Urban Air Pollution

Urban Air Pollution Public and Environmental Health Concerns Elevated levels of toxic compounds Regional and Global Impacts Background Chemistry and Composition Climate Joel Thornton, Asst. Professor ftp.atmos.washington.edu/thornton/atms501

The Urban Smog Problem • Major components: • “invisible”: O3, CO, SO2 • “visible”: PM (aerosols) + some gases (NO2) • Ingredients to Make Smog • Sun (photochemistry) • Stagnation • Sources of NOx, SO2, PM and VOC • Smog – “Smoke” + “Fog” • Coined due to reduced visibility associated with pollution episodes Houston, TX Aug. 2000

Ozone Damage Needle damage (tip necrosis) is a common sign of ozone stress on pines. Often observed in forests downwind of major urban areas—Sierra Nevada, New England, Mexico City, etc.

Large Subset of U.S. Population Exposed #of people living where NAAQS are not attained U.S. NAAQS 1-hr Avg O3 < 125 ppb 8-hr Avg O3 < 85 ppb 24-hr Avg PM2.5 < 60 g/m3 Annual Avg PM2.5 < 15 g/m3

Violation of O3 NAAQS By Region 1hr avg > 125 ppb 8hr avg > 85 ppb

Annual Average PM2.5 in Urban Areas

Surface O3 and Transport 90th percentile O3 concentrations for summers 1991-1995 and mean 850hPa winds on days when O3 > 90th percentile • Stagnation enhances chemistry • Persistent stagnation in regions of strong subsidence: LA, Mexico City, Athens • Air pollution is not just an urban problem

Chemical Production of O3: Main Ingredients Fossil Fuel Combustion and Use Evolution of NOx and O3 in Nashville, TN June 1999 NOx = NO + NO2 CO/VOC hv = uv-vis radiation Biogenic Activity

Cycling of HOx And NOx Leads to NET O3 1. NO + O3 NO2 + O2 k1 2. NO2 + hv  NO + O k2 3. O + O2 + M  O3 k3 predict Often (not always) implies NO + XO2 NO2 + XO

Net O3 Production The Null Cycle NO2 O2 hn O3 NO O3 The Net O3 Production Cycle HO CO NO2 hn HO2 O3 NO CO2

Questions • The rate limiting step in NET O3 production is the conversion of NO to NO2 by peroxy radicals. What is the rate expression for photochemical O3 formation? • Ronald Regan famously noted that “trees pollute too”. What did he mean by this statement?

Biogenic VOC: Major role in O3/PM Pollution O2 Important source of peroxy radicals: enhance O3 production + OH  RO2 In U.S. isoprene emissions > total anthropogenic hydrocarbon emissions GEIA Isoprene Emissions In July, Guenther, et al [1012 atoms C cm-2 s-1]

Global Impacts of Urban Air Pollution CO - MOPPIT Urban areas (mega-cities) are major point sources NO2 - OMI Courtesy of P. Weiss-Penzias and D. Jaffe, UWB Global pollution transport makes meeting own air quality standards more challenging

Questions • A vast majority of NOx is emitted at the surface, and most NOx is a result of human activity. Given that NOx catalyzes O3 production, it is important to consider the global impact of anthropogenic NOx emissions. • NOx is removed from the atmosphere primarily by the reaction NO2 + OH  HNO3, k ~ 1x10-11 cm3 molec-1 s-1 what is a typical lifetime for NOx w.r.t. this process? ([OH] ~ 1x106 molec cm3) • Is this lifetime sufficient to allow NOx to be transported away from an urban area to the remote troposphere? • Do anthropogenic emissions of NOx reach the remote troposphere at all?

Questions • An important biogenic hydrocarbon is isoprene. Isoprene is very reactive towards OH. Given that [Isoprene] ~ 1 ppb, and [propane] < ~ 1 ppb outside of Atlanta, would it make sense to try to regulate propane emissions to combat O3 production? O2 k ~ 1x10-11 cm3 molec-1 s-1 + OH  RO2 O2 k ~ 1x10-13 cm3 molec-1 s-1 CH3CH2CH3 + OH  RO2 + H2O

Urban Air Pollution