Pollutant transport among California regions (original) (raw)
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2014
The performance of the Weather Research and Forecasting regional model with chemistry (WRF-73 Chem) in simulating the spatial and temporal variations in aerosol mass, composition, and size over 74 California is quantified using the extensive meteorological, trace gas, and aerosol measurements collected 75 during the California Nexus of Air Quality and Climate Experiment (CalNex) and the Carbonaceous 76 Aerosol and Radiative Effects Study (CARES) conducted during May and June of 2010. The overall 77 objective of the field campaigns was to obtain data needed to better understand processes that affect both 78 climate and air quality, including emission assessments, transport and chemical aging of aerosols, aerosol 79 radiative effects. Simulations were performed that examined the sensitivity of aerosol concentrations to 80 anthropogenic emissions and to long-range transport of aerosols into the domain obtained from a global 81 model. The configuration of WRF-Chem used in this study is shown to reproduce the overall synoptic 82 conditions, thermally-driven circulations, and boundary layer structure observed in region that controls 83 the transport and mixing of trace gases and aerosols. Reducing the default emissions inventory by 50% 84 led to an overall improvement in many simulated trace gases and black carbon aerosol at most sites and 85 along most aircraft flight paths; however, simulated organic aerosol was closer to observed when there 86 were no adjustments to the primary organic aerosol emissions. We found that sulfate was better 87 simulated over northern California whereas nitrate was better simulated over southern California. While 88 the overall spatial and temporal variability of aerosols and their precursors were simulated reasonably 89 well, we show cases where the local transport of some aerosol plumes were either too slow or too fast, 90 which adversely affects the statistics quantifying the differences between observed and simulated 91 quantities. Comparisons with lidar and in-situ measurements indicate that long-range transport of 92 aerosols from the global model was likely too high in the free troposphere even though their 93 concentrations were relatively low. This bias led to an over-prediction in aerosol optical depth by as 94 much as a factor of two that offset the under-predictions of boundary-layer extinction resulting primarily 95 from local emissions. Lowering the boundary conditions of aerosol concentrations by 50% greatly 96 reduced the bias in simulated aerosol optical depth for all regions of California. This study shows that 97 quantifying regional-scale variations in aerosol radiative forcing and determining the relative role of 98 emissions from local and distant sources is challenging during 'clean' conditions and that a wide array of 99 measurements are needed to ensure model predictions are correct for the right reasons. In this regard, the 100 combined CalNex and CARES datasets are an ideal testbed that can be used to evaluate aerosol models in 101 great detail and develop improved treatments for aerosol processes.
Atmospheric Chemistry and Physics, 2014
The performance of the Weather Research and Forecasting regional model with chemistry (WRF-Chem) in simulating the spatial and temporal variations in aerosol mass, composition, and size over California is quantified using the extensive meteorological, trace gas, and aerosol measurements collected during the California Nexus of Air Quality and Climate Experiment (CalNex) and the Carbonaceous Aerosol and Radiative Effects Study (CARES) conducted during May and June of 2010. The overall objective of the field campaigns was to obtain data needed to better under-stand processes that affect both climate and air quality, including emission assessments, transport and chemical aging of aerosols, aerosol radiative effects. Simulations were performed that examined the sensitivity of aerosol concentrations to anthropogenic emissions and to long-range transport of aerosols into the domain obtained from a global model. The configuration of WRF-Chem used in this study is shown to reproduce the overall synoptic conditions, thermally driven circulations, and boundary layer structure observed in region that controls the transport and mixing of Published by Copernicus Publications on behalf of the European Geosciences Union. Atmos. Chem. Phys., 14, 10013-10060, 2014 www.atmos-chem-phys.net/14/10013/2014/ J. D. Fast et al.: Modeling regional aerosol and aerosol precursor variability over California Atmos.
Atmospheric Chemistry and Physics, 2012
We describe the synoptic and regional-scale meteorological conditions that affected the transport and mixing of trace gases and aerosols in the vicinity of Sacramento, California during June 2010 when the Carbonaceous Aerosol and Radiative Effects Study (CARES) was conducted. The meteorological measurements collected by various instruments deployed during the campaign and the performance of the chemistry version of the Weather Research and Forecasting model (WRF-Chem) are both discussed. WRF-Chem was run daily during the campaign to forecast the spatial and temporal variation of carbon monoxide emitted from 20 anthropogenic source regions in California to guide aircraft sampling. The model is shown to reproduce the overall circulations and boundary-layer characteristics in the region, although errors in the upslope wind speed and boundary-layer depth contribute to differences in the observed and simulated carbon monoxide. Thermally-driven upslope flows that transported pollutants from Sacramento over the foothills of the Sierra Nevada occurred every afternoon, except during three periods when the passage of mid-tropospheric troughs disrupted the regional-scale flow patterns. The meteorological conditions after the passage of the third trough were the most favorable for photochemistry and likely formation of secondary organic aerosols. Meteorological measurements and model forecasts indicate that the Sacramento pollutant plume was likely transported over a downwind site that col-lected trace gas and aerosol measurements during 23 time periods; however, direct transport occurred during only eight of these periods. The model also showed that emissions from the San Francisco Bay area transported by intrusions of marine air contributed a large fraction of the carbon monoxide in the vicinity of Sacramento, suggesting that this source likely affects local chemistry. Contributions from other sources of pollutants, such as those in the Sacramento Valley and San Joaquin Valley, were relatively low. Aerosol layering in the free troposphere was observed during the morning by an airborne Lidar. WRF-Chem forecasts showed that mountain venting processes contributed to aged pollutants aloft in the valley atmosphere that are then entrained into the growing boundary layer the subsequent day.
Atmospheric Chemistry and Physics, 2010
Multi-scale tracer and full-chemistry simulations with the STEM atmospheric chemistry model are used to analyze the effects of transported background ozone (O 3 ) from the eastern Pacific on California air quality during the ARCTAS-CARB experiment conducted in June, 2008. Previous work has focused on the importance of long-range transport of O 3 to North America air quality in springtime. However during this summer experiment the longrange transport of O 3 is also shown to be important. Simulated and observed O 3 transport patterns from the coast to inland northern California are shown to vary based on meteorological conditions and the O 3 profiles over the oceans, which are strongly episodically affected by Asian inflows. Analysis of the correlations of O 3 at various altitudes above the coastal site at Trinidad Head and at a downwind surface site in northern California, show that under long-range transport events, high O 3 air-masses (O 3 >60 ppb) at altitudes between about 2 and 4 km can be transported inland and can Correspondence to: M. Huang (mhuang1@engineering.uiowa.edu) significantly influence surface O 3 20-30 h later. These results show the importance of characterizing the vertical structure of the lateral boundary conditions (LBC) needed in air quality simulations. The importance of the LBC on O 3 prediction during this period is further studied through a series of sensitivity studies using different forms of LBC. It is shown that the use of the LBC downscaled from RAQMS global model that assimilated MLS and OMI data improves the model performance. We also show that the predictions can be further improved through the use of LBC based on NASA DC-8 airborne observations during the ARCTAS-CARB experiment. These results indicate the need to develop observational strategies to provide information on the three-dimensional nature of pollutant distributions, in order to improve our capability to predict pollution levels and to better quantify the influence of these Asian inflows on the US west coast air quality.
Air Quality Modeling in the South Coast Air Basin of California: What Do the Numbers Really Mean?
Journal of the Air & Waste Management Association, 2006
This study evaluates air quality model sensitivity to input and to model components. Simulations are performed using the California Institute of Technology (CIT) airshed model. Results show the impacts on ozone (O 3 ) concentration in the South Coast Air Basin (SCAB) of California because of changes in: (1) input data, including meteorological conditions (temperature, UV radiation, mixing height, and wind speed), boundary conditions, and initial conditions (ICs); and (2) model components, including advection solver and chemical mechanism. O 3 concentrations are strongly affected by meteorological conditions and, in particular, by temperature. ICs also affect O 3 concentrations, especially in the first 2 days of simulation. On the other hand, boundary conditions do not significantly affect the absolute peak O 3 concentration, although they do affect concentrations near the inflow boundaries. Moreover, predicted O 3 concentrations are impacted considerably by the chemical mechanism. In addition, dispersion of pollutants is affected by the advection routine used to calculate its transport. Comparison among CIT, California Photochemical Grid Model (CALGRID), and Urban Airshed Model air quality models suggests that differences in O 3 predictions are mainly caused by the different chemical mechanisms used. Additionally, advection solvers contribute to the differences observed among model predictions. Uncertainty in predicted peak O 3 concentration suggests that air quality evaluation should not be based solely on this single value but also on trends predicted by air quality models using a number of chemical mechanisms and with an advection solver that is mass conservative.
Atmospheric Chemistry and Physics, 2015
In this study, WRF-Chem is utilized at high resolution (1.333 km grid spacing for the innermost domain) to investigate impacts of southern California anthropogenic emissions (SoCal) on Phoenix ground-level ozone concentrations ([O<sub>3</sub>]) for a pair of recent exceedance episodes. First, WRF-Chem control simulations, based on the US Environmental Protection Agency (EPA) 2005 National Emissions Inventories (NEI05), are conducted to evaluate model performance. Compared with surface observations of hourly ozone, CO,…
Atmospheric Chemistry and Physics, 2013
The impacts of transported background (TBG) pollutants on western US ozone (O 3 ) distributions in summer 2008 are studied using the multi-scale Sulfur Transport and dEposition Modeling system. Forward sensitivity simulations show that TBG contributes ∼30-35 ppb to the surface Monthly mean Daily maximum 8-h Average O 3 (MDA8) over Pacific Southwest (US Environmental Protection Agency (EPA) Region 9, including California, Nevada and Arizona) and Pacific Northwest (EPA Region 10, including Washington, Oregon and Idaho), and ∼10-17 ppmh to the secondary standard metric "W126 monthly index" over EPA Region 9 and ∼3-4 ppm-h over Region 10. The strongest TBG impacts on W126 occur over the grass/shrubcovered regions. Among TBG pollutants, O 3 is the major contributor to surface O 3 , while peroxyacetyl nitrate is the most important O 3 precursor species. W126 shows larger responses than MDA8 to perturbations in TBG and stronger non-linearity to the magnitude of perturbations. The TBG impacts on both metrics overall negatively correlate to model vertical resolution and positively correlate to the horizontal resolution.
For the first time, a ∼ decadal (9 years from 2000 to 2008) air quality model simulation with 4 km horizontal resolution over populated regions and daily time resolution has been conducted for California to provide air quality data for health effect studies. Model predictions are compared to measurements to evaluate the accuracy of the simulation with an emphasis on spatial and temporal variations that could be used in epidemiology studies. Better model performance is found at longer averaging times, suggesting that model results with averaging times ≥ 1 month should be the first to be considered in epidemiological studies. The UCD/CIT model predicts spatial and temporal variations in the concentrations of O 3 , PM 2.5 , elemental carbon (EC), organic carbon (OC), nitrate, and ammonium that meet standard modeling performance criteria when compared to monthly-averaged measurements. Predicted sulfate concentrations do not meet target performance metrics due to missing sulfur sources in the emissions. Predicted seasonal and annual variations of PM 2.5 , EC, OC, nitrate, and ammonium have mean fractional biases that meet the model performance criteria in 95, 100, 71, 73, and 92 % of the simulated months, respectively. The base data set provides an improvement for predicted population exposure to PM concentrations in California compared to exposures estimated by central site monitors operated 1 day out of every 3 days at a few urban locations. Uncertainties in the model predictions arise from several issues. Incomplete understanding of secondary organic aerosol formation mechanisms leads to OC bias in the model results in summertime but does not affect OC predictions in winter when concentrations are typically highest. The CO and NO (species dominated by mobile emissions) results reveal temporal and spatial uncertainties associated with the mobile emissions generated by the EMFAC 2007 model. The WRF model tends to overpredict wind speed during stagnation events, leading to underpredictions of high PM concentrations, usually in winter months. The WRF model also generally underpredicts relative humidity, resulting in less particulate nitrate formation, especially during winter months. These limitations must be recognized when using data in health studies. All model results included in the current manuscript can be downloaded free of charge at http: //faculty.engineering.ucdavis.edu/kleeman/.