Evaluating Ambient Concentrations and Local Emissions of Greenhouse Gases (GHGs) in the San Francisco Bay Area of California Using a Comprehensive Fixed-site and Mobile Monitoring Network

The Bay Area Air Quality Management District (hence the Air District) is the greater San Francisco Bay metropolitan region's chief air quality regulatory agency. Aligning itself with Executive Order S-3-05, the Air District's Climate Protection Program has set a goal to reduce the region's GHG emissions by 80% below 1990 levels by the year 2050. In order to achieve this target, the Air District has adopted a 10-point Climate Action Work Program which lays out the agency's priorities, actions and coordination with regional stakeholders. The Program has three core objectives: (1) to develop a technical and monitoring program to document the region's GHG sources and related emissions, (2) to implement a policy and rule-based approach to control and regulate GHG emissions, and finally, (3) to utilize local governance, incentives and partnerships to encourage GHG emissions reductions.

In the first phase of the technical program, the Air District has set up a long term ambient GHG monitoring network at four sites, consistent with protocols of international atmospheric monitoring networks. The first site is located north and upwind of the urban core at Bodega Bay by the Pacific Coast. This site mostly receives clean marine inflow from the west-northwest and hence serves as the regional background site. The other three sites are strategically located at regional exit points for Bay Area plumes that presumably contain GHG enhancements from local sources. These stations are at San Martin, which is located south and frequently downwind of San Jose metropolitan area; at Patterson Pass, which is at the cross section of the eastern edge of the Bay Area with California's Central Valley; and at Bethel Island at the mouth of the Sacramento-San Joaquin Delta. At all sites, carbon dioxide (CO₂) and methane (CH₄) are being measured continuously, along with combustion tracer CO and other air pollutants. The GHG measurements are performed with high precision and fast laser instruments that are based on the Cavity Ringdown Spectroscopy (CRDS; Picarro Inc.) technique. In the longer term, the network will allow the Air District to monitor and study trends in ambient concentrations of GHGs and thus evaluate the effectiveness of its policy, regulation and enforcement efforts. We present data from the sites in their first few months of operation and demonstrate the efficacy and utility of this monitoring network.

We also present our progress on the design and fabrication of a dedicated mobile GHG measurement platform (a research van) equipped with state of the art analyzers capable of measuring isotopic methane (¹³C – CH₄), CH₄, CO₂ and also nitrous oxide (N₂O) in ambient air at fast temporal rates. There are a variety of local stationary GHG sources in the Bay Area like landfills, waste water treatment plants, dairies, oil refineries, natural gas cogeneration plants, gas pipelines etc. Ambient measurements of GHGs conducted upwind and downwind of such target facilities/sources can be combined with short-range inverse-dispersion methods and an atmospheric dispersion model is used to infer GHG emission rates specific to the target source. The isotopic information would aid in CH₄ source attribution. The source attribution will be strengthened by adding simultaneous measurements of more GHG source markers in the mobile laboratory like CO, NO_x (vehicle emissions), black carbon (incomplete combustion of fossil fuels) and potentially ethane (fugitive natural gas). The measured estimates of GHGs from local sources will allow verification and validation of the Air District's regional GHG emissions inventory for the Bay Area.