## Draft

Precursor Effectiveness

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

contributing to ambient PM 2.5 , staff of the Air Resources Board (ARB) and the South

Coast Air Quality Management District (SCAQMD) conducted ai r quality modeling

sensitivity runs using the modeling conducted for the attainment demonstration s

. 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 ea ch 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 PM 2.5 emissions were reduced , on e 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 sce nario (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 PM 2.5 emission

reductions as well as the efficacy of reducing primary PM 2.5 relative to t hat for NOx in

the Bakersfield area

.

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

contains the 200 6 design value (DV) for those sites. The third row contains the

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

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

10 percent emissions reductions in the future year for NOx, and primary PM 2.5 , respectively. The differences between the numbers in these columns and the third

column were then divide d 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 PM 2.5 relative to that of controlling

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

primary PM 2.5 is more effective than controlling NOx.

## Draft

T o develop the ratio, staff focused on the response i n the Bakersfield metropolitan area

where the highest PM 2.5 concentrations in the San Joa quin Valley occur. The

response s at the three sites that operated in Bakersfield were then a veraged. This

analysis showed that controlling primary PM 2.5 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

C ontrols

(ug/m3) 2014 DV 2014 DV

Modeled

2014 DV

With F urther

10% NOx

Reduction

(ug/m3) Modeled

2014 DV

With Further

10% PM 2.5 Reduction

(ug/m3) PM 2.5

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