Urban Meteorology I

October 10, 2007

Urban Meteorology

• Special concerns– Severe weather– Air quality– Urban runoff– Climate change

Air Pollutants

• Air pollutants – airborne substances (solid, liquid, or gas) that occur in concentrations high enough to threaten the health of people and animals.

• Come from natural and anthropogenic sources

• Natural sources – wind, volcanoes, forest fires

Air Pollutants

• Antropogenic pollution is from fixed and mobile sources

• Pollution is either primary or secondary air pollution– Primary enters the atmosphere directly:

smokestack, tail pipes– Secondary forms when a chemical reaction

occurs between a primary pollutant and the air or sun

Particulate Matter Pollution

• The most visible type of pollution in the atmosphere

• Natural and anthropogenic

• Particulates are particles of dust, soot, sulfuric acid, PCBs, asbestos, and ash that are small enough to be suspended in the air

Particulate Matter Pollution

• Particulate are hygroscopic, so water vapor is attracted to them in the atmosphere

• This creates a wet haze in the atmosphere – water vapor condensing around particulates, scattering sunlight, and giving the sky a milky appearance

Carbon Monoxide

• The most plentiful of the primary pollutants

• Produced by the incomplete combustion of carbon

• Thanks to Clean Air laws, has been reduced by 40% since 1970

• Removed from the air naturally by soil microbes (which there aren’t many of in cities)

Carbon Monoxide Pollution

• Why is it a problem?

• In areas with poor ventilation (parking garage, cities with inversion layers close to the surface, valley bottoms), carbon monoxide collects

• Human hemoglobin prefers carbon monoxide to oxygen

• Leads to brain suffocation

Smog

• Smog = smoke + fog

• Large cities experience photochemical smog = smog + sunlight– aka Los Angeles-type smog

• When smog is sulfurous, its London-type smog– Certain types of coal have more sulfur than

other types

Ozone

• Troposheric ozone is BAD!!!– Secondary pollutant– Nitrogen dioxide + sunlight = nitric oxide and

atomic oxygen– Atomic oxygen + oxygen = ozone– Ozone production is higher during the

summer months and in the afternoon when there is sunlight

• Peak between 2 and 4 pm

Air Quality Index

• Includes carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone, and particulates

• Amounts of primary pollutants has decreased

• Harder to decrease secondary pollutants

Air Pollution

• Factors affecting air pollution– Wind– Inversions

• Stable vs unstable• Height of smoke stacks• Radiation inversion – night and early morning• Subsidence inversion – long-lived

– Topography • Inversion created by and pollution trapped in

valleys

Subsidence Inversion

Radiation Inversion

Los Angeles – Subsidence Inversion

Ingredients for Pollution Buildup

• Many sources of air pollution in a small space (like a valley or basin)

• High pressure cell

• Light or absent surface winds

• Subsidence inversion

• Clear skies to promote radiation inversion

• Sunlight

Urban Heat Islands

• More pronounced at night

• Causes thermal low to set up over cities

• Country breeze flows into city from suburbs into the low

• If no inversion present, increases the likelihood of precipitation

Urban Heat Islands

• Humid cities have more rainfall than the surrounding countryside

• Areas downwind from humid cities have increased rainfall, particularly during weekdays

• Caused by increase of airborne pollutants that serve as condensation nuclei

Urban Heat Islands

• Reasons– Pavement absorbs more heat than soil or

plants – releases it slowly at night making cities warmer

– Buildings create urban canyons that disrupts air flow and movement of pollutants

– Pollutants can either trap or block radiation– Excess water vapor produced

Urban Heat Islands

• Change surface air flow – Causes air to flow into cities from the

countryside– If water body nearby, water vapor pulled into

the cities– Atlanta is 5-8ºF warmer than the surrounding

countryside