Air-quality implications of widespread adoption of cool roofs on ozone and particulate matter in southern California.

The installation of roofing materials with increased solar reflectance (i.e., "cool roofs") can mitigate the urban heat island effect and reduce energy use. In addition, meteorological changes, along with the possibility of enhanced UV reflection from these surfaces, can have complex impacts on ozone and PM2.5 concentrations. We aim to evaluate the air-quality impacts of widespread cool-roof installations prescribed by California's Title 24 building energy efficiency standards within the heavily populated and polluted South Coast Air Basin (SoCAB). Development of a comprehensive rooftop area database and evaluation of spectral reflectance measurements of roofing materials allows us to project potential future changes in solar and UV reflectance for simulations using the Weather Research Forecast and Community Multiscale Air Quality (CMAQ) models. 2012 meteorological simulations indicate a decrease in daily maximum temperatures, daily maximum boundary layer heights, and ventilation coefficients throughout the SoCAB upon widespread installation of cool roofs. CMAQ simulations show significant increases in PM2.5 concentrations and policy-relevant design values. Changes in 8-h ozone concentrations depend on the potential change in UV reflectance, ranging from a decrease in population-weighted concentrations when UV reflectance remains unchanged to an increase when changes in UV reflectance are at an upper bound. However, 8-h policy-relevant ozone design values increase in all cases. Although the other benefits of cool roofs could outweigh small air-quality penalties, UV reflectance standards for cool roofing materials could mitigate these negative consequences. Results of this study motivate the careful consideration of future rooftop and pavement solar reflectance modification policies.

A Real-Time Fast-Flow Tube Study of VOC and Particulate Emissions from Electronic, Potentially Reduced-Harm, Conventional, and Reference Cigarettes.

Tobacco-free electronic cigarettes (e-cigarettes), which are currently not regulated by the FDA, have become widespread as a "safe" form of smoking. One approach to evaluate the potential toxicity of e-cigarettes and other types of potentially "reduced-harm" cigarettes is to compare their emissions of volatile organic compounds (VOCs), including reactive organic electrophillic compounds such as acrolein, and particulate matter to those of conventional and reference cigarettes. Our newly designed fast-flow tube system enabled us to analyze VOC composition and particle number concentration in real-time by promptly diluting puffs of mainstream smoke obtained from different brands of combustion cigarettes and e-cigarettes. A proton transfer reaction time-of-flight mass spectrometer (PTRMS) was used to analyze real-time cigarette VOC emissions with a 1 s time resolution. Particles were detected with a condensation particle counter (CPC). This technique offers real-time analysis of VOCs and particles in each puff without sample aging and does not require any sample pretreatment or extra handling. Several important determining factors in VOC and particle concentration were investigated: (1) puff frequency; (2) puff number; (3) tar content; (4) filter type. Results indicate that electronic cigarettes are not free from acrolein and acetaldehyde emissions and produce comparable particle number concentrations to those of combustion cigarettes, more specifically to the 1R5F reference cigarette. Unlike conventional cigarettes, which emit different amounts of particles and VOCs each puff, there was no significant puff dependence in the e-cigarette emissions. Charcoal filter cigarettes did not fully prevent the emission of acrolein and other VOCs.

Direct photolysis of α-pinene ozonolysis secondary organic aerosol: effect on particle mass and peroxide content.

Primary and secondary organic aerosols (POA and SOA) contain a complex mixture of multifunctional chemicals, many of which are photolabile. Much of the previous work that aimed to understand the chemical evolution (aging) of POA and SOA has focused on the reactive uptake of gas-phase oxidants by particles. By stripping volatile compounds and ozone from α-pinene ozonolysis SOA with three 1-m-long denuders, and exposing the residual particles in a flow cell to near-ultraviolet (λ>300 nm) radiation, we find that condensed-phase photochemistry can induce significant changes in SOA particle size and chemical composition. The particle-bound organic peroxides, which are highly abundant in α-pinene ozonolysis SOA (22 ± 5% by weight), have an atmospheric photolysis lifetime of about 6 days at a 24-h average solar zenith angle (SZA) of 65° experienced at 34° latitude (Los Angeles) in the summer. In addition, the particle diameter shrinks 0.56% per day under these irradiation conditions as a result of the loss of volatile photolysis products. Experiments with and without the denuders show similar results, suggesting that condensed-phase processes dominate over heterogeneous reactions of particles with organic vapors, excess ozone, and gas-phase free radicals. These condensed-phase photochemical processes occur on atmospherically relevant time scales and should be considered when modeling the evolution of organic aerosol in the atmosphere.

Experimental and theoretical study of aqueous cis-pinonic acid photolysis.

Direct aqueous photolysis of cis-pinonic acid (PA; 2-(3-acetyl-2,2-dimethylcyclobutyl)acetic acid; CAS Registry No. 473-72-3) by 280-400 nm radiation was investigated. The photolysis resulted in Norrish type II isomerization of PA leading to 3-isopropenyl-6-oxoheptanoic acid (CAS Registry No. 4436-82-2), also known as limononic acid, as the major product, confirmed by (1)H and (13)C NMR analysis, chemical ionization mass spectrometry, and electrospray ionization mass spectrometry. Several minor products resulting from Norrish type I splitting of PA were also detected. The molar extinction coefficients of aqueous PA were measured and used to calculate the photolysis quantum yield of aqueous PA as 0.5 ± 0.3 (effective average value over the 280-400 nm range). The gas-phase photolysis quantum yield of 0.53 ± 0.06 for PA methyl ester (PAMe; CAS Registry No. 16978-11-3) was also measured for comparison. These results indicate that photolysis of PA is not significantly suppressed by the presence of water. This fact was confirmed by photodissociation dynamics simulations of bare PA and of PAMe hydrated with one or five water molecules using on-the-fly dynamics simulations on a semiempirical potential energy surface. The calculations correctly predicted the occurrence of both Norrish type I and Norrish type II photolysis pathways, both driven by the dynamics on the lowest triplet excited state of PA and PAMe. The rate of removal of PA by direct aqueous photolysis in cloudwater and in aerosol water was calculated for a range of solar zenith angles and compared with rates of other removal processes such as gas-phase oxidation by OH, aqueous-phase oxidation by OH, and gas-phase photolysis. Although the direct photolysis mechanism was not the most significant sink for PA in cloud and fog droplets, direct photolysis can be expected to contribute to removal of PA and more soluble/less volatile biogenic oxidation products in wet particulate matter.

Absorption spectra and photolysis of methyl peroxide in liquid and frozen water.

Methyl peroxide (CH(3)OOH) is commonly found in atmospheric waters and ices in significant concentrations. It is the simplest organic peroxide and an important precursor to hydroxyl radical. Many studies have examined the photochemical behavior of gaseous CH(3)OOH; however, the photochemistry of liquid and frozen water solutions is poorly understood. We present a series of experiments and theoretical calculations designed to elucidate the photochemical behavior of CH(3)OOH dissolved in liquid water and ice over a range of temperatures. The molar extinction coefficients of aqueous CH(3)OOH are different from the gas phase, and they do not change upon freezing. Between -12 and 43 °C, the quantum yield of CH(3)OOH photolysis is described by the following equation: Φ(T) = exp((-2175 ± 448)1/T) + 7.66 ± 1.56). We use on-the-fly ab initio molecular dynamics simulations to model structures and absorption spectra of a bare CH(3)OOH molecule and a CH(3)OOH molecule immersed inside 20 water molecules at 50, 200, and 220 K. The simulations predict large sensitivity in the absorption spectrum of CH(3)OOH to temperature, with the spectrum narrowing and shifting to the blue under cryogenic conditions because of constrained dihedral motion around the O-O bond. The shift in the absorption spectrum is not observed in the experiment when the CH(3)OOH solution is frozen suggesting that CH(3)OOH remains in a liquid layer between the ice grains. Using the extinction coefficients and photolysis quantum yields obtained in this work, we show that under conditions with low temperatures, in the presence of clouds with a high liquid-water content and large solar zenith angles, the loss of CH(3)OOH by aqueous photolysis is responsible for up to 20% of the total loss of CH(3)OOH due to photolysis. Gas phase photolysis of CH(3)OOH dominates under all other conditions.

Adventures in ozoneland: down the rabbit-hole.

In this perspective we describe a 15 year pursuit of the Stabilized Criegee Intermediate (SCI). We have conducted several complementary experiments to measure the pressure dependence of product yields-including OH radical and ozonides-on sequences of alkene + ozone systems. In so doing we have been able to bring into gradual focus a succession of weakly bound intermediates, starting with the primary ozonide, then the SCI, and finally a vinyl hydroperoxide (VHP) product of SCI rearrangement. We have narrowed the phase space in our hunt for direct SCI observations to a range of alkene carbon numbers and system pressures, but the system continues to deliver surprises. One surprise is strong evidence that the VHP is a significant bottleneck along the reaction coordinate. These findings support the search for the SCI, build our fundamental understanding of collisional energy transfer in highly excited, multiple-well, chemically activated systems, and finally directly inform atmospheric chemistry on topics including HO(x) radical formation and reactions associated with secondary organic aerosol formation.

Ozonolysis of cyclic alkenes as surrogates for biogenic terpenes: primary ozonide formation and decomposition.

Alkene ozonolysis reactions proceed through an unstable intermediate, the primary ozonide (POZ). POZ decomposition controls the complex mechanism. We probe the kinetics of primary ozonide decomposition using temperature programmed reaction spectroscopy (TPRS), revealing primary ozonide decomposition barrier heights of 9.1 +/- 0.4, 9.4 +/- 0.4, and 11.9 +/- 1.2 kcal mol(-1) for cyclohexene, 1-methyl-cyclohexene, and methylene-cyclohexane, respectively. We compare experimental decomposition spectra with spectral predictions using density functional theory (DFT) to reveal decomposition products resembling vinyl-hydroperoxides and dioxiranes. We do not find evidence of secondary ozonides. Additional computations with DFT, scaled with the TPRS barrier height, yield barrier heights ranging from 9.4 to 12.1 kcal mol(-1) for the four competing decomposition pathways of the 1-methyl-cyclohexene POZ. Entropic differences were minimal, indicating that POZ decomposition branching is controlled purely by enthalpic variations. These kinetic computations were used to calculate a hydroxyl radical yield for 1-methyl-cyclohexene ozonolysis of 0.85 at 298 K.

A semiempirical correlation between enthalpy of vaporization and saturation concentration for organic aerosol.

To model the temperature-induced partitioning of semivolatile organics in laboratory experiments or atmospheric models, one must know the appropriate heats of vaporization. Current treatments typically assume a constant value of the heat of vaporization or else use specific values from a small set of surrogate compounds. With published experimental vapor-pressure data from over 800 organic compounds, we have developed a semiempirical correlation between the saturation concentration (C*, microg m(-3)) and the heat of vaporization (deltaH(VAP), kJ mol(-1)) for organics in the volatility basis set. Near room temperature, deltaH(VAP) = -11 log(10)C(300)(*) + 129. Knowledge of the relationship between C* and deltaH(VAP) constrains a free parameter in thermodenuder data analysis. A thermodenuder model using our deltaH(VAP) values agrees well with thermal behavior observed in laboratory experiments.

The kinetics of tetramethylethene ozonolysis: decomposition of the primary ozonide and subsequent product formation in the condensed phase.

We report data from real-time FTIR temperature programmed reaction spectroscopy on a cryogenic zinc selenide window revealing the intermediates from ozonation of 2,3-dimethyl-2-butene (TME). We have found convincing evidence of a 1,2,3-trioxolane (the primary ozonide, POZ), which decomposes at 185 K to yield a 1,2,4-trioxolane product (the secondary ozonide, SOZ). Computational infrared spectra confirmed the presence of the POZ and SOZ. The barrier height for POZ decomposition, determined experimentally, was found to be 13.8 +/- 1.0 kcal mol(-1), and the A factor calculated with RRKM theory based on density functional reactant and transition state frequencies was found to be 4.16 x 10(13) s(-1). The TME SOZ has not previously been observed without the presence of a polyethylene surface. SOZ formation kinetics from the reaction of the POZ decomposition products along with the competing reaction pathways were examined with computational chemistry calculations using DFT. These calculations confirm our experimental observation of SOZ formation.