90 Matching Annotations
  1. Oct 2018
    1. N. E. Klepeis et al., J. Expo. Anal. Environ. Epidemiol. 11, 231–252 (2001).

      Telephone survey that monitored where people in the United States spent the majority of their time. Found that 87% of time was spent in buildings and 6% of time was spent inside of automobiles.

      Since 93% of human activities are indoors, indoor air quality is extremely important.

    2. G. J. Dollard, P. Dumitrean, S. Telling, J. Dixon, R. G. Derwent, Atmos. Environ. 41, 2559–2569 (2007).

      Study analyzed VOC emissions in the United Kingdom from 1993 to 2004. Found that VOC emissions from automobiles were decreasing at a rate of 20% per year due to more stringent automobile emissions regulations.

    3. B. C. McDonald, D. R. Gentner, A. H. Goldstein, R. A. Harley, Environ. Sci. Technol. 47, 10022–10031 (2013).

      Study looked at emissions from automobile traffic in the United States between 1990 and 2010. Found that fuel consumption increased by 10% - 40% during the time period, but emissions such as carbon monoxide and VOCs decreased by 80% - 90% due to increased fuel efficiency and regulations.

    4. Q. Di et al., N. Engl. J. Med. 376, 2513–2522 (2017).

      The United States' air quality standards sets advisable levels for different pollutants. Human health was shown to be negatively impacted even when fine particulate matter and ozone were present at levels below the current air quality standards.

    5. chlorofluorocarbons

      Compounds that contain chlorine, fluorine, and carbon. These types of compounds have previously been used in refrigerators, air conditioners, and in aerosol spray cans. These molecules were determined to be main contributors to the ozone hole in the stratosphere.

      In 1987 the Montreal Protocol was passed as an international treaty to limit the emissions of chlorofluorocarbons and protect the ozone layer.

      Read more about the ozone layer at the EPA: https://www.epa.gov/ozone-layer-protection

      See how the world might look if the Montreal Protocol did not exist at NASA: https://svs.gsfc.nasa.gov/3586

    6. Consumer uses of VCPs likely remain key sources of human exposure to air toxics relative to fossil fuels, especially because people spend most of their time indoors (62).

      In a 2001 National Human Activity Pattern Survey of 9,386 people in the United States found that on average 87% of time is spent in buildings and 6% of time is spent inside automobiles. Indoor air quality is an important issue, since on average 93% of time is spent indoors.

    7. Volatile methyl siloxanes

      Silicone is a polymer that is made up of siloxanes. Silicone is used in many household products including baby bottles, adhesives, and skin and hair care products. The smaller the polymer is, the more volatile it will be. Many skin and hair care products use smaller siloxanes which are volatile.

    8. Chemical manufacturers have reformulated products to reduce aromatic content, such as in cleaning agents (33). However, single- and multiple-ring aromatics are still present in products and in indoor air (32), and they contribute to SOA outdoors (44, 58).

      Manufacturers of various products have tried to eliminate the use of VOCs that are known air toxins, but many VOCs that are precursors to SOAs are still present in everyday products.

    9. Prior studies often report missing

      Hydroxyl radical (OH) concentrations that are measured have been lower than what is predicted by models. This implies that other species or processes not included in the model must account for the difference between the model and the measurements.

    10. Monte Carlo analysis

      This technique is used in calculations for complicated systems where changes in one variable influences many parts of the calculation. In this technique many calculations are run with each calculation having different beginning conditions. The end results are then analyzed to look at the range of possible outcomes. The error bars in parentheses in Figure 4 are determined by a Monte Carlo simulation.

      See more about Monte Carlo simulations from the Massachusetts Institute of Technology: http://news.mit.edu/2010/exp-monte-carlo-0517

    11. Here, we assess the importance of VCP emissions to ambient air pollution, again using Los Angeles as a test case (Fig. 4). Los Angeles currently violates the U.S. 8-hour O3 standard,

      The air in Los Angeles, California has ozone (\(O_3\)) levels that are higher than those recommended by the Environmental Protection Agency. The amounts and types of VOCs present influences ozone levels.

    12. We tested our indoor model with measurements of residential (32) and commercial buildings (40)

      A previous study on residential buildings combined measurements from 77 separate studies to examine the typical levels of VOCs in average households. The study only used measurements from U.S. households and those of countries with similar styles of living.

      Another study on commercial buildings made measurements of the levels of VOCs in 37 different small and medium-sized commercial buildings in California. This allowed the researchers to quantify typical VOC exposure in commercial buildings.

      This study is using these two experimental studies to test their model.

      A primary instrument used for determining the VOC components in air is a mass spectrometer. Learn more about mass spectrometers by watching the following video. https://www.youtube.com/watch?v=J-wao0O0_qM

    13. IVOCs

      Intermediate Volatile Organic Compounds. Molecules that contain carbon atoms that somewhat evaporate into the gas-phase at typical pressures and temperatures.

    14. Adding emissions from VCPs (Fig. 3B) reduces the model bias in ambient air from –39% to +1%, and the R2 in the box model improves from 0.59 to 0.94.

      The model calculations don't agree well with the experimental data when the contribution from VCPs is ignored. When the contribution from VCPs is included in the model calculations, the model agrees well with the experimental data. This change in model agreement can be seen in the models bias and R-squared values.

      Model bias measures on average the percent error between the model calculations and the experimental measurements. Without the inclusion of VCPs the model was, on average, under predicting the concentrations of VOCs by 39% (Panel A of Figure 3). With the inclusion of VCPs the model was on average over predicting the concentration of VOCs by 1% (Panel B of Figure 3).

    15. A surprising result is that mobile-source emissions of ethanol account for less than 20% of ambient concentrations, even though gasoline blends now routinely include at least 10% ethanol. This suggests that other sources are contributing substantially to ambient ethanol concentrations, which we attribute to VCPs.

      Ethanol was measured in the air. Less than 20% of the ethanol comes from mobile sources (gasoline and other automobiles). The remaining 80% comes from other sources, such as volatile chemical products.

    16. We therefore conclude that large underpredictions are due to missing emission sources.

      Comparisons are made between experimental data and model calculations. The model calculations are influenced by what molecules are emitted, how those emissions chemically react, and how wind and other variables cause emissions to move from one place to another. The calculations use techniques from other researchers (including this paper's co-authors Joost de Gouw and Si-Wan Kim) to take into account chemical reactions and movement of the emissions from one place to another.

      The authors then make different predictions with their model assuming different types of chemical emissions (fossil fuels and/or VCPs).

    17. we found that fossil fuel VOCs [from mobile sources and from local oil and natural gas production and distribution (36)] can only account for 61% of the mass of freshly emitted VOCs measured, and 59% of their variability

      Box model calculations were carried out under various conditions. The authors can make various assumptions about what sources are emitting VOCs into the air (mobile sources or VCPs). The results of the calculations under these various conditions are then compared to experimental data. This allows the researchers to examine what sources are important contributors to VOCs in air.

    18. Previous studies typically relied on ambient VOC measurements mainly of compounds found in fossil fuels, while not including many species found in chemical products (16).

      Researchers have looked at various volatile organic molecules to track emissions. The types of molecules that researchers examine have an influence on conclusions drawn. Many studies have not looked for some of the volatile organic molecules that come from household products. This could explain why many models have underestimated the contribution of household products to volatile organic emissions.

    19. The rest is from upstream sources associated with oil and natural gas production and distribution.

      To see how this finding was covered in traditional media.

      Read more at the Los Angeles Times: http://www.latimes.com/science/sciencenow/la-sci-sn-la-smog-petroleum-20180215-story.html

    20. At national and urban scales, we attribute 15 to 42% of petrochemical VOCs to mobile sources and 39 to 62% of petrochemical VOCs to VCPs.

      To see how this finding was covered in traditional media.

      Read more at The New York Times: https://www.nytimes.com/2018/02/16/climate/perfume-pollution-smog.html

    21. mobile sources

      Produced from transportation-related activities; for example, driving.

      Mobile sources also include off-road engine sources (e.g. construction equipment, lawn mowers, and recreational vehicles), not necessarily related to transportation per se.

    22. acetone

      A common organic solvent. It has a variety of household uses; acetone is the active ingredient in nail polish remover.

    23. he relatively low VOC emission factor for on-road gasoline engines today (Fig. 2) results from (i) combustion oxidizing most hydrocarbons in fuel to carbon dioxide, and (ii) the increasing effectiveness of modern three-way catalytic converters in reducing tailpipe VOC emissions over multiple decades (5–7).

      Air pollution from automobiles has been greatly reduced by the use of catalytic converters. Catalytic converters have an interesting history.

      Read more at the Royal Society of Chemistry:


    24. Most organic compounds in soaps and detergents dissolve in water and end up in sewer systems (20), with negligible amounts emitted from wastewater treatment plants (21).

      Only a very small percentage of organic molecules in soaps that are disposed of in drains have the potential to negatively impact air quality. Overwhelmingly, organic molecules used in soaps and detergents end up being disposed of down the drain into sewer systems. These molecules then end up in sewer treatment plants. Most of these organic molecules are then either physically or chemically removed from the water at the treatment plant.

      Based on Fig. 1, around 97% of the mass of water and organic molecules in soaps and detergents end up being disposed of down the drain.

    25. Chemical feedstocks are almost entirely derived from fossil hydrocarbons (18) and are transformed to chemicals found in everyday household products (tables S1 to S3).

      Chemicals are primarily made with fossil fuels. An ever growing field of research in chemistry, known as Green Chemistry, seeks to find ways of performing chemistry that reduce environmental impacts.

      Read more at Scientific American:


    26. Automotive emissions of VOCs have decreased steadily from efforts to control tailpipe emissions in the United States (5) and Europe (6).

      Changes in how engines operate (air/fuel ratios, use of catalytic converters, etc.) have led to substantial decreases in automotive emissions. In major U.S. cities (New York, Los Angeles, and Houston) from 1990 to 2010 some hazardous emissions have decreased by 80% to 90% even as car traffic has increased. Hazardous emissions from vehicle exhaust are expected to remain at approximately this same level as vehicles continue to utilize technologies to reduce hazardous emissions.

    27. It is thus critical to identify and quantify the most important human-produced sources of VOC emissions to effectively mitigate air pollution and improve human health.

      Regulations on sulfur content of automobiles helps improve air quality.

      Read more at Scientific American:


    28. transitioning away from transportation-related sources and toward VCPs.

      Hear the lead author, Brian McDonald, talk about the findings of this work in the podcast, Science Friday.


    29. tropospheric

      Refers to the lowest portion of the atmosphere, from the surface of Earth out to about 5 miles above the surface.

      The troposphere contains all the breathable oxygen in the atmosphere sustaining all life on Earth.

    30. Existing U.S. regulations on VCPs emphasize mitigating ozone and air toxics

      Volatile organic compounds (VOCs) have been shown to have a negative impact on human health, according to the U.S. Environmental Protection Agency. In October 2018, the EPA announced it would disband its Particulate Matter Review Panel.

      Read more in The New York Times: https://www.nytimes.com/2018/10/11/climate/epa-disbands-pollution-science-panel.html

    31. carbonaceous aerosols

      Particles suspended in the air that contain mostly carbon. Soot is a good example of a carbonaceous aerosol.

    32. A gap in emission inventories

      When we think of air pollution, automobile exhaust is one of the first images that comes to mind, but other sources of air pollution are also responsible.

      Read more at CBS News:


  2. Sep 2018
    1. volatile organic compound (VOC)

      Molecules that contain carbon atoms (i.e. organic) and readily evaporate (i.e. volatile) into the gas-phase at typical pressures and temperatures.

    2. oxidation

      In Earth's atmosphere, oxidation generally involves the reaction of a chemical species (such as a VOC) with an oxygen-containing molecule (such as ozone) or a radical (such as OH). The oxidized molecule will generally be less volatile and more water soluble than the starting material. The oxidized product may condense more easily (due to lowered vapor pressure) and form SOA in the atmosphere.

    3. As diesel particle filters and oxidation catalysts become more widespread, and reduce diesel contributions to PM2.5 (60), the fraction of PM2.5 from VCPs will grow because SOA precursor emissions from VCPs are not decreasing as quickly (7).

      Various technologies are being engineered to reduce particle emissions from diesel engines. Use of VCPs that lead to formation of aerosols is not decreasing as fast. VCPs' contribution to aerosol formation will become increasingly important.

    4. Of the fossil total, ~40%, or ~1.3 μg m−3, is attributed to directly emitted particles (55, 56), mainly from diesel engines (7).

      Aerosols are directly emitted by natural (e.g., wildfires) and human activities (e.g., driving). The fossil total of aerosols refers to those from human activities. In Los Angeles, 40% of human-activity aerosols are soot-like aerosols directly emitted from engine exhaust. Diesel engines are the main contributors. The other 60% form in the atmosphere from emissions of gaseous VOCs, such as from automobiles or VCPs.

    5. The model-observation comparison of fossil-derived SOA improves substantially when we add VCP emissions to traditionally considered transportation emissions (fig. S6). Note that nonfossil contributions to SOA, such as from wood burning, cooking, and biogenic sources, are not considered here. If we consider emissions from mobile sources and upstream emission sources alone, then the amount of fossil SOA predicted by SOM is lower than measurements at the Pasadena ground site by a factor of 3.4 ± 1.7 (55, 56). The inclusion of VCP emissions is required to bring the modeled and measured SOA to agreement, within their respective uncertainties (fig. S6).

      The model calculations for formation of secondary organic aerosols (SOAs) do not agree with experimental data if the contribution from VCPs is ignored. The inclusion of VCP emissions in the model calculations allows the calculations to better agree with the experimental data.

    6. one-dimensional volatility basis set (51) for OVOCs

      A different aerosol formation model, published by Robinson et al. (2007). The authors of this study used both this and the SOM model to predict aerosol formation in their box model.

    7. The aerosol yields used in this study (table S12) are mostly estimated from the Statistical Oxidation Model (SOM)

      Co-author Christopher D. Cappa and other researchers developed the model used in this study to predict the formation of aerosol particles from VOCs. Using data from their aerosol formation model in this study's box model, the authors predicted the amount of aerosol particles that will be formed in Los Angeles.

    8. chemical transport modeling

      Calculations of how molecules move in air from one location to another. Also accounts for how molecules chemically transform in the atmosphere.

    9. upstream

      Refers to emissions that occur in the supply chain prior to use by the consumer.

    10. three-way catalytic converters

      An emission control device that simultaneously removes three air pollutants from tailpipe exhaust by oxidizing carbon monoxide and VOCs, and reducing nitrogen oxides.

    11. fine particulate matter (PM2.5)

      Suspended particles in air (dust, soot, water droplets, etc.) that are 2.5 micrometers or smaller in diameter.

      Fine particulate matter can be dangerous because the particle size is small enough to be inhaled deep into our lungs. https://blissair.com/what-is-pm-2-5.htm

    12. secondary organic aerosols

      Small particles that are formed in the atmosphere from the oxidation (a type of chemical reaction) of carbon-containing gases such as VOCs. Later referred to as SOAs in this paper.

    13. risks of mortality from respiratory diseases

      Ozone in the stratosphere (10,000 - 50,000 km above Earth's surface) is a net positive as it absorbs potentially harmful UV radiation. However, ozone at or near ground level (tropospheric) is a health-risk. Ozone that is inhaled into the body reacts with cells in the respiratory tract. This leads to increased respiratory disease and is correlated with increased mortality rates.

      See more about the hazardous effects of ozone at the EPA: https://www.epa.gov/ozone-pollution-and-your-patients-health/health-effects-ozone-general-population#intro

    14. nitrogen oxides (NOx = NO + NO2)

      Molecular compounds that contain nitrogen and oxygen. These types of molecules are commonly emitted into the atmosphere through automobile exhaust.

  3. Aug 2018
    1. We used energy and chemical production statistics, together with near-roadway and laboratory measurements, to construct the mass balance shown in Fig. 1 (17).

      Economic data was used to estimate the mass of various chemical products used in the United States. You can see how the estimates were derived in Tables S2 and S3 of the supporting information.

      Tables S2 and S3 can be found on pages 23 and 24 of the Supplementary Materials document

    2. epidemiological study

      Research into the amount, source, spread, and control of diseases.

    3. National Research Council, “Air quality management in the United States” (2004).

      This document sets standards for ensuring good air quality in the United States. Most of the recommendations focus on transportation and industrial sources. This paper suggests that household VCPs are becoming an ever increasing source of VOCs.

    4. X. M. Wu, M. G. Apte, R. Maddalena, D. H. Bennett, Environ. Sci. Technol. 45, 9075–9083 (2011).

      Monitored air pollution at various small industrial buildings in California. This monitoring included measurements for VOCs.

      This experimental data was used to compare with the model calculations in this study.

    5. California Air Resources Board, “The California Consumer Products Regulation” (2015).

      California looks to regulate the emission of VOCs and thus regulates and keeps records of VOCs in various products. The authors of this paper were able to utilize that database to know what chemicals are present in various products.

    6. J. M. Logue, T. E. McKone, M. H. Sherman, B. C. Singer, Indoor Air 21, 92–109 (2011).

      Compiled results from 77 separate studies to look at typical levels of indoor air pollution in household environments. The study includes VOCs.

      This experimental data was used to compare with the model calculations in this study.

    7. A. Borbon et al., J. Geophys. Res. Atmos. 118, 2041–2057 (2013).

      An experimental study that monitored the presence of VOCs in the outdoor air near Los Angeles, California during 2010.

      This study uses the experimental data by Bordon to compare with their model calculations.

    8. J. L. Jimenez et al., Science 326, 1525–1529 (2009).

      Jimenez and colleagues developed a model to show and predict how VOCs react to turn into aerosol particles in the atmosphere.

      Understanding VOC emissions is important to understanding aerosol formation.

    9. VCPs have begun to contribute significantly to SOA formation outdoors. Given that global mortality from fine particles is significantly greater than for ambient O3 pollution (1), further study is needed on whether chemical products currently designed to mitigate O3 are also sufficient to protect humans from exposure to fine particles.

      VCPs are becoming more important for understanding air quality. Further research into these products and the potential health risks they pose is needed.

    10. Although fossil fuels remain important sources of urban air pollution, exposure to ambient PM2.5 is increasingly from chemical products as the transportation sector becomes cleaner.

      Learn about the air quality index from the Environmental Protection Agency and monitor the air quality in your area. Learn about air quality index

      United States Air Quality Monitoring

    11. aerosol precursors

      Molecules that lead to the formation of aerosols (suspended particles in air).

    12. In fig. S5, we show that half of measured OH reactivity (21 ± 7 s−1) can be explained by fossil fuel VOC emissions (3.9 ± 1.8 s−1) and other non-VCP sources of OH reactivity (7.3 ± 1.6 s−1). The emissions from use of VCPs contribute an additional 4.8 ± 3.4 s−1, bringing the summed OH reactivity to within ~25% of the observations (fig. S5). Although our inventory slightly underestimates OH reactivity, it is now within uncertainties of measurements. The inclusion of typically unmeasured or unreported oxygenated compounds from VCPs can help to resolve some of the missing OH reactivity observed over cities.

      Predicting reactivity of hydroxyl radical is important for understanding many atmospheric processes. The inclusion of VCPs into the model calculations is needed to get better agreement with experimental data.

      The agreement between experimental data and model calculations is still off by about 25%. The authors suggest that other VCPs that have not been included in the model could account for the difference.

    13. sinks of OH reactivity

      Chemical reactions that remove hydroxyl radicals from the atmosphere.

    14. We attribute half of VOC reactivity (Fig. 4C) from petrochemical sources to VCPs and the other half to mobile and upstream sources. Because the VOC reactivity of VCPs is similar to that of transportation fuels (table S12), the distribution looks similar to that of VOC emissions (Fig. 4B).

      About half of the VOCs in air can be shown to come from VCPs, the other half comes from transportation sources (driving, etc.) and upstream sources (extracting oil, refining gasoline, making chemical products, etc.)

    15. The correspondence between our model predictions and indoor air quality measurements is high (Fig. 3D, R2 = 0.92). The model results are now consistent with typical indoor air concentrations for key markers (e.g., acetone, C9–C11 n-alkanes, ethanol, and dichloromethane) and important classes of SOA precursors, including terpenes (e.g., limonene) (41), glycols and glycol ethers (e.g., 2-butoxyethanol) (42), volatile methyl siloxanes (e.g., D5-siloxane) (43), aromatics (e.g., toluene, xylenes) (44), and heavier alkanes (e.g., C12–C13 n-alkanes) (45).

      If you just assume that VOCs in indoor air come from outdoor air sources, the model calculations and the experimental data have poor agreement.

      The model calculations agree well with experimental data if you account for VCPs that are directly emitted into indoor air.

    16. Except for formaldehyde, primary emissions of aldehydes do not appear to be good markers of fossil fuels (Fig. 3A) or VCPs (Fig. 3B) considered in this study, and are therefore excluded from our model bias and R2 calculations. One possible source of aldehydes is cooking emissions (39).

      The model calculations do not consider certain types of chemicals because they do not correlate well with emissions from fossil fuels or from VCPs. Other sources of VOCs, such as those that come from cooking, are not included in this study.

    17. Nonane, decane, undecane

      Molecules that contain only carbon and hydrogen atoms (hydrocarbons). Nonane has 9 carbon atoms, decane has 10 carbon atoms, and undecane has 11 carbon atoms.

    18. California has an extensive regulatory reporting program for consumer products (34), including residential and commercial uses, which we used to speciate emissions. These speciation profiles provided us with target compounds to characterize in both outdoor and indoor environments. We also accounted for industrial emissions from VCPs (e.g., degreasing, adhesives, and coatings). The reporting data are in agreement with a U.S. database of chemicals (35) used as key constituents in chemical products (table S7). The VOC speciation profiles of VCPs (table S8) are distinguishable from those of fossil fuels (table S9), although there is some overlap in species present.

      Every emission source of VOCs emits different compounds. Many databases of the different emission profiles exist. The emission profiles allow the researchers to determine what activities (driving a car, painting a house, spraying perfume, etc.) lead to the presence of different VOCs in the air. The researchers use these profiles to differentiate between sources that come from fossil fuels (e.g. driving a car) and those that come from VCPs (e.g. painting or perfume).

    19. box model

      Calculation that predicts the chemical make-up of air due to emissions and various reactions that occur to transform those emissions.

    20. If chemical products are an important source of urban air pollution, then their chemical fingerprint (fig. S3) should be consistent with ambient and indoor air quality measurements.

      Previous studies have measured volatile organic compounds in outdoor and indoor air around Los Angeles, California. This study is using that data to see how it agrees with model calculations when accounting for volatile organic compounds from different sources. The video below gives an overview of how atmospheric modeling helps us understand what is occurring.


      The study looks at two different environments (indoor air and outdoor air) that can influence each other.

      See Figure S4 on page 18 from the Supplementary Materials document

    21. marker of coating-related VCPs in this study and in the past (16), increased in ambient air in Los Angeles from 1990 to 2010 (22). This is in sharp contrast to VOCs present in gasoline exhaust, which decreased markedly during the same period (22), except for ethanol (23).

      Organic compounds emitted from gasoline exhaust have decreased from 1990 - 2010. The decrease is due to increased combustion efficiency and catalytic converters. Organic compounds that come from other sources have increased during this same time period.

    22. The fraction that can be emitted to the atmosphere depends strongly on product type and use (table S4).

      Emissions from volatile chemical products was determined by looking at the composition of chemicals in a particular product and how readily those chemicals evaporate into the air. The authors compiled information from various other researchers to perform these calculations.

    23. In 2012, the amount of oil and natural gas used as fuel in the United States was ~15 times the amount used as chemical feedstocks (Fig. 1A).

      United States government data was compiled to look at the amount of oil and natural gas supplied and what these sources are used for. See Table S1 of the supporting information to see the breakdown.

      Table S1 can be found on page 22 of the Supplementary Materials document

    24. Transportation emissions of NOx and VOCs have long been considered major contributors to formation of O3 (8) and SOA (9–11) in urban areas, although recent studies have suggested the importance of nonvehicular sources as major contributors (12–14).

      Vehicle emissions contribute to air pollution. This includes increases in ozone and aerosol concentrations. Vehicular emissions alone cannot account for the increased ozone and and aerosol concentrations measured in the atmosphere. Additional sources of emission must exist to account for the increase, but where those sources come from is still uncertain.

    25. adverse human health effects occur below current U.S. standards for PM2.5 and O3 (4).

      The Environmental Protection Agency (EPA) sets legal limits for exposure to various environmental toxins. In studying Medicare recipients it was found that increases to PM2.5 and ozone exposure cause an increase in mortality rate. A 10 microgram per cubic meter increase of PM2.5 caused a 7.3% increase in mortality.


    26. transport

      Movement of chemicals away from their initial emission source.

    27. Although U.S. sales of VCPs are substantially smaller than for gasoline and diesel fuel, VOC emissions from VCPs (7.6 ± 1.5 Tg) are twice as large as from mobile sources (3.5 ± 1.1 Tg) (Fig. 1E, light green, dark green, and blue bars) because of differences in emission factors.

      The authors use different calculations to determine VOC emissions from mobile sources and VCPs. This is due to the different end uses. Oil and natural gas used for mobile sources undergo combustion that causes much of the carbon to be emitted as carbon dioxide and not VOC. VCPs are primarily used directly and remain as intact organic molecules.

    28. intermediate-volatility organic compounds (IVOCs)

      Molecules that contain carbon atoms that somewhat evaporate into the gas-phase at typical pressures and temperatures.

    29. GBD 2016 Risk Factors Collaborators, Lancet 390, 1345–1422 (2017).

      This study compiled a large number of studies on human health to examine various risk factors for disease and death.

      Air pollution was found to be the fifth leading risk factor for human health.

    30. We consider four key pieces of evidence to support this finding: (i) energy and chemical production statistics; (ii) near-roadway measurements of transportation emissions, together with laboratory testing of chemical products; (iii) ambient air measurements away from roads; and (iv) indoor air measurements.

      Many experiments from other researchers are utilized in this study to compare model calculations with experimental data.

      The authors in this study utilize all of these different sources of information to build a better understanding of the sources of air pollution.

  4. Jul 2018
    1. Exposure to air pollution is the fifth ranking human health risk factor globally, following malnutrition, dietary risks, high blood pressure, and tobacco (1).

      Understanding the causes of disease and death are crucial to better health outcomes. A look at health data from 1990 to 2015 identified factors that lead to negative health outcomes. Exposure to increased air pollution primarily leads to increased respiratory and cardiovascular disease.

    2. aerosol

      Suspended particles in air. Dust, soot, and water droplets are some examples of aerosols.

    3. multigenerational aging schemes

      After a chemical is emitted it undergoes reactions in the atmosphere. The products of the initial chemical reaction then undergo additional reactions in the atmosphere. Multigenerational aging accounts for the reactions that occur after the initial reaction.

    4. hydroxyl radical (OH)

      The primary reactant in the atmosphere that oxidizes hydrocarbons.

    5. fresh emission conditions

      The chemical make-up of an emission before any additional chemical reactions take place.

    6. chemical speciation

      Able to tell the difference between similar chemicals.

    7. source apportionment studies

      Studies designed to determine how various compounds are emitted into the atmosphere.

    8. higher-volatility

      Evaporate more quickly into the gas phase.

    9. anthropogenic

      From human activities.

    10. organic solvents

      Hydrocarbon liquid environments that dissolve chemicals responsible for the functioning of a product.

    11. fossil origin

      Comes from products derived from oil and natural gas.

    12. detailed mass balance

      A way to keep track of where all of the atoms and molecules are formed and where they end up.

    13. volatile chemical products (VCPs)

      Substances used by people that give off gas-phase molecules.

    14. transportation emissions

      Molecules that exit the tailpipe of a vehicle while driving.