# Stockpiles and exposed area wind erosion calculator: guide to reporting

Use the emissions calculator to help you report the releases to air, due to wind erosion from active stockpiles and exposed areas which are disturbed at least once during the reporting year.

## Reporting to the National Pollutant Release Inventory

If your facility meets the National Pollutant Release Inventory (NPRI) reporting requirements, you must consider air emissions from all sources, unless the activities are specifically exempted from reporting. Therefore, you may need to consider emissions from sources other than wind erosion when determining the total air releases for particulate matter, and the annual quantities MPO (Manufactured, Processed, or Otherwise Used) for metals, for comparison to the reporting thresholds.

For more information about NPRI reporting requirements, refer to the guide for reporting to the NPRI .

## Emissions estimates for wind erosion

While there are many acceptable methods for estimating emissions for NPRI, this Emissions Calculator uses site-specific emission factors.

The general equation for emissions estimation using an emission factor is:

Emission rate = activity rate x emission factor

The Emissions Calculator uses published methods to develop site-specific emissions for wind erosion; these are discussed below.  The activity rate is the active area of the stockpile or the exposed area.

## Stockpile active area vs footprint

A distinction must be made between the active area and the stockpile footprint. The active area refers to the area that is disturbed during the reporting year; this may be substantially smaller than the footprint. You should use the average active area over the year when calculating emissions from wind erosion.

The footprint is the total area of the stockpile. This can be determined using the diameter for a circular footprint, or an equivalent diameter for other stockpile shapes. An equivalent diameter is a diameter that would result in a circle having the same footprint area as the non-circular shape. The footprint is the measure that is used to determine whether the stockpile is flat or elevated.

## Elevated versus flat stockpiles

Stockpiles are defined as being either elevated (with a height-to-base ratio greater than 0.2), or non-elevated (flat) (with a height-to-base ratio of 0.2 or less). This distinction is based on the US EPA definitions in AP-42 Section 13.2.5 (AP-42, 2006) [pdf].

## Emission factors for various stockpile configurations

For elevated stockpiles (whether frequently or infrequently disturbed) and infrequently disturbed flat stockpiles (flat stockpiles that are disturbed less than once per week), the Emissions Calculator uses an empirical formulation to estimate the emission factor that is consistent with the approach detailed in the Environment and Climate Change Canada's Pits and quarries reporting guide (Method A).

For large, flat exposed areas that are frequently disturbed (at least once per week), the Emissions Calculator uses the method detailed in US EPA AP-42 Section 13.2.5 (AP-42, 2006) [pdf].  This method (Method B) provides a more accurate estimate as it considers the number of disturbances per year and the erosion potential of the stockpile. The method calculates the total emissions between pile disturbances, and the total annual emission factor is calculated as the sum of the individual erosion events.

## Method A – Annual wind erosion emissions from stockpile surfaces

The Emissions Calculator uses the following equation as the default method to calculate the annual particulate emission factor (EF) for elevated stockpiles and flat stockpiles that are infrequently disturbed:

EF [kg/m2] = 1.2x10-4 x J x 1.7 x (s/1.5) x 365 x ((365-P)/235) x (I/15)

Where:

• J = particulate aerodynamic factor with the following values:
• J(TPM) = 1.0
• J(PM10) = 0.5
• J(PM2.5) = 0.2
• s = average silt loading of storage pile in percent (%)
• P = average number of days during the year with at least 0.254 mm of precipitation
• I = percentage of time with unobstructed wind speed >19.3 km/h in percent (%)

The total emissions are then calculated by multiplying the EF by the exposed, active surface area.

For conical stockpiles, calculate the exposed area using:

A = π x R √(R2 + H2)

Where:

• π = 3.1416 (approximately)
• R = radius of the stockpile (m)
• H = height of the stockpile (m)
• A = exposed surface area of stockpile (m2)

The Emissions Calculator determines the exposed surface area based on the footprint area and defined active area, where applicable.

Default silt content values are included in the calculator, as summarized in Table 1. You are also able to enter a site-specific silt content, if you have this information.

Table 1: Default silt content values used in the calculator for Method A (MDAQMD, 2000)
Stockpile material Average silt content (%)
Limestone 0.5
Crushed limestone 1.5
Asphalt batching 5
Coal 6
Concrete batching 6
Sand and gravel processing 8
Overburden 10
Blend ore and dirt 15
Flue dust 20
Inorganic minerals 30

Method A requires the determination of a site-specific wind factor (I) and precipitation factor (P) for the reporting year.

You can obtain the hourly wind speed data for the nearest weather station from the Government of Canada historical climate data site, or from an on-site weather station. If you are using downloaded climate data, you will need to individually download 12 monthly .csv files to compile the weather data summary for the reporting year. You must then enter this data into the “Hourly wind speed data input” tab. It is possible that some data may be missing from the downloads. If the missing data are less than 10% of the total annual data, it is reasonable to either ignore the missing data, or estimate missing hours by averaging the last measured value immediately prior to the data gap and the first value after the gap. If the data set has substantial gaps (more than 10% missing), you can contact ECCC to determine the best approach to use for the site.

Alternatively, if hourly wind data are not available for the site, you can use the 30-year climate normals for the nearest climate station in order to estimate an average wind factor as described in the following steps. You can then input the information for the wind factor 'I' as a user defined value into the “Input data” tab.

To find the number of days per year with wind speed more than 19.3 km/h:

1. Go to the Government of Canada’s historical climate data portal
2. Click on "Canadian Climate Normals"
3. Click on the search method that you would like to use (Search by Station Name, Province or Proximity), enter the information requested and click ‘Go’
4. Click on the appropriate station from the list displayed (or the nearest one)
5. Click on the tab labelled “Normals Data”
6. Scroll down to the “Wind” table, find the “Wind: Speed (km/h)" row and then go across the month columns
7. Find the months with "Wind: Speed (km/h)" more than 19.3 km/h
8. Divide the number of months with wind speed more than 19.3 km/h per 12 months and multiply by 100, you obtain the average percentage of time 'I' during the year that wind speed exceeded 19.3 km/h

You can obtain the daily precipitation data for the nearest weather station from the Government of Canada historical climate data site , or from an on-site weather station. If using downloaded climate data, you will need to download one annual .csv file with the daily meteorological data. You must then input this data into the “Precipitation data input” tab. It is possible that some data may be missing from the downloads. If the missing data are less than 10% of the total annual data, it is reasonable to either ignore the missing data, or estimate missing daily data by averaging the last measured value immediately prior to the data gap and the first value after the gap. If the data set has substantial gaps (more than 10% missing), you should contact us to determine the best approach for the site.

## Method B - US EPA AP-42 industrial wind erosion

The Emission Calculator uses this method to estimate the total emissions from a flat, exposed area or flat pile that is frequently disturbed. The method provides a more accurate estimate for these scenarios as it considers the number of disturbances per year and the erosion potential of the stockpile. The method calculates the total emissions between pile disturbances, and the total annual emission factor is calculated as the sum of the individual erosion events.

Calculated the EF for each size fraction using the equations:

EF [g/m2] = k x ∑NPi

Where:

• k = Particulate size multiplier with the following values:
• k(TPM) = 1.0
• k(PM10) = 0.5
• k(PM2.5) = 0.075
• Pi = Erosion potential corresponding to the fastest mile of wind between disturbances (g/m2)
• N =  Number of disturbances per year (N = 365 per year where there are daily disturbances)

A disturbance is an action that results in the exposure of fresh surface materials, such as the addition or removal of material.

The emission factor is multiplied by the active area of the flat stockpile or exposed area to obtain the annual particulate emissions that are the result of wind erosion.

In order to estimate the erosion potential between each disturbance, a threshold friction velocity u*t must be established for the aggregate/erodible material in the pile, and a friction velocity u* calculated based upon local wind speed data.

This method assumes zero wind erosion emissions (erosion potential P is zero) when the wind speeds are relatively low, and the calculated friction velocity is less than threshold friction velocity (u* ≤ u*t):

P = 0 when (u* ≤ u*t)

Where:

• P = erosion potential
• u* = Friction velocity (m/s)
• u*t = Threshold friction velocity (m/s)

However, once the friction velocity is greater than the threshold friction velocity, there will be particulate emissions from wind erosion, and the erosion potential P can be calculated using the equation:

P = 58 x (u* - u*t)2 + 25 x (u* - u*t)

Where:

• P = erosion potential
• u* = Friction velocity (m/s)
• u*t = Threshold friction velocity (m/s)

For the threshold friction velocity of a given material, default threshold friction velocity (u*t) values are provided in the Emissions Calculator (Table 2), with the option for the user to enter a site-specific threshold friction velocity if one has been determined. A field procedure for determining the threshold friction velocity is described in US EPA AP-42 13.2.5 [pdf].

Table 2: Default threshold friction velocity used in the calculator for Method B (AP-42, 2006)
Stockpile material Threshold friction velocity (m/s)
Overburden 1.02
Scoria 1.33
Ground coal pile 0.55
Uncrusted coal pile 1.12
Scaper tracks on coal pile 0.62
Fine coal dust on concrete pad 0.54

For a flat stockpile, the friction velocity (u*) is estimated from local hourly wind data using the following equation:

u* = 0.053 x u+10

Where u+10 is the fastest mile for a reference anemometer at 10 metres height for the period between disturbances.

To estimate the fastest mile, use hourly wind data from the nearest weather station from the Government of Canada historical climate data site, or from an on‑site weather station. Multiply the maximum wind speed measured between disturbances by 1.24 to estimate the fastest mile (WMO, 2010).

You can obtain the hourly wind speed data for the nearest weather station from the Government of Canada historical climate data site, or from an on-site weather station. If you are using downloaded data, you will need to individually download 12 monthly .csv files and compile the weather data summary for the reporting year. You must then entered this summary data into the “Hourly wind speed data input” sheet. It is possible that some data is missing from the downloads. If the missing data are less than 10% of the total annual data, it is reasonable to either ignore the missing data, or estimate missing hours by averaging the last measured value immediately prior to the data gap and the first value after the gap.  If the data set has substantial gaps (more than 10% missing), you can contact ECCC to determine the best approach for the site.

## Pollution control techniques and estimated control efficiencies

The formulation of AP-42 methodology for Method B is based on the dry, exposed materials with limited erosion potential. It is reasonable to add an adjustment factor for natural mitigation from precipitation and/or snow on the ground, which is incorporated in the Emissions Calculator. Note that Method A already includes an adjustment for natural mitigation, however, other control techniques can also be applied to the stockpiles assessed using Method A.

In addition to dust mitigation from precipitation, the uncontrolled emission rate may be further adjusted to account for control measures implemented at the site. The Emissions Calculator has a number of default values for control efficiencies from common techniques that may be selected (Table 3), or you may also enter a site-specific control factor. If more than one technique is used for a given area, you should develop a site-specific control efficiency.

Table 3: Emission control techniques and default control efficiencies for common techniques
Control technique Control efficiency % Reference
Water application 50 – 95 (depending on daily application rate) MDAQMD, 2000
Three-sided enclosure 75 WRAP, 2006
Apply suppressant or gravel 84 WRAP, 2006
Revegetate or apply cover 90 WRAP, 2006

## Calculate emissions for Part 1 metals

You may need to account for the metal content of the particulate matter that is released to the air due to wind erosion. The Emissions Calculator provides a list of NPRI Part 1 metals to allow you to input the metal concentration data, either in ppm or %, such that the metal air emissions may be estimated. If the composition of the particulate matter released to air is not available, you may use the metal concentrations of the bulk material to estimate air releases of the metal NPRI substances.

## References

AMEC Earth & Environmental, A division of AMEC Americas Limited. Activity data and emission factors for estimating fugitive particulates from the aggregate mining and rock quarrying sector - Final Report. NPRI, 2007

Australian National Pollutant Inventory, Emission estimation technique manual for fugitive emissions, Version 2.0. 2012

Buonicore and Davis. Air Pollution Engineering Manual Chapter 4, First Edition, A&WMA, New York, USA. 1992

US EPA AP-42. Industrial Wind Erosion, Chapter 13.2.5, AP-42, Compilation of Air Pollutant Emission Factors, Volume 1, Stationary Point and Area Sources. 2006