Suzanne E. Paulson

Aerosol formation in the photooxidation of isoprene and β-pinene

Isoprene and β-pinene, at concentration levels ranging from a few ppb to a few ppm, were reacted photochemically with NO_x in the Caltech outdoor smog chamber facility. Aerosol formation from the isoprene photooxidation is found to be negligible even under extreme ambient conditions due to the relatively high vapor pressure of its products. Aerosol carbon yield from the β-pinene photooxidation is as high as 8% and depends strongly on the initial HC/NO_x ratio. The average vapor pressure of the β-pinene aerosol is estimated to be 37±24 ppt at 31 °C. Monoterpene photooxidation can be a significant source of secondary aerosol in rural environments and in urban areas with extended natural vegetation.

The reactions of ozone with alkenes: An important source of HOx in the boundary layer

The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radi- cals. Organic peroxy radicals (RO2) probably accompany the production of OH. In this paper, we draw attention to the potential importance of these reactions in the primary production of HOx (HOx = OH, HO2 and RO2) radicals in various regions of the boundary layer. The reactions of ozone with anthropogenic alkenes are shown to be the most important source of HOx in many urban settings during the day and evening, and a significant source at night. The majority of HO x comes from trace quantities of alkenes with internal double bonds. Reaction of 03 with isoprene and terpenes can be an important source of HOx in forested regions; we show that these reactions are the dominant radical source in the late afternoon and into the night. This additional HOx source is expected to increase predicted OH concentrations compared to those calculated by models that do not include the O3-alkene source.

Development and evaluation of a photooxidation mechanism for isoprene

A detailed mechanism for isoprene photooxidation is developed that includes the recent developments on each of isoprenes atmospherically important reactions: O[sub 3], OH, O([sup 3]P), and NO[sub 3] as well as the OH reactions of methacrolein and methyl vinyl ketone. The mechanism also attempts to account for the gaps in the product distributions for a few reactions. The CO yield from isoprene oxidation in the presence of NO[sub x], more than about 50 ppt, is calculated as about 3.4. The mechanism is tested against chamber data that include a range of mixtures of the primary oxidants, isoprene concentrations, and hydrocarbon/NO[sub x] ratios. It performs well under conditions where the OH and O([sup 3]P) reactions dominate the predicts peak O[sub 3] concentrations within about 25% under all conditions. The mechanism underpredicts methyl vinyl ketone yields as well as O[sub 3] levels under conditions of low available NO[sub x]. Sensitivity of the mechanism to assumptions about the NO[sub 3] and O[sub 3] reactions are examined, and possible sources of the discrepancies are discussed. A condensed mechanism is developed and compared to the explicit mechanism. 48 refs., 6 figs., 3 tabs.

HDO in the Martian atmosphere - Implications for the abundance of crustal water

The physical and chemical processes that lead to the preferential escape of hydrogen over deuterium in the Martian atmosphere are studied in detail using a one-dimensional photochemical model. Comparison of our theory with recent observations of HDO suggests that, averaged over the planet, Mars contains 0.2 m of crustal water that is exchangeable with the atmosphere. Our estimate is considerably lower than recent estimates of subsurface water on Mars based on geomorphological analysis of Viking images. The estimate can be reconciled if only a small fraction of crustal water can exchange with the atmosphere.

Human Breath Emissions of VOCs

The medical community has long recognized that humans exhale volatile organic compounds (VOCs). Several studies have quantified emissions of VOCs from human breath, with values ranging widely due to variation between and within individuals. The authors have measured human breath concentrations of isoprene and pentane. The major VOCs in the breath of healthy individuals are isoprene (12-580 ppb), acetone (1.2-1,880 ppb), ethanol (13-1,000 ppb), methanol (160-2,000 ppb) and other alcohols. In this study, we give a brief summary of VOC measurements in human breath and discuss their implications for indoor concentrations of these compounds, their contributions to regional and global emissions budgets, and potential ambient air sampling artifacts. Though human breath emissions are a negligible source of VOCs on regional and global scales (less than 4% and 0.3%, respectively), simple box model calculations indicate that they may become an important (and sometimes major) indoor source of VOCs under crowded conditions. Human breath emissions are generally not taken into account in indoor air studies, and results from this study suggest that they should be.

Product studies of the OH‐ and ozone‐initiated oxidation of some monoterpenes

The OH- and O3-initiated oxidation of five monoterpenes (myrcene, terpinolene, Δ3-carene, α-pinene, and β-pinene) has been studied in environmental chambers equipped with either a Fourier transform infrared spectrometer or a gas chromatography/flame ionization detector system. The OH-oxidation of myrcene and terpinolene is shown to lead to substantial yields of acetone (36 and 39%, respectively), while the acetone yield from the pinene compounds is quite small (4% and ∼2%, for α- and β-pinene, respectively). Formaldehyde has been identified as a major product (yields of 20–40%) in the OH-initiated oxidation of all five species. Formic acid was also observed in the OH-initiated oxidation of all five monoterpenes, with yields of 2% from β-pinene and 5–9% from the other species studied. The production of acetone from the reaction of monoterpenes with ozone in the presence of an OH scavenger was measured. The yields of acetone for the O3 reactions were α-pinene, 0.03±0.01; β-pinene, 0.009±0.009; Δ3-carene, 0.10±0.015; myrcene, 0.25±0.06; and terpinolene, 0.50±0.06. The mechanism leading to the production of these compounds is discussed, as is the atmospheric relevance of the results. In particular, an estimate of the contribution of monoterpene oxidation to observed atmospheric levels of acetone and formic acid is made.

Measurement of OH radical formation from the reaction of ozone with several biogenic alkenes

OH radical formation from the ozone reaction with isoprene, α-pinene, and methyl vinyl ketone were measured using the recently developed small-ratio relative-rate technique. This method uses small amounts of fast-reacting aromatics and aliphatic ethers to trace OH formation. Measured OH yields were 0.25±0.06, 0.70±0.17, and 0.16±0.05 for isoprene, α-pinene, and methyl vinyl ketone, respectively. The levels of biogenic alkenes necessary to contribute a non-negligible fraction of new OH, HO2, and RO2 radicals are briefly discussed.

Aircraft Emission Impacts in a Neighborhood Adjacent to a General Aviation Airport in Southern California

Real time air pollutant concentrations were measured downwind of Santa Monica Airport (SMA), using an electric vehicle mobile platform equipped with fast response instruments in spring and summer of 2008. SMA is a general aviation airport operated for private aircraft and corporate jets in Los Angeles County, California. An impact area of elevated ultrafine particle (UFP) concentrations was observed extending beyond 660 m downwind and 250 m perpendicular to the wind on the downwind side of SMA. Aircraft operations resulted in average UFP concentrations elevated by factors of 10 and 2.5 at 100 and 660 m downwind, respectively, over background levels. The long downwind impact distance (i.e., compared to nearby freeways at the same time of day) is likely primarily due to the large volumes of aircraft emissions containing higher initial concentrations of UFP than on-road vehicles. Aircraft did not appreciably elevate average levels of black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (PB-PAH), although spikes in concentration of these pollutants were observed associated with jet takeoffs. Jet departures resulted in peak 60-s average concentrations of up to 2.2 x 10(6) cm(-3), 440 ng m(-3), and 30 microg m(-3) for UFP, PB-PAH, and BC, respectively, 100 m downwind of the takeoff area. These peak levels were elevated by factors of 440, 90, and 100 compared to background concentrations. Peak UFP concentrations were reasonably correlated (r(2) = 0.62) with fuel consumption rates associated with aircraft departures, estimated from aircraft weights and acceleration rates. UFP concentrations remained elevated for extended periods associated particularly with jet departures, but also with jet taxi and idle, and operations of propeller aircraft. UFP measured downwind of SMA had a median mode of about 11 nm (electric mobility diameter), which was about half of the 22 nm median mode associated with UFP from heavy duty diesel trucks. The observation of highly elevated ultrafine particle concentrations in a large residential area downwind of this local airport has potential health implications for persons living near general aviation airports.

Production of stabilized Criegee intermediates and peroxides in the gas phase ozonolysis of alkenes: 1. Ethene, trans‐2‐butene, and 2,3‐dimethyl‐2‐butene

Ozone-alkene reactions generate stabilized Criegee intermediates (of the form R1R2COO), which are believed to react with water molecules to form organic hydroperoxides, hydrogen peroxide and carboxylic acids. These reactions are thought to be significant sources of these environmentally important compounds, yet both the yields of stabilized Criegee intermediates and the branching ratios from their reaction with water are not well known. The formation of hydrogen peroxide and organic hydroperoxides was investigated in the gas phase ozonolysis of ethene, trans-2-butene, and 2,3-dimethyl-2-butene for relative humidities (RH) from 0 and 80% by gas chromatography with flame ionization detection and high-performance liquid chromatography with fluorescence detection. Additionally, yields of acetaldehyde and acetic acid from trans-2-butene and acetone from 2,3-dimethyl-2-butene were measured. The reactions of stabilized Criegee intermediates with water were found to proceed almost entirely via organic hydroperoxide or hydrogen peroxide formation with little acid formation. Stabilized Criegee intermediate yields of 0.39, 0.24, and 0.10 were obtained for ethene, trans-2-butene, and 2,3-dimethyl-2-butene, respectively.

Evidence for formation of OH radicals from the reaction of O3 with alkenes in the gas phase

A series of experiments investigating the formation of OH radicals from O3-alkene reactions are described. Pairs of OH tracer compounds—aromatic and aliphatic hydrocarbons—were added to O3-alkene mixtures in small quantities. Under these conditions, a large fraction of the tracer is consumed, resulting in a much larger signal than in experiments using excess tracer. The average decay ratios of tracer pairs in 30 experiments were within 7% of those predicted by literature rate constants for tracer-OH reactions, strongly supporting the hypothesis that OH is formed directly from O3 reaction with alkenes.