No abstract

Motivated by growing considerations of the scale, severity, and risks associated with human exposure to indoor particulate matter, this work reviewed existing literature to: (i) identify state-of-the-art experimental techniques used for personal exposure assessment; (ii) compare exposure levels reported for domestic/school settings in different countries (excluding exposure to environmental tobacco smoke and particulate matter from biomass cooking in developing countries); (iii) assess the contribution of outdoor background vs indoor sources to personal exposure; and (iv) examine scientific understanding of the risks posed by personal exposure to indoor aerosols. Limited studies assessing integrated daily residential exposure to just one particle size fraction, ultrafine particles, show that the contribution of indoor sources ranged from 19% to 76%. This indicates a strong dependence on resident activities, source events and site specificity, and highlights the importance of indoor sources for total personal exposure. Further, it was assessed that 10-30% of the total burden of disease from particulate matter exposure was due to indoor-generated particles, signifying that indoor environments are likely to be a dominant environmental factor affecting human health. However, due to challenges associated with conducting epidemiological assessments, the role of indoor-generated particles has not been fully acknowledged, and improved exposure/risk assessment methods are still needed, together with a serious focus on exposure control.

SUMMARYA laboratory investigation of electric charge transfer during the impact of vapour-grown ice crystals and supercooled water droplets upon a simulated soft-hailstone target has shown that the magnitude of the charge transferred to the riming surface when crystals separate from it is a function of temperature, crystal dimension, relative velocity, liquid water content, and impurity content of the water droplets and hence the impurity content of the riming target. The sign of the charge transfer depends on temperature, liquid water content and droplet and rime impurity content.In the absence of crystals, no charge transfer was detected during riming. In the absence of supercooled water droplets, crystals impacting at 10m s-on an evaporating rime target produced a small negative charge on the rime of less than -0.25fC per separating crystal. When the target surface grew by vapour diffusion it gained a small positive charge during such interactions. Much larger charges and completely different charge transfer behaviour was noted during riming. The target became positively charged at high liquid water contents and temperatures above a critical value, but negatively charged at lower temperatures or with lower liquid water contents. The critical sign reversal temperature at a liquid water content of 1 g m -3 was about -20°C. At -10°C with a liquid water content of 2gm-3, a 125pm crystal impacting at 3ms-I charged the target by + lOfC upon separation. The charge transfer increased sharply with impact speed and crystal size. Warming the positively charging rime to cause it to evaporate failed to reverse the sign of the charge transfer. Experiments with impurities showed that the sign reversal temperature increased if the droplets contained contaminants at concentrations found in cloud water.It is suggested that there are two distinct charge transfer processes during crystal interactions with an ice target, the dominant one requiring the presence of supercooled water droplets. Careful control and knowledge of the microphysical properties of the clouds used in these experimental simulations has permitted an examination of charge transfer under many of the conditions used in previous studies. The results provide an understanding of the differences and a reconciliation between some of the previously disparate findings in terms of the two distinct charge transfer regimes.

A laboratory investigation of electric charge transfer during the impact of vapour-grown ice crystals and supercooled water droplets upon a simulated soft-hailstone target has shown that the magnitude of the charge transferred to the riming surface when crystals separate from it is a function of temperature, crystal dimension, relative velocity, liquid water content, and impurity content of the water droplets and hence the impurity content of the riming target. The sign of the charge transfer depends on temperature, liquid water content and droplet and rime impurity content.In the absence of crystals, no charge transfer was detected during riming. In the absence of supercooled water droplets, crystals impacting at 10m s-on an evaporating rime target produced a small negative charge on the rime of less than -0.25fC per separating crystal. When the target surface grew by vapour diffusion it gained a small positive charge during such interactions. Much larger charges and completely different charge transfer behaviour was noted during riming. The target became positively charged at high liquid water contents and temperatures above a critical value, but negatively charged at lower temperatures or with lower liquid water contents. The critical sign reversal temperature at a liquid water content of 1 g m -3 was about -20°C. At -10°C with a liquid water content of 2gm-3, a 125pm crystal impacting at 3ms-I charged the target by + lOfC upon separation. The charge transfer increased sharply with impact speed and crystal size. Warming the positively charging rime to cause it to evaporate failed to reverse the sign of the charge transfer. Experiments with impurities showed that the sign reversal temperature increased if the droplets contained contaminants at concentrations found in cloud water.It is suggested that there are two distinct charge transfer processes during crystal interactions with an ice target, the dominant one requiring the presence of supercooled water droplets. Careful control and knowledge of the microphysical properties of the clouds used in these experimental simulations has permitted an examination of charge transfer under many of the conditions used in previous studies. The results provide an understanding of the differences and a reconciliation between some of the previously disparate findings in terms of the two distinct charge transfer regimes.

No abstract

Particle emissions from twelve buses, operating alternately on low sulfur (LS; 500 ppm) and ultralow sulfur (ULS; 50 ppm) diesel fuel, were monitored. The buses were 1-19 years old and had no after-treatment devices fitted. Measurements were carried out at four steady-state operational modes on a chassis dynamometer using a mini dilution tunnel (PM mass measurement) and a Dekati ejector diluter as a secondary diluter (SMPS particle number). The mean particle number emission rate (s(-1)) of the buses, in the size range 8-400 nm, using ULS diesel was 31% to 59% lower than the rate using LS diesel in all four modes. The fractional reduction was highest in the newest buses and decreased with mileage upto about 500,000 km, after which no further decrease was apparent. However, the mean total suspended particle (TSP) mass emission rate did not show a systematic difference between the two fuel types. When the fuel was changed from LS to ULS diesel, the reduction in particle number was mainly in the nanoparticle size range. Over all operational modes, 58% of the particles were smaller than 50 nm with LS fuel as opposed to just 45% with ULS fuel, suggesting that sulfur in diesel fuel was playing a major role in the formation of nanoparticles. The greatest influence of the fuel sulfur content was observed at the highest engine load, where 74% of the particles were smaller than 50 nm with LS diesel compared to 43% with ULS diesel.

No abstract

A comprehensive study of the particle and carbon dioxide emissions from a fleet of six dedicated liquefied petroleum gas (LPG) powered and five unleaded petrol (ULP) powered new Ford Falcon Forte passenger vehicles was carried out on a chassis dynamometer at four different vehicle speeds--0 (idle), 40, 60, 80 and 100 km h(-1). Emission factors and their relative values between the two fuel types together with a statistical significance for any difference were estimated for each parameter. In general, LPG was found to be a 'cleaner' fuel, although in most cases, the differences were not statistically significant owing to the large variations between emissions from different vehicles. The particle number emission factors ranged from 10(11) to 10(13) km(-1) and was over 70% less with LPG compared to ULP. Corresponding differences in particle mass emission factor between the two fuels were small and ranged from the order of 10 microg km(-1) at 40 to about 1000 microg km(-1) at 100 km h(-1). The count median particle diameter (CMD) ranged from 20 to 35 nm and was larger with LPG than with ULP in all modes except the idle mode. Carbon dioxide emission factors ranged from about 300 to 400 g km(-1) at 40 km h(-1), falling with increasing speed to about 200 g km(-1) at 100 km h(-1). At all speeds, the values were 10% to 18% greater with ULP than with LPG.