Air Quality and Railroads in Georgia
Georgia, particularly in and around the Atlanta metropolitan area, has both poor air quality (Figure 1) and a large, active railroad industry ( Figure 2 and Figure 3). Because the city of Atlanta originated at the intersection of railroads, the most densely populated counties occur where there are a large number of tracks and rail yards (Figure 4). While the use of railroads over trucking has a much lower overall impact on air quality when comparing equivalent tons of shipping (AAR, 2007), recent air quality and health evaluation projects indicate that railroad emissions are major contributors to both ozone and PM2.5 concentrations in polluted urban areas (EPA, 2006; STAPPA/ALAPCO, 2006; ED, 2006). Georgia is out of attainment with the federally mandated National Ambient Air Quality Standards (NAAQS) for both of these pollutants (Figure 1). In addition to nonattainment, significant adverse health impacts of both ozone and PM2.5 at levels below the current NAAQS (CASAC, 2006a,b, 2007) and in particular of PM produced by diesel combustion (Garshick et al., 2004; Jerrett et al., 2005; CARB, 2004,2006a; Zeldin et al., 2007) are being increasingly identified. Federal requirements such as those addressing PM Hotspots target some of these impacts, and additional restrictions on PM and ozone concentrations may be required of the states in the future.
Ozone. The formation processes for ozone are now well understood, and sources of NOx emissions, the limiting precursor in Georgia, are fairly well characterized. Attempts to control ozone in Georgia have been underway for a number of years and sources that have not previously been controlled beyond federal requirements now need to be considered for additional NOx reductions. Dispersed, ground level sources of NOx produce ozone most efficiently and rapidly (Ryerson, 2001; Bergin, 2007; EPD, 2007), so enhanced control of emissions from the large diesel engines used by railroads, particularly in and near ozone nonattainment areas, is a promising ozone reduction strategy.
PM2.5. A large fraction of the PM2.5 in Georgia (Figure 5 ) is well characterized (i.e. sulfate and nitrate formation from SO2 and NOx emissions) and control programs are underway to reduce these contributions. However, some components of PM2.5 such as secondary organic aerosols (a fraction of the Organic Carbon category in Figure 1) are still not well understood. Refined ambient measurement techniques are under development and formation processes and emissions sources are under study. The largest anthropogenic contributor to organic carbon is currently believed to be mobile sources, and diesel combustion is also known to emit a large fraction of the observed Elemental Carbon (i.e. primary carbon emissions). Source apportionment of measured PM2.5 indicates that diesel combustion is a large contributor to urban PM2.5 monitoring sites (Figure 6), and the clear historical dominance of measurements from a site near a large railyard (monitor Fire Station #8, Figure 7a and b) supports this finding and further indicates that rail yards contribute a significant fraction of this diesel component in the Atlanta PM2.5 nonattainment area.
Emissions controls in addition to those currently planned are required to meet the NAAQS for annual and daily PM2.5 and ozone. While recently proposed federal regulations for locomotives (EPA, 2006, 2007a) would help improve air quality in some areas if approved, the Atlanta area will not experience much improvement even by 2030, after more than 20 years of implementation ( Figure 8a and Figure 8b). Some states are attempting to reduce impacts from the railroad industry through both voluntary partnerships with railroad companies as well as through regulatory requirement, some with significant and some with limited success (CARB, 2006b; TCEQ, 2006).
In order to design an effective pollution reduction program, we are first evaluating how well the current emissions inventory represents emissions from railroad-associated activities, recognizing that diesel engine emissions are difficult to characterize and railroad operations are not consistently predictable. While specific emissions tests and speciation profiles are not yet available, the large scale of railroad operations and use of fleet-averaged emission factor estimates can give a close enough approximation for impact analysis and reduction planning purposes. To better understand railroad impacts on air quality in Georgia and to identify the potentially most beneficial and cost effective emission reduction options, we are improving the current modeling emissions inventory to the best of our ability for use in air quality impact analysis and preparing a more detailed inventory of major railroad and related equipment likely operating in Georgia.
The three primary goals for an improved railroad emissions inventory in air quality analysis are: (1) to use in photochemical modeling to evaluate source-specific contributions to ozone and PM2.5 formation and to improve the accuracy of model predictions, (2) to use in dispersion modeling to evaluate source-specific contributions to PM monitors, and (3) to quantify potential improvements to air quality from differing air quality management options and to meet federal regulatory requirements for air quality. All three goals are based on protecting public health and the environment. Additional benefits of an improved emissions inventory are gaining a better understanding of the impacts of diesel combustion on air quality and supporting ongoing epidemiological studies that attempt to distinguish impacts of different components of PM2.5.