10.7 Unpaved Roads
Categories 755, 756, 757, and 758
10.7.1 Introduction
Unpaved road dust, the fugitive particulate emissions resulting from motor vehicles traveling on various types of unpaved road surfaces, are included in this section. It’s a major source of particulate matter (PM) in Bay Area and was considered as non-permitted area source. Depending on the unpaved road types, the fugitive dust particles were accounted for by the four categories below.
| Category # | Description | Classification |
|---|---|---|
| 755 | Unpaved City/County Roads | Area |
| 756 | Unpaved Forest/Park Roads | Area |
| 757 | Unpaved Bureau of Land Management (BLM) and Bureau of Indian Affairs (BIA) Roads | Area |
| 758 | Unpaved Farm Roads | Area |
10.7.2 Methodology
The District has estimated the unpaved road dust emissions primarily based on methodology and underlying data adopted by California Air Resource Board (CARB) but applied updates as necessary and appropriate452. A brief description is given in the remaining section.
(a) Activity Data / Throughput
The activity for the categories is vehicle miles traveled (VMT) on the unpaved roads. The District acquired the activity data from CARB’s methodology document1 as summarized below.
For non-farm roads (categories 755, 756, 757), the total VMT for each road type was estimated by assuming 10 miles of travel per day for each mile of unpaved road453. The total mileage of unpaved roads for each road type were acquired from different sources.
category 755: The city and county unpaved road mileage were taken from a 2009 report of California Statewide Local Streets and Roads Needs Assessment (CSLSRNA)454. This report shows that there are no unpaved roads under city and county jurisdiction in the Bay Area starting from 2007.
category 756: For parks and forests, Caltrans’ Maintained Public Record Mileage for 2008 were used.
category 757: The most recent available Caltrans Maintained Public Record Mileage containing BLM/BIA unpaved road mileage for 1998 was used which shows BLM/BIA unpaved roads in Sonoma county only.
For farm roads (category 758), county specific number of acres for various types of crops were taken from California County Agricultural Commissioners reports of 2012 harvested crop acreage. Crop specific VMT were applied to the harvested crop acreage for each county in the Bay Area to estimate county VMT455.
(b) County Distribution / Fractions
For non-farm unpaved roads, the county distribution was calculated based on county specific VMT in CARB’s 2008 unpaved road emissions for PM10 in Table 1 of CARB methodology document2.
For farm unpaved roads, the county distribution was derived from the county-specific VMT calculation as described in the subsection (a) above, which is shown in Table 1 of CARB methodology document4
(c) Emission Factors
For non-farm unpaved roads, the Bay Area specific PM emission factors were back-calculated from 2008 Bay Area total PM emissions and VMT in Table 2 in CARB’s methodology document2. The resulting PM emission factors for categories 756 and 757 have the same value of 2.784 pounds PM per vehicle mile traveled (lb/VMT) for the two categories. This is consistent with CARB’s approach which assumes the same PM10 emission factors for all non-farm unpaved roads. This composite emission factor already accounts for rainfall adjustment.
For farm unpaved roads, we adopted CARB’s statewide emission factor for geologic dust emissions from vehicular travel on all unpaved roads, which is 2.0 lbs PM10 per VMT. Please refer to CARB’s methodology document4 for more details on the data source of the emission factor.
(d) Control Factors
The District doesn’t regulate emissions from on-road mobile sources directly. 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 unpaved road dust.
(e) Speciation
CARB’s chemical speciation profile # 470 based on unpaved road dust sampling conducted by CARB and MRI were used to estimate PM and PM2.5456. According to the profile, PM10 is 59.43% of PM while PM2.5 is 10% of PM10.
10.7.3 Changes in Methodology
For city/county unpaved road dust (category 755), historical emissions prior to year 2007 were assumed to be the same as that in base year 2008 inventory. This is different from the previous base year inventory which applied backcast profile to year 2011 emissions and leads to zero emissions for all historical years from 1990 to 2011. More details are provided in Trends subsection.
10.7.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication. Though with much limited VMT, unpaved road dust is still a major source of PM emissions in Bay Area. Their contribution to entire PM inventory will become more and more significant as the PM emissions of other sources, such as tailpipe emissions from on-road vehicles, continue to decrease due to stringent regulations or advanced technologies. However, its health impact may not as significant as other PM sources as the unpaved roads usually located at remote area with little population.
10.7.5 Trends
Except for city/county roads, the PM emissions from unpaved road dust are the same for all years from 1990 to 2040 as both emission factor and activity level haven’t changed over time and will remain the same until new and better information become available.
(a) Historical Emissions / History
Unlike paved roads, the information on unpaved roads and their traffic level is always limited. Without better data, the current base year inventory followed the same approach as the previous base year inventory, and assumes a flat back-cast profile so that the emissions in the historical years are consistent with the base year. The only exception is for city and county unpaved roads which are reported to be zero miles in Bay Area based on a survey conduced in 2007-2008 in the 2009 CSLSRNA report2. As non-zero miles for city and county unpaved roads were reported in the previous CSLSRNA reports, the annual emissions of year 1990 to 2006 from the base year 2008 inventory were adopted to represent historical emissions.
(b) Future Projections / Growth
It’s assumed that there is no growth in VMT at the absence of better information. In general, The assumption follows CARB’s methodology with the rational that the construction of new unpaved roads in Bay Area is very limited and the existing unpaved roads will be paved over if frequent heavily traffic occurs. The net increase of VMT on unpaved roads is negligible.
10.7.6 Uncertainties
There are unknown magnitude of uncertainties for the unpaved road dust due to the limitations in emission factors, activity data and historical and future trends. The emission factors are from researches more than two decades ago. The assumptions in estimating the activity level and trends both need re-evaluation.
10.7.7 Contact
Author: Yuan Du
Reviewer: Abhinav Guha, Tan Dinh
Last Update: November 06, 2023
10.7.8 References & Footnotes
CARB. CARB Miscellaneous Process Methodologies - Unpaved Road and Traffic Area Dust. [Accessed on January 24, 2020]. https://ww2.arb.ca.gov/carb-miscellaneous-process-methodologies-unpaved-road-and-traffic-area-dust↩︎
CARB. December 2012. Miscellaneous Process Methodology 7.10 Unpaved Road Dust, Non-Farm Roads. https://ww3.arb.ca.gov/ei/areasrc/fullpdf/full7-10_2012.pdf↩︎
California Statewide Local Streets and Roads Needs Assessment (CSLSRNA). October 2009. Final Report for the League of California Cities. Nichols Consulting Engineers, Chtd. http://dpw.lacounty.gov/gmed/slsr2/reports/finalreport.pdf↩︎
CARB. December 2016. Miscellaneous Process Methodology 7.11 Unpaved Road Dust, Farm Roads. https://ww3.arb.ca.gov/ei/areasrc/fullpdf/full7-11_2016.pdf↩︎
CARB. Speciation Profiles Used in CARB Modeling. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎