Environmental Engineering Reference
In-Depth Information
downward trend in NO
x
emissions was not matched by ambient NO
x
trends at all
monitoring sites, the average ambient NO
x
decreases were approximately equal to
the inventory trends. For all NMOC sites and most CO monitoring sites, ambient
pollutant declines were as large as, or larger than, emission decreases reported in
the inventory.
While emissions declined overall, a plausible hypothesis is that ozone precursor
emissions and concentrations did not decline enough in some areas experiencing
substantial growth and development to produce a significant ozone response in
those areas. From 1990 to 2004, domain-wide NMOC, NO
x
, and CO emissions
declined by 33%, 29%, and 52%, respectively. In local (28 × 28 km) areas sur-
rounding ozone-monitoring sites, NMOC and NO
x
emissions exhibited declines of
approximately 10% to 40%. However, 15 of 44 sites exhibited local NO
x
reductions
less than 20% and eight sites showed local NMOC reductions less than 20%; six
sites showed local emission reductions less than 20% for both NMOC and NO
x
.
The geographical range of the six sites includes much of the northern and central
San Joaquin Valley; local NMOC emissions increased at three of the six locations.
In Fresno County, the emission inventory indicates that downward trends in NO
x
emissions from point sources and from gasoline-engine vehicles were partially
offset by upward trends in emissions from diesel-engine vehicles. The lack of
change in local emissions in the northern and central San Joquin Valley is a
potentially significant factor in the amount of local ozone formation. Ozone
decreases, if any, might be expected to result more from regional-scale emission
reductions than from the relatively modest local emission changes. Ambient
measurements showed no direct and simple correlation between local emission
changes and peak ozone trends, however.
Meteorological factors affect peak ozone concentrations and induce year-to-
year variations that result in “good” and “bad” ozone years, but do not account for
the observed ozone trends. Monitoring data from 1990 to 2004 do not show signi-
ficant trends in synoptic-scale meteorological conditions, average upper-air (850
mb) temperature, average daily-maximum surface temperature, or season-average
background ozone concentrations. Changes in background ozone, temperature, or
synoptic-scale meteorological patterns, if any, do not explain the observed spatial
variations in ozone trends. The ozone decreases were larger in the southern San
Francisco Bay Area, the Sierra Nevada foothills, and the northern San Joaquin
Valley than in other parts of central California.
The resistance of peak ozone to substantial (~10-50%) reductions in emissions
of ozone precursors is plausibly due to the complex, nonlinear relationship of
ozone formation to NMOC and NO
x
. The ambient monitoring data show that
changes occurred in ratios of NMOC/NO
x
, ratios of various NMOC species, and
in average NMOC reactivity from 1994 to 2004. The ambient decreases in total
NMOC and sums of species concentrations exceeded 50%. The predominant
change was a reduction in species and total NMOC concentrations, though the
average reactivity (with OH radical as represented by the k
OH
rate constants) also
declined at some sites post-2000. Mean morning NMOC/NO
x
molar ratios
declined from ~6:1-8:1 during 1994-1998 to ~4:1-6:1 during 2000-2004. The
