An experimental investigation of incremental reactivities of volatile organic compounds

California has adopted a set of VOC reactivity factors for regulatory purposes that is based on a model of the ozone formation process. These incremental reactivity factors (derived by Carter) describe the amount of ozone each exhaust VOC will form under a certain set of conditions in an urban atmosphere. The main objective of this study is to measure reactivity factors using smog chamber experiments, and to compare the measurements to the Carter factors. A new facility was constructed explicitly for this study. The facility has four identical smog chambers and a temperature-controlled enclosure for the chambers. The chambers are irradiated using a set of filtered xenon arc lamps to approximate “sunlight”. The reactivities of 14 individual VOCs representative of those found in automobile exhaust and several mixtures of VOCs have been measured. The measured and Carter-reactivity factors were highly correlated, suggesting that the chemical model used by Carter accounts for the reactivities of a wide range of compounds with dramatically different uncertainties in their mechanisms. The measured results, in general, are consistent with the Carter-reactivity factors for comparing the relative reactivities of VOCs in the atmosphere. However, additional kinetic and mechanistic studies of many VOC species including aromatic isomers are needed to improve reactivity scales.     

Photochemical Reactivity of Perchloroethylene: A New Appraisal

Perchloroethylene (PCE), a solvent used in dry cleaning, has been suspected of contributing significantly to photochemical ozone/oxidant (O₃/O_x) problems in urban atmospheres. Past evidence, however, was neither complete nor consistent. To interpret more conclusively the past evidence, and further understand PCE's role in the O₃O_x problem, a smog chamber testing program was conducted. The program's objectives were: (a) to explain the mechanism of the PCE reaction in smog chamber atmospheres, and (b) to extrapolate the smog chamber findings regarding PCE reactivity to the real atmosphere. Results showed that in smog chambers, PCE reacts and forms O₃/O_x following what appears to be a Cl instigated photooxidation mechanism rather than the OH initiated mechanism accepted in current smog chemistry. The evidence, collectively, strongly supported this conclusion even though the source of Cl atoms could not be identified with confidence. It was further concluded that in the real atmosphere neither the Cl instigated nor the OH instigated photooxidations of PCE can generate substantial concentrations of O₃/O_x. In fact, PCE contributes less to the ambient O₃/O_x problem than equal concentrations of ethane.     

A New and More Sensitive Procedure For Analysis of Peroxybenzoyl Nitrate

The existence of the powerful lacrimator peroxybenzoyl nitrate (PBzN) as a laboratory-generated photochemical smog product was first established in 1968. However, the analysis of this compound at the requisite parts-per-billion level is difficult. For example, although earlier research suggested that all mono-alkyl benzenes having at least two benzylic hydrogens form PBzN, PBzN formation fromtoluene-NOz mixtures could not be established in smog chambers due to analytical interferences with the Gas Liquid Chromatography-Electron Capture (GLC-EC) method. The present paper proposes a new procedure for PBzN which is highly sensitive and less subject to interferences. The procedure involves quantitative conversion of PBzN to methyl benzoate (MeOBz) by trapping this compound in basic methanol solution followed by Gas Liquid Chroma-tography-Flame lonization Detection (GLC-FID) determination of the MeOBz. By employing this method, the formation of PBzN from laboratory irradiation of toluene-NO_x in air was demonstrated and quantitatively determined. Furthermore, the absence of PBzN formation from o-xylene and i-propylbenzene was confirmed. The technique has, in principle, unlimited sensitivity. Preliminary atmospheric sampling trials in the San Francisco Bay Area demonstrated that, if PBzN were present, its concentration was ≤0.07 ppb.     

Evaluation of the UNC toluene-SOA mechanism with respect to other chamber studies and key model parameters

In a companion paper by Hu et al. [2007. A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NO_x and natural sunlight. Atmospheric Environment, doi:10.1016/j.atmosenv.2007.04.025], a kinetic mechanism was developed from data generated in the University of North Carolina's (UNC) 270 m³ dual outdoor aerosol smog chamber, to predict secondary organic aerosol (SOA) formation from toluene oxidation in the atmosphere. In this paper, experimental data sets from European Photoreactor (EUPHORE), smog chambers at the California Institute of Technology (Caltech), and the UNC 300 m³ dual-outdoor gas phase chamber were used to evaluate the toluene mechanism. The model simulates SOA formation for the ‘low-NO_x’ and ‘mid-NO_x’ experiments from EUPHORE chambers reasonably well, but over-predicts SOA mass concentrations for the ‘high-NO_x’ run. The model well simulates the SOA mass concentrations observed from the Caltech chambers. Experiments with the three key toluene products, 1,4-butenedial, 4-oxo-2-pentenal and o-cresol in the presence of oxides of nitrogen (NO_x) are also simulated by the developed mechanism. The model well predicts the NO_x time–concentration profiles and the decay of these two carbonyls, but underestimates ozone (O₃) formation for 4-oxo-2-pentenal. It well simulates SOA formation from 1,4-butenedial but overestimates (possibly due to experimental problems) the measured aerosol mass concentrations from 4-oxo-2-pentenal. The model underestimates SOA production from o-cresol, mostly due to its under-prediction of o-cresol decay. The effects of varying temperature, relative humidity, glyoxal uptake, organic nitrate yields, and background seed aerosol concentrations, were also investigated.     

Validation of odor concentration from mechanical-biological treatment piles using static chamber and wind tunnel with different wind speed values

The basic principle of odor sampling from surface sources is based primarily on the amount of air obtained from a specific area of the ground, which acts as a source of malodorous compounds. Wind tunnels and flux chambers are often the only available, direct method of evaluating the odor fluxes from small area sources. There are currently no widely accepted chamber-based methods; thus, there is still a need for standardization of these methods to ensure accuracy and comparability. Previous research has established that there is a significant difference between the odor concentration values obtained using the Lindvall chamber and those obtained by a dynamic flow chamber. Thus, the present study compares sampling methods using a streaming chamber modeled on the Lindvall cover (using different wind speeds), a static chamber, and a direct sampling method without any screens. The volumes of chambers in the current work were similar, ~0.08 m³. This study was conducted at the mechanical-biological treatment plant in Poland. Samples were taken from a pile covered by the membrane. Measured odor concentration values were between 2 and 150 ou_E/m³. Results of the study demonstrated that both chambers can be used interchangeably in the following conditions: odor concentration is below 60 ou_E/m³, wind speed inside the Lindvall chamber is below 0.2 m/sec, and a flow value is below 0.011 m³/sec. Increasing the wind speed above the aforementioned value results in significant differences in the results obtained between those methods. In all experiments, the results of the concentration of odor in the samples using the static chamber were consistently higher than those from the samples measured in the Lindvall chamber. Lastly, the results of experiments were employed to determine a model function of the relationship between wind speed and odor concentration values.

Implications: Several researchers wrote that there are no widely accepted chamber-based methods. Also, there is still a need for standardization to ensure full comparability of these methods. The present study compared the existing methods to improve the standardization of area source sampling. The practical usefulness of the results was proving that both examined chambers can be used interchangeably. Statistically similar results were achieved while odor concentration was below 60 ou_E/m³ and wind speed inside the Lindvall chamber was below 0.2 m/sec. Increasing wind speed over these values results in differences between these methods. A model function of relationship between wind speed and odor concentration value was determined.     

Atmospheric Aerosol Composition and Concentration as a Function of Particle Size and of Time

As a step toward better understanding the reactive Los Angeles air basin atmosphere, a study was undertaken at the University of California—Riverside Campus, to determine the composition and concentration of atmospheric particulate matter as a function of particle size and time. The study involved developing a method for obtaining size-classified, time-fractionated aerosol samples amenable to chemical and physical (including microscopic) analysis. During a 15-day period, samples were obtained over 4-hr periods and subsequently analyzed for nitrate, sulfate, iron, and lead. Concentration of six gaseous pollutantsj total aerosol light-scattering, and several meteorological measurements were simultaneously recorded and averaged over the 4-hr intervals. This data was presented graphically to show the diurnal variation in and relationship between gaseous, particulate, and meteorological measurements. A strong relationship between gaseous peroxyacetyl nitrate, particulate nitrate, and aerosol light-scattering was found. High concentrations of ammonium nitrate particles, mainly in the 0.5-2μ, diameter size range, were found in the atmospheric particulate samples collected on days of very high smog (very limited visibility).     

A Study of Sulfur Dioxide in Photochemical Smog I. Effect of SO2 and Water Vapor Concentration in the 1-Butene/NOx/SO2 System

There is an appreciable chemical interaction between SO₂ and photochemical smog which depends on the concentration of SO₂ and water vapor. The rate of decay of SO₂ concentration is greatly increased in the presence of photochemical smog. With 0.75 ppm SO₂, a light-scattering aerosol is produced in dry systems and systems at 22 and 55% relative humidity (RH). Aerosol is not observed until after the NO₂ peak has been reached and the NO concentration has fallen to a very low value. The formation of aerosol corresponds in time to the region of most rapid decrease in the SO₂ profile. In systems at 65% RH or with smaller amounts of SO₂, no light scattering is observed, but the percentage of SO₂ disappearing is greater. In relatively dry systems the presence of SO₂ results in a general slowing down of the photochemical smog reactions. In systems containing water vapor concentrations comparable to those found in the atmosphere, the inhibiting influence of SO₂ on the smog reaction is less pronounced. However, the maximum concentration of oxidant produced by the photochemical smog reactions is significantly lower when SO₂ is present.     

A Combustion Method for the Determination of Solvent Vapors at Low Concentrations in Air

The nitrogen-containing products of smog chamber reactions have been the subject of much controversy. Concern has arisen over nitrogen products because of the almost universally poor nitrogen balance reported for irradiated mixtures of hydrocarbons and nitrogen oxides. Some possible nitrogen-containing products, such as molecular nitrogen, nitrous oxide, and nitroolefins have been investigated and shown to be unimportant. The nitrogen products most often measured are peroxyacetyl nitrate and residual nitrogen dioxide. These two products rarely comprise more than 70% of the initial nitrogen at the end of an experiment, and often account for less than 50%. Previous experiments in which total nitrate was determined in the gas phase and on the vessel walls at the end of irradiation have shown very good nitrogen balances. The assumption has been made that the nitrate arises from nitric acid formed on the walls by adsorbed N₂O₅.

In the work reported here, all major nitrogen-containing compounds have been monitored continuously for the first time. Nitric oxide and nitrogen dioxide have been monitored by chemilumines-cence and automated Saltzman techniques. Methyl, ethyl, and peroxyacetyl nitrate have been determined by gas chromatogra-phy. Two methods, one continuous and one integrated, have been specially developed to measure nitric acid both in the smog chambers and in the atmosphere. Continuous determination of these compounds yields good nitrogen balances throughout the irradiations.

Profiles of the nitrogen-containing species from irradiated HC/ NO_x mixtures are discussed in terms of nitrogen products and nitrogen balance. Differences in product distribution for different hydrocarbon systems are also considered. Using rate information from the nitrogen compound profiles, important reactions leading to nitrogen-containing products are identified. Interference with the chemiluminescent technique by HON0₂, PAN, and C₂H₅ONO₂ is discussed.     

On-line measurements of α-pinene ozonolysis products using an atmospheric pressure chemical ionisation ion-trap mass spectrometer

An on-line technique to investigate complex organic oxidation reactions in environmental chamber experiments is presented. The method is based on the direct introduction of the chamber air into an atmospheric pressure ion source of a commercial ion-trap mass spectrometer. To demonstrate the analytical potential of the method (atmospheric pressure chemical ionisation/mass spectrometry, APCI/MS), the ozonolysis of α-pinene was investigated in a series of experiments performed in various sized reaction chambers at atmospheric pressure and 296 K in synthetic air. Investigations were focussed on the influence of the water vapour concentration on the formation of the predominant oxidation product, pinonaldehyde, derived from the α-pinene/ozone reaction. Quantification of pinonaldehyde was achieved by conducting a standard addition technique. The molar yield of pinonaldehyde was found to depend strongly on the actual water vapour concentration between <1 and 80% relative humidity. Starting with an average yield of 0.23±0.05 at dry conditions, pinonaldehyde formation was approximately doubled by reaching a yield of 0.53±0.05 at a relative humidity of around 60%. Furthermore, the formation mechanism of pinonaldehyde was investigated in greater detail using isotopically labelled water. Applying on-line APCI/MS, pinonaldehyde formation under incorporation of ¹⁸O was observed, strongly supporting the reaction of the stabilised Criegee radical with water in the gas phase as suggested by Alvarado et al. (Journal of Geophysical Research 103 (1998) 25541–25551). Furthermore, the mass spectra recorded on-line were used to perform a semi-quantitative estimation of the decomposition pathway of the primary ozonide, indicating a branching ratio of 0.35/0.65.     

Comparisons of ambient air particulates concentrations (TSP,PM2.5) and dry depositions during smog and non-smog periods at Luliao,Shalu District sampling site

The concentrations of ambient total suspended particulates (TSP) and PM_2.5, and the dry depositions at a sample site at Luliao Junior High School (Luliao) in central Taiwan were measured during smog and non-smog days between December 2017 and July 2018. The results are compared to those obtained during non-smog periods in the years 2015–2017. The mean TSP and PM_2.5 concentrations and dry deposition flux were 72.41?±?26.40, 41.88?±?23.51?μg/m³, and 797.57?±?731.46?μg/m² min, respectively, on the smog days. The mean TSP and PM_2.5 concentrations and dry deposition flux on the non-smog days were 56.39?±?18.08, 34.81?±?12.59?μg/m³ and 468.93?±?600.57?μg/m² min, respectively. The mean TSP concentration in the smog period was 28% greater than that in the non-smog period, and the mean PM_2.5 concentration was 20% higher. The mean dry deposition flux in the smog period was 70% higher than that in the non-smog period at Luliao. The PM_2.5 concentrations exceeded the standards set by the Taiwan EPA (35?μg/m³ daily, and 15?μg/m³ annually). Therefore, the TSP and PM_2.5 concentrations and dry deposition must be reduced in central Taiwan on smog days. In addition, atmospheric TSP and PM_2.5 concentrations at various sampling sites were compared, and those herein were not higher than those measured in other countries. Finally, apart from the local traffic emissions, during smog periods, the other pollution source originated from the transportation process of traffic pollutants emitted in the northwest side of Taiwan.     

Characteristics of the major chemical constituents of PM2.5 and smog events in Seoul,Korea in 2003 and 2004

One hundred ninety-five chemically speciated samples were collected from March 2003 to February 2005 in the Seoul Metropolitan area to investigate the characteristics of the major components in PM_2.5 and to characterize the chemical variations between smog and non-smog events. The annual average PM_2.5 concentration was 43 μg m⁻³ that is almost three times higher than the US NAAQS annual PM_2.5 standard of 15 μg m⁻³. During this sampling period, smog and yellow sand events were observed on 27 and 10 days, respectively. The PM_2.5 concentrations and its constituents during smog events were about two–three times higher than those during non-smog and yellow sand events. In particular, the mass fractions of secondary aerosols such as sulfate, nitrate, and ammonium during the smog events were higher than those of the other constituents. The mean concentration and mass fraction of secondary organic carbon (SOC) were highest during the winter smog events. Sulfate, nitrate and SOC that can have long residence times were important species during the smog events suggesting that regional scale sources rather than local sources were important. Five-day backward air trajectory analysis showed that the air parcels during smog events passed through the major industrial areas in China more often than those during non-smog events.     

Atmospheric concentrations,gaseous–particulate distribution,and carcinogenic potential of polycyclic aromatic hydrocarbons in Assiut,Egypt

The concentrations of 15 priority PAHs were determined in the atmospheric gaseous and particulate phases from nine sites across Assiut City, Egypt. While naphthalene, acenaphthene, and fluorene were the most abundant in the gaseous phase with average concentrations of 377, 184, and 181 ng/m³, benzo[b]fluoranthene, chrysene, and benzo[g,h,i]perylene showed the highest levels in the particulate phase with average concentrations of 76, 6, and 52 ng/m³. The average total atmospheric concentration of target PAHs (1,590 ng/m³) indicates that Assiut is one of the highest PAH-contaminated areas in the world. Statistical analysis revealed a significant difference between the levels of PAHs in the atmosphere of urban and suburban sites (P?=?0.029 and 0.043 for gaseous and particulate phases, respectively). Investigation of diagnostic PAH concentration ratios revealed vehicular combustion and traffic exhaust emissions as the major sources of PAHs with a higher contribution of gasoline rather than diesel vehicles in the sampled areas. Benzo[a]pyrene has the highest contribution (average?=?32, 4 % for gaseous and particulate phases) to the total carcinogenic activity (TCA) of atmospheric PAHs. While particulate phase PAHs have higher contribution to the TCA, gaseous phase PAHs present at higher concentrations in the atmosphere are more capable of undergoing atmospheric reactions to form more toxic derivatives.     

Public perception of smog: A case study in Ningbo City,China

Smog has become a public environmental crisis in most areas of China, and in response, research efforts have mainly focused on the chemical properties of smog and its impact on human health. However, in-depth research on the public’s perception of smog has not yet been conducted. A survey of residents living around eight state-controlled atmospheric environmental monitoring sites in Ningbo City was conducted using stratified sampling. The data was statistically analyzed to investigate people’s views and behavioral tendencies in smog weather, the influence of different media reports on public outlook, and public opinions on the local atmosphere and pollution management in different areas. The results showed that people’s perception of smog differs greatly from actual conditions, indicating that the public opinion tends to deviate when faced with a public crisis. Mainstream media (TV, newspaper, etc.), accounting for 67% of all media sources, are the main source for dissemination of smog information. The main sources of pollution, in order of decreasing contribution, according to residents of Ningbo City are as follows: motor vehicle exhaust, industrial coal combustion, large-scale construction, biomass burning, and kitchen fumes.

Implications: Since 2011, most areas of China have been affected by frequent smog. Most research on smog has been concentrated on its causes, alert systems, and prevention measures, whereas in-depth research on the public perception of smog has not yet been conducted. When a risky environmental event such as smog occurs, consequences may be more serious than the event itself will cause if people take irrational measures because of lacking relevant knowledge. Therefore, investigating people’s attitude and response to smog is both theoretically and practically significant.     

Development of a versatile experimental setup for the evaluation of the photocatalytic properties of construction materials under realistic outdoor conditions

The interest on outdoor photocatalytic materials is growing in the last years. Nevertheless, most of the experimental devices designed for the assessment of their performance operate at controlled laboratory conditions, i.e., pollutant concentration, temperature, UV irradiation, and water vapor contents, far from those of real outdoor environments. The aim of the present study was the design and development of an experimental device for the continuous test of photocatalytic outdoor materials under sun irradiation using real outdoor air as feed, with the concomitant fluctuation of pollutant concentration, temperature, and water vapor content. A three-port measurement system based on two UV-transparent chambers was designed and built. A test chamber contained the photoactive element and a reference chamber to place the substrate without the photoactive element were employed. The third sampling point, placed outdoors, allowed the characterization of the surrounding air, which feeds the test chambers. Temperature, relative humidity (RH), and UV-A irradiance were monitored at each sampling point with specific sensors. NO _x concentration was measured by a chemiluminescence NO _x analyzer. Three automatic valves allowed the consecutive analysis of the concentration at the three points at fixed time intervals. The reliability of the analytical system was demonstrated by comparing the NO _x concentration data with those obtained at the nearest weather station to the experimental device location. The use of a chamber-based reaction system leads to an attenuation of NO _x and atmospheric parameter profiles, but maintaining the general trends. The air characterization results showed the wide operating window under which the photoactive materials should work outdoors, depending on the traffic intensity and the season, which are reproduced inside the test chambers. The designed system allows the measurement of the photoactivity of outdoor materials or the comparison of several samples at the same time. The suitability of the system for the evaluation of the DeNO _x properties of construction elements at realistic outdoor conditions was demonstrated. The designed experimental device can be used 24/7 for testing materials under real fluctuations of NO _x concentration, temperature, UV irradiation, and relative humidity and the presence of other outdoor air pollutants such as VOCs, SO _x , or NH₃. The chamber-based design allows comparing a photocatalytic material with respect to a reference substrate without the photoactive phase, or even the comparison of several outdoor elements at the same time. Figure

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Effect of Hydrocarbon and NO x on Photochemical Smog Formation under Simulated Transport Conditions

The body of information presented in this paper is directed to those individuals concerned with the effect of urban pollution on downwind areas. Concern has been expressed over the appropriate hydrocarbon and NO _x control strategy to be used in minimizing the effects of ozone and NO₂ on urban population centers and their downwind environs. O₃ and NO₂ formation were studied in smog chamber irradiations as a function of the initial NO _x concentration at three hydrocarbon concentrations. By carrying out the irradiations for a period of time equivalent to one solar day in a continuously diluting system, smog formation in a chemically reacting pollutant system under transport was simulated. The results of this experimental simulation suggest that hydrocarbon reduction reduces O₃ in urban as well as downwind areas while NO _x reduction increases O₃ in the urban area and has little effect on O₃ in downwind areas. Both hydrocarbon and NO _x reduction will reduce atmospheric NO₂ levels, with the effect of NO _x reduction generally being more pronounced.     

Chloroform – concentration gradients in soil air and atmospheric air,and emission fluxes from soil

Since we demonstrated the natural formation of chloroform in soil, the question arose to which extent this contributes to the chloroform present in the atmosphere. Concentration gradients in soil air and atmospheric air of different forests were measured. Chloroform concentration gradients indicating emission occur in forest soils and the atmosphere under the canopy, whereas this was not observed for other chlorinated solvents. Above the canopy all concentration gradients observed for chloroform and 1,1,1-trichloroethane indicate deposition. The emission flux was measured using enclosures and calculated from the observed concentration gradients in soil air and atmospheric air. Wood-degrading areas and soils with a humic layer were found to emit up to 1000 ng chloroform m⁻² h⁻¹ and seem to be larger chloroform sources than the other areas of study. Rather unexpectedly, some points of one sampling site appeared to emit 1,1,1-trichloroethane, tetrachloromethane and tetrachloroethene. A reasonable agreement was found between the fluxes using enclosures and those derived from the concentration gradients in soil air and atmospheric air.     

The photochemical formation and gas–particle partitioning of oxidation products of decamethyl cyclopentasiloxane and decamethyl tetrasiloxane in the atmosphere

Decamethyl cyclopentasiloxane (D₅) and decamethyl tetrasiloxane (MD₂M) were injected into a smog chamber containing fine Arizona road dust particles (95% surface area <2.6 μM) and an urban smog atmosphere in the daytime. A photochemical reaction – gas–particle partitioning reaction scheme, was implemented to simulate the formation and gas–particle partitioning of hydroxyl oxidation products of D₅ and MD₂M. This scheme incorporated the reactions of D₅ and MD₂M into an existing urban smog chemical mechanism carbon bond IV and partitioned the products between gas and particle phase by treating gas–particle partitioning as a kinetic process and specifying an uptake and off-gassing rate. A photochemical model PKSS was used to simulate this set of reactions. A Langmuirian partitioning model was used to convert the measured and estimated mass-based partitioning coefficients (K_P) to a molar or volume-based form. The model simulations indicated that >99% of all product silanol formed in the gas-phase partition immediately to particle phase and the experimental data agreed with model predictions. One product, D₄TOH was observed and confirmed for the D₅ reaction and this system was modeled successfully. Experimental data was inadequate for MD₂M reaction products and it is likely that more than one product formed. The model set up a framework into which more reaction and partitioning steps can be easily added.     

Analysis of chemical and physical effects of ultraviolet bulbs on cooking emissions

There is a growing recognition of the risks to health, fire hazard, and air quality from cooking emissions. Recent research has identified what is emitted when foods are cooked. Some of the emitted mass is captured in the exhaust system. The balance is expelled into the atmosphere. The outlet of the exhaust system is a demarcation point-upstream the captured mass is the operator or building owner's concern, whereas downstream into the atmosphere, it affects air quality. Building codes have long required operators to deal with the upstream section. More recently, regulations are being placed on what kitchens can emit to the atmosphere. The industry is responding to this challenge with product innovations. Recently gained understanding of cooking emissions supports much of the innovation-but not all. This paper evaluates the purported benefit of adding better filtration and ultraviolet C (UVC) bulbs in kitchen hoods. A "UV hood" claims a two-step process to reduce emissions: better filters capture more emitted mass, and UVC photons and ozone drive photo-decomposition and oxidation reactions of some of the remaining greasy constituents. Adding UV to a hood at least doubles the cost compared to an equivalent non-UV hood. There is evidence that UV hoods do reduce some emissions. The essential question is whether improved performance is due to UV or relatively inexpensive, improved filters. Experimentation exposed an oleic acid aerosol, representative of cooking emissions, to UVC energy and ozone at higher concentrations and for longer exposure times than can occur in a UV hood. Particle-size and chemical changes were measured on samples collected with UV bulbs off and on. Results strongly indicate little change is happening and most emission reductions are caused by better filtration and not UV. The conclusion is that UV hoods fall short of claimed performance, and unreacted ozone may increase air pollution.     

Air Pollution in Six Major U. S. Cities as Measured by the Continuous Air Monitoring Program

Data obtained by the Continuous Air Monitoring Program (CAMP) in six cities during two years are summarized. Six gaseous pollutants were monitored in Cincinnati, Chicago, New Orleans, Philadelphia, San Francisco, and Washington, D. C. during 1962 and 1963. The data serve as a basis for describing several contrasts and similarities in the nature of air pollution experienced in six cities, which represent a broad geographical and climatological range of urbayi environments. Specific topics covered are: typical pollutant levels, patterns of daily and seasonal variations, and unusual phenomena such as atmospheric stagnation periods and photochemical smog formation.     

Development of the SAPRC-07 chemical mechanism

An updated version of the SAPRC-99 gas-phase atmospheric chemical mechanism, designated SAPRC-07, is described. The rate constants and reactions have been updated based on current data and evaluations, the aromatics mechanisms have been reformulated and are less parameterized, chlorine chemistry has been added, the method used to represent peroxy reactions has been reformulated to be more appropriate for modeling gas-phase secondary organic aerosol precursors, and representations for many types of VOCs have been added or improved. This mechanism was evaluated against the result of ～2400 environmental chamber experiments carried out in 11 different environmental chambers, including experiments to test mechanisms for over 110 types of VOCs. The performance in simulating the chamber data was generally satisfactory for most types of VOCs but some biases were seen in simulations of some types of experiments. The mechanism was used to derive updated MIR and other ozone reactivity scales for almost 1100 types of VOCs, though in most cases the changes in MIR values relative to SAPRC-99 were not large. This mechanism update results in somewhat lower predictions of ozone in one-day ambient model scenarios under low VOC/NO_x conditions. The files needed to implement the mechanism and additional documentation is available at the SAPRC mechanism web site at http://www.cert.ucr.edu/～carter/SAPRC.