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Workshop on Air Quality Data Analysis and Interpretation Ozone Formation Potential.

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Presentation on theme: "Workshop on Air Quality Data Analysis and Interpretation Ozone Formation Potential."— Presentation transcript:

1 Workshop on Air Quality Data Analysis and Interpretation Ozone Formation Potential

2 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.

3 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.)

4 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?

5 NMHC vs NOx – National Housing 10T 2000-01 The slope of this relationship is 9.3 ppbC/ppb.

6 Frequency Distribution of NMHC/NOx – National Housing 10T 2000-01 The mean ratio is 22.9 and the median is 17.2.

7 NMHC-NO x Discussion  The National Housing 10T results for 2000-01 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.

8 Ozone isopleths 120160 200 240 10/1 15/1

9 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.

10 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.

11 PAN Isopleths 0.25 0.501.0 2.0 4.0

12 HNO 3 Isopleths 510 2030

13 HCHO Isopleths 1510 20

14 OH Isopleths 0.050.080.10

15 HO 2 Isopleths 20 80 40 60

16 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.

17 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.

18 CARTER’S 1994 MAXIMUM INCREMENTAL REACTIVITY (MIR) VALUES FOR HYDROCARBON AND CARBONYL COMPOUNDS Species NameAIRS Nog Ozone/g Cmol Ozone/mol C Acetylene432060.50.14 Ethene432037.42.16 Ethane432020.250.08 Propene432059.42.75 Propane432040.480.15 i-Butane432141.210.37 1-Butene432808.92.6 n-Butane432121.020.31 t-2-Butene43216102.92 c-2-Butene43217102.92 3-methyl-1-butene432826.21.81 i-Pentane432211.380.41 1-Pentene432246.21.81 n-Pentane432201.040.31 Isoprene432439.12.58 t-2-pentene432268.82.57 c-2-pentene432278.82.57 2-methyl-2-butene432286.41.87 2,2-dimethylbutane432440.820.25 Cyclopentene432837.72.19 4-methyl-1-pentene432343.00.87 Cyclopentane432422.40.7

19 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-dimethylbutane432841.070.32 2-methylpentane432851.50.45 3-methylpentane432301.50.45 2-methyl-1-pentene432463.0c0.87 n-hexane432310.980.29 t-2-hexene432896.71.96 c-2-hexene432906.71.96 Methylcyclopentane432622.80.82 2,4-dimethylpentane432471.50.45 Benzene452010.420.11 Cyclohexane432481.280.37 2-methylhexane432631.080.32 2,3-dimethylpentane432911.310.39 3-methylhexane432491.40.42 2,2,4-trimethylpentane432500.930.28 n-Heptane432320.810.24 Methylcyclohexane432611.80.53 2,3,4-trimethylpentane432521.60.48 Toluene452022.70.74 2-methylheptane439600.960.29 3-methylheptane432530.990.29 n-Octane432330.60.18 Ethylbenzene452032.70.75

20 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-Xylenes451097.4d2.05 Styrene452202.20.60 n-nonane432350.540.16 Isopropylbenzene452102.20.6 n-Propylbenzene452092.10.58 1,3,5-trimethylbenzene4520710.12.81 1,2,4-trimethylbenzene452088.82.45 1,2,3-trimethylbenzene452258.92.6 o-Xylene452046.51.8 o-ethyltoluene452115.3c1.48 m-ethyltoluene452125.3c1.48 p-ethyltoluene452135.3c1.48 m-diethylbenzene452184.8c1.33 p-diethylbenzene452194.8c1.33 n-Decane432380.460.17 n-Undecane439540.420.12 Formaldehyde435027.24.5 Acetaldehyde435035.52.52 Acetone435510.560.23 Carbon Monoxide421010.0540.032 Methane432010.0150.005 BOLD Font indicates a reactivity greater than formaldehyde.

21 MIR vs OH Rate Constant

22 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.

23 Concentration and Reactivity

24 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.

25 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.

26 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.

27 Age of Air Mass from ratios of VOCs Sitebenzene toluene m&p-xylene benzene Meaning Bronx, NY0.281.55Fresh E.Hartford, CT0.391.40Fresh Stafford, CT0.670.56Aged Chicopee,MA0.221.59Fresh Lynn, MA0.401.53Fresh Cape Eliz,MA0.740.19aged Fresh Aged ~0.4 >0.4 ~1.5 <1.5

28 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


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