VOCs, NOx, and Ozone: How Industrial Emissions Create Smog
Ground-level ozone — the primary component of smog — is not emitted directly by vehicles or industrial facilities. Instead, it forms through photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of sunlight. This chemistry is why ozone is worst on hot, sunny, stagnant-air days, and why industrial facilities that emit VOCs and NOx contribute to ozone even in cities miles away from the emission source.
Sources of VOC Emissions
VOCs are a diverse category of organic compounds that evaporate readily at room temperature. Industrial sources of VOC emissions include: petroleum refineries (where hydrocarbons evaporate during processing), chemical manufacturing plants, printing facilities (from solvents in inks), paint and coating operations, and dry cleaning facilities. TRI reports VOC releases from facilities above threshold quantities. Browse facilities releasing VOCs at the chemical index, which includes individual VOC compounds like benzene, toluene, xylene, and formaldehyde.
Sources of NOx Emissions
Nitrogen oxides (primarily NO and NO₂) form when any fuel is burned at high temperatures — the heat causes atmospheric nitrogen and oxygen to combine. Industrial sources of NOx include: electric power plants, industrial boilers and furnaces, cement kilns, and internal combustion engines. The Clean Air Act Title IV acid rain program (which uses a cap-and-trade system for NOx and SO₂) has driven significant reductions in power sector NOx emissions since 1990. Find facilities with CAA air violations at the violations filter.
The Ozone Formation Chemistry
When VOCs and NOx are present in the lower atmosphere with sunlight, a complex series of reactions produces ozone. The reaction is nonlinear: reducing either VOCs or NOx by itself may not reduce ozone if the other precursor is abundant. In some urban areas, ozone is "NOx-limited" — more NOx reductions produce more ozone reduction. In others, it\'s "VOC-limited." This chemistry complicates ozone control strategies and requires EPA and states to model precursor emissions and atmospheric chemistry simultaneously.
Health Impacts and Air Quality Standards
Ground-level ozone irritates the respiratory system, exacerbates asthma, and impairs lung development in children. EPA sets National Ambient Air Quality Standards (NAAQS) for ozone; areas that don\'t meet these standards are designated "nonattainment" and must develop State Implementation Plans to reduce emissions. Many of the country\'s most industrialized regions — the Houston Ship Channel, Southern California, the Chicago metro area — have faced persistent ozone nonattainment. Browse industrial facilities by state at our state browser and explore PFAS and carcinogen releasing facilities separately at PFAS and carcinogens.