Photochemical Indicators of Ozone Sensitivity to Precursor Emissions
Gail Tonnesen, College of Engineering
Center for Environmental Research and Technology
University of California, Riverside
The need to determine whether VOC or NOx control strategies are more effective
for attaining ozone (O3) standards has led to a search for observational based methods
that uniquely characterize VOC sensitive versus NOx sensitive conditions. One such
approach is the method of photochemical indicators. These indicators are species or
combinations of species that have characteristic values for conditions in which O3 is
primarily sensitive to emissions reductions in either NOx or VOC. While ambient
measurements of indicator values may not be sufficient for determining the “best” control
strategy, they are useful for evaluating whether models accurately represent O3 sensitivity
to VOC and NOx, and thus they represent an important advance in model evaluation.
The theoretical basis for the usefulness of photochemical indicators has been
derived from analyses of the budgets of radical species termination (Sillman, 1995;
Kleinman et al., 1997), radical propagation efficiency (Tonnesen and Dennis, 2000a,b)
and from analyses of the variability of O3 production efficiency as a function of total NOy
availability (Johnson, 1994; Hess et al., 1992; Chang et al., 1997).
Indicators have been derived both for the sensitivity of peak O3 concentration
(Sillman, 1995, 2000; Chang et al., 1997; Tonnesen and Dennis, 2000b; Blanchard, 1999)
and for the sensitivity of the instantaneous rate of O3 or odd oxygen (Ox) production
(Tonnesen and Dennis, 2000a). The former are useful for investigating the cumulative
sensitivity of O3 production over the history of an air parcel trajectory, while the
instantaneous indicators are useful for evaluating local O3 and Ox production sensitivity
at a particular time and location. The combined use of both cumulative and local
indicators will provide the most rigorous approach for model validation.
There remains considerable uncertainty in the robustness of indicator values for
cumulative indicators of O3 concentration due to variability in heterogeneous chemistry
and deposition (Tonnesen and Dennis, 2000b). There is also uncertainty in the robustness
of indicator values for different geographical regions (Lu and Chang, 1998; Vogel et al.,
1999). Indicators of local O3 production sensitivity may be less susceptible to biases
caused by deposition and heterogeneous chemistry, yet considerable uncertainty exists in
the values of local indicators due to uncertainty in radical budgets (Stevens et al., 1997;
Crosley, 1997; Jacob, 1998; Carpenter et al., 1998).
Table 1 lists those indicators that have been proposed and evaluated in model
simulations. There is not yet a scientific consensus about the particular forms of
indicators that are most useful. For example, the extent parameter has recently been
applied in California (Blanchard et al., 1999), yet Sillman (2000) concluded that the
extent parameter is not useful. In addition, Sillman (1995) has proposed the ratio of
HCHO/NOy as an indicator of O3 concentration sensitivity and this indicator has been
widely used, yet Tonnesen and Dennis (2000b) found that HCHO/NOy performed poorly
and HCHO/NOx performed significantly better in model simulations in the eastern U.S.
To date, indicator values have only been evaluated in modeling studies – there has
been no empirical investigation of the particular values of the indicators that signify NOxsensitive or VOC-sensitive conditions. As a result, there remains considerable uncertainty
in using indicators to determine whether O3 is primarily NOx-sensitive or VOC-sensitive.
Although absolute O3 sensitivity of an air mass can not yet be accurately determined,
indicators can be used more confidently to estimate relative shifts in O3 sensitivity both
spatially and temporally. Comparisons of ambient and model simulated indicator ratios
have been demonstrated to be an important approach for performing diagnostic model
evaluations (Sillman et al., 1997, 1998). Due to the limited number of model studies and
lack of empirical validation, there remains considerable uncertainty as to which of the
indicators listed in Table 1a are most robust and most useful for evaluating model
scenarios.
Table 1a. Modeling Studies of Indicators of Cumulative O3 Concentration Sensitivity.
Reference:
Indicator ratios evaluated
Blanchard et al., 1999; Chang et al, 1997;
extent parameter
Milford et al., 1994; Vogel et al., 1999;
NOy
Sillman et al., 1997, 1998; Kleinman et al., 1997;
NOy, O3/NOy, HCHO/NOy, O3/HNO3, H2O2/HNO3
Lu and Chang, 1998;
O3/NOz, HCHO/NOy, H2O2/HNO3
Tonnesen and Dennis, 2000b;
O3/HNO3, HCHO/NO2, H2O2/(O3+NO2), O3/NOx,
H2O2/HNO3
Table 1b. Modeling Studies of Indicators of Local O3 or Ox Production Sensitivity.
Reference:
Indicator ratios evaluated
Tonnesen and Dennis, 2000a
O3 NOx
P(H2O2) P(HNO3)
HO2
kOH,iVOCi   kOHNO2+z kOH,iVOCi 
kHO2NO   kHO2 (HO2+RO2)+ kHO2O3+ kHO2NO 
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