Nitrogen Dioxide (NO2) is one of a group of highly reactive gases known as “nitrogen oxides” (NOX). There are many nitrogen oxides, but for regulatory purposes NOX is the sum of nitric oxide (NO) and nitrogen dioxide (NO2). Nitrogen oxides are combustion products and can come from engines and boilers.
Nitrogen is abundant as N2 in ambient air and is a constituent in varying forms in fuels. Nitrogen emissions can be controlled before, during, and after combustion. Pre-treatment removes nitrogen from fuels. Specialized low-NOX burners use a controlled mix of air and fuel to minimize NOX during combustion. Post-combustion treatment uses a catalyst to return nitrogen to a non-toxic form (N2), the same as in ambient air.
NO and NO2 are the two main constituents of NOX and exist in a daytime equilibrium. NO can react with atmospheric oxygen to form NO2, while solar radiation can breakdown NO2 to again form NO and oxygen. The oxygen released from NO2 can react with diatomic oxygen (O2) to form ozone (O3), so NOX is one precursor to ozone pollution.
There are two federal standards for nitrogen dioxide: a 1-hour 100 parts per billion (ppb) primary standard and a 53 ppb annual mean primary/secondary standard. Oklahoma is in attainment with both standards.
The division monitors NO2 at three sites in Oklahoma City and one site in Tulsa. The OKC Near Road monitor opened in March 2015 just off the I-44 highway and Will Rogers Park. All NO2 measurements are made using a chemiluminescent (light-producing chemical reaction) method. Monitors report hourly values. All four sites meet the NO2 standard. Current data may be accessed here.
NO2 concentrations continue to decrease as a result of a number of mobile source regulations for light- and heavy-duty vehicles. Implementation of these regulations is staggered over multiple years for manufacturers to design cleaner burning engines. Current air quality monitoring data does not reflect all the benefits of vehicles that meet these strict NOX standards because many older vehicles remain. These reductions are even more significant because NOX is a precursor to ozone, another air pollutant.
The 100 ppb standard is measured based on the 98th percentile of 1-hour daily maximum concentrations, averaged over three years. The 53 ppb standard is an annual mean. The OKC Near Road NO2 monitor was added in March 2015 and cannot be used for official comparison to the 100 ppb standard until it records 3 years of complete data.
2016 NO2 Values vs. Annual Arithmetic Mean NAAQS
Annual Arithmetic Mean NO2 concentration (ppb)
2016 NO2 Values vs. 1 Hour NAAQS
3 yr avg ('14-'16) 98th percentile
1 hr avg NO2 concentration (ppb)
*Incomplete Data Set (site opened March 2015)