Assessing the Ozone Formation Potential

Introduction
Assessment Techniques and Example Analyses

NMOC/NO_x and NMOC/NO_yRatios
- Examples of NMOC_NOx_analyses
- Analyses Along Transport Path

Reactivity of Identified Hydrocarbons
- Carters' 1994_Maxiumum Incremental Reaction...
- Examples of Reactivity Assessment
- Ten Most Abundant Hydrocarbon Species

Relative Age of Hydrocarbon Mixture
- Analysis Examples
- Assessing the Age of an Air Mass

Summary
References

• Photochemical interaction of VOC 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
• Reactivity of identified hydrocarbons
• Relative age of hydrocarbon mixture
• NO_y comparison to NO_x
• Observational-based modeling
• Biogenic contribution to NMHC
• Other methods: ozone and NO_y, other organic species relationships

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.

• 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 standard for the ratio - standard or local?
  • Time of day of the ratio?
  • Subtract out the background concentrations?

• Frequency distributions: all ratios, by time of day
• Scatter plots of NMOC and NO_x
• Spatial and temporal variations in ratios
• Analysis of the ratio as a function of time of day or along a trajectory
• Handling instrument detection limit and background concentrations

Frequency of LMOS surface NMOC/NO_x ratios in (a) all data and (b) 0700-0900 CDT data. Only ratios with NO_x concentrations > 8 ppb were included (Main and Roberts, 1993). NMOC = NMHC + formaldehyde and acetaldehyde.

NMOC/NO_x, and ozone and NO_x concentrations, for selected NMOC samples collected on June 26, 1991. Samples were selected along an estimated time/distance path similar to an estimated trajectory for a polluted air parcel which might have arrived at the location of the maximum ozone concentration. The extent of reaction was about one (indicating NO_x limitations) for those samples with * in the NMOC/NO_x column. (Roberts et al., 1995b). 

 NMOC = NMHC + formaldehyde and acetaldehyde

NMOC/NO_x, and ozone and NO_x concentrations, for all LMOS NMOC samples with ozone concentrations greater than 125 ppb. The mean NMOC/NO_x ratio is 10 and the median ratio is 9. The extent of reaction was about one (indicating NO_x limitations) for those samples with * in the NMOC/NO_x column. (Roberts et al., 1995b) 

 NMOC = NMHC + formaldehyde and acetaldehyde

• 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.

Bold indicates reactivity > formaldehyde.

^a Carter (1994). Note that the paper provides only units of g Ozone per g C.

^b Calculated from g Ozone/g C values:

Where MWo₃ = Molecular weight of Ozone (48 g/mol), MW_voc = molecular weight of the VOC, and #C_voc is the number of carbons in the VOC. 

^c Carter (1991). Note that the Carter (1994) reference did not include an updated value for this species.

 ^d Average of m-xylene and p-xylene values.

• Most abundant species evaluation using concentration and reactivity-weighted data: many less-abundant species based on concentration become important when reactivity is considered.
• Frequency distribution (histogram): assess temporal changes in the total reactivity.
• Summary statistics by time of day: assess change in total reactivity by time of day.
• Fingerprint plots: perform inter-site comparisons, assess changes in composition with time of day.
• Bar plots: assess spatial and temporal changes of the total reactivity, assess reactivity by species groups.

 Data Source: Level 1, AIRS data.
Bold indicates species on both concentration and reactivity-scaled abundance lists.

Chicago-Jardine, IL
June 1996

Concentration or Wt. Fraction	Reactivity-Scaled Data
Ethane	Ethene
i-Pentane	Propene
Propane	m&p-Xylenes
Toluene	Toluene
n-Butane	1-Butene
Ethene	i-Pentane
n-Pentane	o-Xylene
Acetylene	n-Butane
Benzene	m-Ethyltoluene
2-Methylpentane	n-Pentane

Data Source: Level 1, AIRS data.
Bold indicates species on both concentration and reactivity-scaled abundance lists.

Stafford, CT
June 1995

Concentration or Wt. Fraction	Reactivity-Scaled Data
Isoprene	Isoprene
Ethane	Ethene
i-Pentane	m&p-Xylenes
Toluene	Toluene
Propane	Propene
n-Butane	1,2,4-Trimethylbenzene
n-Pentane	i-Pentane
Benzene	o-Xylene
Acetylene	1,3,5-Trimethylbenzene
Ethene/m&p-Xylenes	Styrene/t-2-Butene

Data Source: Level 0, preliminary data.
Bold indicates species on both concentration and reactivity-scaled abundance lists.

East Hartford, CT
June 1995

Concentration or Wt. Fraction	Reactivity-Scaled Data
i-Pentane	m&p-xylenes
Toluene	1,2,4-trimethylbenzene
n-Butane	Propene
Propane	i-Pentane
m&p-xylenes	Isoprene
Ethane	Ethene
Acetylene	Toluene
n-Pentane	1-pentene
1-pentene	3-methyl-2-butene
Ethene/Isoprene	Cyclopentene/o-xylene

Data Source: Level 0, preliminary data, CT DEP.
Bold indicates species on both concentration and reactivity-scaled abundance lists.

Concentration or Wt. Fraction	Reactivity-Scaled Data
i-Pentane	m&p-xylenes
Toluene	Ethene
Propane	Propene
n-Butane	1,2,4-Trimethylbenzene
Ethene	Toluene
Ethane	o-Xylene
Acetylene	1,3,5-Trimethylbenzene
m&p-Xylenes	i-Pentane
n-Hexane	t-2-Butene
Benzene	t-2-Hexene

• VOC 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.

• Summary statistics
• Scatter plots
• Diurnal plots of ratios
• Nighttime vs. daytime ratios
• Comparison of ratios among sites

^a Average over 1600-1800 EST unless otherwise noted.
^b Average over 1500-1700 EST.
^c Shows evidence of local enrichment of toluene in comparison to other areas.

Data source: Level 0, preliminary data from CT DEP, MA DEP, NY DEC, and ME DEP

These analyses provide information which assist in the decisions on which species are the most important to ozone formation and begin to address the question of NO_x versus hydrocarbon emission controls.

California Air Resources Board (1994) California phase 2 reformulated gasoline news. RFG Forum, No. 1, December.

Carter W.P.L. (1991) Development of ozone reactivity scales for volatile organic compounds. Report prepared for the U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA-600/3-91-050.

Carter W.P.L. (1994) Development of ozone reactivity scales for volatile organic compounds. J. Air & Waste Manag. Assoc. 44, 881-899.

Carter W.P.L. (1995) Computer modeling of environmental chamber measurements of maximum incremental reactivities of volatile organic compounds. Atmos. Environ. 29, 2513-2527.

Carter W.P.L., Pierce J. A., Luo D., and Malkina I. L. (1995) Environmental chamber study of maximum incremental reactivities of volatile organic compounds. Atmos. Environ. 29, 2499-2511.

Grosjean E., Grosjean D., Fraser M.P., and Cass G.R. (1996) Air quality model evaluation data for organics. 2. C₁ - C₁₄ carbonyls in Los Angeles air. Environ. Sci. Technol. 30, 2687-2703. 

Harley R.A., Hannglan M.P., and Cass G.R. (1992) Respeciation of organic gas emissions and the detection of excess unburned gasoline in the atmosphere. Environ. Sci. Technol. 26, 2395-2408.

Kelly T.J., Ward G.F., and Satola J. (1995) A comparison of NO_y and conventional "NO_x" measurements at a rural site in Pennsylvania. Paper presented at the Air & Waste Management Association and U.S. Environmental Protection Agency Measurement of Toxic and Related Air Pollutants Conference, Research Triangle Park, NC, May 16-19.

Korc M.E. and Chinkin L.R. (1993) Improvement of the speciation profiles used in the development of the 1991 LMOS emission inventory. Draft final report prepared for the Lake Michigan Air Directors Consortium, Des Plaines, IL, by Sonoma Technology, Inc., Santa Rosa, CA, STI-92324-1394-DFR; December.

LADCO (1995) Lake Michigan Ozone Study. 1994 data analysis report, version 1.1. Report prepared by Lake Michigan Air Directors Consortium, Des Plaines, IL, May.

Lindsey C.G., Dye T.S., Main H.H., Korc M.E., Blumenthal D.L., Roberts P.T., Ray S.E., and Arthur M. (1995) Air quality and meteorological data analyses for the 1994 NARSTO-Northeast Air Quality Study. Draft final report prepared for Electric Power Research Institute, Palo Alto, CA by Sonoma Technology, Inc., Santa Rosa, CA, STI-94362-1511-DFR, July.

Lurmann F.W. and Main H.H. (1992) Analysis of the ambient VOC data collected in the Southern California Air Quality Study. Report prepared for the California Air Resources Board, Sacramento, CA by Sonoma Technology, Inc., Santa Rosa, CA, STI-99120-1161-FR, Contract No. A823-130, February.

Main H.H. and Roberts P.T. (1993) Validation and analysis of the Lake Michigan Ozone Study ambient VOC data. Draft final report prepared for the Lake Michigan Air Directors Consortium, Des Plaines, IL by Sonoma Technology, Inc., Santa Rosa, CA, STI-90217-1352-DFR, April.

Nelson P.F. and Quigley S.M. (1983) The m, p-xylenes: ethylbenzene ratio, a technique for estimating hydrocarbon age in ambient atmospheres. Atmos. Environ. 17, 659-662.

NESCAUM (1995) Preview of the 1994 ozone precursor concentrations in the northeastern U.S. 5/1/94 draft report prepared by the Ambient Monitoring and Assessment Committee of the Northeast States for Coordinated Air Use Management, Boston, MA.

Roberts P.T., Roth P.M., Blanchard C.L., Korc M.E., and Lurmann (1995b) Characteristics of VOC-limited and NO_x-limited areas within the Lake Michigan air quality region. Technical memorandum prepared for Lake Michigan Air Directors Consortium, Des Plaines, IL by Sonoma Technology, Inc., Santa Rosa, CA and Envair, Albany, CA, STI-92322-1504-TM, May.

Stoeckenius T.E., Ligocki M.P., Cohen B.L., Rosenbaum A.S., and Douglas S.G. (1994b) Recommendations for analysis of PAMS data. Final report prepared by Systems Applications International, San Rafael, CA, SYSAPP94-94/011r1, February.

Systems Applications International, Sonoma Technology Inc., Earth Tech, and Alpine Geophysics (1995) Gulf of Mexico Air Quality Study. Vol 1: Summary of data analysis and modeling. Final report prepared for U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA, OCS Study, MMS 95-0038.

Yarwood G., Gray H.A., Ligocki M.P., and Whitten G.Z. (1994) Evaluation of ambient species profiles, ambient versus modeled NMHC:NO_x and CO:NO_x ratios, and source receptor analyses. Final report prepared for U.S. Environmental Protection Agency, Office of Mobil Sources, Research Triangle Park, NC, by Systems Applications International, San Rafael, CA, SYSAPP94-94/081, September.