C. P. Meyer

Trends of ozone in the troposphere

Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. From two stations at high northern hemisphere (NH) latitudes there has been a significant decline in ozone amounts throughout the troposphere since the early 1980s. At midlatitudes of the NH where data are the most abundant, on the other hand, important regional differences prevail. The two stations in the eastern United States show that changes in ozone concentrations since the early 1970s have been relatively small. At the two sites in Europe, however, ozone amounts increased rapidly into the mid-1980s, but have increased less rapidly (or in some places not at all) since then. Increases at the Japanese ozonesonde station have been largest in the lower troposphere, but have slowed in the recent decade. The tropics are sparsely sampled but do not show significant changes. Small increases are suggested at southern hemisphere (SH) midlatitudes by the two surface data records. In Antarctica large declines in the ozone concentration are noted in the South Pole data, and like those at high latitudes of the NH, seem to parallel the large decreases in the stratosphere.

A study of peroxy radicals and ozone photochemistry at coastal sites in the northern and southern hemispheres

Peroxy radicals and other important species relevant to ozone photochemistry, including ozone, its photolysis rate coefficient jO(1D), NOx (NO + NO2), and peroxides, were measured at the coastal sites of Cape Grim, Tasmania, in January/February 1995 during the Southern Ocean Atmospheric Photochemistry Experiment (SOAPEX 1) and Mace Head, Western Ireland, in May 1995 during the Atlantic Atmospheric Photochemistry Experiment (ATAPEX 1). At both sites it was observed that the relationship between peroxy radical (HO2 + RO2) concentrations and jO(1D) switched from a square root dependence in clean oceanic or “baseline” air to a first-order dependence in more polluted air. Simple algorithms derived from a photochemical reaction scheme indicate that this switch-over occurs when atmospheric NO levels are sufficient for peroxy radical reaction with NO to compete with radical recombination reactions. At this crucial point, net tropospheric ozone production is expected to occur and was observed in the ozone diurnal cycles when the peroxy radical/jO(1D) dependencies became first order. The peroxy radical/jO(1D) relationships imply that ozone production exceeds destruction at NO levels of 55±30 parts per trillion by volume (pptv) at Mace Head during late spring and 23±20 pptv at Cape Grim during summer, suggesting that the tropospheric ozone production potential of the southern hemisphere is more responsive to the availability of NO than that of the northern hemisphere.

Relationships between ozone photolysis rates and peroxy radical concentrations in clean marine air over the Southern Ocean

Measurements of the sum of inorganic and organic peroxy radicals (RO2) and photolysis rate coefficients J(NO2) and J(O1D) have been made at Cape Grim, Tasmania in the course of a comprehensive experiment which studied photochemistry in the unpolluted marine boundary layer. The SOAPEX (Southern Ocean Atmospheric Photochemistry Experiment) campaign included measurements of ozone, peroxides, nitrogen oxides, water vapor, and many other parameters. This first full length paper concerned with the experiment focuses on the types of relationships observed between peroxy radicals and J(NO2), J(O1D) and √[J(O1D)] in different air masses in which ozone is either produced or destroyed by photochemistry. It was found that in baseline air with ozone loss, RO2 was proportional to √[J(O1D)], whereas in more polluted air RO2 was proportional to J(O1D). Simple algorithms were derived to explain these relationships and also to calculate the concentrations of OH radicals in baseline air from the instantaneous RO2 concentrations. The signal to noise ratio of the peroxy radical measurements was up to 10 for 1-min values and much higher than in other previous deployments of the instrument in the northern hemisphere, leading to the confident determination of the relationships between RO2 and J(O1D) in different conditions. The absolute concentration Of RO2 determined in these experiments is in some doubt, but this does not affect our conclusions concerned either with the behavior of peroxy radicals with changing light levels or with the concentrations of OH calculated from RO2. The results provide confidence that the level of understanding of the photochemistry of ozone leading to the production of peroxide via recombination of peroxy radicals in clean air environments is well advanced.

Mid‐latitude marine boundary‐layer ozone destruction at visible sunrise observed at Cape Grim, Tasmania, 41°S

An analysis is made of 13 years of observations of ozone concentrations in the remote marine boundary layer at Cape Grim, Tasmania 41°S. These data reveal a decrease in ozone concentration in the first few hours following sunrise at a rate of around 0.1 ppb h−1 in mid-summer and in mid-winter. This ozone destruction phenomenon causes an asymmetry in the daily ozone loss rate with enhanced destruction following sunrise, is statistically distinguishable from the O3-HOx destruction cycle that peaks at mid-day, and occurs at similar rates in mid-summer and in mid-winter. The cause of this sunrise ozone decrease is examined using the conservation equation for ozone. We speculate that ozone destruction at sunrise arises due to halogen chemistry. The absence of sunrise ozone decrease in models of marine boundary-layer photochemistry means that the ozone destruction rate in the remote marine boundary layer is underestimated by perhaps a factor of two.

Comparisons of field measurements of carbon dioxide and nitrous oxide fluxes with model simulations for a legume pasture in southeast Australia

We measured the fluxes of carbon dioxide (CC2) and nitrous oxide (N2O) and relevant environmental variables for two treatments in a legume pasture system on an acid soil in southeast Australia during 1993. The two treatments were control and liming (to reduce soil acidity). These trace gas fluxes were also simulated using a revised version of the process-based model DNDC. Our version of the DNDC (Denitrification and Decomposition) model has been significantly modified by including a surface energy balance submodel; by using more comprehensive formulations for soil evaporation, plant transpiration, plant growth, and plant nitrogen uptake; by using an implicit difference scheme to solve the diffusion equations governing heat and water fluxes in the soil; and by initializing the soil organic carbon in different pools using their relative proportions at the steady state. The simulated average nighttime CO2 fluxes from the control plot were 0.07, 0.05, and 0.16 g C m−2 h−1 in March, August, and October of 1993, respectively, as compared with the measured average nighttime CO2 fluxes of 0.10, 0.07, and 0.14 g C m−2 h−1 for the corresponding periods. The simulated average daily N2O fluxes from the control plot were 0.09, 0.03, and 0.04 mg N m−2 d−1 in March, August, and October of 1993, respectively, as compared with the measured average daily N2O fluxes of 0.07, 0.03, and 0.16 mg N m−2 d−1 for the corresponding periods. Similar agreements between model simulations of CO2 and N2O fluxes and measurements were also obtained for the limed plot. We find that the simulations by model DNDC and field observations of gaseous N2O emissions agree well over a range of 3 orders of magnitude. We conclude that the daily and seasonal variations of CO2 and N2O fluxes on a plot scale can be reasonably simulated by this process-based model. The model shows that the fraction of N2O produced aerobically from nitrification was 73% for the control plot and 55% for the limed plots in 1993; therefore mitigation strategies for reducing N2O emission from such temperate or semiarid nitrogen-limited systems in Australia should be focused on the N2O loss from nitrification.

Exposure to bushfire smoke during prescribed burns and wildfires: Firefighters’ exposure risks and options

Firefighters are exposed to known health-damaging air pollutants present in bushfire smoke and poorly managed exposure can result in serious health issues. A better understanding of exposure levels and the major factors influencing exposures is crucial for the development of mitigation strategies to minimise exposure risks and adverse health impacts. This study monitored air toxics within the breathing zone of firefighters at prescribed burns and at wildfires in Australia. The results showed that exposure levels were highly variable, with higher exposures (sometimes exceeding occupational exposure standards) associated with particular work tasks (such as patrol and suppression) and with certain burn conditions. The majority of firefighters exposures were at low and moderate levels (~60%), however considerable attention should be given to the high (~30%) and very high (6%) exposure risk situations for which acute and chronic health risks are very likely and for which control strategies should be developed and implemented to minimise health risks.

Impact of smoke from prescribed burning: Is it a public health concern?

Given the increase in wildfire intensity and frequency worldwide, prescribed burning is becoming a more common and widespread practice. Prescribed burning is a fire management tool used to reduce fuel loads for wildfire suppression purposes and occurs on an annual basis in many parts of the world. Smoke from prescribed burning can have a substantial impact on air quality and the environment. Prescribed burning is a significant source of fine particulate matter (PM2.5 aerodynamic diameter < 2.5µm) and these particulates are found to be consistently elevated during smoke events. Due to their fine nature PM2.5 are particularly harmful to human health. Here we discuss the impact of prescribed burning on air quality particularly focussing on PM2.5. We have summarised available case studies from Australia including a recent study we conducted in regional Victoria, Australia during the prescribed burning season in 2013. The studies reported very high short-term (hourly) concentrations of PM2.5 during prescribed burning. Given the increase in PM2.5 concentrations during smoke events, there is a need to understand the influence of prescribed burning smoke exposure on human health. This is important especially since adverse health impacts have been observed during wildfire events when PM2.5 concentrations were similar to those observed during prescribed burning events. Robust research is required to quantify and determine health impacts from prescribed burning smoke exposure and derive evidence based interventions for managing the risk. Implications: Given the increase in PM2.5 concentrations during PB smoke events and its impact on the local air quality, the need to understand the influence of PB smoke exposure on human health is important. This knowledge will be important to inform policy and practice of the integrated, consistent, and adaptive approach to the appropriate planning and implementation of public health strategies during PB events. This will also have important implications for land management and public health organizations in developing evidence based objectives to minimize the risk of PB smoke exposure.

Release of native and mass labelled PCDD/PCDF from soil heated to simulate bushfires

Soil is an important reservoir of PCDD/PCDF, which can be released when environmental conditions change. Fire is an extreme event that can increase the surface temperatures of soil substantially, yet little is known of the role soil plays in the emission of PCDD/PCDF. Soil containing native PCDD/PCDF was fortified with a mixture of mass labelled PCDD/PCDF and heated between 150 °C and 400 °C. Both native and mass labelled PCDD/PCDF were released from the soil beyond 200 °C. Release of the mass labelled compounds was linearly related to temperature with up to 9 % found in the air stream at 400 °C. The release of some native PCDD/PCDF was much greater. At 400 °C, emission of 1,2,3,7,8-Cl(5)DD was 300% compared to pre-experimental soil. Emission of PCDD/PCDF from soil during bushfires is a relevant process and may originate from both volatilization and formation via de novo or precursor pathways, or dechlorination.

Formation of artefacts while sampling emissions of PCDD/PCDF from open burning of biomass.

Emission factors for PCDD/PCDF determined from open combustion are used to estimate national emission budgets; therefore, it is important to have confidence in their accuracy. It has been suspected that artefacts may form due to the presence of hot metal surfaces of sampling equipment, thus skewing emission factors. In this study, emissions of PCDD/PCDF from open burning of forest biomass over a brick hearth were sampled. Five experiments were carried out using a portable sampler. Experiments were designed where the key variable, sample hood and inlet temperatures were manipulated. Other variables such as fuel origin, type and density were consistent. The measured concentration of PCDD/PCDF in the smoke samples ranged from 0.01 μg TEQ (t fuel)(-1) at the lowest maximum hood temperature (185°C) to 15 μg TEQ (t fuel)(-1) at the highest maximum hood temperature (598°C). when hood inlet temperatures exceeded 400°C emission factors were significantly elevated and this is attributed to the formation of artefacts that can cause the over estimation of emission factors. The increase in hood temperature also resulted in a change in the PCDD/PCDF congener and homologue profile of the emissions. For example at the lowest temperature (Fire 1) the PCDD/PCDF ratio measured was 50:1, whereas at the highest temperature (Fire 5) this ratio was about 0.53:1. When the sampler hood and inlet temperatures were kept in the normal operating range of <200°C, emission factors were comparable to those observed in many previous studies in Australia with emissions dominated by PCDD.

Emission Factors for Selected Semivolatile Organic Chemicals from Burning of Tropical Biomass Fuels and Estimation of Annual Australian Emissions

This study reveals that open-field biomass burning can be an important source of various semivolatile organic chemicals (SVOCs) to the atmosphere including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and a range of pesticides. Emission factors (EFs) for 39 individual SVOCs are determined from burning of various fuel types that are common in tropical Australia. Emissions of PAHs are found to be sensitive to differences in combustion efficiencies rather than fuel types, reflecting a de novo formation mechanism. In contrast, revolatilization may be important for other SVOCs such as PCBs. On the basis of the EFs determined in this work, estimates of the annual emissions of these SVOCs from Australian bushfires/wildfires are achieved, including, for example, ∑PAHs (160 (min)-1100 (max) Mg), ∑PCBs (14-300 kg), ∑PBDEs (8.8-590 kg), α-endosulfan (6.5-200 kg), and chlorpyrifos (up to 1400 kg), as well as dioxin toxic equivalents (TEQs) of ∑dioxin-like-PCBs (0.018-1.4 g). Emissions of SVOCs that are predominantly revolatilized appear to be related to their use history, with higher emissions estimated for chemicals that had a greater historical usage and were banned only recently or are still in use.