Background Air Quality

A demonstration of compliance with ambient air quality standards must account for background air quality levels. Background air quality means ambient levels of pollutants not associated with any of the sources explicitly included in the modeling analysis.

Background levels are established for each regulated pollutant and averaging period being assessed. These values are added to modeled impacts to obtain total impacts, which are then compared with applicable ambient air quality standards.

Conservative rural and urban background values are available for all four Air Quality Control Regions (AQCR) of the state:

RURAL BACKGROUND VALUES
URBAN BACKGROUND VALUES

In addition, the applicant may propose background values determined from a database(s) collected at a monitoring site considered to be representative of a topographic, geographic, and/or demographic setting similar to that of the applicant's impact areas.

One should recognize that more refined data analysis may have to be conducted if the data contained in this table does not permit an attainment demonstration based on modeling. If this proves to be the case, appropriate raw data should be obtained from MEDEP - Field Services Division for the pollutant in question. These data can then be more thoroughly reviewed and a more refined background data determination made.

General guidance on determining background determinations based on monitoring data is provided in the most recent version of the Guideline Document For Background Air Quality Determinations (MEDEP-BAQ, 1993). Prior approval from the MEDEP-BAQ is required for this method and will be granted on a case-by-case basis.

The tables that follows lists the background data values by region on a pollutant/averaging period basis. The data presented here is typically considered to be conservative. Many of the data values for a given region were obtained from a monitoring station outside of the region, as not all regions of the state have (or had) monitoring stations for the necessary parameters. Consequently, the listed data value for a given parameter in a particular region may not be truly representative of the background air quality for that region. In those instances where data could not be obtained from within a region, an alternative sampling site was chosen based on community size and the character of the sources impacting the alternative site being relatively similar to the region of interest.

Rural Background Data Values

Pollutant	Southern Maine	Central Maine	Eastern Maine	Northern Maine
SO2
1-Hour	471	471	471	471
3-Hour	182	182	182	182
24-Hour	112	112	112	112
Annual	12	12	12	12
PM10
24-Hour	413	474	425	326
Annual	93	114	105	126
PM2.5
24-Hour	-	-	-	-
Annual	-	-	-	-
CO
1-Hour	6567	6567	6567	6567
8-Hour	4797	4797	4797	4797
NOx
1-Hour	478	478	478	478
Annual	38	38	38	38

Urban Background Data Values

Pollutant	Southern Maine	Central Maine	Eastern Maine	Northern Maine
SO2
1-Hour	1071	1071	1071	1071
3-Hour	732	633	633	694
24-Hour	442	293	293	294
Annual	102	93	93	54
PM10
24-Hour	465	513	636	757
Annual	245	213	216	177
PM2.5
24-Hour	-	-	-	-
Annual	-	-	-	-
CO
1-Hour	36008	36008	36008	36008
8-Hour	24598	24598	24598	24598
NOx
1-Hour	949	949	949	949
Annual	3210	3210	3210	3210

KEY:
1. EMPACT Site - Portland (Site #23-005-0027), 2004-2006 data, 1-hour SO₂ data is based upon design value for urban SO₂
2. EMPACT Site - Portland (Site #23-005-0027), 2003-2006 data
3. CKP Site - Lewiston (Site #23-001-0011), 2002 data
4. Richardson Site - Easton (Site #23-003-1018), 2003-2005 data
5. EMPACT Site - Portland (Site #23-005-0027), 2003 data
6. Bangor KPS Site (Site #23-019-0002), 2003-2004 data
7. Riverside Site - Presque Isle (Site #23-003-1011), 2006 data
8. Chittendon Site - Burlington, VT (Site #50-007-0014), 2004-2007 data, urban CO site that is comparable to urban areas in Maine sites
9. EMPACT Site - Portland (Site # 23-005-0027), 2004-2006 data, 1-hour NO_x Data is based upon design value for urban NO₂
10. EMPACT Site - Portland (Site # 23-005-0027), 2004-2006 data