In central California, five different non-attainment areas for the 24-hr PM2.5 standard have recently been designated by the US EPA, including the San Francisco Bay Area (SFBA). As a result, the Bay Area Air Quality Management District (BAAQMD) is required to develop a State Implementation Plan (SIP) tentatively scheduled for 2013. The SIP development process will largely be guided by results obtained from current scientific studies. They include the analysis of ambient measurements, emissions inventory development and air quality modeling.
Nearly all SFBA exceedances of the daily NAAQS occur during the winter season (November-February). Episodes usually develop under: stable atmospheric conditions inhibiting vertical dispersion; clear and sunny skies favoring enhanced secondary PM2.5 formation; and pronounced overnight drainage (downslope) flows off the Central Valley rims, causing low-level air in the Central Valley to empty through the Delta and into the SFBA along its eastern boundary. Atmospheric transitions of aloft weather systems profoundly influence the surface winds that determine PM2.5 levels. Surface conditions stagnate when an aloft high pressure system moves over Central California. Persisting high pressure conditions allow PM2.5 to accumulate to the exceedance level in the SFBA, typically after 2-4 days.
This abstract is Part II of a two-part series presenting preliminary PM2.5 modeling. Part I (Rogers et al., 2010) describes meteorological modeling using the Weather Research and Forecasting (WRF) model. Part II describes the emissions inventory development and air quality modeling using the Community Multiscale Air Quality (CMAQ) model.
Central California is a large mountain-valley area with complex terrain, airflow features and air quality patterns. The air quality modeling domain is likewise large. In addition to the SFBA, the domain includes the entire Central Valley (CV) comprising the Sacramento Valley (SV) to the north and the San Joaquin Valley (SJV) to the south. Outlying areas within the modeling domain include areas over the Pacific Ocean, coastal locations along the Coast Range, and the inland Sierra Nevada.
The BAAQMD has conducted preliminary winter-season PM2.5 photochemical modeling. Dates 1 December through 31 January were simulated for 2000-01, for 62 total days. Meteorological fields were prepared using WRF. Emissions were prepared mostly from an annual inventory for year 2000 supplied by the California Air Resources Board (ARB). The SFBA portion of this inventory was replaced by the BAAQMD planning inventory that was augmented by Sonoma Technology, Inc. to reflect regional data. Emissions were adjusted for the winter season and respective years, the latter using ARB Almanac Emission Projection Data. They were gridded using the Models-3 SMOKE Modeling System. The meteorological fields and emissions inventory were used as inputs to the Models-3 Community Multiscale Air Quality (CMAQ) model. CMAQ was implemented using the SAPRC99 chemical mechanism, the Models-3 AE3 aerosol module, and the RADM aqueous-phase chemistry model. Initial and boundary conditions for CMAQ were obtained from specialized measurements obtained during the California Regional Particulate Air Quality Study (CRPAQS) conducted over 1999-2001. The horizontal grid resolutions of CMAQ and emissions inputs were 4 km, matching the grid resolution of the inner most domain of the WRF model. There were 20 vertical layers in CMAQ. The bottom layer was approximately 20 m thick.
"Base case" PM2.5 simulation results were evaluated against PM2.5 measurements. CMAQ usually adequately represented PM2.5 levels in the SFBA and Delta region. A 12-day episode during 2000-01, however, exhibited poor model performance. Simulated ammonia and nitric acid levels were in agreement with measurements.
Primary PM2.5 levels were elevated locally around PM sources. In San Francisco and San Jose, primary PM2.5 alone could trigger an exceedance. Secondary PM2.5, mostly ammonium nitrate, accumulated regionally. Secondary PM2.5 levels were very high in the CV and decreased westward through the SFBA. In the CV, secondary PM2.5 alone could trigger an exceedance. Ammonium nitrate PM2.5 buildup around the SFBA appeared limited by nitric acid, especially near San Francisco and San Jose.
"Sensitivity" PM2.5 simulations were performed by reducing SFBA anthropogenic emissions by 20% relative to the base case simulation for: NOx and VOC combined; gaseous sulfur species; ammonia; directly emitted PM; and these four classes combined (all anthropogenic emissions). Reducing the directly emitted PM reduced peak PM2.5 levels nearly ten times more efficiently than reducing precursor emissions. Reducing ammonia emissions was the most effective of the precursor emissions reductions. Ammonia emissions reductions were relatively ineffective around San Francisco and San Jose, where nitric acid was nearly completely depleted. NOx, VOC, and sulfur emissions reductions generally had minimal impacts but were occasionally significant.
"Transport" PM2.5 simulations were performed by zeroing out anthropogenic emissions within the SFBA. Transport impacts were highest through the Carquinez Strait and Altamont Pass which connect the SFBA with the CV. Transport impacts of secondary ammonium nitrate PM2.5 during SFBA episodic conditions averaged 13 µg/m3 along the SFBA eastern boundary.
Initial research has built on U.S. EPA and California ARB efforts. Preliminary results explored contributions of primary and secondary PM2.5; characterized secondary PM2.5 buildup pathways; estimated PM2.5 sensitivities to emissions reductions; and assessed transport impacts.
References: Rogers, R., Deng, A., Stauffer, D., Jia, Y., Soong, S.-T., Tanrikulu, S., Beaver, S., Tran, C., 2010. Fine Particulate Matter Modeling in Central California, Part I: Application of the Weather Research and Forecasting Model. Abstract submitted to AMS 2011 Annual Meeting, 13th Conference on Atmospheric Chemistry.