Qualitative Episode Analysis for 2002 Ozone Season

Episode of June 10-12, 2002

June 10: At 0000 UTC, at cold front is analyzed in northern PA. This boundary drifts slightly

south and becomes stationary along a Dover-Baltimore-Pittsburgh line by 1200 UTC. Visible

images show clear skies with hints of haze at 1600 UTC with only shallow cumulus developing

by 1800 UTC. Surface reports at 1800 UTC have scattered reports of haze in the Washington

area and more widespread reports west of the Appalachians. The upper air pattern is conducive

to the development of a high ozone period with high pressure at 850 mb centered over KY and a

modest ridge west of the region at 500 mb. The forecast back trajectories are consistent with

standard high ozone cases with westerly flow. Upstream ozone at 1500 UTC at the source of the

back trajectories on June 9 is strongly enhanced – in the 70-80 ppb (1-hour average) range.

Widespread Code Orange concentrations are found south of the frontal boundary. 

June 11: The slow moving/near stationary frontal boundary has now taken the form of a

retreating warm front and is well into New England at 1800 UTC with an Appalachian lee trough

(ALT) analyzed from Baltimore to northern NC. A broad ridge at 850 mb stretches from AL to

the Delmarva with the 500 mb ridge axis remaining just west of the mid-Atlantic. High ozone

concentrations are reported right along the I-95 Corridor with scattered Code Red concentrations.

The forecast back trajectories show a shift from west-northwesterly flow to along-Corridor. The

1200 UTC sounding at Dulles Airport, VA (IAD) showed continuing westerly flow through the

depth of the boundary layer with a very strong cap at 660 mb. A residual mixed layer, often

found in association with high ozone cases, is also seen. 

June 12: Ozone concentrations fall region-wide on June 12 with strong southwest winds

observed. Forecast back trajectories suggest very fast flow backing further to the southwest than

the previous two days. Convection develops by afternoon across central PA with a substantial

cirrus cloud shield moving into the mid-Atlantic in advance of the rain.

Episode of June 22-26:

During this episode, the mid-Atlantic is sandwiched between two systems. First, a vigorous low

that crosses southern Canada on June 20-21 and, second, an upper level low that develops over

the southeastern US and then drifts westward with time. A small area of high pressure is wedged

between the systems and the highest ozone concentrations are found beneath this high pressure

wedge. 

June 22: The strong system that crossed Canada earlier in the period has now weakened and

moved just northeast of ME (500 mb). A sprawling area of high pressure at 850 mb is found

from IN east to just south of Long Island. At the surface at 1200 UTC, high pressure is located

over WV. The frontal boundary associated with the departing Canadian system is quasistationary over New England and northern NY. Further south, the upper level low over the

eastern Gulf of Mexico has resulted in the development of a coastal trough with sustained

easterly winds reaching as far north as NC. The ozone pattern this day follows the synoptic

situation closely with good air quality north of the frontal boundary and south across VA and NC
where the maritime inflow is strongest. In the Washington, DC area a wide range of

concentrations is found. Concentrations range from Code Green in the northwest suburbs to

upper Code Orange in the near southern suburbs.

Skies are generally clear across the Washington region with some shallow cumulus developing

by afternoon. The 1200 UTC IAD sounding showed light southwest winds beneath the surface

based inversion with easterly flow aloft and a very strong cap at 820 mb. The air mass is

relatively dry for the season (62-63 ○F).

June 23: Areas of good air quality (Code Green) are again found in the Washington area with

no 8-hour ozone violations on this day. The very clean maritime air mass associated with the

coastal trough and associated upper level low is seen over VA. The forecast back trajectories

show a complex transport pattern with an offshore component at 500 m. Back trajectories based

on analysis field are roughly similar with a weaker maritime component. The morning IAD

sounding again has a strong inversion based at 850 mb with light and variable winds beneath it. 

June 24: By mid-afternoon on June 23, low pressure develops along a frontal boundary north of

the Washington area. This system develops quickly so that, by 1200 UTC on June 24, a

reinforced cold front has pushed to just north of New York City. By 1800 UTC, the boundary is

quasi-stationary across northern NJ and central PA with scattered convection occurring across

central PA. Scattered haze is reported south of this frontal boundary in the morning hours,

persisting into afternoon. The 1200 UTC IAD sounding shows a residual inversion present at

800 mb though much weaker than the two previous days. An elevated mixed layer is evident just

beneath the inversion. The highest ozone concentrations are organized in a west-to-east band

across OH and PA into NJ and MD. Scattered Code Red concentrations are found east and

northeast of Washington, DC. 

June 25: The frontal boundary that was driven southward on June 24 reaches as far as southern

NJ by 1200 UTC before returning northward by mid-afternoon. Scattered Code Red

concentrations are present in the Washington area. The most unusual aspect of the ozone field

on this day is the presence of Code Red concentrations over the urban center of Washington.

Haze reports are widespread and forecast back trajectories are less complex than previous days

with standard west-northwest flow within the boundary layer.

June 26: The episode ends on June 26 when the upper level ridge begins to flatten. This allows

for increased boundary layer winds and a much more unstable atmosphere. The 1200 UTC IAD

sounding is nearly dry adiabatic on June 26 in the boundary layer with strong SW winds

throughout (10-15 knots range). The unstable air mass results in convection developing along

the Blue Ridges by 1900 UTC with widespread convection later in the afternoon.

Episode of July 1-3, 2002 

A relatively mild ozone episode with strong local peaks on July 2, 2002. The peak 1-hour ozone

concentration in the Washington area was 158 ppbv in the Washington area on July 2, 2002.

Widespread one-hour exceedances on July 2, 2002 were reported near Washington, DC.

Throughout the episode, the highest concentrations were found along and east of the I-95

Corridor with lower concentrations further west.
The high ozone event of June 22-26, 2002 ended with an upper level trough crossing the

northeastern US. The trough exited the region fairly rapidly but the transition to a ridging

pattern was complicated by a small “cut off’ low left behind by the trough over New England.

This low drifted only slowly eastward to near Nova Scotia by July 2, 2002. Surface winds were

north to northeast on June 29, 2002 becoming southwest early on June 30, 2002.

July 1: The 1200 UTC surface analysis shows a fairly standard high ozone pattern. The center

of high pressure is located in western VA with a very weak pressure gradient across the region.

At 850 mb, high pressure is centered further west over western TN with a weak trough exiting

New England. A similar pattern is present at 500 mb although the presence of a lingering closed

low just east of Maine suggests that the pattern will remain stationary in the short term not

allowing the ridge to build quickly east.

Ozone concentrations in the Washington, DC region were relatively high (86-87 ppbv 8-hour

average) on the preceding day (June 30), and back trajectories suggested slow transport along

and slightly west of the I-95 Corridor. Peak 1-hour ozone concentrations rose into the 87-99

ppbv range in the Washington area. The bulk of the monitors exceeding the 8-hour standard

were in the 90-100 ppbv range, reflecting a rising regional ozone load. 

The morning IAD sounding showed a layer of strong southwest winds and the Fort Meade, MD

profiler showed steady WSW winds through the day. As reflected in the afternoon visible

image, there was significant boundary layer overturning producing widespread, though shallow,

cumulus beneath a subsidence inversion based at ~ 770 mb. The 0000 UTC IAD sounding for

July 2 shows a residual mixed layer to ~ 800 mb corroborating that deep boundary layer mixing

occurred.

July 2: The most unusual aspect of this day was the abrupt decrease in visibility beginning in

the late morning and continuing through the afternoon and the speed with which the haze layer

moved northeastward into New England. Typically, visibility reaches its maximum with mixing

and increased winds in the afternoon and then reaches a minimum just before sunrise. This

occurred in the context of continuing brisk southwest winds with 10-13 knots reported by

afternoon. The regional surface wind field was complex with variable winds through the early

afternoon becoming southeast. 

The upper level analyses at 850 mb and 500 mb are barely distinguishable from the previous day.

Of most interest is the position of high pressure at 850 mb that is slightly further north over IL at

1200 UTC. Unlike the standard mid-Atlantic pollution case, however, this continental high has

not linked up with the semi-permanent Bermuda High as a lingering trough is present along the

Eastern Seaboard. As a result, 850 mb winds reported at IAD were north to north-northwest

early on July 2, 2002. At IAD, this flow pattern is consistent to the base of a strong capping

inversion at ~ 800 mb. The IAD sounding at 0000 UTC on July 3 showed the boundary layer

was further suppressed during the day with mixing only to ~ 1500 m. As a result, only shallow

cumulus develops over the mid-Atlantic during the afternoon with the exception of scattered

strong convective activity across western PA south into WV.

Back trajectories, and regional surface observations, suggest transport of pollutants from
locations northwest of the I-95 Corridor. Upstream ozone at 1600 UTC across northwestern PA

was on the order of 70-80 ppbv. 

July 3: Although temperatures continued very warm in the upper 90’s ○F, the characteristics of

the air mass appear to change rapidly yet again. Ozone concentrations fell across the region with

peaks reaching the Code Orange range. 

Forecast back trajectories for July 3 were quite similar to the preceding day and verify well with

analysis trajectories. Ozone concentrations upstream, however, ran ~ 10 ppbv lower than on the

preceding day. The morning sounding for July 3 was more unstable with a much reduced cap

from the previous day although only shallow cumulus formed during the afternoon hours.

The ALT, which was analyzed along the I-95 Corridor for much of July 2, slips slowly eastward

and the region of highest ozone concentrations is roughly aligned with its later afternoon

position.

Episode of July 31-August 5

July 31: Scattered Code Orange reported in the southern mid-Atlantic. Generally moderate

ozone is reported along the I-95 Corridor. Forecast back trajectories suggest fairly fast boundary

layer flow with the air mass origination in southern Ontario. Ozone concentrations at 1600 UTC

on July 30th in that region were about average (45-50 ppbv). The cold front, located near

Norfolk, VA at 0000 UTC, drifts a bit further south by 1200 UTC and then dissipates by 1800

UTC. 

August 1: Ozone concentrations rise region-wide. Forecast back trajectories show a strong

anti-cyclonic curvature from near Lake Ontario through northern NJ. Again, upstream ozone is

relatively low (40-50 ppbv) in southern Canada and northern PA but emissions along the path are

likely quite high as the air parcels cross the metropolitan New York area. An ALT is analyzed

overnight along and east of the I-95 Corridor. By 1200 UTC, high pressure is centered over WV

a climatologically favored location for high ozone in the I-95 Corridor. A very strong low-level

inversion is observed with northeast winds in the layer beneath 800 mb. 

August 2: Highest O3 concentrations are reported right along the I-95 Corridor with scattered

Code Red observations in the Washington area. The highest ozone concentrations are found

south and west of Washington DC. Forecast back trajectories show a good deal of variability.

At 1200 UTC, a back door front is analyzed near NYC with surface high pressure still centered

near WV. As on August 1, the sounding at IAD shows a strong low-level inversion. Early

morning observations show haze along the I-95 Corridor with the most numerous observations in

VA and NC. As afternoon mixing occurs, only widely scattered haze reports by afternoon.

August 3: High ozone levels are concentrated along the I-95 Corridor again on August 3. The

forecast back trajectories are the standard west-northwest flow although mid-day ozone across

northern OH on the preceding day was not extreme (47-56 ppbv). This likely reflects difficulties

in the trajectory model in the vicinity of a frontal boundary which reaches central PA by 0000

UTC and then just N of PHL by 1200 UTC where it stalls. Scattered haze is reported across PA,

MD and VA lingering into the early afternoon hours. 
August 4: Ozone increases along a band from just north of Washington, DC to just north of

New York City with widespread Code Red concentrations in a pattern characterized by recirculation and stagnation. The cold front that reached into eastern PA on the previous day

becomes stationary along a line from Portland, ME to just north of New York City and then

across central PA to near Pittsburgh. This boundary washes out by 1200 UTC with surface high

pressure remaining in place over WV and an ALT analyzed along the I-95 Corridor at 1800

UTC.

August 5: Scattered Code Red ozone concentrations near Washington DC. The presence of

significant cloud cover north of the Mason Dixon Line reduces peak ozone in that region. The

frontal boundary has washed out over New England with remnants still quasi-stationary over

northern PA. The next cold front reaches northwestern PA by 1200 UTC. Again there is

considerable vertical shear noted by the forecast back trajectories with southerly flow at the

lowest (500 m) layer.

Episode of August 10-14

August 10: High pressure is directly over the mid-Atlantic with dew points in the mid-50’s ○F,

and clear skies. Temperatures are generally in the upper 80s across the Washington, D.C. region.

With high pressure overhead, the forecast back trajectories indicate very light winds and

recirculation. Highly variable ozone field with concentrations are present on August 10thin the

Washington, D.C area. Scattered Code Orange peaks were reported along the I-95 Corridor.

While the Dulles, Virginia (IAD) sounding at 1200 UTC did not show a very strong low-level

inversion, with 950 mb temperatures only 21○C, there was a very strong cap at 805 mb with

absolutely stable conditions above this level. The presence of a deep residual layer (975-805

mb) suggests the presence of stagnation. Peak eight-hour average ozone exceedances in the

Washington, DC region ranged from 87 ppbv (Arlington and Loudoun Counties, VA) to 93 ppbv

(Fairfax County, Annandale, VA).

August 11: Surface high pressure drops slowly southeastward across the mid-Atlantic with the

center analyzed in western NC at 1200 UTC drifting to coastal SC by 1800 UTC. The upper

level ridge has also moved east and is located over the mid-Atlantic at 1200 UTC. The ozone

map for the Washington area shows another day of highly variable peak ozone. A peak

concentration of 120 ppbv occurred along the I-95 Corridor northeast of Washington DC. Again,

a very clear day in the mid-Atlantic. Winds are generally south to southwest as is reflected in the

boundary layer back trajectories. The key factor driving local ozone production appears to be a

very stable boundary layer. The 1200 UTC sounding at IAD shows a very strong low-level

inversion from 950-900 mb with a deep residual layer beneath a continuing strong subsidence

inversion, now based at 760 mb. Peak eight-hour average exceedances ranged from 88 ppbv

(Prince Georges County, MD) to 106 ppbv (Washington, D.C., McMillan).

August 12: The upper level ridge remains quasi-stationary with its axis over the mid-Atlantic.

The center of high pressure at 850 mb is over NC/GA. At the surface, the characteristic

Appalachian lee trough (ALT) is analyzed at 0000 UTC and remains in place through 1800 UTC,

continuing into August 13. Clear skies remain, although haze is seen in the 1600 UTC visible
image. The surface observations show a rapid and widespread decrease in visibility west and

northwest of the I-95 Corridor, shifting further east by early afternoon. The 1200 UTC IAD

sounding is similar to the preceding in several respects: a slightly deeper and continuing strong

low level inversion, now from 975-925 mb with a strong cap at ~ 770 mb. Winds are fairly

strong from the NW. This is reflected in the forecast back trajectories that show a shift to

westerly transport. The upwind O3 concentrations at 1600 UTC on August 11 in the vicinity of

the origin of the forecast back trajectories is enhanced, on the order of 78-86 ppbv. Ozone

concentrations fall this day west of the Appalachians but increase markedly across the midAtlantic with widespread Code Red observations from NC to Boston. Widespread exceedances

are found in the Washington area, ranging from 85 ppbv (Rockville, MD) to 114 ppbv (Arlington

County, VA).

August 13: Periods of calm winds are reported overnight. Significant haze has now spread all

along the I-95 Corridor and remains into the afternoon hours. The regional ozone map shows an

extensive episode. Skies are again clear with shallow convection developing later in the

afternoon. The 1200 UTC IAD sounding showed a continuing strong low level inversion with a

residual mixed layer to 850 mb ending just beneath a weak secondary inversion. The cap aloft

has lifted to ~ 630 mb and the sounding is more unstable compared to previous day’s between

the two inversion layers. In addition to a very stable, and hazy, boundary layer, the surface

analyses show that the ALT continues in place from late on August 12, to a position slightly

further east at 1200 UTC and then backing west at 1800 UTC. As is typically the case, the

highest ozone concentrations are found in proximity to this boundary. Eight-hour exceedances

ranged from 87 ppbv (Loudoun County, VA) to 125 ppbv (Fairfax County, Mt. Vernon, VA).

August 14: Back trajectories forecast fairly fast flow along the I-95 Corridor. The increase in

southerly wind component results as the upper level ridge axis finally moves offshore. On the

back side of the upper level ridge, low level southerly winds increase as the Bermuda High

circulation pushes maritime air northward. The haze reports corroborate a slow clean out from

south to north with morning haze reported north of Washington, D.C. This episode ends in a

very different manner than the standard high ozone episode. Instead of the passage of a sharp

cold front, this episode ends gradually as cleaner air sweeps north, winds increase and the

atmosphere steadily destabilizes. On August 15, concentrations fall across the region as low

level flow becomes more southeast and the Bermuda high fills in westward. Cloud cover spreads

over the region on August 16 with ozone concentration continuing to decrease. Peak eight-hour

exceedances ranged from 87 ppbv (Arlington County, VA) to 101 ppbv (Fort Meade, MD).

Episode of September 9-10

Summary: A two-day ozone episode with northeasterly flow due to high pressure to the north

and west and Tropical Storm Gustav to the southeast. Strong subsidence occurs over the region

as Gustav approached and moves along the North Carolina coast. Northeasterly winds occurs

due to clockwise flow around high pressure over the Ohio Valley and counter-clockwise flow

around Gustav as it neared the Outer Banks. Highest ozone concentrations on September 9

th

occur in central Pennsylvania, northeast of Baltimore, near Annapolis, MD and west of Fairfax

County, VA. A sharp ozone gradient was evident on the 10th with low concentrations east of a
Baltimore-Rockville, MD-Fairfax City, VA line while eight-hour exceedances occurred in

counties immediately west and south of Fairfax.

September 9: At 1200 UTC, Tropical Storm Gustav is southeast of Cape Hatteras, NC with high

pressure was over Ohio and Indiana. Widespread surface haze observed in Ohio, Michigan,

Indiana, and Illinois. Very warm air aloft over Pennsylvania and West Virginia enhanced stable

atmospheric dispersion conditions. The upper air pattern indicates a large ridge over the eastern

half of the US, with a closed low associated with Gustav off the southeast coast. Strong

subsidence occurs over Mid-Atlantic region, centered over south-central Pennsylvania. Forecast

back trajectories indicate flow from the northeast megalopolis (Philadelphia to New York City).

Code Orange concentrations are observed throughout much of New Jersey, northeast Maryland,

and the western Virginia suburbs of Washington, DC. Code Red was reached northeast of

Baltimore. Code Green conditions occurred in coastal southern Delaware and Maryland due to

the influence of clean air from Gustav.

September 10: Tropical Storm Gustav nearer Cape Hatteras with heavy precipitation along the

Outer Banks. Widespread surface haze was again observed throughout the Ohio Valley into

Indiana, Illinois, and Michigan. As Gustav moves closer to the North Carolina coast, the region

of strong subsidence moves further west, along and to the west of the Appalachians. Forecast

back trajectories indicate the influence from Gustav as the flow was generally from the east. The

maritime tropical air from Gustav reaches as far inland as Washington, D.C. but stable,

continental air is quite close to the west as indicated by the 8-hour ozone peak concentration

map. A very sharp ozone gradient is evident. Conditions range from Code Green to Code Red

just several miles apart in Fairfax County, VA. Code Orange conditions occur west of Baltimore

and in the western Virginia suburbs of DC while Code Green conditions occur along I-95 and

eastward. Code Red conditions occur just west of I-95 at three Virginia locations in the

Washington, D.C. region.