Source Contribution to Connecticut Ozone

Source Contribution to Connecticut’s Ozone Problem

Connecticut uses emissions inventories, such as the National Emissions Inventory (NEI), to assess sector contributions to Connecticut’s ozone problem. Based on inventory results, control strategies can be developed to reduce nitrogen oxides (NOx) and volatile organic compounds (VOCs), which form ozone in the presence of sunlight. The sectors and subcategories of the inventory are:

• NEI point sources, which include emissions estimates for larger sources that are located at a fixed, stationary location.  Point sources in the NEI include large industrial facilities and electric power plants, airports, and smaller industrial, non-industrial and commercial facilities.  
• NEI nonpoint sources, which include emissions estimates for sources which individually are too small in magnitude to report as point sources.  These emissions sources are included in the NEI as a county total or tribal total (for participating tribes).  Examples include residential heating, commercial combustion, asphalt paving, and commercial and consumer solvent use.
• NEI onroad sources, which include emissions from onroad vehicles that use gasoline, diesel, and other fuels. These sources include light-duty and heavy-duty vehicle emissions from operation on roads, highway ramps, and during idling. 
• NEI nonroad sources, which include off-road mobile sources that use gasoline, diesel, and other fuels. Source types include construction equipment, lawn and garden equipment, aircraft ground support equipment, locomotives, and commercial marine vessels.
• NEI "event" sources, which include wildfires and prescribed burns that are reported in a day-specific format.

Past ozone control strategies for NOx have centered around point source Electrical Generating Units (EGUs). Until recently, EGU based strategies have been effective in reducing long-range air pollutant transport into Connecticut, but increasingly, nonpoint, onroad, and nonroad sectors have become the dominant source of NOx production. The chart below compares NOx and VOC emissions in Connecticut from the 2017 and 2020 NEIs.

The data show that nonpoint and onroad sources have the largest NOx emissions, while nonpoint, onroad and nonroad sources have the largest VOC emissions. From 2017 to 2020, Connecticut saw a 21% decrease in NOx emissions and a 10% decrease in VOC emissions across all four sectors of the inventory. Looking more closely, the nonroad and nonpoint sectors saw increases in VOC emissions while every other sector saw decreases in both ozone precursor emissions.

Another important emissions inventory is the Neighborhood Emission Mapping Operation (NEMO), which is a high-resolution gridded inventory (1km) that has been produced to be used in photochemical modeling. The 2019 NEMO NOx inventory, shown below, is mapped for southwest Connecticut and New York City (without EGU point sources) to show where the nonEGU emissions are concentrated. At this resolution, the NOx emissions along the major highways are evident. Since the onroad NOx emissions account for a large percentage of ozone precursors, this is a source category that needs further reductions.

Satellite technology has evolved to a point where nitrogen dioxide emissions (NO2) can be resolved on an urban and hourly scale using the Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument. The following TEMPO satellite tile plot shows hourly tropospheric NO2 concentrations on June 4, 2026, from 9 AM to 4 PM with white areas representing clouds. The TEMPO NO2 subplots indicate the presence of the New York City (NYC) metropolitan NO2 plume extending into southern New England in the morning and decreasing throughout the day as ozone formation occurs. By observing hourly NO2 concentrations, we can understand the diurnal cycle of NO2 and track the movement of the NO2 plume contributing to elevated ozone levels in Connecticut.

In 2023, the Synergistic TEMPO Air Quality Science (STAQS) study was conducted over four domains, including Los Angeles, New York City, Chicago, and Toronto. A high resolution NO2 instrument (GCAS) was used to locate sources of NO2 emissions. The last map shows some of the point sources upwind of Connecticut on July 26, 2023, at 9 AM (13z). Individual plumes from larger point sources can be identified within the NYC metropolitan emissions plume as winds carry the plume toward Long Island Sound and Connecticut.

Content last Updated July 7, 2026