10.11 Windblown Dust
Categories 764 and 765
10.11.1 Introduction
Fugitive dust emissions (particulate matter) resulting from wind erosion across agricultural fields (category 764), and from wind erosion of soil on unpaved roads and associated areas (category 765) are discussed in the section. They’re considered as non-permitted area sources.
10.11.2 Methodology
Emissions of the two categories have been developed primarily based on the methodology adopted by California Air Resource Board (CARB)469. In general, particulate matter (PM) emissions from agricultural windblown dust were calculated by multiplying the crop acreage by the emission factors derived from an equation developed by the U.S. Department of Agriculture and adjusted by CARB with local parameters470. PM emissions from unpaved roads windblown dust were calculated by multiplying the unpaved acreage by the emission factors derived from the same equation but with parameters specific to unpaved roads471. More details are provided in the following sections.
(a) Activity Data / Throughput
Total acreage for field crops, vegetable crops, fruit and nut crops, nursery crops, plants, flowers and miscellaneous crops were obtained for each county as reported in the County’s Agricultural Crops Report472. The estimated crop acres were used as throughput for the calculation of wind erosion from agricultural fields (category 764).
The number of miles of unpaved road for each county were estimated from the California Statistical Abstracts473, and converted into acreage. It was assumed the typical width of a road was 25 feet. The acreage of unpaved road was used as throughput for the calculation of windblown dust from unpaved roads (category 765).
(b) County Distribution / Fractions
The throughput of both categories are reported or estimated for each county and are used in distributing emissions throughout the counties.
(c) Emission Factors
PM emission factor from dust emissions were developed based on the U.S. Department of Agriculture’s equation:
\[ \text{Emission Factor} = E \times I \times C \times K \times L \times V \] where
\(E\) = portion of total wind erosion loses as suspended particulates
\(I\) = soil erodibility, t/acre/yr
\(C\) = climatic factor
\(K\) = surface roughness factor
\(L\) = unsheltered field width factor
\(V\) = vegetative cover factor
(\(K\), \(L\), \(V\) depends on the crop type)
The following parameters specific to Bay Area Counties were used to derive emission factors for agricultural lands and unpaved roads respectively.
(d) Control Factors
No control factors were applied as currently there is no active regulations in the District or at State level to mitigate the PM emissions from windblown dust.
(e) Speciation
For windblown dust from agricultural lands, the PM size fractions of CARB’s chemical speciation profile # 418 were used to estimate PM10 and PM2.5 emissions from PM emissions. This profile reflects CARB’s latest understanding for PM size fractions of windblown dust from agricultural land compared to the speciation profile # 411 that was used for the previous base year inventory 474. According to the profile, PM10 is 45.43% of PM while PM2.5 is 7.86% of PM.
For windblown dust from unpaved roads, the PM size fractions of CARB’s chemical speciation profile # 416 were used to estimate PM10 and PM2.5 emissions from PM emissions6. According to the profile, PM10 is 59.43% of PM while PM2.5 is 7.86% of PM.
(f) Monthly Variation
For both categories, emission distribution through the months was estimated to be equal throughout the year.
10.11.3 Changes in Methodology
There is no major change in the current inventory methodology comparing with the previous methodology except that the PM size fractions used to derive PM10 and PM2.5 emissions of windblown dust from agricultural lands have been updated.
10.11.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication. Windblown dust from agricultural lands is a major source of PM10 emissions. Its contribution to entire PM inventory will become more and more important as the PM emissions of other combustion sources continue to decrease due to stringent regulations or advanced control technologies, e.g., tailpipe emissions from on-road vehicles and off-road equipment. However, its health impact may not be as significant as other PM sources, as agricultural lands and unpaved roads where windblown dust usually occurs are located in remote areas with little population.
10.11.5 Trends
With the emission factors consistent over time, the emissions trend is determined by the trend of throughput.
(a) Historical Emissions / History
For windblown dust from agricultural lands, whose historical emissions were estimated based on the annual report of crop acreage, the trend of historical emissions reflects the decreasing acreage of exposed agricultural lands in Bay Area.
(b) Future Projections / Growth
For both categories, future year emissions were estimated by holding the base year 2015 emissions constant till year 2040 due to lack of better information on future crop acreage and reliable statistics on unpaved roads. The rational of “no-growth” is that: 1) there is a decreasing trend of the agricultural lands in years prior to 2015; 2) the construction of new unpaved roads in Bay Area is very limited.
10.11.6 Uncertainties
There are unknown magnitude of uncertainties for the windblown dust emissions due to the limitations in emission factors, and trends in activity data. The emission factors are derived based on an equation which has not been updated for more than two decades, along with some key parameters. The assumptions in estimating the future trend of agricultural land acreage and historical trend of unpaved road acreage both need re-evaluation, and may be improved with more accurate projection of agricultural land acreage in annual reports and updated data on unpaved roads statistics. Moreover, current assumption on monthly variation of emissions doesn’t account for the impact from different precipitation in rain and dry seasons in Bay Area.
10.11.7 Contact
Author: Yuan Du
Reviewer: Abhinav Guha, Tan Dinh
Last Update: November 06, 2023
10.11.8 References & Footnotes
CARB. CARB Miscellaneous Process Methodologies - Fugitive Windblown Dust. [Accessed on January 27, 2020]. https://ww2.arb.ca.gov/carb-miscellaneous-process-methodologies-fugitive-windblown-dust↩︎
CARB. Section 7.12 Windblown Dust - Agricultural Lands. July 1999. https://ww3.arb.ca.gov/ei/areasrc/fullpdf/full7-12.pdf↩︎
CARB. Section 7.13 Windblown Dust - Unpaved Roads. August 1999. https://ww3.arb.ca.gov/ei/areasrc/fullpdf/full7-13.pdf↩︎
California Department of Food and Agriculture (CDFA). Links to County Crop Reports. https://www.cdfa.ca.gov/exec/county/CountyCropReports.html↩︎
California Department of Finance. Statistical Abstracts. https://archive.org/search.php?query=%22california%20statistical%20abstract%22↩︎
CARB. Particle size fraction data for source categories. https://ww2.arb.ca.gov/sites/default/files/2021-11/pmsizeprofile03nov21.xlsx↩︎