William P. L. Carter

Development of ozone reactivity scales for volatile organic compounds

Abstract This paper discusses methods for ranking photochemical ozone formation reactivities of volatile organic compounds (VOCs). Photochemical mechanisms for the atmospheric reactions of 118 VOCs were used to calculate their effects on ozone formation under various NOx conditions in model scenarios representing 39 different urban areas. Their effects on ozone were used to derive 18 different ozone reactivity scales, one of which is the Maximum Incremental Reactivity (MIR) scale used in the new California Low Emission Vehicle and Clean Fuel Regulations. These scales are based on three different methods for quantifying ozone impacts and on six different approaches for dealing with the dependencies of reactivity on NOx. The predictions of the scales are compared, the reasons for their similarities and differences are discussed, and the sensitivities of the scales to NOx and other scenario conditions are examined. Scales based on peak ozone levels were highly dependent on NOx, but those based on integrated ...

Aggregation and analysis of volatile organic compound emissions for regional modeling

Abstract A general two-step procedure for aggregating the hundreds of reported volatile organic compounds (VOCs) into a much smaller set of lumped classes appropriate for regional airshed modeling is described. In the first step, the compounds are condensed into a manageable number of emission categories which could be adapted to a variety of molecularly-based lumped chemical mechanisms. In the second step, the emissions are further aggregated into a smaller set of VOC classes which directly correspond to those in a particular models mechanism. The application of this procedure is illustrated by aggregating the National Acid Precipitation Assessment Program (NAPAP) anthropogenic VOC emissions inventory the U.S. first into the 32-class system, and then into the groups of model species used in the latest version of the Regional Acid Deposition Model (RADM2.0). The importance of different VOC categories and source types on regional pollution production is explored by comparing the contributions of each of the emissions groupings, RADM model species, and major emissions sources, to total moles carbon VOC reacted in model simulations. For this particular anthropogenic inventory and chemical mechanism, it is found that over 50% of the moles carbon reacted is associated with mobile sources. Such analysis can help indicate which uncertainties in anthropogenic emissions inventories may have the greatest impact on results of regional simulations.

Observations of nitrous acid in the Los Angeles atmosphere and implications for predictions of ozone-precursor relationships.

Direct measurements of nitrous acid (HONO) were made in downtown Los Angeles and Riverside, CA, during night and early morning hours of July/August 1980 using a long-path differential optical absorption spectrometer. Up to 8 ppb of HONO were observed in Los Angeles, approximately twice the maximum levels previously measured in Riverside during the summer of 1979. Possible sources of the observed HONO are discussed. If the observed HONO levels are included in initial NO, concentration, EKMA isopleth calculations predict that more rigorous control of NO, emissions (especially a t low HC/NO, levels) or of hydrocarbons emissions is necessary to reduce ozone maxima by a given amount compared with predictions based on calculations neglecting initial HONO. Moreover, including HONO in the starting NO, leads to predictions of accelerated rates of oxidant production which results in much larger predicted O3 doses at elevated O3 levels. For example, the predicted O3 dosage at levels above 0.3 ppm ozone in the case of NMHC = 1 ppm and [NO,], = 0.12 ppm is increased by over 250% when 10 ppb of HONO is taken to be initially present.

DEVELOPMENT AND EVALUATION OF A DETAILED MECHANISM FOR THE ATMOSPHERIC REACTIONS OF ISOPRENE AND NOX

A detailed photochemical mechanism for the atmospheric reactions of isoprene and its major oxidation products in the presence of NOx, which incorporates the most recent laboratory results and our current understanding of the system, is described. It is evaluated by comparing its predictions against results of NOx-air irradiations of isoprene and its two major products, methacrolein, and methyl vinyl ketone (MVK), in five different types of environmental chambers at two different laboratories. In most cases it simulated experimental results within the uncertainty of the data and the chamber and run characterization model. However, the photodecomposition quantum yields of methacrolein and MVK and the organic nitrate yield from the OH + isoprene reaction had to be adjusted to obtain satisfactory simulations of the data. The major discrepancy observed was that the model tended to underpredict PAN by ca. 40% in the isoprene experiments, despite the fact that the model predicted PAN from methacrolein and MVK reasonably well. The uncertainties and additional data needed to completely characterize the isoprene atmospheric photooxidation system are discussed.

Condensed atmospheric photooxidation mechanisms for isoprene

Abstract Two condensed mechanisms for the atmospheric reactions of isoprene, which differ in the number of species used to represent isoprenes reactive products, have been developed for use in ambient air quality modehng. They are based on a detailed isoprene mechanism that has recently been developed and extensively evaluated against environmental chamber data. The new condensed mechanisms give very close predictions to those of the detailed mechanism for ozone, OH radicals, nitric acid, H2O2, formaldehyde, total PANS, and for incremental effects of isoprene on ozone formation in one day simulations. The effects of the condensations become somewhat greater in multi-day simulations, particularly in cases where NO3 reactions are important at nighttime, but the ozone predictions are still very close. On the other hand, the SAPRC-90, RADM-2, and Carbon Bond IV isoprene mechanisms give quite different predictions of these quantities. It is recommended that the new mechanisms replace those currently used in airshed simulations where isoprene emissions are important.

Computer modeling of environmental chamber measurements of maximum incremental reactivities of volatile organic compounds

Abstract A detailed atmospheric photochemical mechanism which had been previously used in model calculations for developing ozone reactivity scales for volatile organic compounds (VOCs) was evaluated by comparing its predictions with measurements of incremental reactivities in an environmental chamber system. An updated version of this mechanism is also described and evaluated. The experiments consisted of determining the effects of adding representative alkanes, alkenes, aromatic hydrocarbons, aldehydes or CO on NO oxidation, ozone formation and radical levels in a simplified model photochemical smog system representing; conditions where ozone formation is most sensitive to VOCs. The published mechanism correctly simulated the observed qualitative reactivity trends, but overpredicted the effect of adding formaldehyde early in the experiments, performed poorly in simulating reactivities of branched alkanes, tended to underpredict the reactivities of alkenes, and did not simulate differences in reactivities of aromatic isomers. The updates to the mechanism improved the simulation results for the branched alkanes and the alkenes, but not for formaldehyde and the aromatics. The implications of these results concerning the development of atmospheric mechanisms for VOCs are discussed.

Hydroxyl radical rate constants and photolysis rates of .alpha.-dicarbonyls

38, 811. (37) Matthess, G.; Pekdeger, A. GWF, GasWasserfach: WasserlAbwasser 1980, 121, 214. (38) Davis, J. In “Contaminants and Sediments”: Baker, R. A,, Ed.; Ann Arbor Science: Ann Arbor, MI 1980; p 279. (39) Hansch, C.; Leo, A. In “Substituent Constants for Correlation Analysis in Chemistry and Biology”; Elsevier: Amsterdam, 1979. (40) Tute, M. S. Adv. Drug. Res. 1971, 6 , 1. (41) Mackay, D.; Bobra, A,; Shin, W. Y.; Yalkowsky, S. H.

.alpha.-Dicarbonyl yields from the NOx-air photooxidations of a series of aromatic hydrocarbons in air

The yields of the ring-cleavage products glyoxal, methylglyoxal and biacetyl from the reactions of OH radicals with benzene, toluene, o-, m- and p-xylene and 1,2,3-, 1,2,4- and 1,3,5-trimethylbenzene in the presence of part-per-million concentrations of NO have been determined in one atmosphere of air using in-situ long pathlength Fourier-transform infrared absorption spectroscopy and differential optical absorption spectroscopy with supplementary gas-chromatographic analyses. The yields of glyoxal and methylglyoxal were respectively: from toluene, 0.105 plus or minus 0.019 and 0.146 plus or minus 0.006; from o-xylene, 0.087 plus or minus 0.012 and 0.246 plus or minus 0.020; from m-xylene, 0.086 plus or minus 0.011 and 0.319 plus or minus 0.009; from p-xylene, 0.225 plus or minus 0.039 and 0.105 plus or minus 0.034; from 1,2,3-trimethylbenzene, 0.058 plus or minus 0.008 and 0.152 plus or minus 0.025; and from 1,2,4-trimethylbenzene, 0.048 plus or minus 0.005 and 0.357 plus or minus 0.017.

Alkyl nitrate formation from the atmospheric photoxidation of alkanes; a revised estimation method

The available experimental data concerning the yields of alkyl nitrates in the reactions of alkyl peroxy radicals with NO have been used to derive a revised expression for the estimation of alkyl nitrate yields in the atmospheric photooxidation of alkanes as a function of temperature and pressure. This revised expression gives more reasonable predictions of alkyl nitrate yields under high altitude tropospheric conditions than that which has been previously published.

Impact of an Updated Carbon Bond Mechanism on Predictions from the CMAQ Modeling System: Preliminary Assessment

Abstract An updated and expanded version of the Carbon Bond mechanism (CB05) has been incorporated into the Community Multiscale Air Quality (CMAQ) modeling system to more accurately simulate wintertime, pristine, and high-altitude situations. The CB05 mechanism has nearly 2 times the number of reactions relative to the previous version of the Carbon Bond mechanism (CB-IV). While the expansions do provide more detailed treatment of urban areas, most of the new reactions involve biogenics, toxics, and species potentially important to particulate formation and acid deposition. Model simulations were performed using the CB05 and the CB-IV mechanisms for the winter and summer of 2001. For winter with the CB05 mechanism, ozone, aerosol nitrate, and aerosol sulfate concentrations were within 1% of the results obtained with the CB-IV mechanism. Organic carbon concentrations were within 2% of the results obtained with the CB-IV mechanism. However, formaldehyde and hydrogen peroxide concentrations were lower by 25...