Weekend/Weekday Ozone Observations in the South Coast Air Basin

transported from an urban center to a downwind location of maximum ozone concentrations.
Multiple simulations are performed with varying initial concentrations of NOx and
anthropogenic VOC. The ozone ridge in the isopleth diagrams corresponds to the maximum O 3
concentration that can be achieved at a given VOC level. The VOC/NOx ratio at the ridgeline is
about 10 to 12. The HO radical chain length, which is the number of times a newly formed HO
radical is regenerated through radical chain propagation before it is destroyed, reaches a
maximum at this VOC/NOx ratio. Thus, the ridgeline corresponds to the VOC/NOx ratio at
which O 3 is most efficiently formed. Above the ridgeline, a reduction of NOx lowers the rate at
which HO and NO 2 are removed by formation of HNO 3 and leads to an increase in maximum O 3 .
This region is commonly described as “VOC-limited” and “VOC-sensitive” (i.e., lowering VOC
most effectively reduces O 3 ). “NOx-disbenefit” refers to a situation when NOx reduction leads
to an increase in ozone. This disbenefit occurs only in the VOC-limited region. Below the
ridgeline at low NOx concentrations, there is a large region where lowering NOx most
effectively reduces O 3 and large reductions in VOC have practically no effect on maximum O 3 .
This region is described as “NOx-limited”. Note that unlike VOC-limitation, “NOx-limited” is
not synonymous with “NOx-sensitive” because peak ozone concentrations are sensitive to NOx
concentrations both above and below the ridgeline. A decrease in NOx above the ridgeline
increases ozone while a decrease in NOx below the ridgeline decreases ozone.
In addition to the rate of ozone formation, the intensity and spatial extent of the weekend
(WE) effect also depend upon the degree of inhibition of ozone accumulation due to titration of
O 3 with NO. NO exists in excess of O 3 in the urban center overnight, and suppresses the
concentration of O 3 to zero or near zero in the surface layer. Fresh NO emissions during the
morning commute prolong the inhibition of ozone accumulation after sunrise. During this
inhibition period, the photolysis of carbonyl compounds and smaller contributions of nitrous acid
(HONO) and other radical precursors are the primary source of HO radicals until a sufficient
amount of NO has been converted to NO 2 . O 3 carried over aloft from the previous day can mix
down in the morning and contribute O 3 and radicals to the developing surface ozone chemistry.
Lower NOx emissions on weekends decrease NO titration of the O 3 newly formed at the surface
and the ozone transported from aloft. Accordingly, ozone formation begins earlier on weekends.
VOC/NOx ratios vary within air basins, resulting in either NOx-limited or VOC-limited
areas depending upon the time of day, the mix and type of emission sources, timing of additional
fresh emissions, and pattern of pollutant transport. The instantaneous VOC/NOx ratios tend to
increase during transport (absent injection of fresh emissions) because HO reacts more rapidly
with NO 2 than with VOCs. Thus, NOx is removed more rapidly from the system than VOCs. It is
possible that a pollutant mix that is initially VOC-limited will become NOx-limited in downwind
areas. Addition of dispersed NOx sources in downwind suburban areas may extend the area of
VOC limitation further downwind. An increase in the rate of ozone formation due to the increase
in VOC/NOx ratio during transport may be offset by dilution in the absence of fresh emissions.
1.4.2 Evolution of the Weekend Effect in the South Coast Air Basin
Southern California has historically experienced the most severe smog in the United
States. High ambient levels of ozone result from the combination of emissions from the second
largest urban area in the U.S., high mountains that restrain transport of air pollutants, and adverse
meteorology that limit atmospheric dispersion. Prior to the implementation of emission reduction
1-11