Urban and rural ozone concentrations in Alberta, Canada

Environmental controls on ozone fluxes in a poplar plantation in Western Europe

Tropospheric O3 is a strong oxidant that may affect vegetation and human health. Here we report on the O3 fluxes from a poplar plantation in Belgium during one year. Surprisingly, the winter and autumn O3 fluxes were of similar magnitude to ones observed during most of the peak vegetation development. Largest O3 uptakes were recorded at the beginning of the growing season in correspondence to a minimum stomatal uptake. Wind speed was the most important control and explained 44% of the variability in the nighttime O3 fluxes, suggesting that turbulent mixing and the mechanical destruction of O3 played a substantial role in the O3 fluxes. The stomatal O3 uptake accounted for a seasonal average of 59% of the total O3 uptake. Multiple regression and partial correlation analyses showed that net ecosystem exchange was not affected by the stomatal O3 uptake.

"Virtual injector" flow tube method for measuring relative rates kinetics of gas-phase and aerosol species

A new method for measuring gas-phase and aerosol reaction kinetics is described in which the gas flow, itself, acts as a "virtual injector" continuously increasing the contact time in analogy to conventional movable-injector kinetics techniques. In this method a laser is directed down the length of a flow tube, instantly initiating reaction by photodissociation of a precursor species at every point throughout the flow tube. Key tropospheric reactants such as OH, Cl, NO(3), and O(3) can be generated with nearly uniform concentrations along the length of the flow tube in this manner using 355 nm radiation from the third harmonic of a Nd:YAG laser. As the flow travels down the flow tube, both the gas-phase and particle-phase species react with the photogenerated radicals or O(3) for increasingly longer time before exiting and being detected. The advantages of this method are that (1) any wall loss of gas-phase and particle species is automatically accounted for, (2) the reactions are conducted under nearly pseudo-first-order conditions, (3) the progress of the reaction is followed as a continuous function of reaction time instead of reactant concentration, (4) data collection is quick with an entire decay trace being collected in as little as 1 min, (5) relative rates of several species can be measured simultaneously, and (6) bimolecular rate constants at least as small as k = 10(-17) (cm(3)/molecule)/s, or aerosol uptake coefficients at least as small as γ = 10(-4), can be measured. Using the virtual injector technique with an aerosol chemical ionization mass spectrometer (CIMS) as a detector, examples of gas-phase relative rates and uptake by oleic acid particles are given for OH, Cl, NO(3), and O(3) reactions with most agreeing to within 20% of published values, where available.

Surface ozone trends in Hong Kong in 1985-1995

Ozone trends in Hong Kong were assessed from the ozone data recorded at three urban stations and one station in the new development area. Three parameters were selected as the robust trend indicators in the study: monthly mean, monthly averaged daily 1-h maximum, and number of annual hours > 120 microg m(-3). As the ozone data displayed obvious seasonal variation, using deseasonalized monthly average parameters to estimate the ozone trends could smooth out the influence of seasonal fluctuations. The detection of an increase of more than 10% per year in ozone concentration at one station was particularly alarming. Effects of meteorological factors, control programmes of ozone precursors, and long-range transport on the ambient ozone level were examined in the study.

Surface ozone exposures measured at clean locations around the world

For assessing the effects of air pollution on vegetation, some researchers have used control chambers as the basis of comparison between crops and trees grown in contemporary polluted rural locations and those grown in a clean environment. There has been some concern whether the arbitrary ozone level of 0.025 ppm and below, often used in charcoal-filtration chambers to simulate the natural background concentration of ozone, is appropriate. Because of the many complex and man-made factors that influence ozone levels, it is difficult to determine natural background. To identify a range of ozone exposures that occur at 'clean' sites, we have calculated ozone exposures observed at a number of 'clean' monitoring sites located in the United States and Canada. We do not claim that these sites are totally free from human influence, but rather than the ozone concentrations observed at these 'clean' sites may be appropriate for use by vegetation researchers in control chambers as pragmatic and defensible surrogates for natural background. For comparison, we have also calculated ozone exposures observed at four 'clean' remote sites in the Northern and Southern Hemispheres and at two remote sites (Whiteface Mountain, NY and Hohenpeissenberg, FRG) that are considered to be more polluted. Exposure indices relevant for describing the relationship between ozone and vegetation effects were applied. For studying the effects of ozone on vegetation, the higher concentrations are of interest. The sigmoidally-weighted index appeared to best separate those sites that experienced frequent high concentration exposures from those that experienced few high concentrations. Although there was a consistent seasonal pattern for the National Oceanic and Atmospheric Administration (NOAA) Geophysical Monitoring for Climate Change (GMCC) sites indicating a winter/spring maximum, this was not the case for the other remote sites. Some sites in the continental United States and southern Canada experienced ozone exposures in the range between those values experienced at the South Pole and Mauna Loa NOAA GMCC sites. The 7-month average of the daily 7 h average ozone concentration at 'clean' sites located in the continental United States and southern Canada ranged from 0.028 to 0.050 ppm. Our analysis indicates that seasonal 7 h average values of 0.025 ppm and below, used by some vegetation researchers as a reference point, may be too low and that estimates of crop losses and tree damage in many locations may have been too high. Our analysis indicates that a more appropriate reference point in North America might be between 0.030 and 0.045 ppm. We have observed that the subtle effects of changing distribution patterns of hourly average ozone concentrations may be obscured with the use of exposure indices such as the monthly average. Future assessments of the effects associated with ground-level ozone should involve the use of exposure indices sensitive to changes in the distribution patterns of hourly average ozone concentrations.