# Presentation: 065482

## A Variable Averaging Period Scheme For Real-Time Mapping of PM2.5

- Lewis Weinstock
- Forsyth County Environmental Affairs Department
- Winston-Salem, North Carolina
- 336-727-8060
- weinstl1@co.forsyth.nc.us

## The National Ambient Air Quality Standard for PM2.5 requires averaging over a 24-hour period.
Continuous monitors have shown that short term levels of PM2.5 react quickly to changes in meteorology and source emissions.
The result is potential for a “discontinuity” between 24-hr PM2.5 based AQI readings and real time conditions that can be confusing to the public.

- Continuous monitors have shown that short term levels of PM2.5 react quickly to changes in meteorology and source emissions.
- The result is potential for a “discontinuity” between 24-hr PM2.5 based AQI readings and real time conditions that can be confusing to the public.

- Real Time Mapping of PM2.5
- _The Problem_

- Continuous PM2.5 monitor

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## When Is This a Problem?

**AQI Category**

- “Dead Zone”

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## Why Do We Care?

- AQI data systems will indicate conditions much worse (or better) than short term levels.
- The public may take unnecessary health precautions after high levels have passed.
- Web cam pictures and real time maps may conflict with the “look” of ambient conditions.
- Try explaining this to your TV meteorologist.

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## Shorten PM2.5 Averaging Period

- 3hr
- Response includes brief, local effects like mowing, instrument “burps”, and precipitation events.
- Rush hour mobile source peaks can dominate response.
- 6hr
- Tracks rapid air mass changes nicely.
- Still far from health based averaging period.
- 12hr
- Balances quicker response with closeness to NAAQS.
- Can lag rapid ambient changes.
- 24hr
- Only period consistent with health based research in the NAAQS.
- Badly lags even moderately paced ambient changes.

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## Proposal: Design a Variable Averaging Period Scheme for Mapping PM2.5

- Utilize the **RATIO** of running (6hr/24hr) PM2.5 averages to assess the rate of change of ambient concentrations and display a map more “in-synch” with real time.

- DMC Requirements
- Calculate running 6hr, 12 hr, 24hr averages  for each PM2.5 site on hourly basis.
- Calculate the (6hr/24hr) ratio “on the fly” each hour.
- Map the appropriate averaging interval for each monitor according to decision rule.

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## Decision Rule

| (6hr/24hr) ratio of PM2.5 | Mapping Period on animations | PM2.5 Levels |
| --- | --- | --- |
| 0.90 to 1.10 | 24 hour | Slow |
| 1.11 to 1.75
0.50 to 0.89 | 12 hour | Moderate increase
Moderate decrease |
| >1.75 
<0.50 | 6 hour | Rapid increase
Rapid decrease |

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## Example Data

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## The variable averaging scheme “anticipates” the true 24 hour

**7-15-02**

**7-17-02**

**7-18-02**

**7-16-02**

**7-19-02**

-  running average during rapid ambient changes to present
-  a more dynamic indicator of PM2.5 levels for mapping use.

**EST**

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## Animations courtesy of Alan Chan & Craig Anderson (Sonoma) and John White (EPA).

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## Variable Averaging Period Scheme

- Advantages
- More representative of real time PM2.5 levels.
- Maps and derived AQI may be truer to ambient conditions.
- Provides balance between very short averaging times and actual 24-hr NAAQS.

- Disadvantages
- Maps are combination of different averaging times for each monitor.
- Uses non-NAAQS averaging period.
- Health interpretations uncertain.
- Requires server resources for calculations.
- Too complex for public explanation?

- Questions?

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