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

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.

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?

Pollution in the Mid-Atlantic Region

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.

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 www.epa.gov

Not Every Hot day Results in Poor Air Quality

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

A Typical Day in a Pollution Episode H

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

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

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

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 > ~ 200- 800m (???) Small scale < 200m (???)

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 80-99 100-110 111-124 125+ 61-79

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

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.

Free Atmosphere Height (km) 2 Layer Planetary Boundary Residual Layer 1 Mixed Layer Residual Layer Mixed Layer Low Level Jet Stable Nocturnal Boundary Layer surface layer surface layer surface layer afternoon sunset midnight sunrise noon

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.

A Typical Day in a Pollution Episode

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.

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.

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.

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

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.

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."

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

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

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.

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.

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

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)

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

General Characteristics of a Low Level Jet Maximum winds are typically observed between 300-500 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

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.

NOAA Forecast Systems Laboratory Profiler Network

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

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

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.

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

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

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

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

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.

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

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.