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 http://www.meteo.psu.edu/~wfryan/mount-holly-2010.pptx. What Do We Forecast?. Ozone (O 3 ) – since 1996
Air Quality Forecasting in thePhiladelphia Metropolitan Area
Department of Meteorology
National Weather Service, Mount Holly, April 28, 2010
O3 monitors shown in black
PM2.5 monitors generally
in same locations but about
half the density.
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).
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.
2009 is not blank, there just weren’t any Code Red cases!
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.
Note: Data for 2004-2008 uses gravimetric filter monitors (FRM) while 2009
uses 24-average from continuous monitors.
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).
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.
Example: Surface Analysis During a Poor Air Quality Event
An Appalachian Lee Trough is commonly
associated with poor air quality cases.
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
Emissions controls enacted
in 2003 in that region have
reduced O3 significantly.
Average 850 mb Winds
July 12-17, 1997
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.
NOAA ARL HYSPLIT Model
Fair Hill, MD
The effect of thunderstorms on local O3 can be
remarkable even at its periphery.
June 26, 1998
Hourly Ozone Concentrations
Fair Hill, MD
June 25-26, 1998
24-h back trajectories at 500,
1000 and 1500 m
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!
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