ATTACHMENT B: 

CLEAN AIR ACT § 110(a)(2)(D)(i)(I) – Interstate Transport Provisions

“Each such plan shall [...] contain adequate provisions 

(i) Prohibiting, consistent with the provisions of this subchapter, any source or other 

type of emissions activity within the state from emitting any air pollutant in 

amounts which will:

(I) Contribute significantly to nonattainment in, or interfere with 

maintenance by, any other State with respect to any such national primary 

or secondary ambient air quality standard, or

The District of Columbia (District) does not contribute to nonattainment in, or interference with 

maintenance by, any other state with respect to the 2008 8-hour ozone national ambient air 

quality standards (NAAQS). As explained below, emissions of oxides of nitrogen (NOx) from 

sources in the District are small relative to those from other states. State Implementation Plan 

(SIP)-approved regulations to reduce emissions from sources that emit NOx and volatile organic 

compounds (VOCs) are in place as appropriate. The District’s ozone air quality is significantly 

impacted by transported pollution from outside its boundaries and is heavily reliant on federal 

measures and reductions from other states.

Ozone Trends

Ambient measurements for ground-level ozone (O3) are conducted at three air monitoring sites in 

the District. The 8-hour O3 concentrations have generally dropped over time at the District’s 

monitors. The chart in Figure 1 below shows the 8-hour O3 design concentrations from the 

District’s lead monitor and demonstrates how, over time, O3 air quality has improved in the 

District.
Figure 1: Design Values at the McMillan Monitoring Station Over Time

The trend is similar to design value trends in the Washington DC-MD-VA ozone nonattainment 

area as a whole (shown in Figure 2 below).

Figure 2: Design Values in the Washington DC-MD-VA Area Over Time

0.060

0.065

0.070

0.075

0.080

0.085

0.090

0.095

0.100

0.105

8-Hour O3 Design Values at McMillan 

2008 8-hr Ozone NAAQS: 0.075 ppm

1997 8-hr Ozone NAAQS: 0.084 ppm
The District’s Contribution and SIP-Approved Emissions Reduction Efforts

Emissions of NOx and efforts to control NOx are generally the primary focus of interstate 

transport related to O3 precursor pollution. Of all NOx emissions generated in the Washington 

DC-MD-VA nonattainment area, the District contributes a nominal portion – about nine percent 

(2011 NEI Version 1). 

EGUs: Electric generating units (EGU) are a significant source of NOx and this sector is a major 

contributor to interstate transport of O3 precursor pollution. Currently, there are no EGUs or 

other large industrial sources of NOx emissions in the District. The District’s last remaining 

EGUs were decommissioned in 2012, in part to meet permit requirements incorporated into the 

District’s Regional Haze State Implementation Plan (SIP) (77 Fed. Reg. 5191; codified at 40 

C.F.R. § 52.470(e)). 

Non-EGUs: The District has only one “non-EGU” facility with an ozone season cap applicable 

under EPA’s NOx SIP Call: the United States General Services Administration (GSA). GSA’s 

three regulated units are required to meet a combined stringent NOx emissions limit of 25 tons 

per ozone season. None of the other stationary point sources regulated under the Clean Air Act 

Title V program in the District have units large enough (over 250 mmBTU/hr in size) to require 

NOx controls specifically to address interstate transport. 

In general, point sources in the District contribute a small amount of NOx compared to other 

sectors, as illustrated in Figure 3. 

Figure 3. NOx Emissions by Sector

In the District, the primary source of point and area source NOx emissions is fuel combustion, 

mainly from smaller boilers. Most fuel burning equipment in the District requires a permit; only 

fuel burning equipment with a capacity of five million British thermal units or less per hour of 

heat input is exempt from general permitting requirements at 20 DCMR Chapter 2 (see § 

200.12). Lowest Achievable Emission Rate (LAER) controls and emission offsets are required 

point

7%

area

17%

nonroad

24%

onroad

52%

2011 NOx Emissions in the District
on sources that trigger the District’s revised nonattainment NSR requirements (§ 204). (Revised 

Chapter 2 regulations were proposed to EPA as a SIP revision on April 5, 2013.) Under new 

minor NSR requirements (§ 209), new or modified stationary sources with potential emissions 

exceeding certain thresholds must meet a minimal level of control consistent with state-of-the-art 

technology for that type of equipment.

Mobile Sources: The District implements an Enhanced Vehicle Inspection and Maintenance 

(I/M) program to control emissions from onroad vehicles. The District also does extensive 

outreach to educate tour bus drivers and others who may be impacted by the District’s strict

engine idling time limits at 20 DCMR § 900.1. Otherwise, the District relies heavily on federal 

measures to reduce NOx emissions from onroad and nonroad engines. 

Presented in Figure 4 below are the emission inventories for the District by source category 

based on official National Emissions Inventory (NEI) estimates by the U.S. Environmental 

Protection Agency (EPA). 

Figure 4. NOx Emissions in the District by Source Sector Over Time.

The District’s emissions show a significant downward trend despite increases in population, 

employment, households, and VMT over time.

Contributions from Other States

EPA’s modeling analysis for the proposed Cross State Air Pollution Rule (CSAPR)1projected 

that in 2012, states outside of the Washington DC-MD-VA nonattainment area would account

 

1 On August 21, 2012, the U.S. Court of Appeals for the District of Columbia Circuit vacated CSAPR. EME Homer 

City Generation, L.P. v. EPA (696 F. 3d 7 (D.C. Cir. 2012)). On June 24, 2013, the U.S. Supreme Court granted 

EPA’s petition for certiorari. The Supreme Court heard arguments on December 10, 2013.

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

1996 1999 2002 2005 2008 2011

NOx Emissions By Sector

Point Area Nonroad Onroad
for 74 percent of the region’s ozone pollution. Actual monitored design values in 2012 at the 

District’s three monitoring sites for ozone were higher than projected.

District of Columbia

Monitor Site

2012 Base 

Case Average 

Values (ppb)

2012 Base Case 

Maximum 

Values (ppb)

Actual 

DVs, 2010 

(ppb)

Actual 

DVs, 2011 

(ppb)

Actual 

DVs, 2012 

(ppb)

110010043 (McMillan) 76.9 79.0 79 79 84

110010041 (River 

Terrace) 72.9 77.2 77 76 80

110010025 (Takoma) 72.7 73.9 75 75* N/A*

* There was a fire at the Takoma monitoring station in 2011, so measurements ceased at that location.

In the CSAPR preamble, EPA suggested that the District does not have sources of NOx

emissions within its boundaries that contribute to transported NOx pollution in other states. 

Hence, EPA did not propose to establish a NOx budget or finalize a FIP for the District upon 

conclusion that, “there are no emissions reductions available for EGUs...at the cost threshold 

deemed sufficient to eliminate significant contribution to nonattainment and interference with 

maintenance of the NAAQS...” (76 Fed. Reg. 48262; August 8, 2011). Since EPA’s analysis for 

the proposed CSPAR rule2, the District’s stationary point sector NOx emissions have actually 

reduced further because of the decommissioning of facilities.

In its analysis regarding the District’s designation recommendation for the 2008 8-hour ozone 

NAAQS, EPA noted that it “preliminarily agrees...that an appreciable part of the air quality 

problem within the District of Columbia is due to emissions outside its borders.” Reasons cited 

in EPA’s analysis include the fact that the District is surrounded by three of the “top five” 

counties for emissions in the Washington-Baltimore-Northern Virginia Combined Statistical 

Area (CSA) and the top three in the DC-MD-VA nonattainment area: Fairfax County (VA), 

Montgomery County (MD), and Prince George’s County (MD). At the time of EPA’s analysis, 

the District’s VMT was only one-twelfth that of the Washington DC-MD-VA nonattainment 

area. A large percentage of the time, adjacent and geographically close counties and cities in 

Virginia are upwind of the District.3 

Conclusion

The District of Columbia is a small jurisdiction with no significant sources of NOx and VOC 

emissions and little opportunity to reduce precursor emissions of ground-level ozone. A 

significant part of the air quality problem within the District is due to transported pollution from 

outside of its borders. While the District does what it can to adopt new control measures, it 

historically has and will likely continue to rely heavily on Federal measures particularly from the 

point and (on-road and nonroad) mobile sectors, and measures in other states.

 

2 U.S. EPA, “Air Quality Modeling Final Rule TSD: Contributions of 8-hour ozone, annual PM2.5, and 24-hour 

PM2.5 from each state to each monitoring site,” found at: http://www.epa.gov/crossstaterule/techinfo.html

3 U.S. EPA, “EPA Response” letter for the District of Columbia (December 9, 2011), found at: 

http://www.epa.gov/ozonedesignations/2008standards/rec/region3R.htm