C. Mihele

Direct polymerization of isoprene and α‐pinene on acidic aerosols

[1] The direct polymerization of isoprene and α-pinene on acidic sulfate aerosols has been studied in a reaction chamber utilizing aerosol mass spectrometry. Results indicated that both species can be directly taken up into acidic aerosols to a significant extent, forming polymers that contain at least 4 isoprene or 2 α-pinene repeating units. Aerosol mass spectra indicate that double bonds in the polymers hydrate under acid catalysis, leading to partial oxygenation of the polymers. This reactive uptake depends highly upon relative humidity and particle acidity. This process is rapid and reaches equilibrium in less than 50 minutes, with effective partition coefficients (K p,eff ) between 1.2-14.1 × 10 -6 m 3 μg -1 , from which it is estimated <0.5-5 ng m -3 of polymers may be present from both species in acidic aerosols. The formation of biogenic polymers is an important mechanism for incorporating hydrophobic, unsaturated species into polar aerosols and enhanced SOA formation.

Evaluating Multipollutant Exposure and Urban Air Quality: Pollutant Interrelationships, Neighborhood Variability, and Nitrogen Dioxide as a Proxy Pollutant

Background: Although urban air pollution is a complex mix containing multiple constituents, studies of the health effects of long-term exposure often focus on a single pollutant as a proxy for the entire mixture. A better understanding of the component pollutant concentrations and interrelationships would be useful in epidemiological studies that exploit spatial differences in exposure by clarifying the extent to which measures of individual pollutants, particularly nitrogen dioxide (NO2), represent spatial patterns in the multipollutant mixture. Objectives: We examined air pollutant concentrations and interrelationships at the intraurban scale to obtain insight into the nature of the urban mixture of air pollutants. Methods: Mobile measurements of 23 air pollutants were taken systematically at high resolution in Montreal, Quebec, Canada, over 34 days in the winter, summer, and autumn of 2009. Results: We observed variability in pollution levels and in the statistical correlations between different pollutants according to season and neighborhood. Nitrogen oxide species (nitric oxide, NO2, nitrogen oxides, and total oxidized nitrogen species) had the highest overall spatial correlations with the suite of pollutants measured. Ultrafine particles and hydrocarbon-like organic aerosol concentration, a derived measure used as a specific indicator of traffic particles, also had very high correlations. Conclusions: Our findings indicate that the multipollutant mix varies considerably throughout the city, both in time and in space, and thus, no single pollutant would be a perfect proxy measure for the entire mix under all circumstances. However, based on overall average spatial correlations with the suite of pollutants measured, nitrogen oxide species appeared to be the best available indicators of spatial variation in exposure to the outdoor urban air pollutant mixture. Citation: Levy I, Mihele C, Lu G, Narayan J, Brook JR. 2014. Evaluating multipollutant exposure and urban air quality: pollutant interrelationships, neighborhood variability, and nitrogen dioxide as a proxy pollutant. Environ Health Perspect 122:65–72; http://dx.doi.org/10.1289/ehp.1306518

Particle Formation and Gas/Particle Partition Measurements of the Products of the Naphthalene-OH Radical Reaction in a Smog Chamber

The formation of particles during the reaction of naphthalene with OH radicals in a smog chamber was followed by means of a scanning particle-sizing system and aerosol spectrometer probe. The initial formation of nucleation-sized particles (0.03-0.09 w m) quickly grew into an accumulation mode (0.1-3.0 w m) during the photolysis. A small nucleation mode component, however, persisted throughout the experiment, although the major portion of the mass is found in the accumulation mode at later stages in the reaction. Multichannel, annular, diffusion denuders have been used to determine the gas/particle partition factors for many of the observed products. While 1,4-naphthoquinone appears predominantly in the gas phase, the 1- and 2-naphthols and the nitronaphthalenes were found predominantly in the particle phase. Although 2-formylcinnamaldehyde, the most abundant product in the naphthalene-OH photolysis, was split approximately evenly between the gaseous and the particulate phases, the 2-formylcinnamic acid was found almost exclusively in the particle phase.

Reactions of Methylated Naphthalenes with Hydroxyl Radicals Under Simulated Atmospheric Conditions

Abstract The gas-phase reactions of OH with 1- and 2-methylnaphthalene, 1,3- and 2,3-dimethylnaphthalene, acenaphthylene and acenaphthene were investigated under simulated atmospheric conditions in a 10m3 smog chamber. The rates of reaction, relative to naphthalene = 1.0, were 1-methylnaphthalene, 1.5; 2-methylnaphthalene, 2.1; 1,3-dimethylnaphthalene, 0.9; 2,3-dimethylnaphthalene, 0.9; acenaphthene, 2.9; and acenaphthylene, 5.8. Solid phase microextraction (SPME) in combination with gas chromatography/mass spectrometry (GC/MS) was used to analyze the organic reaction products, thus avoiding the production of artifacts observed in the previously used cold finger trapping procedure. Reaction products were mainly homologs of those formed in the reaction of naphthalene with OH radicals.

Rapid organic aerosol formation downwind of a highway: Measured and model results from the FEVER study

The Fast Evolution of Vehicle Emissions from Roadway (FEVER) study was undertaken to strategically measure pollutant gradients perpendicular to a major highway north of Toronto, Canada. A case study period was analyzed when there was an average perpendicular wind direction. Two independent, fast response measurements were used to infer rapid organic aerosol (OA) growth on a spatial scale from 34 m to 285 m at the same time as a decrease was observed in the mixing ratio of primary emitted species, such as CO2 and NOx. An integrated organic gas and particle sampler also showed that near the highway, the aerosol had a larger semivolatile fraction than lower volatile fraction, but over a relatively short distance downwind of the highway, the aerosol transformed to being more low volatile with the change being driven by both evaporation of semivolatile and production of lower volatile organic aerosol. A new 1-D column Lagrangian atmospheric chemistry model was developed to help interpret the measured increase in the ∆OA/∆CO2 curve from 34 m to 285 m downwind of highway, where the ∆ refers to background-corrected concentrations. The model was sensitive to the assumptions for semivolatile organic compounds (SVOCs). Different combinations of SVOC emissions and background mixing ratios were able to yield similar model curves and reproduce the observations. Future measurements of total gas-phase SVOC in equilibrium with aerosol both upwind and downwind of the highway would be helpful to constrain the model.

Robustness of Land-Use Regression Models Developed from Mobile Air Pollutant Measurements

Land-use regression (LUR) models are useful for resolving fine scale spatial variations in average air pollutant concentrations across urban areas. With the rise of mobile air pollution campaigns, characterized by short-term monitoring and large spatial extents, it is important to investigate the effects of sampling protocols on the resulting LUR. In this study a mobile lab was used to repeatedly visit a large number of locations (∼1800), defined by road segments, to derive average concentrations across the city of Montreal, Canada. We hypothesize that the robustness of the LUR from these data depends upon how many independent, random times each location is visited (Nvis) and the number of locations (Nloc) used in model development and that these parameters can be optimized. By performing multiple LURs on random sets of locations, we assessed the robustness of the LUR through consistency in adjusted R2 (i.e., coefficient of variation, CV) and in regression coefficients among different models. As Nloc increased, R2adj became less variable; for Nloc = 100 vs Nloc = 300 the CV in R2adj for ultrafine particles decreased from 0.088 to 0.029 and from 0.115 to 0.076 for NO2. The CV in the R2adj also decreased as Nvis increased from 6 to 16; from 0.090 to 0.014 for UFP. As Nloc and Nvis increase, the variability in the coefficient sizes across the different model realizations were also seen to decrease.