Air quality forecasting in the philadelphia metropolitan area
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Air Quality Forecasting in the Philadelphia Metropolitan Area. Bill Ryan Department of Meteorology Penn State [email protected] National Weather Service, Mount Holly, April 28, 2010 What Do We Forecast?. Ozone (O 3 ) – since 1996

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Air quality forecasting in the philadelphia metropolitan area

Air Quality Forecasting in thePhiladelphia Metropolitan Area

Bill Ryan

Department of Meteorology

Penn State

[email protected]

National Weather Service, Mount Holly, April 28, 2010

What do we forecast

What Do We Forecast?

  • Ozone (O3) – since 1996

    • Photochemical pollutant, requires hydrocarbons, oxides of nitrogen (NOx) and UV sunlight.

    • Season: April-October, peak in June-August.

  • Fine Particles (PM2.5) – since 2003

    • Many sources, primarily products of combustion, many formation mechanisms.

    • Year round pollutant, peaks in summer season (sulfate) with a secondary peak in deep winter.

Why do we forecast

Why Do We Forecast?

  • O3

    • At high concentrations, causes “sunburn” to lungs

    • Most affected:

      • Children: They process a lot of air, affect growth of lungs and therefore has lifelong impact on lung capacity.

      • Elderly and anyone with a compromised respiratory system. Ozone amplifies response to other allergens.

  • PM2.5

    • Reduces lung function similar to O3 but particles can also cross the boundary from the lungs into the bloodstream and cause cardio-pulmonary problems.

Who uses our forecasts

Who Uses Our Forecasts?

  • Forecasts are directed to two purposes:

    • Protect public health

      • Issue watches and warnings well in advance so citizens can plan their activities.

    • Reduce emissions of O3 and PM2.5 “precursors”:

      • “Air Quality Action Day”

      • Voluntary program of large employers to reduce emissions across the region on forecast bad air days.

  • The lag time required between warnings and action is why forecasts are issued ~ 2-3 pm daily and valid the following day (12-36 h forecast).

Where do we forecast

Where Do We Forecast?

O3 monitors shown in black


PM2.5 monitors generally

in same locations but about

half the density.

Air quality alerts

Air Quality Alerts

Beginning last summer, Air

Quality Alerts were posted to the

NWS forecast page.

The Philadelphia metropolitan

area, as far as the EPA is

concerned, includes portions

of three states.

This poses a challenge for forecast

coordination as the states are

ultimately responsible for meeting

clean air standards.

Warning are coordinated between

neighboring states (PA, NJ, DE).

What are the forecast metrics official

What Are the Forecast Metrics (Official)?

  • Units of Measure

    • O3: parts per billion by volume (ppbv)

    • PM2.5: micrograms per cubic meter (µgm-3)

  • Warning Criteria

    • Pegged to National Ambient Air Quality Standard (NAAQS), standard re-evaluated every 5 years.

    • O3: 8-hour running average ≥ 76 ppbv. Currently under review.

    • PM2.5: 24-hour (midnight-midnight) average ≥ 35 µgm-3.

How are metrics reported to public

How Are Metrics Reported to Public?

  • Air Quality Index (AQI)

    • Dimensionless parameter where > 100 is the Warning Criteria level.

    • AQI for any given day is the highest of either O3 or PM2.5 AQI.

  • Color Codes

    • Green (“Good”, 0-50 AQI), Yellow (“Moderate”, 51-100 AQI)

    • Orange (“Unhealthy for Sensitive Groups”, 101-150 AQI), similar to Watch. Air Quality Alert issued.

    • Red (“Unhealthy”, > 151 AQI), similar to Warning.

Climatology of color codes for o 3 in phl summer season jja

Climatology of Color Codes for O3 in PHL: Summer Season (JJA)

Ozone climatology (?) is never static. For example, significant regional scale

controls of power plant NOx emissions were introduced In the 2002-2003 time period.

As result, we have observed a “step down” in O3 to cleaner levels since 2003.

Changes in o 3 climatology frequency of severe o 3 cases in phl

Changes in O3 Climatology:Frequency of Severe O3 Cases in PHL

Code Red

2009 is not blank, there just weren’t any Code Red cases!

Climatology of color codes for o 3 in phl

Climatology of Color Codes for O3 in PHL

Year to year variations in O3 respond to weather and non-weather factors.

For example, 2009 was cool and wet, but the recession

further reduced O3 precursor emissions of hydrocarbons and NOx.

Phl pm 2 5 concentrations remarkably low in 2009 function of lower energy usage

PHL PM2.5 Concentrations Remarkably Low in 2009:Function of Lower Energy Usage

Note: Data for 2004-2008 uses gravimetric filter monitors (FRM) while 2009

uses 24-average from continuous monitors.

What are our forecast tools

What Are Our Forecast Tools?

  • Persistence

    • Both O3 and PM2.5, and some of their precursors, have long lifetimes in the troposphere (on order of days to week)

    • As a result, local air quality is a combination of local and transported pollutants.

    • What is the direction of transport and the concentrations of pollutants being transported?

  • Weather Parameters

    • O3 is associated with high temperature, few clouds.

    • PM2.5 has many sources and formation routes, therefore, no simple weather associations. In summer, humidity increasessulfate load.

What are our forecast tools1

What Are Our Forecast Tools?

  • Statistical Models

    • Useful for O3, cost-effective though limited because need long training period.

    • Not useful for PM2.5.

  • Numerical Models

    • NOAA-EPA Operational O3 model in use since 2005 ( Several other models, see Appendix A.

    • PM2.5 model in developmental stage at NOAA. Other PM2.5 models are available but skill limited.

What is our forecast skill

What is Our Forecast Skill?

  • For O3:

    • Median Absolute Error: 6.2 ppbv, or ± 10%

    • Bias: +1.7 ppbv

  • For PM2.5:

    • Median Absolute Error:

      • Summer Season: 4.0 µgm-3, or ± 20%

      • Overall: 3.5 µgm-3

    • Bias

      • Summer Season: + 0.4 µgm-3

      • Overall: : + 0.4 µgm-3

Forecast skill for air quality alerts may september 2004 2009

Forecast Skill for Air Quality Alerts (May-September, 2004-2009)

  • Standard Measures

    • Hit Rate: 74%

    • False Alarm: 31%

    • Accuracy: 87%

  • Skill Measures:

    • Threat Score: 0.56, Range: [0,1]

    • Heidke Score: 0.63, Range: [-1,1]

  • Skill Score with Reference to Persistence:

    • Improvements of 40-53% depending on measures selected for comparison.

Forecast skill for air quality alerts may september 2004 20091

Forecast Skill for Air Quality Alerts (May-September, 2004-2009)

  • Forecast skill primarily driven by good O3 forecasts. PM2.5 forecasts are less skillful:

    • Hit Rate for PM2.5 is only 43%, but 82% of the “missed” PM2.5 cases were covered by an Air Quality Alert for O3 already in place.

    • Frequency of Code Orange PM2.5 cases in warm season is 2.3 times less than Code Orange O3.

    • Worth noting: Most bad PM2.5 days are also bad O3 days (84%), but bad O3 days are not usually bad PM2.5 days (23%).

What does a poor air quality day look like

What Does a Poor Air Quality Day Look Like?

  • O3

    • Full sun, long day length (high SZA)

    • Light winds, transport aloft (0.5-2 km) from west; pollutant concentrations in the residual layer is critical

    • Limited vertical mixing

  • PM2.5

    • Light winds

    • Transport aloft from west

    • Strong morning inversion and limited mixing later

    • High humidity

Air quality forecasting in the philadelphia metropolitan area

Poor air quality events are linked to

the synoptic cycle. The majority

of bad air days occur in multi-day

episodes (2-5 days).

In the standard summer season

pollution episode, an upper air

ridge, with its axis over or west of

the mid-Atlantic, is in place.

A cold front passes bringing clean

air, then, as surface high pressure

migrates slowly from the Midwest,

it becomes modified (dirty).



Air quality forecasting in the philadelphia metropolitan area

Eventually the continental high pressure

center stalls and links up with the semi-

permanent Bermuda High circulation.

This is a common summer season PM2.5

and O3 episode pattern. High pressure

overhead leads to clear skies, and light


This is not, by itself, enough to lead to

a bad air day. Hot weather and light

winds are necessary, but not

sufficient, for poor air quality.



Air quality forecasting in the philadelphia metropolitan area

Example: Surface Analysis During a Poor Air Quality Event

An Appalachian Lee Trough is commonly

associated with poor air quality cases.

Other necessary ingredients

Other Necessary Ingredients

  • In addition to sunny skies and light winds, need:

    • Limited Vertical Mixing

      • Some kind of trapping inversion. In the summer season, surface-based inversions break early in the day. A secondary inversion aloft (950-850 mb) or strong warm air advection is often necessary.

    • Transport of Pollutants into Region

      • The Ohio River Valley is the largest source of power plant emissions of NOx (for O3) and sulfate (for PM2.5).

Westerly winds in residual layer bring pollutants to our forecast area

Westerly Winds in Residual Layer Bring Pollutants to Our Forecast Area

Vector averaged winds at

850 mb during an unhealthy

O3 episode in 1997.

Midwest and Ohio River

Valley are source of high

emissions of oxides of

nitrogen (forms O3 and

PM2.5) and sulfates

(forms PM2.5).

Emissions controls enacted

in 2003 in that region have

reduced O3 significantly.

Average 850 mb Winds

July 12-17, 1997

Back trajectory forecast models help identify source regions for tomorrow s air quality

Back Trajectory Forecast Models Help Identify Source Regions for Tomorrow’s Air Quality

HYSPLIT Back Trajectory

Colored lines show the forecast path

of air parcels reaching PHL at 1200 UTC

on July 16 at three levels above

the ground (500, 1000, 1500 m).

The path is 24 hours in duration

with dots giving position at

6 hour intervals.

The bottom panel shows the forecast

vertical motion of the parcels.

Trajectories are coupled with real time

AQ data to estimate upwind contribution

to tomorrow’s air quality.


When good forecasts go bad

When Good Forecasts Go Bad

  • Convection

    • Local Convection: Timing is critical for O3. Convection after ~ 6 pm is usually too late to clean the air.

    • “Near-Local” Convection: Downdrafts within outflow boundaries contain very clean air.

    • Regional Convection: Reduces transported pollutants and increases high clouds; e.g., evening/overnight MCS upwind.

  • Stalled/Reversing Frontal Boundaries

  • Sea/Bay Breeze Re-circulations

Effect of outflow boundaries on o 3

Effect of Outflow Boundaries on O3

Fair Hill, MD

The effect of thunderstorms on local O3 can be

remarkable even at its periphery.

GOES Visible

1902 UTC

June 26, 1998

Hourly Ozone Concentrations

Fair Hill, MD

June 25-26, 1998

Regional scale convection and local o 3

Regional Scale Convection and Local O3

24-h back trajectories at 500,

1000 and 1500 m

Aug 4

Aug 5

O3 Concentrations, 2 pm, August 4

August 5thwas forecast to be sunny and very warm - conducive to O3 formation. Back trajectories are from climatologically “dirty” location. But, early afternoon O3 concentrations on August 4th along the path of trajectory are clean!

Low upwind o 3 concentrations due to organized thunderstorm system mcs on august 4

Low Upwind O3 Concentrations Due to Organized Thunderstorm System (MCS) on August 4


1515 UTC, August 4

Result: While O3 was high in PHL the

following day, no locations reached the

8-hour average Code Orange threshold.

MCS moved through OH on the

morning of the previous day

Summary 1 of 2

Summary (1 of 2)

  • Air quality forecasts (O3 and PM2.5) are issued daily for the Philadelphia metropolitan area, and surrounding states.

  • Forecasts are issued ~ 2pm in a color code format with Alerts issued for Code Orange or higher.

  • The peak season for poor air quality is June-August although PM2.5 can reach Code Orange levels year round.

Summary 2 of 2

Summary (2 of 2)

  • Poor air quality typically occurs in multi-day episodes linked to the synoptic cycle.

  • Warm temperatures, light winds, sunny skies, limited vertical mixing, with westerly winds in the residual layer, are associated with poor air quality episodes.

  • Forecasts are reasonably accurate but become more uncertain in the presence of convection (local and regional) as well as stalled frontal boundaries and sea breeze fronts.



  • Air quality forecasting in the Philadelphia area is funded by the Delaware Valley Regional Planning Commission (DVRPC) in association with the states of Pennsylvania, Delaware and New Jersey.

  • Special thanks to Sean Greene of DVRPC as well as my forecasting colleagues in the mid-Atlantic region.

Appendix a useful web sites

Appendix A: Useful Web Sites

  • Current Conditions

    • EPA AirNow

  • Forecast Models

    • NOAA-EPA O3 Model

    • NC DENR Model (off line at this time, back in May?)

    • Environment Canada

  • Forecasts and Blogs

    • Philadelphia Metropolitan Area Air Quality Forecast

    • Mid-Atlantic Medium Range Air Quality Outlook

    • The Smog Blog

    • Twitter: @aqforecast

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