Ra. Harley et al., UPDATED PHOTOCHEMICAL MODELING FOR CALIFORNIA SOUTH COAST AIR BASIN -COMPARISON OF CHEMICAL MECHANISMS AND MOTOR-VEHICLE EMISSION INVENTORIES, Environmental science & technology, 31(10), 1997, pp. 2829-2839
Large uncertainties remain in photochemical models used to relate emis
sions of VOC and NOx to ambient O-3 concentrations. Bias in motor vehi
cle emission estimates for VOC has been a long-standing concern. An im
proved Eulerian photochemical model is described and applied to the Au
gust 27-28, 1987, period in southern California. The chemical mechanis
m used in the model is SAPRC93, which predicts peak ozone concentratio
ns 22% higher on average than the LCC mechanism used previously. A rev
ised motor vehicle emission inventory was developed using gasoline sal
es and infrared remote sensing data for CO and measured ambient NMOC/C
O and NOx/CO ratios. On-road vehicle emissions for the South Coast Air
Basin in summer 1987 were estimated to be (1800 +/- 400) x 10(3) and
(710 +/- 160) x 10(3) kg day(-1) for NMOC and NOx, respectively. These
values are 2.4 and 1.0 times, respectively, the corresponding current
official inventory estimates (MVEI 7G). Ozone concentrations predicte
d using the CIT airshed model matched observations more closely when t
he revised inventory was used in place of official emission estimates.
If the vehicle fleet in 1987 were operating with no emission controls
, NMOC and NO, emissions would have been 5900 x 10(3) and 1200 x 10(3)
kg day(-1) respectively. On average, peak predicted ozone concentrati
ons for the controlled vehicle fleet operating in 1987 were 43% lower
than values predicted for the uncontrolled vehicle fleet. The peak pre
dicted ozone concentration with the uncontrolled vehicle fleet was 500
ppb.