14-26 Photochemical smog

APES 2024

Primary pollutant

Primary pollutant

A polluting compound that comes directly out of a smokestack, exhaust pipe, or natural emission source, ex. CO, CO2 , SO2 , NOx, and most
suspended particulate matter, volatile organic compounds (VOCs)

Secondary pollutant

A primary pollutant that has undergone transformation in the presence of sunlight, water, oxygen, or other compounds, Ex. O3 , sulfate, and nitrate

The transformation from primary to secondary pollutant requires

sunlight, water (clouds), and appropriate temperature

Air Pollution Control Act (1963)

funded research and encouraged emissions standards

Clean Air Act of 1970

• Set standards for air quality, limits on emissions

• Provided funds for pollution-control research

• Allowed citizens to sue parties violating the standards

Clean Air Act of 1990

• strengthened regulations for auto
  emissions, toxic air pollutants, acidic deposition, stratospheric ozone depletion

• Introduced emissions trading for sulfur dioxide

• Through the National Ambient Air Quality Standards (NAAQS) the EPA periodically specifies concentration limits for each air pollutant

Coal combustion releases the air pollutants

• carbon dioxide

• sulfur dioxide

• toxic metals

• particulates

Combustion of fossil fuels releases

nitrogen oxides (NOx) into the atmosphere

Release of sulfur dioxide during burning of diesel fuels can lead to

poor air quality

Other pollutants from fossil fuel combustion

carbon monoxide, hydrocarbons and particulate matter

In the US, recent EPA data show that

on-road vehicles (transportation) are the largest
source of carbon monoxide and nitrogen oxides

The major source of anthropogenic sulfur dioxide is

the generation of electricity, primarily from coal

Among the sources of particulate matter are

• road dust

• industrial processes

• electricity generation

• natural and human-made fires

In the US, between 1990-2020

All criteria air pollutants have decreased and lead decreased the most

Smog

A type of air pollution that is a mixture of oxidants and particulate matter

Photochemical oxidant

A class of air pollutants formed as a result of sunlight acting on compounds, ex. nitrogen oxides

Los Angeles/brown type/photochemical smog

dominated by oxidants, ex. ozone

Sulfurous/London-type/Gray/Industrial smog

dominated by sulfur dioxide, sulfate compounds and particulate matter

Tropospheric ozone and photochemical smog
formation

• Nitrogen oxide is produced early in the day
  from combustion of fossil fuels (cars driving
  to work)

• Broken by sunlight in NO and O

• Sunlight – drives O3 formation by breaking down NO2 into NO and O, then the free O combines with O2 to make O3

Hotter atmosphere speeds up

ozone formation, evaporation of VOCs and smog formation

Ozone concentrations peak in the

afternoon and are higher in the summer

Factors that increase smog formation

• Higher VOCs emissions (gas stations, laundromats, plastic factories

• Increased traffic leads to more NO2 emissions and more O3 formation

• Urban areas have more smog

Photochemical Smog impacts

• Reduces sunlight

• O3 damages plant stomata

• O3 irritates animal respiratory tracts

• Worsens asthma and bronchitis

• Irritates eyes

• Increased healthcare costs

• Lost productivity due to sick days

Ways to reduce photochemical smog

• Decrease number of vehicles on the road = less NO2 emissions

• Fewer cars means less VOCs

• Carpooling, public transportation, work from home, biking

• Using more renewable energy sources

• Natural gas releases less NOx than coal