Weekend/Weekday Ozone Observations in the South Coast Air Basin

HO and HO 2 the contour plots were made for solar noon while all other plots were made for
sunset of the first simulated day.
1.4 Results and Discussion
The weekend ozone effect is rooted in ozone’s complex photochemistry, and arises from
the day-of-the-week differences in the temporal and spatial patterns of VOC and NOx emissions.
These emission-activity differences alter the diurnal and spatial variations in VOC and NOx
concentrations and VOC/NOx ratios, which affect the diurnal evolution of ozone chemistry.
Day-of-the-week differences in the concentrations of ozone were examined with respect to its
diurnal patterns, the diurnal patterns of NO, ratios of peak ozone to potential ozone, VOC
reactivity, photochemical aging, and NO 2 photolysis rates. The results of these analyses were
synthesized with the fundamental knowledge of ozone formation, theoretical analysis of ozone
and its precursors, and time-variations in the source contributions of VOC and NOx to develop a
self-consistent explanation for the weekend ozone effect.
1.4.1 Fundamentals of Ozone Formations
The chemistry of ozone formation is well established. Ozone is produced in the
atmosphere by the reaction of a ground state oxygen atom, O ( 3 P), and molecular oxygen (O 2 ).
While O 2 is abundant in the atmosphere, free oxygen atoms are not. At lower altitudes, where
only UV radiation with wavelengths greater than 280 nm is present, the only significant net
oxygen atom production is from photodissociation of NO 2 into NO and ground state oxygen
atoms, Reaction (1). The ground state oxygen atoms react with molecular oxygen to produce O 3 ,
Reaction (2), (where M is a third body such as N 2 or O 2 ).
NO 2 + hν → NO + O( 3 P) (1)
O( 3 P) + O 2 + M → O 3 + M (2)
When nitric oxide molecules are present, O 3 reacts rapidly with NO to regenerate NO 2 , Reaction
(3).
O 3 + NO → NO 2 + O 2 (3)
The first and third reactions occur rapidly, establishing a steady-state equilibrium ozone
concentration [O 3 ] that is determined by the "NO-photostationary state equation," Equation (4)
[O 3 ] = J 1[NO 2 ]
k 3 [NO]
(4)
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