6.17 Asphalt Paving

Categories 94, 95, 96, and 97

6.17.1 Introduction

Categories 94, 95, 96, and 97 account for organic emissions (TOG and ROG) from asphalt paving using various asphalt based materials.

Asphalt paving is the act of applying a combination of asphalt binder and aggregate (sand, gravel, etc.) to pave, repair, or seal surfaces such as roads, parking lots, and airport runways. The base constituent of asphalt binder is asphalt cement, the residue of the distillation of crude oil, which is a semisolid substance. Due to the semisolid features of asphalt cement, it must be heated or mixed with a water/solvent solution before it can be applied, which results in asphalt binder.

Asphalt binder can be categorized as follows:

  • Hot Mix: asphalt cement is heated and mixed with aggregate at a plant and transferred via truck to the application site
  • Liquid (Cutback): asphalt cement is mixed with light petroleum solvent
  • Emulsified: asphalt cement is blended/suspended in a mixture of water and emulsifying agent

Liquid asphalt can be further categorized by the volatility of the petroleum solvent used:

  • Rapid-cure - high volatility
  • Medium-cure - medium volatility
  • Slow-cure (Road Oil) - low volatility

Road Oils (Slow-cure Liquid Asphalt), Hot Mix Asphalt, and Emulsified Asphalt, categories 95 – 97, respectively, account for the organic emissions from asphalt paving materials during and after the paving of roads, streets, and parking lots in the Bay Area.

Rapid-cure Liquid Asphalt (Rapid-cure Cutback Asphalt) is no longer used in the Bay Area. Rapid-cure Liquid Asphalt was formerly Category 94.

Medium-cure Liquid Asphalt (Medium-cure Cutback Asphalt) may be used in the Bay Area provided the atmospheric temperature does not exceed 50 degrees F for 24 hours after application219. This restriction greatly limits the application of this type of asphalt; therefore, the organic emissions from Medium-cure Liquid Asphalt are considered negligible.

Slow-cure Liquid Asphalt (Slow-cure Cutback Asphalt),synonymous with “Road Oils” (category 95), may also be used in the Bay Area with restrictions (see Rule 8-15).

6.17.2 Methodology

These categories are considered area source categories as they cover sources that are not explicitly permitted or individually cataloged by the District. Emissions for area source categories are determined using the formula:

Current Year Emissions = Base Year Emission X Growth Profile, and,

Base Year Emission = Throughput X Control Factor X Emission Factor

where,

  • throughput or activity data for applicable base year(s) is determined using a top-down approach (e.g. state-, national-level data);
  • emission factor is derived from general literature, specific literature and reports, and/or source testing results provided by Air District staff;
  • control factor (if applicable) is determined by District and state rules and regulations in effect;
  • and, historical backcasting and forecasting of emissions is based on growth profiles as outlined in the Trends section of this chapter

Emissions for category 935 were estimated using the base years of 2011-2015 and applying emissions factors based on published literature. This procedure is similar to the California Air Resources Board’s (CARB) methodology for emission estimates220. This method involves a top-down approach where data is collected at a statewide or national scale and proportioned to the Bay Area based on miles of paved road. Detailed background on the determination of throughput, controls, and emission factors is provided in the following sections.

(a) Activity Data / Throughput

Asphalt use in barrels for paving and roofing in California was collected for the The Energy Information Administration (EIA) Office of Oil and Gas. Of the total California use, it was assumed 80% of the asphalt was used for road paving and 20% was used for roofing. To calculate asphalt usage in tons per year, an asphalt density of 8 lbs/gal and a barrel equivalent to 42 gals was assumed. The statewide asphalt use for paving applications is estimated to be the following:

  • Hot-mix: 88%
  • Emulsified: 9%
  • Road Oils: 2%
  • Cutback: <1%

Note: It is assumed some districts, including the BAAQMD, do not use Rapid-cure Cutback Asphalt (see Introduction).

Monthly variation of emissions is based on estimates of construction activities in a year.

(b) County Distribution / Fractions

County activity was based on miles of paved roads in each county as acquired from the State’s Teale Data Center 221.

(c) Emission Factors

The organic emission factors used for hot-mix, emulsified, and road oils are 0.04, 17.9, and 2.19 lb/ton of asphalt applied, respectively (as provided by the Sacramento Metropolitan Air Quality Management District (SMAQMD)). (Note—the initial emission factor used for road oils was 88 lbs/ton. This was an EPA recommended value; however, regulatory controls reduced this value to the level stated above.) The total emissions for area source categories are determined by multiplying the emission factor, throughput, and rule effectiveness factor, if applicable.

(d) Control Factors

Due to Rule 8-15, starting in 1982, Rapid-cure Cutback Asphalt is no longer in use in the Bay Area. The emissions from Slow-cure Liquid asphalt (Road Oils) category were reduced by approximately 75% in 1988 and 97.5% in 1989. Similarly, emissions from using emulsified asphalt were reduced by 49% in 1988 and 55.3% in 1989. No controls were applied to Hot Mix asphalt as this type has significantly lower emissions when compared to Liquid or Emulsified and no regulatory limitations apply to its use.

(e) Speciation

The speciation applied to TOG to determine ROG is consistent with the size fractions of speciation profiles developed by the California Air Resources Board (CARB) and published on their emissions inventory web-page 222. All organic emissions are assumed to be reactive, therefore reactive organics is equal total organics.

(f) Sample Calculations

Below is a sample calculation for emissions from Hot-mix operations:

Organic Emissions (tpy) =

bbl/year of asphalt supplied * 0.88 (fraction allocated to Hot-mix)* 42 gals/bbl * 8 lb/gal / 2000 lb/ton * 0.04 lb/ton

6.17.3 Changes in Methodology

No major changes in methodology were made in this version of the base year emissions inventory as compared to the previous inventory.

Updates to base year data from the previous inventories include the following:

  • Employment projection data from Plan Bay Area 2040, a publication by ABAG and the Metropolitan Transportation Commission (MTC), is used for emissions forecasting223

6.17.4 Emissions

A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

6.17.6 Uncertainties

Uncertainties in this category can be attributed largely to the percentage use of each asphalt type. This was initially determined using previously reported material usages as reported by individual Air Districts in 2000. This could be refined by conducting another survey of material usage per Air District.

6.17.7 Contact

Author: Ariana Husain

Reviewer: Sukarn Claire

Last Update: November 06, 2023

6.17.8 References & Footnotes

  1. Bay Area Air Quality District (BAAQMD). 1987. Regulation 8, Rule 15 Organic Compounds, Emulsified and Liquid Asphalt↩︎

  2. CARB. 2003. Attachment C: Asphalt Paving and Roofing, from STI’s Area Source Emissions Updates↩︎

  3. Funk et al. 2001. Teale Data Center↩︎

  4. CARB. 2022. ORGPROF. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎

  5. Association of Bay Area Governments (ABAG). Forecasts and Projections. [accessed 2023 Feb 22]. https://abag.ca.gov/our-work/land-use/forecasts-projections↩︎