10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Comprehensive Organic Emission Profiles for Mobile Sources: Integration of VOC, IVOC, SVOC and Lower Volatility Organics
QUANYANG LU, Yunliang Zhao, Albert Presto, Andrew May, Tim Gordon, Allen Robinson, Carnegie Mellon University
Abstract Number: 1383 Working Group: Aerosol Modeling
Abstract Mobile sources contribute about one-third of anthropogenic organic emissions in the 2014 EPA National Emission Inventory (NEI); they are an important source of POA (primary organic aerosol) and SOA (secondary organic aerosol) precursor gases, especially in urban environments. Traditional emissions inventories account for two types of organics: volatile organic compounds (VOCs) in the gas-phase and particulate matter (PM). However, this is an overly simplistic representation.
We compiled recently published data to create comprehensive model-ready organic emission profiles for gasoline, gas-turbine, and diesel sources. Our source profiles span the entire volatility range, from nonvolatile organic compounds (NVOC) to low-volatile organic compounds (LVOC), semi-volatile organic compounds (SVOC), intermediate-volatile organic compounds (IVOC) and VOC. The overall mass closure of integrated organics to bulk measurement of non-methane organic gases and organic carbon (NMOG+1.2*OC) is 0.83 for on-road gasoline source, and ~1 for on-road diesel source.
The fractions of IVOC and SVOC to the total organic emissions (NMOG+1.2*OC) are consistent within source type: contributing about 4.5% (2.4%-9.6% as 10th to 90th percentile) and 1.1% (0.4%-3.8%) for gasoline engines, 20-27% and 4.5-7.5% for gas-turbine engine, and 52.5% (45.5%-60.0%) and 4.0% (3.0%-4.6%) for diesel engines.
Organic emissions from all source categories have tri-modal volatility distributions. The largest model is ‘fuel’ mode, which contributes 72.6% (66.5%-77.6%) total organic emissions in gasoline, 65.2% (59.8%-79.6%) in diesel exhaust, and 37.5-38.5% for gas-turbine. The ‘fuel’ mode mirrors the trends in fuel volatility. Emissions from each source also has a low-volatility mode, comprised of SVOCs and even less volatile organics, which peaks at a C* of 10 µg m-3. The final mode is the most volatile, peaking at a C* of 1010 or 1011 µg m-3. It contributes about 25% of the total organics in gasoline and diesel exhaust, and 50-57% for gas-turbine engine exhaust. The composition of this mode is primarily comprised of alkenes and carbonyls.
IVOCs are enriched in the exhaust relative to the fuel, especially for gasoline sources. Sources using more volatile fuel have higher IVOC enrichment factors. Gasoline engine exhaust has a median enrichment factor of 8.5 versus no enrichment for diesel exhaust. Gas turbine exhaust falls in between.
Three approaches are used to parameterize IVOC emission for use in SOA models, which are 1-group, 4-group and source-specific parameterizations. First, using a single lumped group for all sources, we obtained the relative error is ≤20% for gasoline and diesel source, and ≤40% for gas-turbine source, within OA concentration from 0 to 20 µg m-3. Second, when using 4-group parameterization, the relative error is reduced to ≤15% for gasoline and diesel source, and ≤35% for gas-turbine source. Third, by fitting for each source separately, relative error for all sources could be bounded within 10%.
The new profiles predict that IVOCs and SVOC vapors contribute significantly to SOA production, especially for sources using lower volatility fuels. For gasoline sources, IVOCs and SVOC vapors contribute ~80% SOA than tradition VOC precursors. For gas-turbine and diesel sources, IVOCs and SVOC vapors contributions exceeds the SOA production from VOC emissions by a factor of 18 and 38, respectively. Also, the new profiles treat POA as semi-volatile, and they predict that 40%-50% of POA mass evaporates at ambient conditions. There is a large change in the composition of the OA, changing from 50% (traditional) to more than 80% (new) of gasoline OA mass is SOA.