VOC emissions from gasoline sources

Estimated primary emissions of total organic gases from gasoline-related sources declined by 68% statewide between 1996 and 2012 (see Fig. 1). This significant reduction is attributable primarily to the decline in on-road mobile source emissions that occurred even while gasoline sales remained steady and California’s population continued to grow [26, 61]. The 10 chemicals that contributed most to this decline were isopentane, methyl t-butyl ether (MTBE), methane, n-butane, toluene, ethylene, n-pentane, m-xylene, 2-methylpentane, and isobutene. By 2012, emissions from on-road motor vehicles had decreased so much that they were approaching levels emitted by other gasoline-related mobile sources, including lawn and garden equipment, recreational boats and off-road vehicles. Less apparent in Fig. 1 is the decline in emissions from these other mobile sources, dropping almost in half between 1999 and 2012.

Fig. 1: Statewide trends in gasoline-related emissions and gasoline use.

Total organic gases (left axis) from all gasoline-related sources and component categories are plotted for the years 1996 to 2012. Net taxable gasoline gallons sold in California (right axis) are also shown for this same period.

We identified more than 350 VOCs in primary emissions from gasoline-related sources based on the 2012 Emission Inventory. Table 1 lists gasoline-related VOCs we identified as being of concern for exposure and/or health hazards. The top 25 most emitted VOCs made up more than 75% of all gasoline-related TOG emissions in 2012. Isopentane has consistently been the most highly emitted chemical from gasoline-related sources. Between 1996 and 2003, the second most highly emitted gasoline-related VOC was MTBE. In 2004, MTBE was phased out and MTBE emissions dropped to nearly zero. Ethanol replaced MTBE as the oxygenate of choice and by 2012 it became the second most highly emitted gasoline-related VOC. After accounting for secondary atmospheric formation, PAN, formaldehyde, and acetaldehyde were in the top five most highly emitted VOCs based on 2012 data.

Among the top 100 VOCs emitted in 1996, 98 had lower gasoline-related emissions in 2012 (refer to the SM for complete details). Reductions in emissions from on-road gasoline-powered vehicles accounted for the bulk of the decline. Two VOCs with significant health concerns, benzene and 1,3-butadiene, had large declines in gasoline-related emissions, 74% and 75%, respectively. Ethanol showed a large increase in gasoline-related emissions between 1996 (2.2 tons/day) and 2012 (39 tons/day), which was expected due to its use as a replacement oxygenate for MTBE.

Hazard identification – VOCs in primary emissions

Table 1 summarizes the known or suspected toxicities for gasoline-related VOCs. A number of highly emitted hydrocarbons in Table 1 are not of significant concern for human health as the parent compound (e.g., isopentane); however, their atmospheric transformation products (e.g., acetaldehyde) can pose health hazards.

Ethanol has only been identified as a carcinogen or developmental toxicant in association with consumption of alcoholic beverages (see footnote 3 to Table 1 for more details), and no chronic non-cancer inhalation reference values (i.e., cREL or RfC) are available.

The most significant toxicants in the top 25 gasoline-related VOCs include the BTEX compounds (i.e., benzene, rank 15; toluene, rank 3; ethylbenzene, rank 23; and xylenes [(m-xylene, rank 9; o-xylene, rank 22]), with endpoints including cancer, developmental and reproductive toxicity, respiratory toxicity, and neurotoxicity, depending on the specific chemical. Other highly emitted gasoline-related chemicals of note for toxicity include propylene (rank 13; respiratory toxicant), n-hexane (rank 18; male reproductive toxicant and neurotoxicant), and formaldehyde (rank 21; carcinogen and respiratory toxicant).

Table 1 also includes VOCs that had lower levels of primary emissions, but still pose potential concerns due to toxicity. Some of these also have substantial secondary formation. The lower-emitted VOCs of concern included 1,3-butadiene (rank 45), acetaldehyde (rank 47), acrolein (rank 94), benzaldehyde (rank 62), isoprene (rank 91), styrene (rank 93), naphthalene (rank 95), and cumene (rank 140).

Table 1 indicates the VOCs for which we were able to complete a screening level risk assessment. Of the chemicals with known or suspected toxicities, the majority that could not be assessed were the lower ranked aldehydes, which did not have available health reference values and/or sufficient ambient air data for modeling.

Hazard identification – atmospheric transformation products

Many gasoline-related VOCs undergo atmospheric transformations to form secondary products [62], and these can pose potential toxicity concerns. We had sufficient data on emissions, ambient air concentrations, and health reference values to conduct the screening risk assessment for only a few atmospheric products, including acetaldehyde, acrolein and formaldehyde. Some of the unassessed atmospheric products of concern included additional aldehydes (e.g., benzaldehyde, crotonaldehyde, hexaldehyde, tolualdehyde), dicarbonyls (e.g., diacetyl, glyoxal, ethylglyoxal, methylglyoxal, malonaldehyde), cresols, furan, PAN analogs, PAH products (e.g., 2-formylcinnamaldehyde, naphthols, quinones) and nitro-PAHs.

Gasoline-attributable fractions for VOCs

Table 2 presents the 2012 regional and statewide gasoline-attributable fractions for VOCs identified as having health concerns. These fractions ranged from less than 1% to over 90% and varied across the five air basins. As an example regional difference, the fractions for 1,3-butadiene were 47% in the South Coast Air Basin and 24% in the San Joaquin Valley Air Basin. Sources of 1,3-butadiene in both regions included mobile sources, plastic product manufacturing, and wildfires, with proportionally more gasoline-related mobile sources in the South Coast.

Table 2 Regional and statewide gasoline-attributable fractions based on 2012 emissions for chemicals with health concerns.

For the chemicals identified as being gasoline-related, the percentage of total primary emissions from gasoline-related sources was 32% in 1996 and 11% in 2012. The specific time trends in gasoline-attributable fractions were different for each chemical, primarily due to changes in emissions from non-gasoline sources. For example, non-gasoline emissions of benzene (e.g., industrial processes, other mobile sources, and waste disposal) were roughly constant between 1996 and 2012, while non-gasoline emissions of toluene (e.g., cleaning and surface coatings and solvent evaporation) declined. In addition, the approach used by CARB to estimate natural source emissions (e.g., wildfires) was changed in 2002 and later years, which affected the temporal trend in gasoline-attributable fractions for VOCs like acetaldehyde and formaldehyde [46].

Gasoline-attributable air concentrations

Gasoline-attributable ambient concentrations were calculated for VOCs with identified health hazards and available monitoring data. Table 3 summarizes changes between 1996 and 2014 for gasoline-attributable concentrations in the South Coast Air Basin and statewide (where available). In the South Coast Air Basin, gasoline-attributable ambient air concentrations declined by 70 to 90% between 1996 and 2014 for the VOCs we could assess in detail. Statewide gasoline-attributable concentrations also declined by a similar amount between 1996 and 2014. Some chemicals could only be assessed over a shorter time period and are not shown in Table 3. Statewide gasoline-attributable concentrations for acrolein declined by 28% from 2004 to 2014 (the years with acrolein ambient air monitoring data) but were roughly constant in the South Coast Air Basin during the same time period. Statewide gasoline-attributable concentrations of acetaldehyde and formaldehyde declined by 47% and 59% between 2002 and 2014 (the years with stable natural source emission estimates; see discussion above).

Table 3 Percentage decline in gasoline-attributable concentrations between 1996 and 2014.

We were not able to generate gasoline-related concentrations for a number of chemicals of interest. For example, many pollutants with health concerns, such as butyraldehyde, crotonaldehyde, hexaldehyde, isovaleraldehyde, propionaldehyde and tolualdehyde, could not be assessed because ambient air monitoring data were insufficient.

Health risk assessment for VOCs and PAHs

Screening level assessments were carried out to estimate average gasoline-attributable cancer risks on a population-weighted basis for selected VOCs. Table 4 compares the gasoline-attributable cancer risks statewide and in the South Coast Air Basin for 1996 and 2014. In the South Coast Air Basin, the combined gasoline-attributable cancer risk from acetaldehyde, benzene, 1,3-butadiene, ethylbenzene, formaldehyde, naphthalene and styrene declined by 84%, from 9.1 × 10−4 in 1996 to 1.4 × 10−4 in 2014 (Fig. 2). The majority of the gasoline-attributable cancer risk in the South Coast Air Basin came from benzene, 1,3-butadiene and formaldehyde exposures. The naphthalene gasoline-attributable cancer risks, based on modeled ambient concentrations, declined 3-fold between 1996 and 2014.

Table 4 Gasoline-attributable cancer risk in the South Coast Air Basin and statewide.Fig. 2: Gasoline-related cancer risks for VOCs in the South Coast Air Basin.

Cancer risks based on gasolineattributable population-weighted annual average ambient air concentrations are plotted for benzene, 1,3-butadiene, formaldehyde, acetaldehyde, ethylbenzene, naphthalene, and styrene in the South Coast Air Basin between 1996 and 2014.

Fewer chemicals had sufficient statewide ambient air monitoring data to estimate cancer risks. The combined statewide gasoline-attributable cancer risk for acetaldehyde, benzene, 1,3-butadiene and formaldehyde was 4.9 × 10−4 in 1996, declining more than 80% to 8.2 × 10−5 by 2014. Statewide, gasoline-attributable cancer risk in 2014 was primarily attributable to benzene and 1,3-butadiene (5.3 × 10−5 and 1.9 × 10−5, respectively). For comparison, CARB estimated statewide cancer risks from all emission sources of these carcinogens, reporting somewhat lower declines between 1996 and 2007 for benzene (59%) and 1,3-butadiene (66%) [63].

We identified seven gasoline-related particle-bound PAHs that had ambient air data and cancer potencies or potency equivalency factors: benz[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, dibenz[ah]anthracene, indeno[1,2,3-cd]pyrene. We were not able to estimate gasoline-attributable fractions for these, so we estimated total cancer risks at two sites in the South Coast Air Basin based on 2014 data. Even without adjusting for gasoline-only exposures, the total cancer risk for these seven PAHs was still about an order of magnitude lower than the estimated gasoline-attributable cancer risk for the volatile PAH naphthalene.

To assess non-cancer risks, we calculated hazard quotients for VOCs with available chronic health reference values and for which we could estimate gasoline-attributable annual average ambient air concentrations in the South Coast Air Basin and statewide. These included acetaldehyde, acrolein, benzene, formaldehyde, n-hexane, naphthalene, propylene, styrene, toluene, trimethylbenzenes, and xylenes. Only acrolein had a clearly elevated hazard quotient, which was for chronic respiratory toxicity. During 2004–2014 (the time period with adequate data for acrolein), the hazard quotient in the South Coast Air Basin varied between 3 and 4, but did not follow a time trend and remained elevated at 3 in 2014. The gasoline-attributable hazard quotient for acrolein based on average statewide exposures was slightly elevated (1.2) in 2014. Acrolein also accounted for the majority of the 2014 respiratory toxicity hazard indices in the South Coast Air Basin and statewide, with a small contribution (<5%) from formaldehyde.