Challenges and Issues with Reporting the AQI for PM 2.5 on a Real-time Basis David Conroy, EPA New England Anne McWilliams, EPA New England January 30, 2001

1.Issues with reporting raw hourly PM 2.5 data. 2.Issues with reporting the Air Quality Index (AQI) for PM 2.5 as the previously measured 24-hour average. 3.Issues with reporting unadjusted PM 2.5 data from TEOM continuous monitors. 4.Issues with reporting the PM 2.5 AQI for prior reporting periods. Issues with Reporting Real-time PM 2.5 Information Today, I will discuss:

Questions: Are hourly PM 2.5 concentrations alone understandable to the public? Can the public be expected to take appropriate actions to protect their health based on hourly PM 2.5 concentrations? Good ModerateUnhealthy for Sensitive GroupsReporting the Air Quality Index value for PM 2.5 ; the Level of Health Concern (e.g., Good, Moderate, Unhealthy for Sensitive Groups, etc.); and the associated Cautionary Statement are probably much more understandable. Reporting Only Hourly PM 2.5 Data on a Real-time Basis

Example Where AQI Doesn’t Change with Peaks in Hourly PM 2.5 Data If only hourly data is reported, how is the public supposed to know when curtailment of certain activities is recommended? Data not quality assured.

Example Where AQI Doesn’t Change with Peaks in Hourly PM 2.5 Data In this circumstance, the rolling 24-hour average never exceeds 40.4  g/m 3 and the AQI stays in moderate range throughout the period. Moderate Data not quality assured.

Example Where AQI Breaks Into Higher Category Data not quality assured.

Example Where AQI Breaks Into Higher Category In this circumstance, the rolling 24-hour average exceeds 40.4  g/m 3 and the AQI breaks into the Unhealthy for Sensitive Groups range for some duration. Unhealthy for Sensitive Groups Data not quality assured.

Reporting the AQI for PM 2.5 as the Previously Measured 24-hour Average When real-time PM 2.5 data is reported as the average for the previous 24-hour period, the reported AQI may may be out of sync with the hourly values currently being measured. For real-time reporting to be useful, it needs to be able to represent unhealthy air while it’s occurring; not after a high 24-hour average has occurred. Similarly, when hourly values drop dramatically due to a change in meteorology (e.g., a front came through), reporting the AQI as the previous 24-hour average may significantly overestimate current and near-future air quality levels.

Data not quality assured. Moderate In this example, the ending hour 24-hr average closely tracks the hourly values until a front comes through with a cleaner air mass.

Satellite Photograph of the Northeast from September 21, 2000 at 11:45 a.m.

Moderate Data not quality assured. After front comes through, hourly values decrease significantly, but ending- hour 24-hr averages remain in the moderate range.

* The above hourly data are not quality assured. Here is Hourly Data for a Recent 60 Hour Period in Boston Data not quality assured.

In the Winter in Boston, PM 2.5 Levels Related to Wind Speed In this circumstance, hourly PM 2.5 levels increase rapidly with calm conditions and peak during the morning rush hour. In such a circumstance, a timely warning to the public would be useful. Wind Speed (mi/hr) Data not quality assured.

Photograph of Boston Skyline taken at 10 a.m. on January 8, 2001 PM 2.5 concentration at this time in the  g/m 3 range Photograph from CAMNET web site (

Photograph of Boston Skyline taken at 10 a.m. on January 12, 2001 Photograph from CAMNET web site ( PM 2.5 concentration at this time in the 9-11  g/m 3 range

The AQI for this Site is Reported Based on Previous 24-hours Unhealthy for Sensitive Groups Moderate Moderate Here, real-time reporting is not responsive to increasing PM 2.5 levels and results in peak AQI levels being reported after hourly PM 2.5 levels have subsided.

Overall Air Quality:Today This WeekThis Month Specific Pollutants:Today This WeekThis Month Explanation of Terms Today's Air Pollution As of 5:00 PM 1/08/01 Text Only

Overall Air Quality:Today This WeekThis Month Specific Pollutants:Today This WeekThis Month Explanation of Terms Today's Air Pollution As of 8:00 AM 1/09/01 Text Only

Ideal Way for Reporting Would be by Mid-hour 24-hr Average Data not quality assured.

Ideal Way for Reporting Would be by Mid-hour 24-hr Average Unhealthy for Sensitive Groups Moderate Moderate The mid-hour 24 hour average will generally be in much better sync with the hourly data. Data not quality assured.

Reporting Mid Hour 24-hr Averages on a Real-time Basis Mid hour 24-hour averages cannot be reported on a real-time basis based on a full 24 hours of collected data. Mid hour 24-hour averages can only be reported 12 hours after the fact, which is no better than reporting ending hour 24-hour averages. However, mid hour 24-hour averages can be used as the standard to judge methodologies that attempt to predict the “current AQI” using a combination recently collected data (e.g., the most 12 hours) and other parameters.

Reporting Mid Hour 24-hr Averages on a Real-time Basis Using hourly PM 2.5 data from the CAMM monitor in Boston, we have investigated a number of methods that could be utilized to report the “current AQI.” For a recent 3 month period (Sept through Nov. 2000), we looked at the average hourly error between ending hour 24-hr averages and mid hour 24-hr averages. The average error during this period was approximately 25%. The method that we are promoting as having good potential for use in real-time AQI reporting has an average hourly error of approximately 15-16%.

Reporting Mid Hour 24-hr Averages on a Real-time Basis Initial methods investigated looked at the feasibility of just looking at recent periods of data (e.g., the last 4 hours, the last 8 hours) for averaging and converting those averages to the AQI scale. Although these methods proved to be very responsive to the current hourly data (i.e., the AQI rises and falls quickly with the hourly data), there is quite often not a good match to the mid hour rolling 24-hr average.

Here’s Where Just the Most Recent 6 Hours are Averaged A reoccurring problem with methods that rely solely on recent hourly data is that they always overestimate the AQI during peak hourly periods. Data not quality assured.

Here’s Where the Most Recent 12 Hours are Averaged Data not quality assured. Here again, the AQI is overestimated when the hourly PM 2.5 data peaks.

Reporting Mid Hour 24-hr Averages on a Real-time Basis Other methods we investigated looked at using the most recent period of data (e.g., the most recent 4 hour average) as a predictor of future values. In these methods, the most recent period of data is heavily weighted in the calculation of a “predicted” mid hour 24-hour average. This results in the AQI being responsive to the hourly data. When high hourly values occur, however, they should not be weighed as heavily in the “predicted” mid hour 24-hour average or you will over predict the AQI. Very high hourly values should be adjusted downward based on the correlation between daily peak hourly values and daily peak 24-hour averages.

The weighting of the last 4 hours is dependent on the magnitude of the hourly data (values over 30  g/m 3 treated differently) and the trend in the data. Here’s a Technique That Averages the Most Recent 12 Hours and a Weighted Value for the Last 4 Hours Data not quality assured.

Reporting the PM 2.5 AQI from an Unadjusted TEOM Monitor Appears to Significantly Underestimate Air Quality At the Roxbury site in Boston, a TEOM (Tapered Element Oscillating Microbalance) PM 2.5 monitor; a CAMM (Continuous Ambient Mass Monitor) PM 2.5 monitor; and a PM 2.5 monitor FRM are all co-located. Presently, the inlet to the TEOM is heated to 50 o C. The CAMM, on the other hand, samples at ambient temperature to minimize losses due to volatilization. Work done by George Allen, Harvard School of Public Heath, shows that the CAMM compares well with FRM-type measurements, and the TEOM does not. Thus, unadjusted TEOM values have the potential to underestimate PM 2.5 air quality.

Slide courtesy of George Allen, Harvard School of Public Health This graph shows that the TEOM data is consistently lower than the Harvard Impactor sampler, which has been shown to compare very well to the FRM.

Slide courtesy of George Allen, Harvard School of Public Health

This graph shows that the CAMM compares very well to the Harvard Impactor sampler, and appears to be adequately capturing ambient PM 2.5 for direct reporting purposes.

Slide courtesy of George Allen, Harvard School of Public Health A comparison of the CAMM versus TEOM illustrates that, day in and day out, the TEOM is recording much lower levels of ambient PM 2.5. In Boston, the CAMM monitor is exclusively used for AQI reporting purposes.

Slide courtesy of George Allen, Harvard School of Public Health This graph also illustrates that the TEOM monitor is recording much lower levels of ambient PM 2.5 than the CAMM monitor.

Unhealthy for Sensitive Groups Unhealthy for Sensitive Groups Only the AQI derived from the CAMM data breaks into the Unhealthy for Sensitive Groups category.

Reporting the PM 2.5 AQI for Prior Reporting Periods Question: What is the best way to report the PM 2.5 AQI for a prior reporting period (e.g., yesterday) using continuous PM 2.5 data. For example, should the AQI always be based on the midnight to midnight 24 hour average similar to what the FRM records? Alternatively, do you want to report the AQI based on the highest rolling 24-hour average during the reporting period to capture the worst 24 hour period? If reporting is done in this fashion, is it done based on beginning-hour, ending-hour or mid-hour 24-hour averages? Each will give you a different value for a given day.

Midnight to midnight AQI values

Peak AQI for Ending hour 24- hour average

Peak AQI for Beginning hour 24-hour average

Peak AQI for Mid hour 24-hour average

1.Reporting the AQI for PM 2.5, instead of hourly data alone, is probably most understandable to the public. 2.There are issues with reporting the AQI as the previously measured 24-hour average. Other techniques need to be developed for real-time reporting to be effective. 3.There are issues with reporting unadjusted PM 2.5 data from TEOMs. 4.There unresolved questions as to how to report the PM 2.5 AQI for prior reporting periods. Reporting based on peak mid hour 24-hour average is probably the best way to do it. Conclusions