Draft 

Precursor Effectiveness

In order to understand the relative effectiveness of reductions in different precursors

contributing to ambient PM2.5, staff of the Air Resources Board (ARB) and the South 

Coast Air Quality Management District (SCAQMD) conducted air quality modeling 

sensitivity runs using the modeling conducted for the attainment demonstrations. These 

relative effectiveness ratios were used to guide control strategy development as well as 

to normalize the benefits of multiple precursors so they can be reflected in terms of a 

single equivalent precursor. This method also provides a uniform metric for tracking 

progress relative to the attainment emissions targets. Documentation on the 

methodologies used for calculating the effectiveness ratios in each area is provided

below.

South Coast

A description of the methodology used by SCAQMD staff is provided in Appendix C of 

ARB’s staff report on the SCAQMD 2007 State Implementation Plan which can be found 

at:

http://www.arb.ca.gov/planning/sip/2007sip/southcoast/staffrepappc.pdf

San Joaquin Valley 

ARB staff conducted two grid-based modeling sensitivity runs where the NOx and 

primary PM2.5 emissions were reduced, one at a time, by 10 percent relative to the 

future-year modeled attainment scenario. From these sensitivity runs, staff calculated a 

future year modeled design value reflecting the further 10 percent emission reduction 

assumptions. The difference between the attainment scenario (before 10 percent

reduction in emissions) and adjusted (after 10 percent reduction in emissions) design 

values were then compared. Table 1 shows results for NOx and primary PM2.5 emission 

reductions as well as the efficacy of reducing primary PM2.5 relative to that for NOx in 

the Bakersfield area.

The first column of Table 1 lists the Bakersfield monitoring sites and the second column 

contains the 2006 design value (DV) for those sites. The third row contains the 

modeled future 2014 DV with all the planned emissions controls included in the 

simulations. The fourth and fifth columns list the adjusted design values with additional 

10 percent emissions reductions in the future year for NOx, and primary PM2.5, 

respectively. The differences between the numbers in these columns and the third

column were then divided by the tonnage of each precursor corresponding to a 10

percent domain-wide reduction in emissions. The results are displayed in the sixth

column as the effectiveness of controlling primary PM2.5 relative to that of controlling 

NOx. A number greater than unity in column six means that controlling the emissions of 

primary PM2.5 is more effective than controlling NOx. 
Draft 

To develop the ratio, staff focused on the response in the Bakersfield metropolitan area 

where the highest PM2.5 concentrations in the San Joaquin Valley occur. The 

responses at the three sites that operated in Bakersfield were then averaged. This 

analysis showed that controlling primary PM2.5 is approximately nine times more 

effective than controlling NOx. 

Table 1: The effectiveness of precursor controls on the 2014 design value.

Site Measured

2006 DV

(ug/m3) 

Modeled 

2014 DV

With

Controls

 (ug/m3) 2014 DV 2014 DV

Modeled

2014 DV

With Further 

10% NOx

Reduction

(ug/m3)

Modeled 

2014 DV

With Further 

10% PM2.5

Reduction

(ug/m3)

PM2.5 

Effectiveness 

Relative to 

NOx

Site 1

(Bakersfield-California 18.5 14.3 14.1 13.6 8.7

Site 2

(Bakersfield-Planz) 18.9 14.7 14.6 14.0 9.3

Site 3

(Bakersfield-Golden) 18.6 14.4 14.2 13.7 8.6