10.4% and −65.2%, respectively. However, as with O 3, both models had similar NMEs of 68.0% and 70.6%, respectively. In general, AURAMS performance was better than CMAQ for all major PM 2.5 species except nitrate and elemental carbon. Both models significantly under-predicted total organic aerosols (TOAs), although the mean AURAMS concentration was over four times larger than CMAQ's. The under-prediction of TOA was partly due to the exclusion of forest-fire emissions. Sea-salt aerosol made up approximately 50.2% of the AURAMS total PM 2.5 surface concentration versus only 6.2% in CMAQ when averaged over all grid cells. When averaged over land cells only, sea-salt still contributed 13.9% to the total PM 2.5 mass in AURAMS versus 2.0% in CMAQ.

OH-reactivity of volatile organic compounds at urban and rural sites across Canada: Evaluation of air quality model predictions using speciated VOC measurements

The total OH-reactivity resulting from volatile organic compound (VOC) oxidation (defined here as ∑ k OH[VOC]) is a measure of the potential of an air mass to produce organic peroxy radicals. Organic peroxy radicals are key intermediate species in the production of ozone and secondary organic aerosol. In this study, routine VOC measurements from 18 selected urban and rural sites across Canada (data from the National Air Pollution Surveillance (NAPS) network) were used to calculate OH-reactivity. Observationally-derived OH-reactivity for VOCs was compared to predictions from Environment Canada's unified regional air quality modelling system (AURAMS). The model was run for two summer periods: between July 8 and August 19, 2004 for evaluation over an eastern Canada domain and between August 23 and September 9, 2005 for evaluation over a western Canada domain. Total OH-reactivity was predicted within a factor of two for most sites; however, OH-reactivity model-measurement differences for individual model VOC species were often larger and offsetting contributions were common. In general, total OH-reactivity was under-predicted in urban centres with the exception of the site in Calgary, Alberta. The higher alkene species, ALKE, was largely responsible for the model under-prediction at urban sites. Calgary, Alberta showed an over-prediction in the higher alkane species, ALKA. The observationally-derived OH-reactivities for aromatic compounds were in good agreement with the two model aromatic VOC species (AROM, TOLU). In rural areas in eastern Canada, total OH-reactivity was over-predicted largely due to over-predictions in the contribution from isoprene (ISOP). A sensitivity modelling run with a new Canadian vegetation database improved ISOP predictions in the rural east; however, systematic ISOP over-predictions were still apparent.

Regional modeling of organic aerosols over China in summertime

A comprehensive aerosol model was developed and incorporated into a regional air quality model system (RAQMS). It was used to study the behavior and the regional characteristics of carbonaceous aerosols over China in summertime with focus on organic carbon aerosols (OC), especially the secondary organic carbon aerosol (SOC). The whole month of July and August 2003 were selected as simulation period when biogenic volatile organic compounds (VOCs) emitted from vegetation reach the annual maximum. A number of measurements for organic and elemental carbon (EC) aerosols were conducted simultaneously over China during this period, which allow for a rigorous evaluation of model performance and an in‐depth investigation of organic aerosols. The comparison of model results with observations indicates that the developed model system is able to represent the atmospheric oxidative capacity and the major evolutionary features of OC and EC reasonably well, but a tendency was found toward underprediction of both OC and EC aerosols. Sensitivity simulations regarding absorbing matter for condensation (AMC) and enthalpy of vaporization (EOV) were conducted to understand the effects of key factors on secondary organic aerosol (SOA) formation and to provide possible ranges of SOA yields in the troposphere of East Asia. It is found that the overall SOA formation tends to increase with increasing EOV and AMC, but the sensitivity of SOA formation to EOV depends on ambient temperature as well. High OC concentrations mainly occurred in the areas along the Yangtze River, in central China and most parts of north China including Beijing and Tianjin districts, with the maximum monthly mean as high as 15 μg m−3. SOC concentration was high in the forest areas of southeastern, southwestern and northeastern China, with maximum concentration exceeding 5.0 μg m−3 to the south of the Yangtze Delta. The contribution of SOC to OC is around 50% in these areas, whereas in the broad areas north of the Yangtze River, primary organic carbon aerosol (POC) accounts for a high fraction of the total OC (>60%). The estimated regional burdens of EC, OC, POC, and SOC for base case are 28.13, 61.16, 46.05, and 15.11 Gg, respectively. Of the SOC burden, contributions from oxidation of anthropogenic aromatics, monoterpene and isoprene are 17% (2.58 Gg), 48% (7.32 Gg) and 35% (5.21 Gg), respectively. The regional SOC burden varies from 15.1 to 40.3 Gg in different scenarios. For the base case, the total burdens of EC, OC, POC and SOC are estimated to be 13.8, 37.3, 25.6, and 11.8 Gg, respectively, while using emissions solely from China (20°N∼50°N and west of 100°E) in model simulation.

Transport of airborne particulate matters originating from Mentougou, Beijing, China

In this study, a coupled regional air quality modeling system is applied to investigate the time spatial variations in airborne particulate matters (PM 10), originating from Mentougou to Beijing municipal area in the period of April 1–7, 2004, and the influences of complex terrain and meteorological conditions upon boundary layer structure and PM 10 concentration distributions. An intercomparison of the performance with CALPUFF against the observed data is presented and an examination of scatter plots is provided. The statistics show that the correlation coefficient and STD between the modeled and observed data are 0.86 and 0.03, respectively. Analysis of model results illustrates that the pollutants emitted from Mentougou can be transported to Beijing municipal area along certain transport pathways, and PM 10 concentration distributions show heterogeneity characteristics. Contributions of the Mentougou sources to the PM 10 concentrations in Beijing municipal area are up to 0.1–15 μg/m 3.

Synoptic-scale transport of ozone into Southern Ontario

This study focuses on synoptic-scale transport of ozone as it affects Southern Ontario. This process has been analyzed for the summer in 2001, as an example period of a frequent event that usually occurs during summer in this region. The work was carried out using the mesoscale modeling system generation 5 (MM5)/sparse matrix operator kernel emission modeling system (SMOKE)/community multiscale air quality (CMAQ) regional air quality modeling system, together with observational data from monitoring stations located throughout the modeling domain. Other different analyses have been carried out to supply more information apart from that obtained by the modeling system. A back-trajectory cluster methodology was used to evaluate the magnitude of the effects studied and an analysis of wind direction and cloud cover revealed a significant correlation with ozone concentration ( R 2=0.5–0.6). Synoptic sea-surface level pressure (SLP) patterns were also analyzed to examine other meteorological aspects. The contribution of natural background ozone to the total amount within the region was compared with that from synoptic-scale transport. The influence of emission of pollutants from selected areas on ozone concentrations in Southern Ontario was also analyzed. As relevant results of these analyses, the model predicts that background ozone is the largest contribution to the ground-level ozone concentration during days in which low values were recorded. However, when smog episodes occurred, the model predicts that around 60% of the ozone formed by anthropogenic emissions of pollutants is due to releases from nearby US states.

Seasonal Simulation of Tropospheric Ozone over the Midwestern and Northeastern United States: An Application of a Coupled Regional Climate and Air Quality Modeling System

Abstract The impacts of air pollution on the environment and human health could increase as a result of potential climate change. To assess such possible changes, model simulations of pollutant concentrations need to be performed at climatic (seasonal) rather than episodic (days) time scales, using future climate projections from a general circulation model. Such a modeling system was employed here, consisting of a regional climate model (RCM), an emissions model, and an air quality model. To assess overall model performance with one-way coupling, this system was used to simulate tropospheric ozone concentrations in the midwestern and northeastern United States for summer seasons between 1995 and 2000. The RCM meteorological conditions were driven by the National Centers for Environmental Prediction/Department of Energy global reanalysis (R-2) using the same procedure that integrates future climate model projections. Based on analyses for several urban and rural areas and regional domains, fairly good agreement with observations was found for the diurnal cycle and for several multiday periods of high ozone episodes. Even better agreement occurred between monthly and seasonal mean quantities of observed and model-simulated values. This is consistent with an RCM designed primarily to produce good simulations of monthly and seasonal mean statistics of weather systems.

A regional modelling study of the high ozone episode of June 2001 in southern Ontario

High ozone levels were observed in southern Ontario in the summer of 2001, particularly in June, when the observed maximum was 137 ppb at Long Point. Development of effective ozone abatement strategies to prevent such episodes requires acknowledge of the chemistry in the appropriate source regions. Comprehensive high-resolution Eulerian chemical transport models, when used with accurate emissions data and meteorology, can elucidate the atmospheric chemical and physical processes responsible for episodes like these. In this work, the MM5 /SMOKE/CMAQ regional air quality modelling system was used to investigate the chemistry involved in ozone formation during the episode in question and also more generally in the target domain. Some of the important simulations were further developed using Taylor diagrams to explore the ozone background and understand the sensitivity of ozone to NO X and VOC concentrations. Results from an arbitrary reduction of road traffic are discussed, based on NO X and VOC species in the traffic emission inventory. The ozone production rate was extracted from the model and mapped for June 2001 to assist in the identification of the source regions contributing to the ozone episode.

Cloud processing of gases and aerosols in a regional air quality model (AURAMS)

The representation of cloud chemistry and scavenging processes in a new multiple-pollutant (unified) regional air-quality modelling system, AURAMS, is described. Aqueous-phase chemistry and scavenging processes are coupled explicitly with microphysical fields from the meteorological driver model and the size- and chemical-composition-resolved aerosols in AURAMS. The impact of aqueous-phase oxidation on regional aerosols (primarily sulphate), both in terms of mass and size distribution, is examined based on model simulations of a 1-week period over eastern North America. It is shown that aqueous-phase oxidation contributes about 30% to 40% of the total atmospheric sulphate production in this case. Cloud chemistry is also shown to modify the aerosol size distribution, which in turn can either enhance or reduce aerosol scattering efficiency in different geographic regions depending on where on the aerosol size spectrum the mass is added. The study also indicates that precipitation evaporation can be an important process in terms of tracer redistribution in the vertical. Whether and how to treat tracer release from precipitation evaporation can have a significant impact on model predictions of near-surface ambient tracer concentrations and wet deposition fluxes. Comparison between observations and AURAMS predictions shows that modelling cloud processing of gas and aerosols depends critically on the meteorological driver model's ability to predict cloud microphysics fields. In this case, the model underpredicted precipitation amount for the study period, which contributed to the underestimation of wet deposition and in turn may also have impacted the modelled ambient tracer concentrations.

Regional Air Quality Modeling System (RAQMS) predictions of the tropospheric ozone budget over east Asia

The NASA Langley Research Center and University of Wisconsin Regional Air Quality Modeling System (RAQMS) is used to estimate the tropospheric ozone budget over east Asia during the NASA Global Tropospheric Experiment (GTE) Transport and Chemical Evolution over the Pacific (TRACE‐P) mission. The computed ozone budget explicitly accounts for stratosphere/troposphere exchange (STE) and in situ ozone production using on‐line chemical calculations. The east Asian O3 budget is computed during the period from 7 March to 12 April 2001. Gross formation dominates STE by a ratio of 7 to 1 in east Asia during TRACE‐P. However, this ratio is strongly influenced by altitude of the tropopause. Approximately 30% of the ozone that is advected across the tropopause over east Asia is subsequently advected out over the western Pacific within the upper 4 km of the troposphere by the Japan jet. The average net photochemical production (gross formation‐gross destruction) within the regional domain is 0.37 Tg d−1 or 7% of the average flux at the eastern boundary of the domain during the TRACE‐P time period. The budget analysis shows a very close balance between sources and sinks within the RAQMS regional domain during the TRACE‐P time period. This balance results in very small average accumulation (∼1 Tg) of O3 in the east Asian region and very little net export averaged over the period (0.03 Tg d−1). The low ozone export from east Asia predicted by RAQMS during TRACE‐P is a consequence of relatively high dry deposition rates, which are 37% of the gross ozone formation (1.469 Tg d−1) within the TRACE‐P regional domain.

A summer simulation of biogenic contributions to ground‐level ozone over the continental United States

The role of biogenic emissions in the production of ground‐level ozone has been the subject of considerable scientific investigation. However, because existing studies generally draw their conclusions from simulations of episodes lasting days to a week, there is a need to evaluate the biogenic impact over a relatively long timescale. Moreover, the magnitude of synergistic interaction between anthropogenic and biogenic emissions should be carefully quantified, and this issue is not accounted for in most previous investigations. In this study, we performed a summer seasonal (June to August 1995) model evaluation of surface ozone across the continental United States. A three‐dimensional regional climate, emissions, and air quality modeling system was used to do the simulations. The factor separation (FS) technique was applied to quantify the contributions from biogenic emissions alone and those from the synergy between anthropogenic and biogenic emissions. In the first step of this study, U.S. Environmental Protection Agency's National Emission Trends (NET) 1996 and 2020 “control case” raw anthropogenic emissions inventories were processed through Sparse Matrix Operator Kernel Emissions (SMOKE), an emissions model, to generate the speciated, gridded, and hourly emissions data needed for the air quality model. Next, six air quality simulations were carried out assuming zero emissions, biogenic only emissions, 1995 anthropogenic only emissions, biogenic plus 1995 anthropogenic emissions, 2020 anthropogenic only emissions, and biogenic plus 2020 anthropogenic emissions. The model results show that ground‐level ozone concentrations decrease moderately under the EPA's 2020 emissions scenario for many areas within the continental United States, with large reductions in vast areas of the eastern United States. They also show that the 1995 summer average “total biogenic contribution” to daily maximum surface ozone concentrations can reach 34 ppb. Biogenic emissions are associated with at least 20% of surface ozone concentrations for the most areas of the continental United States, with the peaks reaching more than 40% in California coastal areas, the southeastern states, and northeastern areas. A sizable portion of this “total biogenic contribution,” however, (up to 80% in some areas) is due to the synergy between anthropogenic and biogenic emissions and would thus be influenced by controls on anthropogenic source emissions.

Modelling gaseous dry deposition in AURAMS: a unified regional air-quality modelling system

An upgraded parameterization scheme for gaseous dry-deposition velocities has been developed for a new regional air-quality model with a 91-species gas-phase chemistry mechanism, of which 48 species are “transported” species. The well-known resistance analogy to dry deposition is adopted in the present scheme, with both O 3 and SO 2 taken as base species. Stomatal resistances are calculated for all dry-depositing species using a “sunlit/shaded big-leaf” canopy stomatal resistance submodel. Dry-ground, wet-ground, dry-cuticle, and wet-cuticle resistances for O 3 and SO 2, and parameters for calculating canopy stomatal resistance and aerodynamic resistance for these two base species are given as input parameters for each of the 15 land-use categories and/or five seasonal categories considered by the scheme. Dry-ground, wet-ground, dry-cuticle, and wet-cuticle resistances for the other 29 model species for which dry deposition is considered to be a significant process are scaled to the resistances of O 3 and SO 2 based on published measurements of their dry deposition and/or their aqueous solubility and oxidizing capacity. Mesophyll resistances are treated as dependent only on chemical species. Field experimental data have then been used to evaluate the scheme's performance for O 3 and SO 2. Example sets of modelled dry-deposition velocities have also been calculated for all 31 dry-deposited species and 15 land-use categories for different environmental conditions. This new scheme incorporates updated information on dry-deposition measurements and is able to predict deposition velocities for 31 gaseous species for different land-use types, seasons, and meteorological conditions.