1. AOSC 200 Lesson 27

2. A Typical Day in a Pollution Episode A common severe pollution weather pattern occurs when high pressure is centered just west of the Mid Atlantic region. Circulation around the high pressure center moves pollution from points west into the mid-Atlantic. H

3. Temperature inversions produce very stable atmospheric conditions in which mixing is greatly reduced. There are two general types of inversions: nocturnal (radiative) inversions at the ground and subsidence inversions aloft. Nocturnal inversions are the result of differential radiative properties of the Earth’s surface and the air above. The Earth is a much better absorber and radiator of energy than air; thus, in the late morning and afternoon hours the lower atmosphere is unstable. The opposite is true in the evening; a stable atmosphere with little vertical mixing prevails. Role of atmospheric stability

4. Temperature Inversion Temperature Temperature Inversion Altitude Pollution trapped below inversion

5. 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.

6. Inversions aloft (subsidence inversions) are associated with prolonged, severe pollution episodes. These types of inversions are caused by the sinking air associated with the center of high pressure systems (subsidence). As the air sinks it is warmed adiabatically. Turbulence at the very lowest part of the atmosphere prevents subsidence from warming that portion of the atmosphere. Los Angles pollution episodes as well as those over the Mid- Atlantic region are the result of inversions aloft associated with strong high pressure systems. Role of Atmospheric Stability

7. Fort Meade profile 6/19/2001

8. Westerly transport is often present when the highest ozone is observed in the mid-Atlantic. 24 hr. Back-trajectories on days of 1-hr. ozone exceedances from 1997 through 2002, Baltimore area

9. Aircraft measurements of ozone

10. Aircraft measurements of Sulfur Dioxide

11. Warmer >> << CoolerTemperature Assume we start at noon on “Day 1” with a relatively clean air mass

12. Warmer >><< CoolerTemperature As the sun sets, the surface begins to cool and a transition takes place

13. O3 Profile Elev (ft) 1,000 2,000 4,000 6,500 Wind Warmer >><< CoolerTemperature The surface cooling continues overnight

14. Warmer >><< CoolerTemperature On Day 2, the sun rises and the nocturnal inversion begins to erode

15. Warmer >><< CoolerTemperature By Noon on Day 3, Local Emissions and High “Background” O 3 Combine

16. This hourly O 3 graph for a summer day near Frederick, Maryland shows O 3 concentrations reaching a minimum in the early morning hours. O 3 Times Series

17. 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.

18. All three modes of transport are important when the highest pollution values are observed in the mid-Atlantic. Large scale ~ 800 km (~70-100 ppbv) (Much of the Eastern US) Medium scale ~ 200- 800 km (Carolinas to New England Region) Small scale ~ 100 km (N. Virginia to Baltimore, Research Triangle to NC/VA border) Modes of Transport

19. Sources Different types of transport imply different types of sources –Local transport Cars, industry, and other sources in the local area –Long distance transport Primarily power plants to W, though plumes from cities certainly contribute. –Low level jet Primarily cars and other low level sources; moves from SW to NE

20. Effects of the August 15, 2003 Blackout on Air Quality Compared with Aug 4, 2002 Selinsgrove, PA

21. Effects of the 2003 Blackout on Air Quality CEM data indicate reductions of 60-80%

22. Effects of the 2003 Blackout on Air Quality

23. In summary It isn’t all transport… It isn’t all local… It isn’t all power plants… It isn’t all automobiles… The problem will not be solved by addressing any one of these problems individually. ALL of these will have to show significant reductions for us to breathe clean air.