Alkylperoxy radicals are responsible for the formation of oxygenated primary organic aerosol.

Organic aerosol (OA) is an air pollutant ubiquitous in urban atmospheres. Urban OA is usually apportioned into primary OA (POA), mostly emitted by mobile sources, and secondary OA (SOA), which forms in the atmosphere due to oxidation of gas-phase precursors from anthropogenic and biogenic sources. By performing coordinated measurements in the particle phase and the gas phase, we show that the alkylperoxy radical chemistry that is responsible for low-temperature ignition also leads to the formation of oxygenated POA (OxyPOA). OxyPOA is distinct from POA emitted during high-temperature ignition and is chemically similar to SOA. We present evidence for the prevalence of OxyPOA in emissions of a spark-ignition engine and a next-generation advanced compression-ignition engine, highlighting the importance of understanding OxyPOA for predicting urban air pollution patterns in current and future atmospheres.

Biomass burning aerosols in most climate models are too absorbing.

Uncertainty in the representation of biomass burning (BB) aerosol composition and optical properties in climate models contributes to a range in modeled aerosol effects on incoming solar radiation. Depending on the model, the top-of-the-atmosphere BB aerosol effect can range from cooling to warming. By relating aerosol absorption relative to extinction and carbonaceous aerosol composition from 12 observational datasets to nine state-of-the-art Earth system models/chemical transport models, we identify varying degrees of overestimation in BB aerosol absorptivity by these models. Modifications to BB aerosol refractive index, size, and mixing state improve the Community Atmosphere Model version 5 (CAM5) agreement with observations, leading to a global change in BB direct radiative effect of -0.07 W m-2, and regional changes of -2 W m-2 (Africa) and -0.5 W m-2 (South America/Temperate). Our findings suggest that current modeled BB contributes less to warming than previously thought, largely due to treatments of aerosol mixing state.

Direct Radiative Effect and Public Health Implications of Aerosol Emissions Associated with Shifting to Gasoline Direct Injection (GDI) Technologies in Light-Duty Vehicles in the United States.

Due to their enhanced fuel economy, the market share of gasoline direct injection (GDI) vehicles has increased significantly over the past decade. However, GDI engines emit higher levels of black carbon (BC) aerosols compared to traditional port fuel injection (PFI) engines. Here, we performed coupled chemical transport and radiative transfer simulations to estimate the aerosol-induced public health and direct radiative effects of shifting the U.S. fleet from PFI to GDI technology. By comparing simulations with current emission profiles and emission profiles modified to reflect a shift from PFI to GDI, we calculated the change in aerosol (mostly BC) concentrations associated with the fleet change. Standard concentration-response calculations indicated that the total annual deaths in the U.S. attributed to particulate gasoline-vehicle emissions would increase from 855 to 1599 due to shifting from PFI to GDI. Furthermore, the increase in BC associated with the shift would lead to an annual average positive radiative effect over the U.S. of approximately +0.075 W/m2, with values as large as +0.45 W/m2 over urban regions. On the other hand, the reduction in CO2 emissions associated with the enhanced fuel economy of GDI vehicles would yield a globally uniform negative radiative effect, estimated to be -0.013 W/m2 over a 20 year time horizon. Therefore, the climate burden of the increase in BC emissions dominates over the U.S., especially over source regions.

Secondary Organic Aerosol Production from Gasoline Vehicle Exhaust: Effects of Engine Technology, Cold Start, and Emission Certification Standard.

Secondary organic aerosol (SOA) formation from dilute exhaust from 16 gasoline vehicles was investigated using a potential aerosol mass (PAM) oxidation flow reactor during chassis dynamometer testing using the cold-start unified cycle (UC). Ten vehicles were equipped with gasoline direct injection engines (GDI vehicles) and six with port fuel injection engines (PFI vehicles) certified to a wide range of emissions standards. We measured similar SOA production from GDI and PFI vehicles certified to the same emissions standard; less SOA production from vehicles certified to stricter emissions standards; and, after accounting for differences in gas-particle partitioning, similar effective SOA yields across different engine technologies and certification standards. Therefore the ongoing, dramatic shift from PFI to GDI vehicles in the United States should not alter the contribution of gasoline vehicles to ambient SOA and the natural replacement of older vehicles with newer ones certified to stricter emissions standards should reduce atmospheric SOA levels. Compared to hot operations, cold-start exhaust had lower effective SOA yields, but still contributed more SOA overall because of substantially higher organic gas emissions. We demonstrate that the PAM reactor can be used as a screening tool for vehicle SOA production by carefully accounting for the effects of the large variations in emission rates.

Reducing secondary organic aerosol formation from gasoline vehicle exhaust.

On-road gasoline vehicles are a major source of secondary organic aerosol (SOA) in urban areas. We investigated SOA formation by oxidizing dilute, ambient-level exhaust concentrations from a fleet of on-road gasoline vehicles in a smog chamber. We measured less SOA formation from newer vehicles meeting more stringent emissions standards. This suggests that the natural replacement of older vehicles with newer ones that meet more stringent emissions standards should reduce SOA levels in urban environments. However, SOA production depends on both precursor concentrations (emissions) and atmospheric chemistry (SOA yields). We found a strongly nonlinear relationship between SOA formation and the ratio of nonmethane organic gas to oxides of nitrogen (NOx) (NMOG:NOx), which affects the fate of peroxy radicals. For example, changing the NMOG:NOx from 4 to 10 ppbC/ppbNOx increased the SOA yield from dilute gasoline vehicle exhaust by a factor of 8. We investigated the implications of this relationship for the Los Angeles area. Although organic gas emissions from gasoline vehicles in Los Angeles are expected to fall by almost 80% over the next two decades, we predict no reduction in SOA production from these emissions due to the effects of rising NMOG:NOx on SOA yields. This highlights the importance of integrated emission control policies for NOx and organic gases.

Comparison of Gasoline Direct-Injection (GDI) and Port Fuel Injection (PFI) Vehicle Emissions: Emission Certification Standards, Cold-Start, Secondary Organic Aerosol Formation Potential, and Potential Climate Impacts.

Recent increases in the Corporate Average Fuel Economy standards have led to widespread adoption of vehicles equipped with gasoline direct-injection (GDI) engines. Changes in engine technologies can alter emissions. To quantify these effects, we measured gas- and particle-phase emissions from 82 light-duty gasoline vehicles recruited from the California in-use fleet tested on a chassis dynamometer using the cold-start unified cycle. The fleet included 15 GDI vehicles, including 8 GDIs certified to the most-stringent emissions standard, superultra-low-emission vehicles (SULEV). We quantified the effects of engine technology, emission certification standards, and cold-start on emissions. For vehicles certified to the same emissions standard, there is no statistical difference of regulated gas-phase pollutant emissions between PFIs and GDIs. However, GDIs had, on average, a factor of 2 higher particulate matter (PM) mass emissions than PFIs due to higher elemental carbon (EC) emissions. SULEV certified GDIs have a factor of 2 lower PM mass emissions than GDIs certified as ultralow-emission vehicles (3.0 ± 1.1 versus 6.3 ± 1.1 mg/mi), suggesting improvements in engine design and calibration. Comprehensive organic speciation revealed no statistically significant differences in the composition of the volatile organic compounds emissions between PFI and GDIs, including benzene, toluene, ethylbenzene, and xylenes (BTEX). Therefore, the secondary organic aerosol and ozone formation potential of the exhaust does not depend on engine technology. Cold-start contributes a larger fraction of the total unified cycle emissions for vehicles meeting more-stringent emission standards. Organic gas emissions were the most sensitive to cold-start compared to the other pollutants tested here. There were no statistically significant differences in the effects of cold-start on GDIs and PFIs. For our test fleet, the measured 14.5% decrease in CO2 emissions from GDIs was much greater than the potential climate forcing associated with higher black carbon emissions. Thus, switching from PFI to GDI vehicles will likely lead to a reduction in net global warming.

Clouds and "throat hit": Effects of liquid composition on nicotine emissions and physical characteristics of electronic cigarette aerosols.

Electronic cigarettes (ECIGs) heat and vaporize a liquid mixture to produce an inhalable aerosol that can deliver nicotine to the user. The liquid mixture is typically composed of propylene glycol (PG) and vegetable glycerin (VG), in which are dissolved trace quantities of flavorants and, usually, nicotine. Due to their different chemical and thermodynamic properties, the proportions of PG and VG in the liquid solution may affect nicotine delivery and user sensory experience. In social media and popular culture, greater PG fraction is associated with greater "throat-hit", a sensation that has been attributed in cigarette smokers to increased presence of vapor-phase nicotine. VG, on the other hand, is associated with thicker and larger exhaled "clouds". In this study, we aim to investigate how PG/VG ratio influences variables that relate to nicotine delivery and plume visibility. Aerosols from varying PG/VG liquids were generated using a digitally controlled vaping instrument and a commercially available ECIG, and analyzed for nicotine content by GC-MS. Particle mass and number distribution were determined using a six-stage cascade impactor and a fast particle spectrometer (TSI EEPS), with tightly controlled dilution and sampling biases. A Mie theory model was used to compute the aerosol scattering coefficients in the visible spectrum. Decreasing the PG/VG ratio resulted in a decrease in total particulate matter (TPM) and nicotine yield (R2 > 0.9, p<.0001). Measured particle count median diameter ranged between 44-97nm, and was significantly smaller for PG liquids. Although the particle mass concentration was lower, aerosols produced using liquids that contained VG had an order of magnitude greater light scattering coefficients. These findings indicate that PG/VG ratio is a strong determinant of both nicotine delivery and user sensory experience.

Probing the Evaporation Dynamics of Mixed SOA/Squalane Particles Using Size-Resolved Composition and Single-Particle Measurements.

An analysis of the formation and evaporation of mixed-particles containing squalane (a surrogate for hydrophobic primary organic aerosol, POA) and secondary organic aerosol (SOA) is presented. In these experiments, one material (D62-squalane or SOA from α-pinene + O3) was prepared first to serve as surface area for condensation of the other, forming the mixed-particles. The mixed-particles were then subjected to a heating-ramp from 22 to 44 °C. We were able to determine that (1) almost all of the SOA mass is comprised of material less volatile than D62-squalane; (2) AMS collection efficiency in these mixed-particle systems can be parametrized as a function of the relative mass fraction of the components; and (3) the vast majority of D62-squalane is able to evaporate from the mixed particles, and does so on the same time scale regardless of the order of preparation. We also performed two-population mixing experiments to directly test whether D62-squalane and SOA from α-pinene + O3 form a single solution or two separate phases. We find that these two OA types are immiscible, which informs our inference of the morphology of the mixed-particles. If the morphology is core-shell and dictated by the order of preparation, these data indicate that squalane is able to diffuse relatively quickly through the SOA shell, implying that there are no major diffusion limitations.

Characterizing the spatial variation of air pollutants and the contributions of high emitting vehicles in Pittsburgh, PA.

We used a mobile measurement platform to characterize a suite of air pollutants (black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (PB-PAH), benzene, and toluene) in the city of Pittsburgh and surrounding areas. More than 270 h of data were collected from forty-two sites which were selected based on analysis in the geographic information system (GIS). Mobile measurements were performed during three different times of day (mornings, afternoons/evenings, and overnight) in both winter (November 2011 to February 2012) and summer (June 2012 to August 2012). Pollutant concentrations were elevated in river valleys by 9% (benzene) to 30% (PB-PAH) relative to upland areas. Traffic had strong impacts on measured pollutants. PB-PAH and BC concentrations at high traffic sites were a factor of 2 and 30% higher than at low traffic sites, respectively. Pollutant concentrations were highest in the morning sessions due to a combination of traffic and meteorological conditions. The highly time-resolved data indicated that elevated pollutant concentrations at high traffic sites were due to short duration plume events associated with high emitting vehicles. High emitting vehicles contributed up to 70% of the near road PB-PAH and 30% of BC; emissions from these vehicles drove substantial spatial variations in BC and PB-PAH concentrations. Many high emitting vehicles were presumably diesel trucks or buses, because plumes were strongly correlated with truck traffic volume. In contrast, PB-PAH and BC in the nonplume background air was weakly correlated with traffic, and their spatial patterns were more influenced by terrain and point source emissions. The spatial variability in contributions of high emitting vehicles suggests that the effect of potential control strategies vary for different pollutants and environments.

Organic aerosol mixing observed by single-particle mass spectrometry.

We present direct measurements of mixing between separately prepared organic aerosol populations in a smog chamber using single-particle mass spectra from the high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Docosane and docosane-d46 (22 carbon linear solid alkane) did not show any signs of mixing, but squalane and squalane-d62 (30 carbon branched liquid alkane) mixed on the time scale expected from a condensational-mixing model. Docosane and docosane-d46 were driven to mix when the chamber temperature was elevated above the melting point for docosane. Docosane vapors were shown to mix into squalane-d62, but not the other way around. These results are consistent with low diffusivity in the solid phase of docosane particles. We performed mixing experiments on secondary organic aerosol (SOA) surrogate systems finding that SOA derived from toluene-d8 (a surrogate for anthropogenic SOA (aSOA)) does not mix into squalane (a surrogate for hydrophobic primary organic aerosol (POA)) but does mix into SOA derived from α-pinene (biogenic SOA (bSOA) surrogate). For the aSOA/POA, the volatility of either aerosol does not limit gas-phase diffusion, indicating that the two particle populations do not mix simply because they are immiscible. In the aSOA/bSOA system, the presence of toluene-d8-derived SOA molecules in the α-pinene-derived SOA provides evidence that the diffusion coefficient in α-pinene-derived SOA is high enough for mixing on the time scale of 1 min. The observations from all of these mixing experiments are generally invisible to bulk aerosol composition measurements but are made possible with single-particle composition data.

Time scales for gas-particle partitioning equilibration of secondary organic aerosol formed from alpha-pinene ozonolysis.

Most chemical transport models assume instantaneous equilibrium to represent gas-particle partitioning of semivolatile organic aerosol. This approach has been challenged by recent studies suggesting that secondary organic aerosol (SOA) cannot reach equilibrium within atmospheric time scales. The emergent hypothesis is that gas-particle partitioning rates are limited by diffusion within the condensed phase, which is thought to be "glassy." Here, we investigate the equilibration time scales of SOA formed from α-pinene ozonolysis by measuring the dynamic response to a modest step-change in temperature. Upon heating, equilibrium is disturbed, and the particles evaporate to restore equilibrium at the new temperature, which is attained when evaporation ceases. The SOA was formed at 10 °C and then heated to near room temperature (30 °C) so that the phase state (viscosity) of the condensed-phase after heating is similar to how it would be in the atmosphere. Experiments were performed in both a thermodenuder, with SOA loading of 350 µg/m(3), and in a smog chamber, with SOA loading of 2-12 µg/m(3). Both experiments show, contrary to previous findings, that the SOA achieves equilibrium with dynamic responses consistent with a mass accommodation coefficient of order 0.1. For typical atmospheric conditions, this translates into equilibration time scales on the order of minutes to tens of minutes, supporting the use of equilibrium partitioning in chemical transport models.

Volatility of organic molecular markers used for source apportionment analysis: measurements and implications for atmospheric lifetime.

Molecular markers are organic species used to define fingerprints for source apportionment of ambient fine particulate matter. Traditionally, these markers have been assumed to be stable in the atmosphere. This work investigates the gas-particle partitioning of eight organic species used as molecular markers in receptor models for biomass burning (levoglucosan), motor vehicles (5α-cholestane, n-hexacosane, n-triacontane, 1,2-benz[a]anthracene, coronene), and meat cooking (cholesterol, oleic acid). Experiments were conducted using a thermodenuder to measure the evaporation of single component particles. The data were analyzed using the integrated volume method to determine saturation concentrations and enthalpies of vaporization for each compound. The results indicate that appreciable quantities (>10%) of most of these markers exist in the gas phase under typical atmospheric conditions. Therefore, these species should be considered semivolatile. Predictions from a chemical kinetics model indicate that gas-particle partitioning has important effects on the atmospheric lifetime of these species. The atmospheric decay of semivolatile compounds proceeds much more rapidly than nonvolatile compounds because gas-phase oxidation induces evaporation of particle-phase material. Therefore, both gas-particle partitioning and chemical reactions need to be accounted for when semivolatile molecular markers are used for source apportionment studies.

Comparison of carcinogen, carbon monoxide, and ultrafine particle emissions from narghile waterpipe and cigarette smoking: Sidestream smoke measurements and assessment of second-hand smoke emission factors.

The lack of scientific evidence on the constituents, properties, and health effects of second-hand waterpipe smoke has fueled controversy over whether public smoking bans should include the waterpipe. The purpose of this study was to investigate and compare emissions of ultrafine particles (UFP, <100 nm), carcinogenic polyaromatic hydrocarbons (PAH), volatile aldehydes, and carbon monoxide (CO) for cigarettes and narghile (shisha, hookah) waterpipes. These smoke constituents are associated with a variety of cancers, and heart and pulmonary diseases, and span the volatility range found in tobacco smoke.Sidestream cigarette and waterpipe smoke was captured and aged in a 1 m(3) Teflon-coated chamber operating at 1.5 air changes per hour (ACH). The chamber was characterized for particle mass and number surface deposition rates. UFP and CO concentrations were measured online using a fast particle spectrometer (TSI 3090 Engine Exhaust Particle Sizer), and an indoor air quality monitor. Particulate PAH and gaseous volatile aldehydes were captured on glass fiber filters and DNPH-coated SPE cartridges, respectively, and analyzed off-line using GC-MS and HPLC-MS. PAH compounds quantified were the 5- and 6-ring compounds of the EPA priority list. Measured aldehydes consisted of formaldehyde, acetaldehyde, acrolein, methacrolein, and propionaldehyde.We found that a single waterpipe use session emits in the sidestream smoke approximately four times the carcinogenic PAH, four times the volatile aldehydes, and 30 times the CO of a single cigarette. Accounting for exhaled mainstream smoke, and given a habitual smoker smoking rate of 2 cigarettes per hour, during a typical one-hour waterpipe use session a waterpipe smoker likely generates ambient carcinogens and toxicants equivalent to 2-10 cigarette smokers, depending on the compound in question. There is therefore good reason to include waterpipe tobacco smoking in public smoking bans.

Elevated toxicant yields with narghile waterpipes smoked using a plastic hose.

The effect of hose permeability on toxicant yields for the narghile waterpipe is investigated, with special reference to the recent adoption of plastic as a hose construction material. Measurements of air infiltration rates for 23 leather and plastic hoses representing 11 types commonly available in Beirut, Lebanon were made, revealing that while leather hoses allowed significant outside air infiltration during a puff constituting up to 31% of the puff volume, plastic hoses were found to be air-tight, indicating that the smoke reaching the waterpipe user can be considerably more concentrated when delivered via a plastic hose. Total particulate matter (TPM), nicotine and carbon monoxide (CO) yields were compared when a waterpipe was machine smoked using a highly permeable leather and an air-tight plastic hose. It was found that the plastic hose resulted in similar yields of nicotine, but more than double the CO yielded with the highly permeable leather hose. Thus, even if narghile smokers titrate for nicotine intake, the use of a plastic hose will likely greatly increase the exposure to CO, a major causative agent in cardiovascular disease.

Polycyclic aromatic hydrocarbons, carbon monoxide, "tar", and nicotine in the mainstream smoke aerosol of the narghile water pipe.

A smoking machine protocol and yields for "tar", nicotine, PAH, and CO are presented for the standard 171-puff steady periodic smoking regimen proposed by Shihadeh et al. [Shihadeh, A., Azar, S., Antonios, C., Haddad, A., 2004b. Towards a topographical model of narghile water-pipe cafe smoking: A pilot study in a high socioeconomic status neighborhood of Beirut, Lebanon. Pharmacology Biochemistry and Behavior 79(1), 75]. Results show that smokers are likely exposed to more "tar" and nicotine than previously thought, and that pyronsynthesized PAH are present in the "tar" despite the low temperatures characteristic of the tobacco in narghile smoking. With a smoking regimen consisting of 171 puffs each of 0.53l volume and 2.6s duration with a 17 s interpuff interval, the following results were obtained for a single smoking session of 10 g of mo'assel tobacco paste with 1.5 quick-lighting charcoal disks applied to the narghile head: 2.94 mg nicotine, 802 mg "tar", 145 mg CO, and relative to the smoke of a single cigarette, greater quantities of chrysene, phenanthrene, and fluoranthene. Anthracene and pyrene were also identified but not quantified. The results indicate that narghile smoke likely contains an abundance of several of the chemicals thought to be causal factors in the elevated incidence of cancer, cardiovascular disease and addiction in cigarette smokers.