“Two Exceptional Events Concerning Ozone
and Particle Pollution in the Southeast”
Chief Meteorologist, Georgia EPD
Research Scientist, Georgia Institute of Technology
What is an Exceptional Event?
U.S. EPA defines the term "exceptional event" to mean an
Affects air quality;
1.Is not reasonably controllable or preventable;
2.Is an event caused by human activity that is unlikely to
recur at a particular location or a natural event; and
3.Is determined by U.S. EPA through the process
established in the regulations, 40 CFR Part 50.14.
Exceptional Events Examples
Case 1 - Elevated ozone levels at three high
altitude Smoky Mountain monitors and possible
contribution from stratospheric ozone intrusion
(Feb 24, 2008 - case study)
Case 2 - Recent PM2.5 smoke episode from Arabia
Bay Fire activity in South Georgia (November 12, 2010
- case study)
Case 1: Tropopause Fold
Schematic diagram illustrating the important features of a typical mid-latitude jet stream
tropopause fold (as pioneered by Danielson, 1968). Light arrows indicate cyclonic motion around
jet core (J), while dark arrows show vertical motion of tropopause (TROP).
1. Note clouds in warm air sector with ascending motion, rising tropopause, and decreasing total
column ozone amount.
2. Descending motion, a lowered tropopause, drier air, and increased total column ozone is found
behind the front and on the south side of the jet.
O3 Measurements at Smoky Mts.
Data provided by Jim Renfro
High ozone (70s and 80s ppb) measured at 3 high elevation Smoky Mountain sites
(near 850mb). Two other lower elevation sites stayed in the 17-23 ppb range. All sites
were in the 20's ppb near midnight out ahead of a cold front.
Mechanisms of Transport Across the
a) The jetstream tropopause fold (due to the
presence of a strong jet aloft and short wave).
b) Subsidence (due to strong Arctic anticyclone)
c) Cutoff Lows (tropopause fold occurs during
formation of closed Low, typically have
interaction of jet streak with a short wave)
(a) was the mechanism in this particular case.
Relative Humidity (850mb) – Smoky Mts.
300 MB Chart – Feb. 24
500 MB Chart – Feb. 24
HYSPLIT Back Trajectory - Feb. 24
TOMS Ozone Data for Feb. 21-25
The TOMS instrument ( OMI/AURA) measures the differential absorption
of backscattered UV radiation from the earth's atmosphere absorbed by
ozone and the other weakly absorbed. The unit used for ozone measurement
in a vertical column having a base of one cm (squared) at STP is defined the
Dobson unit (DU). Drier air and lowered tropopause accompanied the
increase on 2/24.
Case 2 – Arabia Bay Fire
The Arabia Bay fire, located six to
ten miles northwest of Homerville
in Clinch County (south Georgia),
had burned nearly 3000 acres
from Nov 9-29th, as reported the
Georgia Forestry Commission.
Satellite photo showing the area
in which the Arabia Bay swamp
fire was burning. Mainly
composed of brush and leaf litter,
the fire was believed to be caused
by an arsonist. Fire caused dense
smoke and reduced visibility in
Homerville and Valdosta areas.
Elevated PM2.5 Hourly Values
Known Fire Data with MODIS Imagery
Satellite Surface Map – Nov 12, 2010
• High Pressure system centered over the mid-Atlantic moved eastward,
keeping mostly dry stable conditions over central and south GA, along with
mostly clear skies
• Good pre-frontal build-up of PM2.5 ahead of the approaching cold front
(stagnant conditions) may have helped concentrations at VLD go even
Objective WRF Mesoanalysis
• Local model run internally
• Domain chosen for fire
region over South Georgia.
• Wind (barbs), mean
(contours), and Relative
Humidity (color fill)
• Winds veer from northerly
to northeasterly from
11/13 into 11/14.
• PM 2.5 concentrations
rapidly increase on 11/14,
as smoke is transported
from the fire region (near
Homerville) towards the
Valdosta monitoring site.
EPD’s WRF-ARW is initialized with NAM12 data
RUC Forecast Time-Height Section
For Homerville, GA
Light low-level northeasterly flow transported smoke into Valdosta from the fire region
Visible Imagery/RUC Analysis
Aqua Modis RGB – Southeast (11/14)
CALIPSO – Backscatter Plot
Additional Exceptional Event
Warehouse fire in South Fulton County (GA) caused hourly PM2.5
values to increase at EPD Monitoring stations
Several meteorological tools (Trajectories, satellite
imagery, synoptic conditions) could be utilized
when characterizing exceptional events.
In case of a tropopause fold, it is important to recognize
the type of dynamic mechanism and to investigate total
column ozone variability. This helps validate surface and
low-level observations. Difficult to forecast, however,
due to short duration and lack of upper air data.
In fire/smoke activity, other meteorological factors could
have contributed to enhanced particle pollution levels,
such as approaching frontal systems and pre-frontal
pooling. Easier to predict due to long duration and
availability of surface data.
Some exceptional events are more frequent and clear-cut
than others, such as the Arabia fire/smoke case.
• Amy K. Huff, Battelle Memorial Institute
• Nyasha Dunkley, Georgia Environmental
• Sean Miller, Georgia Environmental Protection
• Jim Renfro, National Park Service