8. Q. Name 3 factors that cause the reduction of hurricane intensity after it makes landfall.
loss of warm moist air
reduced temperatures
greater friction

9. Q. How does the solar radiation present on a clear day contribute to air pollution? Assume sources of pollutants are present?
A.It forms secondary pollutants by photochemical reactions. Ozone and PAN would be examples of this.

10. Q. H ow do mountain ranges alter precipitation patterns?
A. If the prevailing wind direction is against the mountain range, there will be an area of relatively abundant rainfall on the windward slopes where the air is forced to rise. A rain shadow, or arid region, will be found on the lee side?

11. Pollution in the Mid-Atlantic Region

12. Recipe for Ozone Emissions of O3 “Precursors” A Sunny Day
Hot Temperatures (typically in the 90’s)
Moderate or Light Surface Winds
Note here that O3 is a secondary pollutant. It is not emitted directely into
The atmosphere but is formed by a series of reactions involving primary
Emissions of NOx and VOC, there are called “precursors”
More on thiws latere.

13. An Example of the Effects of Haze in the Mid-Atlantic
The Great Smoky Mountains National Park
A Clear Day
A Hazy Day
Photos from

14. Not Every Hot day Results in Poor Air Quality

15. Most Unhealthy Air Quality Days Occur in the Summer Season
Most Unhealthy Air Quality Days Occur in the Summer Season. Summer Weather in the Mid-Atlantic Can be Characterized by the “4 H’s”
Hot
Humid
Hazy
High Pressure
These weather conditions occur frequently in mid-Atlantic summers and are often but not always associated with unhealthy air quality.
HAZY
HOT
Humid

16. A Typical Day in a Pollution EpisodeH

17. Westerly Transport is often present when the highest Ozone are Observed in Maryland.

18. Early Morning Vertical Profiles of Ozone with strong nocturnal temperature inversion
Gettysburg, PA, 1995

19. The strength of the Inversion(s) often
dictates how deep Mixing Will Be.
ESN
ESN
6/27
6/28
Afternoon
Ozone (ppbv)
Ozone (ppbv)

20. Transport of Pollutants
There are three scales of transport important to Air Quality in Maryland. All three contribute to poorer air quality than might otherwise be observed.
Large scale > ~ 800m (~70-90 ppbv) H
Medium scale > ~ m (???)
Small scale < 200m (???)

21. Animation of 1hour ozone concentrations over the Eastern US
Ozone follows a diurnal
cycle because its production
is mainly a a function of
temperature and sunlight.
Other factors dictate the
amount of ozone produced:
-wind speed
-cloud cover
-depth of the boundary layer
0-60
80-99
125+
61-79

22. Daily Ozone Cycle Ozone production follows a daily cycle with maximum
Ozone Concentration
Ozone production
follows a daily
cycle with maximum
concentrations
typically observed
in the late afternoon.
Sunrise
Sunset
Time of day

23. O3 Times Series This hourly O3 graph for a summer day near Frederick,
Maryland shows O3 concentrations reaching a minimum
in the early morning hours.

24. Free Atmosphere Height (km) 2 Layer Planetary Boundary Residual Layer1
Mixed
Layer
Residual Layer
Mixed
Layer
Low Level Jet
Stable Nocturnal Boundary Layer
surface layer
surface layer
surface layer
afternoon
sunset
midnight
sunrise
noon

25. Nocturnal Inversion ABOVE BELOW
O3 concentrations remains relatively high.
Winds are moderate with some localized higher winds.
Nocturnal Inversion
BELOW
Ozone reacts with substances near to and deposits onto the earth’s surface – its concentration virtually disappears.
More pollution is released at the surface and is trapped under the inversion – haze increases.

26. A Typical Day in a Pollution Episode

27. A Day in a Pollution Episode
The most severe episodes typically occur over multiple days, building up on the first day and tapering off on the last.
As an introduction, a day in the “middle” of a pollution episode is discussed.
While PM, O3 and haze events typically occur in conjunction with one another, we
will focus here on an O3 event.

28. Before Sunrise In the late night and early morning hours during a
pollution episode certain effects are commonly found:
O3 concentrations are at a minimum, particularly near the urban centers.
Winds are light and variable.
Haze levels are at a maximum with visibility often reduced to a few miles or less.
These effects are due to the development of the nocturnal inversion.

29. The Nocturnal Inversion
On clear nights, a temperature inversion develops near the surface.
- Air temperature usually decreases with height.
An inversion is a layer of air where temperature
increases with height.
- Because the layer of air in the inversion is warmer than the air below it, the cooler air below the inversion cannot rise above it. Pollutants near the surface are therefore trapped below the inversion in the overnight hours.

30. Temperature Inversion
Altitude
The trapping of emissions beneath the noccturnal inversion is the reason why haze concentrations are highest in the late night and early morning hours.
Temperature Inversion
Pollution trapped below inversion
Temperature

31. What causes the nocturnal inversion?
While inversions can occur at various levels
in the atmosphere (and we will see more
examples later) and can be due to a variety
of effects, the nocturnal inversion is caused by surface (or radiational) cooling.

32. Nocturnal Inversion
After sunset on clear nights, the ground surface cools rapidly. However, air is not a very good conductor of heat. As a result, only the layer of air in the first few hundred meters from the surface cools.
The air further aloft remains warm creating what is called the "nocturnal inversion."

33. Late Morning
The ground heats up the air beneath the nocturnal inversion. This air becomes warmer than the air aloft, rises and mixes. The inversion layer disappears.
Ozone and other pollutants above the inversion layer mix with the pollution under the layer
This causes a dramatic increase in ground-level ozone, beginning around 10 AM
Altitude
Temperature

34. O3 concentrations along the western boundary of the I-95 Corridor on
Regional Scale O3
The air that is mixed downward during the late morning and early afternoon hours is typically high in O3 and other pollutants and concentrations are often uniform over large distances.
O3 concentrations along the western
boundary of the I-95 Corridor on
August 17, 1999

35. Regional Scale O3
In this case from July, 1999, the high elevation monitor at Methodist Hill in southern PA is above the nocturnal inversion. By late morning, mixing has brought all monitors to the common regional level.

36. Afternoon
By late morning, downward mixing of O3 leads to relatively uniform concentrations across the region.
Local effects, related to emissions available, then dominate in the early afternoon hours.
O3 is formed as UV radiation drive reactions of O3 precursors.
Depending on a variety of factors, peak O3 concentrations are reached during the mid to late afternoon hours.
The highest concentrations occur downwind of the urban center.

37. What modulates O3 concentrations?
The amount of O3 produced each day depends on a variety of factors including:
Temperature
Concentrations of O3 and precursors
mixed downward during the late morning.
Wind speeds.
Local emissions of O3 precursors
Amount of available sunlight
Depth of vertical mixing

38. Theoretical Surface Ozone During Low Level Jet Periods
figure 3
(Adapted from Corsmeier et al., 1997, figure 1)
Theoretical Surface Ozone During Low Level Jet Periods 1
secondary
ozone maxima .5
Normalized Ozone Concentration
06:00
09:00
12:00
15:00
18:00
21:00
00:00
03:00
06:00
Time of Day (Local)

39. Boundary Layer Winds During a Nocturnal Low Level Jet
figure 4 2
Height (km) 1
turbulence induced
downward mixing
associated with wind sheer
shear
Wind Speed
Wind Direction

40. General Characteristics of a Low Level Jet
Maximum winds are typically observed between meters.
Wind direction often SW-NW, but can be E.
Minimum wind speed 10ms-1 (typical range is 10-20ms-1).
Winds observed as high as 30 ms-1.
Low Level Jets can have a width of a few hundred kilometers
and a length of a thousand kilometers.
Best conceptualized like thin sheet of fast wind rather
than a ribbon of fast moving air (like the polar jet).
polar jet stream
low level let

41. Investigations have shown that there are many possible
causes for the low level jet:
synoptic-scale baroclinicity
baroclinicity associated with sloping terrain
fronts
mountain and valley winds
internal oscillations
land and sea breezes
More than one factor can contribute to jet formation.
There are many different types of low level jets.

42. NOAA Forecast Systems Laboratory Profiler Network

43. Boundary Layer Temperature 21-27 June 2002 Forte Meade, Maryland
which of the days are cloudy?
cloudy
clear, light winds

46. 7/17/99
7/18/99
7/19/99
Time (EDT)

48. Modeling the Low Level Jet
The ability of any grid model to reproduce the observed ozone concentration depends to a large extent on the accuracy of the meteorological fields used as inputs.
Work (using the MM5) by Kesu Zhang et al (SUNY, Albany) has suggested that the selection of boundary layer schemes is important in properly simulating the evolution of the LLJ. (Compared results to boundary layer measurements from Wind Profilers, RASS, Lidar, Aircraft, tethered balloon platforms).
Da-Lin Zhang (UMD, College Park) is also looking at a number
of low-level wind profiles from the Fort Meade Profile.

49. Vertical Winds Plots from the RUC2 model during haze/ozone event of August 2002
850 mb
1000 mb
850 mb
1000 mb
10 ms

50. 960mb winds predicted by the RUC2 model during a high ozone period (12 Aug, 2002)

51. Vertical x-section of wind speed predicted by the RUC2 model during a high ozone period (12 Aug, 2002)

52. Ozone concentration diminishing
After Sunset
If meteorological conditions remain the same, the temperature inversion forms again after dark as the ground cools faster than the air above.
Ozone concentration above the inversion comes to equilibrium with other pollutants and then remains at a constant, relatively high level.
Ozone trapped under the inversion reacts with other pollutants, particles and the surface; the ozone concentration diminishes.
Ozone concentration remaining constant
Altitude
Temperature Inversion
Ozone concentration diminishing
Temperature

53. The End of a High Ozone Episode
An ozone episode usually ends with the arrival of a ‘clean’ air mass:
This can occur with a cold front or other low-pressure system like a tropical storm.
An episode may also end prior to the passage of a cold front if widespread thunderstorms develop ahead of the front.

54. Thunderstorms This is an example of a strong
squall line bringing a high O3
event to a end.

55. Longer Pollution Episodes
While in this example, the episode lasted three days, it is not uncommon for high O3 events to persist for longer periods.
This episode in July, 1997 lasted 7 days in the Baltimore metropolitan area.