28.2 EPA-developed estimates
The calculations for estimating the emissions from the burning of yard waste first involve estimating the amount of leaf and brush waste generated in each county. The amount of county-level yard waste burned is estimated by calculating the per capita amount of leaf and brush waste generated and multiplying that by the number of people likely to burn waste in each county. The number of people likely to burn waste is based on the rural population in each county. The total amount of yard waste burned is multiplied by emissions factors for criteria air pollutants (CAPs) and hazardous air pollutants (HAPs) to estimate emissions of these pollutants from yard waste burning.
28.2.1 Activity Data
The activity data for this source category is the amount of leaf and brush waste generated, which is estimated using data from the EPA report Advancing Sustainable Materials Management [ref 1]. The report presents the total mass of waste generated from the residential and commercial sectors in the United States by type of waste for the calendar year 2018.
The per capita value of yard waste subject to burning was developed based on EPA’s total amount of waste generated [Table 1 in ref 1]. According to the 2010 version of the same EPA report, residential waste generation accounts for 55-65% of the total waste from the residential and commercial sectors [ref 2]; for the per capita calculation, the median value of 60% of total waste generated is assumed. This number is multiplied by the amount of yard waste generated and divided by the U.S. population in 2018 [ref 3] to determine the per capita amount of yard waste generated in the United States.
\[\begin{equation} PC_{yw} = \frac{YW \times 0.60}{P_{US}} \tag{28.1} \end{equation}\]
Where:
\(PC_{yw}\) = Per-capita value of yard waste generated in the US, in tons per person per year
\(YW\) = Annual yard waste generated, in million tons
\(P_{US}\) = US population
As open burning of yard waste is generally not practiced in urban areas. Therefore, only the rural population in each county is assumed to practice open burning. The rural and urban populations are taken from U.S. Census data for each county [ref 3]. It is assumed that 24% of the rural population burns yard waste [ref 4].
\[\begin{equation} PBurn_{c} = RPop_{c} \times 0.24 \tag{28.2} \end{equation}\]
Where:
\(PBurn_{c}\) = Population likely to burn yard waste in county c
\(RPop_{c}\) = Rural population in county c
The number of people likely to burn waste in each county (from Equation (28.2)) is then used with the values of per-capita yard waste generated to determine the amount of household yard waste burned. The first assumption concerns the composition of yard waste; of the total amount of yard waste generated, yard waste composition is assumed to be 25 percent leaves, 25 percent brush, and 50 percent grass by weight [ref 5]. However, open burning of grass clippings is not typically practiced by homeowners, and as such only estimates for leaf burning and brush burning are developed.
The second assumption adjusts for variations in vegetation; the percentage of forested acres (including rural forest and urban forest) is determined using Version 2 of the Biogenic Emission Landuse Database (BELD2) within the Biogenic Emissions Inventory System (BEIS) [ref 6]. Based on this percentage, county-level yard waste values are adjusted according to the values in Table 28.2. To better account for the native vegetation that likely occurs in residential yards of farming states, agricultural land acreage is subtracted before calculating the percentage of forested acres. All municipios in Puerto Rico and counties in the U.S. Virgin Islands, Hawaii, and Alaska were assumed to have greater than 50 percent forested acres.
\[\begin{equation} LW_{c} = PBurn_{c} \times PC_{yw} \times YWFr_{t} \times AF_{fa,c} \tag{28.3} \end{equation}\]
\[\begin{equation} BW_{c} = PBurn_{c} \times PC_{yw} \times YWFr_{t} \times AF_{fa,c} \tag{28.4} \end{equation}\]
Where:
\(LW_{c}\) = Annual leaf waste burned in county c, in tons
\(BW_{c}\) = Annual brush waste burned in county c, in tons
\(PBurn_{c}\) = Population likely to burn yard waste in county c
\(PC_{yw}\) = Per-capita value of yard waste in the U.S., in tons per person
\(YWFr_{t}\) = Fraction of total yard wate for waste type t (leaf or brush)
\(AF_{fa,c}\) = Adjustment factor based on percent of forested acres in county c
| Percent Forested Acres per County | Adjustment for Yard Waste Generated |
|---|---|
| < 10% | 0% generated |
| ≥ 10% & < 50% | 50% generated |
| ≥ 50% | 100% generated |
28.2.2 Allocation Procedure
National values for the amount of waste generated are distributed to the counties based on rural population, as described in the previous section.
28.2.3 Emission Factors
The emissions factors for CAPs are from AP-42 [ref 7], the emissions inventory improvement program [ref 8], and an ERTAC workgroup [ref 9]. For burning of leaves, emissions factors for PM_{2.5} are calculated by multiplying the PM_{10} emissions factor by a ratio of 0.7709. Emission factors for NH3 were derived from the 2002 NEI crop residue emission estimates using the ratio of NH3/NOx for pasture grass from Pouliot et al. (2017) [ref 10] and the NOx emission factor from AP-42. Emissions factors for HAPs are from an EPA Control Technology Center report [ref 11].
28.2.4 Controls
Controls for residential yard waste burning are generally in the form of a ban on open burning of waste in a given municipality or county. However, literature suggests that burn bans are not 100% effective. It is therefore assumed that approximately 25% of the residents that may burn yard waste would burn do so even if a ban is in place. For counties that have burn bans, the assumption is applied by multiplying 0.25 by the annual waste burned. Currently no counties are assumed to have burn bans in place.
If county c has a burn ban, then: \[\begin{equation} LW_{c} = LW_{c} \times 0.25 \tag{28.5} \end{equation}\]
\[\begin{equation} BW_{c} = BW_{c} \times 0.25 \tag{28.6} \end{equation}\]
Where:
\(LW_{c}\) = Annual leaf waste burned in county c, in tons
\(BW_{c}\) = Annual brush waste burned in county c, in tons
28.2.5 Emissions
The annual amount of combustible lead and brush waste burned in each county is multiplied by the appropriate emissions factors:
\[\begin{equation} E_{p,c} = EF_{p} \times LW_{c} \tag{28.7} \end{equation}\]
\[\begin{equation} E_{p,c} = EF_{p} \times BW_{c} \tag{28.8} \end{equation}\]
Where:
\(E_{p,c}\) = Annual emissions of pollutant p in county c
\(EF_{p}\) = Emissions factor for pollutant p, in lbs./ton of waste burned
\(LW_{c}\) = Annual leaf waste burned in county c, in tons
\(BW_{c}\) = Annual brush waste burned in county c, in tons
28.2.6 Sample Calculations
Table 28.3 lists sample calculations to determine the CO emissions from open burning of yard waste. The values in these equations are demonstrating program logic and are not representative of any specific NEI year or county.
| Eq. # | Equation | Values | Result |
|---|---|---|---|
| 1 | \(PC_{yw} = \frac{YW \times 0.60}{P_{US}}\) | \(\frac{34.5\text{ million tons} \times 0.60}{318.85\text{ million people}}\) | 0.065 tons yard waste per person per year |
| 2 | \(PBurn_{c} = RPop_{c} \times 0.24\) | \(22,921\text{ people} \times 0.24\) | 5,501 people likely to burn waste |
| 3 | \(LW_{c} = PBurn_{c} \times PC_{yw} \times YWFr_{t} \times AF_{fa,c}\) | \(5,501\text{ people} \times 0.065\text{ tons} \times 0.25 \times 1\) | 89.39 tons of leaf waste burned |
| 4 | \(BW_{c} = PBurn_{c} \times PC_{yw} \times YWFr_{t} \times AF_{fa,c}\) | \(5,501\text{ people} \times 0.065\text{ tons} \times 0.25 \times 1\) | 89.39 tons of brush waste burned |
| 5 | \(LW_{c} = LW_{c} \times 0.25\) | \(\text{N/A}\) | County does not have a burn ban |
| 6 | \(BW_{c} = BW_{c} \times 0.25\) | \(\text{N/A}\) | County does not have a burn ban |
| 7 | \(E_{p,c} = EF_{p} \times LW_{c}\) | \(89.39\text{ tons leaf waste} \times 112\text{ lbs. per ton}\) | 5.01 tons CO emissions from burning of leaf waste |
| 8 | \(E_{p,c} = EF_{p} \times BW_{c}\) | \(89.39\text{ tons leaf waste} \times 140\text{ lbs. per ton}\) | 6.26 tons CO emissions from burning of brush waste |
28.2.7 Improvements/Changes in the 2023 NEI
There have been no significant changes to this category.
28.2.8 Puerto Rico and U.S. Virgin Islands
Emissions from Puerto Rico are calculated using the same method described above. For the U.S. Virgin Islands, emissions are calculated using 2020 population data [ref 3], since 2023 Census Data does not exist for the U.S. Virgin Islands.