NO₂ to AQI Converter

Nitrogen dioxide is a reddish-brown irritant gas formed mainly by atmospheric oxidation of nitric oxide from high-temperature combustion, so concentrations peak beside busy roads and fall off sharply within 100 to 200 meters of a highway. Its sub-index is built like any other AQI component: locate the breakpoint pair that brackets the measurement and interpolate linearly between the matching index values. The EPA breakpoints are defined on a 1-hour average in parts per billion, with 0 to 53 ppb mapping to Good, 54 to 100 ppb to Moderate, 101 to 360 ppb to Unhealthy for Sensitive Groups, and the scale continuing to 2,049 ppb at index 500. The two lowest anchors are not arbitrary: 53 ppb is the annual primary standard set in 1971 and 100 ppb is the 1-hour standard added in 2010. Converting to mass units needs the molar volume, which varies with temperature; at 25 degrees C and one atmosphere it is about 24.45 liters per mole, so 1 ppb of NO2 at 46.01 grams per mole equals roughly 1.88 micro-g/m3. Two caveats dominate. Any averaging period other than one hour makes these breakpoints inapplicable, and chemiluminescence monitors using heated molybdenum converters overstate NO2 because they also reduce peroxyacetyl nitrate and nitric acid.

Reliable measurement long predates the index built on it. Wet chemical determination of nitrogen oxides descends from the diazotization reaction described by Peter Griess in the 1850s and 1860s, adapted for air sampling as the Saltzman method by Bernard Saltzman of the US Public Health Service in 1954, which trapped NO2 in an azo dye reagent read colorimetrically. Continuous monitoring became practical after 1970, when the chemiluminescent reaction of nitric oxide with ozone was developed into an analyzer; paired with a converter that reduces NO2 to NO, it became the federal reference method. Nitrogen dioxide was named a criteria pollutant under the 1970 Clean Air Act, and the EPA set the annual standard of 0.053 parts per million in 1971. It was carried into the Pollutant Standards Index in 1976, but for decades the NO2 sub-index was rarely the controlling pollutant in the United States because ambient levels stayed far below the alert breakpoints. That changed in January 2010, when the EPA added a 1-hour standard of 100 ppb aimed at short-term near-roadway exposures and required near-road monitoring stations in larger urban areas. The current breakpoint table, including the 54 to 100 ppb Moderate band, follows from that rulemaking.