VOC/NOx ratio

A second reason for the response of 03 to NOx at low VOC/NOx ratios is that at these high NOx concentrations, N02 competes with VOC for the OH radical by forming HN03 (reaction (13)). This terminates the chain oxidation of VOC and removes N02 from the system without forming 03. This chemistry has been confirmed by direct OH radical measurements thus, Eisele et al. (1997) report that OH concentrations increase with the NO concentration up to l-2 ppb but decrease thereafter due to the OH + N02 reaction. This is consistent with model calculations of isopleths of OH concentrations as a function of the VOC/NOx ratio, similar to the ozone isopleths in Fig. 16.14 (e.g., Kley, 1997) in the high NOx-low VOC... 

In short, the effectiveness of VOC versus NOx controls depends critically on the VOC/NOx ratio. 

While the results in Fig. 16.16 were developed for the Los Angeles area, the same general features have been found in a number of other studies as well (e.g., see National Research Council, 1991 and Tesche and McNally, 1991). The approach and issues are thus qualitatively applicable to urban/suburban areas worldwide where pollutants are transported downwind to less densely populated areas and the VOC/NOx ratio changes as the air mass is transported. 

TABLE 16.8 Typical Calculated Incremental Reactivities and Maximum Ozone as a Function of the VOC/NOx Ratio"... 

At a given level of VOC, there exists a NOx concentration at which a maximum amount of ozone is produced, an optimum VOC NOx ratio. For ratios less than this optimum ratio, NOx increases lead to ozone decreases conversely, for ratios larger than this optimum ratio, NOx increases lead to ozone increases. 

The reduction procedure was applied to three sets of initial conditions which were taken from environmental chamber experiments, 134P, 136P and 137P of Hess et al [6]. Without isoprene this gave initial VOC NOx ratios of 3.5, 8.7 and 15.6 [N C) / A (NOx). The concentrations of the species were simulated for a 48 hour period in which the temperature and solar flux were varied as for a typical diurnal cycle in July at a latitude of 50° N [7]. No further emission or deposition of species were included in the simulation. At each stage of the reduction the ozone profile calculated from the smaller mechanism was compared with that for the CBMEX profile. 

In Fig. 7.8, it took nearly 10 hours for O3 to reach its maximum, but it is generally known that the O3 formation rate is roughly proportional to VOC concentration (Akimoto and Sakamaki 1983). According to Eq. (7.52), the O3 formation rate P 0-i) is proportional to the concentration of HO2 and RO2, and the reaction of OH with RH, reaction (7.20), is the rate-determining step for the formation rate of the peroxy radicals. As deduced by the chamber experiment, OH concentration is nearly constant when the VOC/NOx ratio is higher than a certain value (VOC-excess or NOx-limited), so that the reaction rate of OH and VOC is nearly proportional to the VOC concentration (Akimoto et al. 1980). 

For the single VOC - NOx or VOC - CO - NOx experiments, the model is able to simulate the A([03]-[N0]) to within 25% or better in most cases, which is better than the 30% seen in previous mechanism evaluations with the older chamber data (Carter and Lurmann, 1990, 1991 Gery et al., 1989, Carter, 2000). However, there are indications of non-negligible biases in model simulations of certain classes of experiments. The cleaner conditions and the relatively lower magnitude of the chamber effects may make the nm-to-run scatter in the model performance may be less than in the simulations of the previous data, and this tends to make relatively small biases in the model performance more evident. There are, for example, definite biases in the model to under predict O3 formation and NO oxidation in the surrogate - NOx experiments carried out at lower ROG/NOx ratios. These cases are discussed further elsewhere (Carter, 2004b). 

The two large open circles on Figure 1 show the ROG and NOx levels chosen to serve as the base case in the incremental reactivity experiments that are currently underway in this chamber to assess ozone impacts of various different types of VOCs (Carter, 2004c). The 25-30 ppb NOx levels were chosen to be representative of pollution episodes of interest in California, based on input provided by the staff of the California Air Resources Board, which is funding most of the current reactivity studies. The ROG/NOx ratios were chosen to represent two sets of conditions of NOx availability relevant to VOC reactivity. The lower ROG/NOx ratio was chosen to represent the relatively higher NOx conditions of maximum incremental reactivity (MIR) where ozone is most sensitive to VOCs, to approximate the conditions used to derive the widely-used MIR ozone reactivity scale (Carter, 1994). The higher ROG/NOx ratio was chosen to be one-half that yielding maximum ozone levels, and is used to represent conditions where ozone is NOx-limited, but not so NOx-limited that VOC reactivity is irrelevant. These are referred to in the subsequent discussion as the MIR and MOIR/2 base eases, respeetively. 

Although the UCR EPA chamber has only been in operation for a relatively short time, it has already obtained usefiil information concerning the performance of current mechanism in predicting the effects of VOCs and NOx on ozone formation. As discussed here and also in our companion paper (Carter, 2004a), the SAPRC-99 mechanism predicts O3 formation reasonably well in low NOx experiments, and in ambient simulation experiments at the high ROG/NOx levels where maximum ozone formation potentials are achieved. However, die new data indicate problems with the mechanisms that were not previously realized. The SAPRC-99 mechanism consistently under predicts O3 formation in the lower ROG/NOx experiments where O3 formation is most sensitive to VOCs, and the problem is even worse for CB4. Other experiments indicate that there are problems with the formulation wifli die current aromatics photooxidation mechanisms. It is possible that the problems with the under prediction at low ROG/NOx ratios may be caused by problems with the aromatics mechanisms. Experimental and mechanism development work to investigate and hopefully resolve these problems is underway... 

Johnson Q., P. C. Nancarrow, A.Quintanar, and M. Azzi Smog chamber data for testing chemical mechanisms at low VOC to NOX ratio conditions. Final Report to EPA Cooperative Agreement CR-821420, Atmospheric Research and Exposure Assessment Laboratory, U.S. EPA. (1997)... 

Imagine starting with a given mixture of VOCs and NOx. Because OH reacts about 5.5 times more rapidly with NO2 than with VOCs, NOx tends to be removed from the system faster than VOCs.4 In the absence of fresh NOx emissions, as the system reacts, NOx is depleted more rapidly than VOCs, and the instantaneous VOC N02 ratio will increase with time. Eventually the concentration of NOx becomes sufficiently low as a result of the continual removal of NO by the 0H-N02 reaction that OH reacts preferentially with VOCs to keep the ozone-forming cycle going. At very low NOx concentrations, peroxy radical-peroxy radical reactions begin to become important. 

VOC-to-NOx ratios sufficiently low to retard ozone formation from an optimum ratio (represented in the upper left quadrant of Figure 17.4.1) can occur in central eities and in plumes immediately downwind of strong NO, sourees. Rural environments tend to be ehar-acterized by fairly high VOC-to-NO, ratios beeause of die relatively rapid removal of NO ... 

VOC-to-NOx ratios sufficiently low to retard ozone formation from an optimum ratio (represented in the upper left quadrant of Figure 16.3.1) can occur in central cities and in plumes immediately downwind of strong NO somces. Rural environments tend to be characterized by fairly high VOC-to-NOx ratios because of the relatively rapid removal of NOx from non-local sources as compared to that of VOCs, coupled with the usual absence of strong local NO sources and the presence of natural VOC sources. In such rural environments, the formation of ozone is limited more by the absence of emissions of NO, and most ozone present was directly transported from upwind. Indeed, in most of the troposphere, except in areas of strong NO sources, the availability of NO governs ozone production. 

Within the measurement eampaign, several smog experiments were carried out. Two reference experiments (base mixture I and II) were foeused on a VOC "only" mixture containing -butane, ethene and toluene in the presence of NO, representing the polluted troposphere. Both experiments differed in the initial NO/NO2 ratio, while the total amount of initial NOx was more or less similar (about 200 ppb). For each of the 5 diesel fuels investigated, a smog experiment was performed, where diesel exhaust was injected into the chamber up to a constant NOx level. After exhaust injection, about 1 ppm of the VOC mixture was added and the chamber was opened for irradiation. 

The agreement between the measured and calculated values of these radicals is, however, still a necessary but not a sufficient condition of the accuracy of the chemical reaction mechanism. As for these radicals, their concentrations are determined by the ratio of formation and loss rate, and the possibility cannot be excluded that if the error of formation and loss rates are in the similar magnitude, agreement is obtained when these factors are compensated. For further validation of the HOx chemical reaction processes in the atmosphere, a method of checking of the unknown loss processes of OH has been developed, which produces OH in pulse in the air and measures its time decay to compare with the decay rate calculated firom the simultaneously measured VOCs and NOx concentrations. This method is called the measurement of OH reactivity and it is an effective method for checking the OH loss process relating to the OH budget directly. 

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