Workshop on Air Quality Data Analysis and Interpretation Ozone Formation Potential

Assessing reactivity of individual compounds is important!  Photochemical interaction of VOCs and NO x form ozone.  Each VOC reacts at a different rate and with different reaction mechanisms. Therefore, VOCs can differ significantly in their influence on ozone formation.  Recently, control strategies have encouraged the use of a "less-reactive" VOC to achieve ozone reductions.  Emission control strategies are developed based on an assessment of whether or not an area is "VOC- limited" or NO x -limited".  No single analysis should form the basis for decisions on control strategies; rather, several analyses should be performed to form a consensus.

NMOC/NO x AND NMOC/NO y RATIOS  The ratio of NMOC to NO x or NO y in the morning is an important parameter for photochemical systems. The ratio characterizes the efficiency of ozone formation in NMOC- NO x -air mixtures.  At low ratios (< 5 ppbC/ppb), ozone formation is slow and inefficient (hydrocarbon-limited). Decreasing NO x levels may result in increased ozone formation.  At high ratios (> 15 to 20 ppbC/ppb), ozone formation is limited by availability of NO x rather than NMOC (NO x - limited).  Ratios between 5 and 15 are considered transitional, and both NO x and NMOC controls may be effective.  If NO x -limited, generally NO x controls would be effective in decreasing ozone (and VOC controls would not be effective). If VOC-limited, VOC controls would be effective in decreasing ozone (and NO x controls would not.)

NMOC/NO x AND NMOC/NO y RATIOS (continued)  Ratios may change during transport of air parcels - consider the effects of controls on both nearby areas and areas far downwind.  When pollutant transport is a significant or dominant factor in high ambient concentrations at a site, precursor concentrations at upwind locations along the transport path need to be determined.  Identify ozone contributions from local precursor emissions, transported ozone formed in upwind locations, in-situ ozone production from transported upwind precursors.  What comprises NMOC and NO x in NMOC:NO x ? Methane or not? Biogenics (e.g., isoprene, terpenes)? Carbonyl compounds? Unidentified hydrocarbon mass? Adjusted NO x ? NO y ? Only NO x or NO y above a cut-off limit? Time of day of the ratio? Subtract out the background concentrations?

NMHC vs NOx – National Housing 10T The slope of this relationship is 9.3 ppbC/ppb.

Frequency Distribution of NMHC/NOx – National Housing 10T The mean ratio is 22.9 and the median is 17.2.

NMHC-NO x Discussion  The National Housing 10T results for have a questionable interpretation.  The slope = 9.3 would suggest ozone formation is in a transitional region.  The mean ratio = 22.9 and median ratio = 17.2 ppbC/ppb are sufficiently high to suggest ozone formation is in a NO x -limited region.  National Housing is probably not an appropriate location to do the evaluation. This site is somewhat distant from the source region, and probably not in the primary downwind direction.

Ozone isopleths /1 15/1

Ozone Isopleths  Show contours corresponding to the maximum ozone that can be produced in a one day photochemical period at varying initial concentrations of NMOC and NO x.  The straight lines show NMOC/NO x ratios of 5/1, 10/1 and 15/1.

EKMA (Empirical Kinetic Modeling Approach)  EKMA is a procedure for using O 3 isopleths to estimate the effects of controlling NMOC and/or NO x.  The maximum O 3 concentration reached in an area and the morning NMOC/NOx ratio are used to specify the design value for the area.  Relative changes in NMOC and/or NO x from the design value can be followed to estimate the change in the maximum O 3 with these trial controls.  This is a technique that can be used when one does not have data available to run a complete urban airshed model.

PAN Isopleths

HNO 3 Isopleths

HCHO Isopleths

OH Isopleths

HO 2 Isopleths

Other Isopleths  Provide estimates of the maximum concentrations of other modeled species that might be reached in the modeled area.  The starting hydrocarbon mix, temperatures, photolysis rates, etc. can be adapted to particular applications.  OZIPR – is the name of the package, which comes with the CB4 and RADM chemical mechanisms, but can be adapted to others, such as SAPRC.

Reactivity of hydrocarbons  Incremental reactivity may be used to assess effect of changing emissions of a given VOC on ozone formation.  Incremental reactivity is the change in ozone caused by adding a small amount of test VOC to the emission in an episode, divided by the amount of test VOC added: g ozone/g C or mols ozone/mol C  MIR scale was developed by W.P.L. Carter and used in "low emission vehicles and clean fuels" regulations in California.  Most useful in a relative rather than absolute manner.  Uncertainty associated with MIR scale values and the notion that total reactivity equals the sum of individual species incremental reactivities is unverified.  MIR scale values for >C4 aldehydes not yet available.  Need low unidentified fraction of total NMOC to best assess the potential reactivity of a hydrocarbon mixture.

CARTER’S 1994 MAXIMUM INCREMENTAL REACTIVITY (MIR) VALUES FOR HYDROCARBON AND CARBONYL COMPOUNDS Species NameAIRS Nog Ozone/g Cmol Ozone/mol C Acetylene Ethene Ethane Propene Propane i-Butane Butene n-Butane t-2-Butene c-2-Butene methyl-1-butene i-Pentane Pentene n-Pentane Isoprene t-2-pentene c-2-pentene methyl-2-butene ,2-dimethylbutane Cyclopentene methyl-1-pentene Cyclopentane

CARTER’S 1994 MAXIMUM INCREMENTAL REACTIVITY (MIR) VALUES FOR HYDROCARBON AND CARBONYL COMPOUNDS (continued) Species NameAIRS Nog Ozone/g Cmol Ozone/mol C 2,3-dimethylbutane methylpentane methylpentane methyl-1-pentene c0.87 n-hexane t-2-hexene c-2-hexene Methylcyclopentane ,4-dimethylpentane Benzene Cyclohexane methylhexane ,3-dimethylpentane methylhexane ,2,4-trimethylpentane n-Heptane Methylcyclohexane ,3,4-trimethylpentane Toluene methylheptane methylheptane n-Octane Ethylbenzene

CARTER’S 1994 MAXIMUM INCREMENTAL REACTIVITY (MIR) VALUES FOR HYDROCARBON AND CARBONYL COMPOUNDS (continued) Species NameAIRS Nog Ozone/g Cmol Ozone/mol C m&p-Xylenes d2.05 Styrene n-nonane Isopropylbenzene n-Propylbenzene ,3,5-trimethylbenzene ,2,4-trimethylbenzene ,2,3-trimethylbenzene o-Xylene o-ethyltoluene c1.48 m-ethyltoluene c1.48 p-ethyltoluene c1.48 m-diethylbenzene c1.33 p-diethylbenzene c1.33 n-Decane n-Undecane Formaldehyde Acetaldehyde Acetone Carbon Monoxide Methane BOLD Font indicates a reactivity greater than formaldehyde.

MIR vs OH Rate Constant

MIR contains more information than just OH reactivity  At low OH rate constants, the MIR and rate constant are approximately linearly related.  At high OH rate constants, the MIR seems to approach a maximum value. Increasing the rate constant does not affect MIR.  Formaldehyde has a much higher MIR than reflected by OH rate constant.

Concentration and Reactivity

Highest concentration does not necessarily most important on reactivity basis Pico Rivera, CA July-August 1995 ConcentrationReactivity-Scaled Data Propane1,3,5-Trimethylbenzene Toluenem&p-Xylenes i-Pentanem-Diethylbenzene n-UndecaneToluene m&p-XylenesEthene m-Diethylbenzeneo-Xylene EthanePropene n-Butanep-Diethylbenzene n-Nonanei-Pentane 1,3,5-Trimethylbenzeneo-Ethyltoluene Data Source: Level 1, AIRS data. Bold indicates species on both concentration and reactivity- scaled abundance lists.

Relative Age of the Hydrocarbon Mixture  VOCs may be used as indicators of ozone formation potential and tracers of urban emissions.  Relative abundance of more-reactive species (olefins, xylenes) should decrease with time during the day, while less-reactive species (paraffins, benzene) will appear to increase.  This may provide information about fresh sources of pollutants in an air mass.

Age and relative reactivity  Both toluene and m,p-xylene are more reactive than benzene.  When, the quantity of benzene in the sample relative to toluene or xylene increases, relative to fresh emissions, this is evidence of aging.  Following based on benzene/toluene = 0.4 for fresh emissions, increasing with age.  m,p-xylene/benzene = 1.5 for fresh emissions, decreasing with age.

Age of Air Mass from ratios of VOCs Sitebenzene toluene m&p-xylene benzene Meaning Bronx, NY Fresh E.Hartford, CT Fresh Stafford, CT Aged Chicopee,MA Fresh Lynn, MA Fresh Cape Eliz,MA aged Fresh Aged ~0.4 >0.4 ~1.5 <1.5

Age of Air Mass from ratios of VOCs Site – Denver,CObenzene toluene m&p-xylene Benzene Meaning Rose Hill0.32HighFresh South Adams0.65HighAged Swansea0.31HighFresh Regis0.42HighSl Aged Welby0.30HighFresh Auraria0.29HighFresh Aged ~0.4 >0.4 ~1.5 <1.5