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