ALBERT

Description: A recent version (4.6) of the Community Multiscale Air Quality (CMAQ) model was used as the basis for testing model revisions for including reactions involving chlorine (HCl, ClNO2) and reduced sulfur (dimethylsulfide, or DMS, and H2S) species not normally treated in the CB05 gas chemical mechanism and cloud chemistry module. Model chemistry revisions were based on published reaction kinetic data and a recent cloud chemistry model that includes heterogeneous reactions of organic sulfur compounds. Testing of the revised model was conducted using a recently enhanced data base of natural emissions that includes ocean and continental sources of DMS, H2S, chlorinated gases and lightning NOx for the continental United States and surrounding regions. Results using 2002 meteorology and emissions indicated that most simulated chemical and aerosol species exhibit the expected seasonal variations in grid-average surface concentrations. Ozone exhibits a winter and early spring maximum – reasonably consistent with ozone data and model results produced by others – in a pattern that reflects the influences of atmospheric dynamics and pollutant background levels imposed on the CMAQ simulation by boundary conditions derived from a global model. A series of experimental model simulations reveals that the addition of gas phase organic sulfur chemistry leads to sulfate aerosol increases over most of the continental United States. Modifications to the cloud chemistry module result in widespread decreases in SO2 across the modeling domain and a mix of sulfate increases and decreases. Most cloud-mediated sulfate increases occurred over the Pacific Ocean (up to about 0.1 μg m-3) and at slightly lesser amounts over and downwind from the Gulf of Mexico (including portions of the Eastern US). Variations in the chemical response are due to the link between DMS/H2S and their byproduct SO2, the heterogeneity of cloud cover and precipitation (precipitating clouds act as net sinks for SO2 and sulfate), and the persistence of cloud cover (the largest relative sulfate increases occurred over the persistently cloudy Gulf of Mexico and western Atlantic Ocean). Overall, the addition of organic sulfur chemistry increased surface hourly sulfate levels by as much as 1–2 μg m-3 in selected grid cells. The added chemistry produced significantly less sulfate in the vicinity of high SO2 emissions (e.g., wildfires), perhaps in response to lower OH from competing reactions with DMS and its derivatives. Simulated surface levels of DMS compare favorably with published observations made in the marine boundary layer. However, DMS derivatives are lower than observed implying either less chemical reactivity in the model or a low bias in the boundary conditions for DMS derivatives such as dimethylsulfoxide. The sensitivity of sulfate to cloud cover and the aqueous sulfate radical is also explored. This revised version of CMAQ provides a tool for more realistically evaluating the influence of natural emissions on air quality.

Description: The Community Multiscale Air Quality (CMAQ) model version 4.6 has been revised with regard to the representation of chlorine (HCl, ClNO2) and sulfur (dimethylsulfide, or DMS, and H2S), and evaluated against observations and earlier published models. Chemistry parameterizations were based on published reaction kinetic data and a recently developed cloud chemistry model that includes heterogeneous reactions of organic sulfur compounds. Evaluation of the revised model was conducted using a recently enhanced data base of natural emissions that includes ocean and continental sources of DMS, H2S, chlorinated gases and lightning NOx for the continental United States and surrounding regions. Results using 2002 meteorology and emissions indicated that most simulated "natural" (plus background) chemical and aerosol species exhibit the expected seasonal variations at the surface. Ozone exhibits a winter and early spring maximum consistent with ozone data and an earlier published model. Ozone distributions reflect the influences of atmospheric dynamics and pollutant background levels imposed on the CMAQ simulation by boundary conditions derived from a global model. A series of model experiments reveals that the consideration of gas-phase organic sulfur chemistry leads to sulfate aerosol increases over most of the continental United States. Cloud chemistry parameterization changes result in widespread decreases in SO2 across the modeling domain and both increases and decreases in sulfate. Most cloud-mediated sulfate increases occurred mainly over the Pacific Ocean (up to about 0.1 μg m−3) but also over and downwind from the Gulf of Mexico (including parts of the eastern US). Geographic variations in simulated SO2 and sulfate are due to the link between DMS/H2S and their byproduct SO2, the heterogeneity of cloud cover and precipitation (precipitating clouds act as net sinks for SO2 and sulfate), and the persistence of cloud cover (the largest relative sulfate increases occurred over the persistently cloudy Gulf of Mexico and western Atlantic Ocean). Overall, the addition of organic sulfur chemistry increased hourly surface sulfate levels by up to 1–2 μg m−3 but reduced sulfate levels in the vicinity of high SO2 emissions (e.g., wildfires). Simulated surface levels of DMS compare reasonably well with observations in the marine boundary layer where DMS oxidation product levels are lower than observed. This implies either a low bias in model oxidation rates of organic sulfur species or a low bias in the boundary conditions for DMS oxidation products. This revised version of CMAQ provides a tool for realistically simulating the influence of natural emissions on air quality.

Description: A natural emissions inventory for the continental United States and surrounding territories is needed in order to use the US Environmental Protection Agency Community Multiscale Air Quality (CMAQ) Model for simulating natural air quality. The CMAQ air modeling system (including the Sparse Matrix Operator Kernel Emissions (SMOKE) emissions processing system) currently estimates non-methane volatile organic compound (NMVOC) emissions from biogenic sources, nitrogen oxide (NOx) emissions from soils, ammonia from animals, several types of particulate and reactive gas emissions from fires, as well as sea salt emissions. However, there are several emission categories that are not commonly treated by the standard CMAQ Model system. Most notable among these are nitrogen oxide emissions from lightning, reduced sulfur emissions from oceans, geothermal features and other continental sources, windblown dust particulate, and reactive chlorine gas emissions linked with sea salt chloride. A review of past emissions modeling work and existing global emissions data bases provides information and data necessary for preparing a more complete natural emissions data base for CMAQ applications. A model-ready natural emissions data base is developed to complement the anthropogenic emissions inventory used by the VISTAS Regional Planning Organization in its work analyzing regional haze based on the year 2002. This new data base covers a modeling domain that includes the continental United States plus large portions of Canada, Mexico and surrounding oceans. Comparing July 2002 source data reveals that natural emissions account for 16% of total gaseous sulfur (sulfur dioxide, dimethylsulfide and hydrogen sulfide), 44% of total NOx, 80% of reactive carbonaceous gases (NMVOCs and carbon monoxide), 28% of ammonia, 96% of total chlorine (hydrochloric acid, nitryl chloride and sea salt chloride), and 84% of fine particles (i.e., those smaller than 2.5 μm in size) released into the atmosphere. The seasonality and relative importance of the various natural emissions categories are described.

Description: A natural emissions inventory for the continental United States and surrounding territories is needed in order to use the US Environmental Protection Agency Community Multiscale Air Quality (CMAQ) Model for simulating natural air quality. The CMAQ air modeling system (including the Sparse Matrix Operator Kernel Emissions (SMOKE) emissions processing system) currently estimates volatile organic compound (VOC) emissions from biogenic sources, nitrogen oxide (NOx) emissions from soils, ammonia from animals, several types of particulate and reactive gas emissions from fires, as well as windblown dust and sea salt emissions. However, there are several emission categories that are not commonly treated by the standard CMAQ Model system. Most notable among these are nitrogen oxide emissions from lightning, reduced sulfur emissions from oceans, geothermal features and other continental sources, and reactive chlorine gas emissions linked with sea salt chloride. A review of past emissions modeling work and existing global emissions data bases provides information and data necessary for preparing a more complete natural emissions data base for CMAQ applications. A model-ready natural emissions data base is developed to complement the anthropogenic emissions inventory used by the VISTAS Regional Planning Organization in its work analyzing regional haze based on the year 2002. This new data base covers a modeling domain that includes the continental United States plus large portions of Canada, Mexico and surrounding oceans. Comparing July 2002 source data reveals that natural emissions account for 16% of total gaseous sulfur (sulfur dioxide, dimethylsulfide and hydrogen sulfide), 44% of total NOx, 80% of reactive carbonaceous gases (VOCs and carbon monoxide), 28% of ammonia, 96% of total chlorine (hydrochloric acid, nitryl chloride and sea salt chloride), and 84% of fine particles (i.e., those smaller than 2.5 μm in size) released into the atmosphere. The seasonality and relative importance of the various natural emissions categories are described.

Description: Air quality measurements at Look Rock, Tennessee – on the western edge of the Great Smoky Mountains National Park – were begun in 1980 and expanded during the 1980s to a National Park Service (NPS) IMPROVE network station. Measurements were expanded again by the Tennessee Valley Authority (TVA, 1999–2007) to examine the effects of electric generating unit (EGU) emission reductions of SO2 and NOx on air quality at the station. Analysis of temporal trends (1999–2013) has been conducted at the site in collaboration with activities related to the 2013 Southeast Atmosphere Study (SAS) at Look Rock and other southeastern US locations. Key findings from these trend studies include the observation that primary pollutant levels have consistently tracked emissions reductions from EGUs and other primary sources in the region but reductions in secondary pollutants such as particulate sulfate and, specifically, ozone have been smaller compared to reductions in primary emissions. Organic carbonaceous material (OM) remains a major contributor (30–40% in the period 2009–2013) to fine particulate mass at the site, as confirmed by ACSM measurements at the site in 2013. A large portion (65–85%) of carbon in OM derives from modern carbon sources based on 14C measurements. Important parameters affecting ozone levels, fine mass and visibility also include the specific diurnal meteorology at this ridge-top site, its location in a predominantly mixed-deciduous forest, and the presence of primary sources of precursors at distances of 50–500 km from the site in all directions.

Description: Air quality measurements at Look Rock, Tennessee – on the western edge of the Great Smoky Mountains National Park – were begun in 1980 and expanded during the 1980s to a National Park Service (NPS) IMPROVE network station. Measurements were expanded again by the Tennessee Valley Authority (TVA, 1999–2007) to examine the effects of electric generating unit (EGU) emission reductions of SO2 and NOx on air quality at the station. Analysis of temporal trends (1999–2013) has been conducted at the site in collaboration with activities related to the 2013 Southeast Atmosphere Study (SAS) at Look Rock and other southeastern US locations. Key findings from these trend studies include the observation that primary pollutant levels have consistently tracked emission reductions from EGUs and other primary sources in the region, but reductions in secondary pollutants such as particulate sulfate and, specifically, ozone have been smaller compared to reductions in primary emissions. Organic carbonaceous material (OM) remains a major contributor (30–40 % in the period 2009–2013) to fine particulate mass at the site, as confirmed by ACSM measurements at the site in 2013. A large portion (65–85 %) of carbon in OM derives from modern carbon sources based on 14C measurements. Important parameters affecting ozone levels, fine mass, and visibility also include the specific diurnal meteorology at this ridge-top site, its location in a predominantly mixed-deciduous forest, and the presence of primary sources of precursors at distances of 50–500 km from the site in all directions.