Pairing aermod concentrations with the 50th percentile monitored value

PAIRING AERMOD CONCENTRATIONS WITH THE 50TH PERCENTILE MONITORED VALUEBackground Concentrations Workgroup for Air Dispersion ModelingMinnesota Pollution Control Agency

March 25, 2014

Sergio A. Guerra - Wenck Associates, Inc.

Roadmap

• Temporal pairing• Statistical description• Examples

AERMOD Model AccuracyAppendix W: 9.1.2 Studies of Model Accuracy a. A number of studies have been conducted to examine model accuracy,

particularly with respect to the reliability of short-term concentrations required for ambient standard and increment evaluations. The results of these studies are not surprising. Basically, they confirm what expert atmospheric scientists have said for some time: (1) Models are more reliable for estimating longer time-averaged concentrations than for estimating short-term concentrations at specific locations; and (2) the models are reasonably reliable in estimating the magnitude of highest concentrations occurring sometime, somewhere within an area. For example, errors in highest estimated concentrations of ± 10 to 40 percent are found to be typical, i.e., certainly well within the often quoted factor-of-two accuracy that has long been recognized for these models. However, estimates of concentrations that occur at a specific time and site, are poorly correlated with actually observed concentrations and are much less reliable.

• Bowne, N.E. and R.J. Londergan, 1983. Overview, Results, and Conclusions for the EPRI Plume Model Validation and Development Project: Plains Site. EPRI EA–3074. Electric Power Research Institute, Palo Alto, CA.

• Moore, G.E., T.E. Stoeckenius and D.A. Stewart, 1982. A Survey of Statistical Measures of Model Performance and Accuracy for Several Air Quality Models. Publication No. EPA–450/4–83–001. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

Monitored vs Modeled Data:Paired in time and space

AERMOD performance evaluation of three coal-fired electrical generating units in Southwest IndianaKali D. Frost Journal of the Air & Waste Management Association Vol. 64, Iss. 3, 2014

SO2 Concentrations Paired in Time & Space

Probability analyses of combining background concentrations with model-predicted concentrationsDouglas R. Murray, Michael B. Newman Journal of the Air & Waste Management Association Vol. 64, Iss. 3, 2014

SO2 Concentrations Paired in Time Only

Probability analyses of combining background concentrations with model-predicted concentrationsDouglas R. Murray, Michael B. Newman Journal of the Air & Waste Management Association Vol. 64, Iss. 3, 2014

Hit it Big!!! Example• You have 2 chances out of 100 to win the Powerball. Or

you have 98 chances out of a 100 of not winning the power ball.

• You have 2 chances out of 100 to win the Mega Millions. Or you have 98 chances out of a 100 of not winning the Mega Millions.

• What are the chances of winning both the Powerball andthe Mega Millions?

Marginal ProbabilityP(PB ∩ Mega) = P(PB) * P(Mega)

Where:P(PB ∩ Mega)= the marginal probability of winning the

PowerBall and at the same time winning the Mega.

P(PB) = the marginal probability of winning the Powerball (98th percentile).

P(Mega) = the marginal probability of winning the Mega (98th percentile).

Probability of Winning both LottosP(PB ∩ Mega) = P(PB) * P(Mega)

= (1-0.98) * (1-0.98)

= (0.02) * (0.02) = (1/50) * (1/50)

= 0.0004 = 1 / 2,500

= 99.96th percentile of the combined distribution

Combining 98th percentile Pre and Bkg (1-hr NO2 and 24-hr PM2.5)

P(Pre ∩ Bkg) = P(Pre) * P(Bkg)

= (1-0.98) * (1-0.98)

= (0.02) * (0.02)

= 0.0004 = 1 / 2,500


Combining 99th percentile Pre and Bkg (1-hr SO2)

P(Pre ∩ Bkg) = P(Pre) * P(Bkg)

= (1-0.99) * (1-0.99)

= (0.01) * (0.01)

= 0.0001 = 1 / 10,000


Proposed Approach to Combine Modeled and Monitored Concentrations• Combining the 98th (or 99th for 1-hr SO2) % monitored

concentration with the 98th % predicted concentration is too conservative.

• A more reasonable approach is to use a monitored value closer to the main distribution (i.e., the median).

Evaluation of the SO2 and NOX offset ratio method to account for secondary PM2.5 formationSergio A. Guerra, Shannon R. Olsen, Jared J. Anderson Journal of the Air & Waste Management Association Vol. 64, Iss. 3, 2014

Combining 98th Pre and 50th Bkg P(Pre ∩ Bkg) = P(Pre) * P(Bkg)

= (1-0.98) * (1-0.50)

= (0.02) * (0.50)

= 0.01 = 1 / 100

= 99th percentile of the combined distribution


Combining 99th Pre and 50th Bkg P(Pre ∩ Bkg) = P(Pre) * P(Bkg)

= (1-0.99) * (1-0.50)

= (0.01) * (0.50)

= 0.005 = 1 / 200



Positively Skewed Distribution

http://www.agilegeoscience.com

24-hr PM2.5 observations at Shakopee 2008-2010


Blaine ambient monitor location.

Histogram of 1-hour NO2 observations

Conclusion• Use of 50th % monitiored concentration is statistically conservative when pairing it with the 98th (or 99th) % predicted concentration

• Method is simple and statistically sound • Method is protective of the NAAQS while providing a reasonable level of conservatism

QUESTIONS…

Sergio A. Guerra, PhDEnvironmental EngineerPhone: (952) [email protected]

www.SergioAGuerra.com

Engineering

Pairing aermod concentrations with the 50th percentile monitored value