Introduction 1,3 butadiene projections 
Benzene projections for comparison with the NAQS objective
Table 3 shows a comparison of the number of urban major road links projected to exceed the NAQS objective for benzene of 5 ppb as a maximum running annual mean. We have taken a threshold for annual mean concentration of 3.5 ppb to represent a risk of exceeding a maximum running annual mean concentration of 5 ppb. This takes into account the difference between annual and maximum running annual means and also provides an approximate safety factor of 20 % for uncertainties in the mapping methods (Stedman and Dore, 1998).
Table 3. The number of urban major road links projected to exceed the NAQS objective for benzene (7508 road links examined).
|
1996 NAEI |
1997 NAEI |
|||
|
Year |
number of road links |
max. conc. (ppb) |
number of road links |
max. conc. (ppb) |
|
1996 |
571 |
7.7 |
571 |
7.7 |
|
1997 |
292 |
6.9 |
282 |
6.8 |
|
1998 |
129 |
6.1 |
140 |
6.1 |
|
1999 |
38 |
5.3 |
53 |
5.5 |
|
2000 |
15 |
4.6 |
15 |
4.5 |
|
2001 |
2 |
3.9 |
2 |
3.9 |
|
2002 |
0 |
3.3 |
0 |
3.5 |
|
2003 |
0 |
2.7 |
0 |
3.1 |
|
2004 |
0 |
2.3 |
0 |
2.7 |
|
2005 |
0 |
1.9 |
0 |
2.5 |
Projected concentrations in 2000 and 2001 are very similar for the 1996 NAEI and 1997 NAEI based emissions estimates. Projected concentrations in 2003 and 2005 are higher for the 1997 NAEI based estimates but are still well within the objective.
Introduction 1,3 butadiene projections