Source Contribution to Connecticut’s Ozone Problem

Photochemical models use meteorological data and emissions information from the National Emissions Inventory (NEI) to assess current and predicted future ozone concentrations.  Based on results from these models, control strategies can be developed to reduce the pollutants, generally nitrogen oxides (NOx) and volatile organic compounds (VOCs), which in the presence of sunlight, form ozone.  Photochemical models can provide ozone contributions from subcategories of sectors within the emission inventory to assess the impacts of potential control strategies on specific source categories. 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 nitrogen oxides (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, area sources and on-road and non-road the mobile source sectors have become the dominant source of NOx production. The pie chart below is an example of modeling done by the Ozone Transport Commission (OTC) modeling centers and shows the source of modeled contributions to ozone from each source sector at the Westport Connecticut coastal ozone monitor. Inventory Sector Contribution to Westport Ozone

In this example, The EGU sector emissions percentage contribution to the ozone production is 14% of the total. If you add up all the mobile sectors, both on-road and non-road, the percentage contribution reaches 43% of the total. This chart shows the contributions from everywhere in the modeling domain, however, it is also possible to produce a chart with the contributions from all the States.

The next pie chart shows the percent contributions by state, with the model boundary conditions (BC) and biogenic emissions (such as isoprenes from trees) included. For these modeled top ten ozone days, Connecticut sources contributed only about 3% to the ozone production impacting the Westport Connecticut ozone monitor. The largest State contributions were from New York, Pennsylvania and New Jersey, with 16%, 9% and 8% respectively.

State Contribution to Westport Ozone

The EPA recently conducted ozone modeling for the “Good Neighbor Plan for 2015 Ozone NAAQS” to determine which States were the largest contributors to ozone at non-attainment monitors. The following map shows the parts per billion (ppb) of ozone that each State contributed to our Westport monitor on the ten highest ozone days. Connecticut sources contributed less than 3 ppb to the ozone, while New York State contributed over 14 ppb on those days.

2023 Modeled Ozone Contributions to the Westport Monitor

Although EPA is proposing further reductions for EGU NOx and some very large non-EGU sources, it is likely that these reductions will not be enough to put the Connecticut coastal monitors in attainment for the 2015 ozone NAAQS by August 3, 2024. Thus, reductions for other source categories must be considered.

A high-resolution gridded NOx inventory (1km) has been produced to be used in photochemical modeling. This inventory was mapped for southwest Connecticut and New York City (without EGU point sources) to show where the non-EGU emissions are concentrated. At this resolution, the NOx emissions along the major highways are evident. Since the on-road mobile emissions account for a large percentage of the ozone produced at the Westport monitor, this is a source category that needs further reductions.

High Resolution NOx Inventory Map

Satellite technology has evolved to a point where nitrogen dioxide emissions (NO2) can be resolved on an urban scale. The following TROPOMI satellite map shows averaged NO2 concentrations for the summer of 2021.  It plainly shows that urban areas have concentrated areas of NO2 emissions. The map also shows outlines of ozone non-attainment areas in relation to the nearby upwind areas of elevated NO2 levels.

Summer 2021 TROPOMI Satellite NO2

In 2018, the LISTOS study was conducted over Long Island Sound. 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.  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.

2018 LISTOS NO2 Plume Image

Content last Updated May 12, 2022