Sulfur dioxide air quality designations

EPA evaluates how each area of the country meets the national air quality standards for sulfur dioxide (SO2) and other air pollution through a process called “area designations” or “air quality designations.”

EPA and states determine whether an area meets the standards by using a specified value, called a design value. In plain terms, the lower the design value, the cleaner the air. Nearly all Washington counties — 36 out of the state's 39 counties — meet national air quality standards for SO2. Ecology is reviewing the remaining three counties.

Federal air quality classifications

Under the federal Clean Air Act, if the air quality in a geographic area meets or is cleaner than the national standard, it is called an attainment area; areas that don't meet the national standard are called nonattainment areas. In some cases, EPA is not able to determine an area's status after evaluating the available information. Those areas are designated "unclassifiable."

If an area does not meet a standard, then the state is required to create and follow a state implementation plan to bring that area back into attainment.

EPA and states determine whether an area meets this standard by using a specified value, called a Design Value. It is based on a three-year average of the fourth-highest, daily maximum 1-hour concentration recorded at a monitoring site. That three-year average number must be less than or equal to 75 parts per billion (ppb) of SO2. In plain terms, the lower the Design Value, the cleaner the air.

Only design values calculated based on the regulatory monitors that meet Ecology’s and EPA’s data quality system requirements are used to determine if an area is in violation of the standard.

Air quality designation process

In 2010, EPA established a new, lower SO2 standard. EPA evaluates and designates areas within three years of establishing a new standard. When the EPA’s 2010 designation process for SO2 was delayed, a federal court mandated deadlines to complete the process.

The area designation process involves collecting relevant air quality data such as emissions, monitoring values, and modeling. States provide a recommendation to EPA on how each area should be designated. EPA considers that recommendation and public comments to make a final determination. If an area is designated as being in nonattainment, the state must meet federally mandated deadlines for fixing the air quality issues. For areas designated as attainment or unclassifiable there are no additional requirements besides ongoing regular monitoring.

Washington area designations for SO2 

In December 2017, EPA designated 36 of Washington state's 39 counties:

  • EPA agreed with our recommendation for 34 counties and designated these counties as being in "attainment/unclassifiable."
  • EPA designated two counties, Lewis and Thurston, as "unclassifiable," because of limited information.
  • The three remaining counties (Chelan, Douglas, and Whatcom) needed further investigation and were not a part of the 2017 designations.

In Lewis and Thurston counties, Ecology modeled emissions from the TransAlta coal-fired power plant to demonstrate the area continues to meet the standard. However, the EPA requested additional evaluations to support the proposed attainment designation. Due to resource constraints and the low risk of the area violating the standard, we chose to concur with EPA’s decision to designate the area as "unclassifiable."

Recommendation for Chelan and Douglas counties

EPA required us to establish new SO2 monitoring sites near facilities that emit more than 2,000 tons of SO2 a year and to collect data for three years (2017-2019). As a result, Ecology began monitoring SO2 concentrations around two aluminum smelters, one located in Whatcom County near the city of Ferndale, and the other located in Chelan County in Malaga, near the border with Douglas County.

Ecology outlined the siting of the new monitors in the 2016 Annual Monitoring Network Plan and solicited public comments in May-June 2016. 

Monitoring ambient air tells us how much SO2 is in the air at any given moment for a given location. However, advanced modeling and other technical analysis is required to fill in gaps and determine what sources are contributing to the SO2 levels.

We received questions about why monitoring SO2 pollution near the aluminum smelter in Malaga, Chelan County, was needed since the plant curtailed its operations starting in 2016-2017. Regulatory requirements do not allow us to postpone monitoring, and the facility may begin operating again. At this time, the monitor near the facility shows that SO2 readings are well below the NAAQS.

Chelan and Douglas counties meet SO2 standard

In January 2016, Ecology notified EPA that we would be evaluating levels of SO2 around the Alcoa Primary Metal Works Wenatchee (Alcoa), an aluminum smelter in Chelan County, on the border with Douglas County. In 2015, Alcoa emitted 2,741 tons of SO— above EPA’s 2,000-ton per year threshold requiring additional evaluation. However, in 2016, Alcoa announced that it was curtailing operations at the smelter, meaning that production would cease, but that the company would maintain the facility in case conditions warranted reopening. Because curtailment is different from a permanent shutdown and the facility continues to maintain its permit allowing for maximum SO2 emissions, EPA required us to proceed with additional monitoring.

We collected monitoring data in 2017-2019 to identify a Design Value at the Malaga site — a regulatory criteria measuring the three-year average of the fourth-highest concentrations of SO2. We found that Malaga’s SO2 design value for 2017-2019 is 1 part per billion (ppb). The yearly fourth-highest concentrations were: 2017 – 1.1 ppb; 2018 – 1.2 ppb; and 2019 – 1.0 ppb. The Design Value of 1 ppb is substantially below the 75 ppb allowed under the federal SO standard. The Design Value for Malaga indicates that the area is attaining the standard.

There are no other industrial sources of SO2 in Chelan County. In Douglas County, Ecology permits SO2 emissions from the Greater Wenatchee Regional Landfill & Recycling Center. Its yearly SO2 emissions between 2011 and 2017 averaged 4.8 tons, with a maximum of 6.2 tons in 2016. This is well-below EPA’s threshold of 2,000 tons of SO2 emissions a year and does not trigger a need for additional monitoring.

Between the very low Design Value at the Malaga monitoring site and only one other significant SO2 source in the two counties, Ecology recommends that EPA designate the two counties to as being "in attainment / unclassifiable," meaning that they comply with the standard. Should Alcoa restart its operations and increase its emissions, ongoing federal requirements will ensure appropriate verification of the attainment status, via monitoring or modeling, in the future.

Additional information

Whatcom County designation process

We worked with the Northwest Clean Air Agency to evaluate air quality data from Whatcom County and found that most of the county meets federal air quality standards for SO2. However, data from monitors near the Intalco aluminum smelter in the Cherry Point Industrial Area indicated SO2 levels that exceeded the standard. 

Ecology and the Northwest Clean Air Agency prepared a technical report on our findings and submitted the report to EPA: 

To learn more about our efforts to monitor and evaluate SO2 levels in Washington, sign up to Ecology’s SO2 attainment email list.

Public health impacts

Breathing elevated levels of SO2 causes inflammation and tightening of airways, and can trigger asthma symptoms. SO2 typically dissipates very quickly, but people living or working close to the plant may have been exposed to short-term levels that pose a risk to those with preexisting breathing or health issues.

EPA established Air Quality Index (AQI) ratings for SO2 pollution to identify when the air quality becomes unhealthy. The table below provides a summary of number of days in each AQI category in 2017 and in 2018. 

Environmental impacts

Sulfur dioxide decreases visibility in the form of haze and contributes to acid rain.