Chemosphere - Global Change Science最新文献

To test the recent hypothesis of a possible non-photochemical, NO₃-initiated production of peroxy radicals, differential optical absorption spectroscopy (DOAS) measurements of NO₃ and its precursor species were made at a mediterranean Eucalyptus forest site in the framework of the CEC FIELDVOC'94 project. Our data set is presented in combination with other FIELDVOC'94 results for various important reactants in night-time radical chemistry. Despite a high NO₃ production (as calculated from O₃ and NO₂ measurements), its concentration remained below the detection limit of 6 pptv. This is most likely due to the large abundance of olefins, which are very effective NO₃ scavengers. In fact, it is shown that among the known NO₃ sinks, the reaction with olefins was by far the most important NO₃ loss process during our measurements. It was suggested that this reaction could be a non-photochemical source of peroxy radicals (RO₂) and probably OH. Kinetic model calculations based on our observations lead to night-time RO₂ concentrations between 10⁸ and 10¹⁰ molecule $\text{cm}{}^{\mathrm{-3}}$. While during most nights observed RO₂ values were considerably lower, elevated RO₂ concentrations in the low ppt range were occasionally found in the late evening, demonstrating the presence of a night-time RO₂ source. The data set suggests that heterogeneous scavenging of peroxy radicals (which is not accounted for in our model) may be the dominant night-time sink for peroxy radicals.

A field campaign was carried out in a eucalyptus forest in Portugal to identify and quantify ambient VOCs in a rural area, with the emphasis on isoprene and it's reaction products. In conjunction with this study, the higher molecular weight VOCs were also investigated. In total, 53 higher molecular weight compounds were identified at the site, 44 of which were quantified and found to be between 0.002 and 8 ppbv. The range of compounds found suggested that there were both biogenic and anthropogenic sources of VOCs, which indicated that transport effects were of importance at the site. A definite diurnal variation of the monoterpenes was observed. Comparison of the ambient levels of the monoterpenes with emission data from the eucalyptus obtained at the site suggested that there was a source other than the eucalyptus trees for monoterpenes.

During FIELDVOC'94, isoprene and its main degradation products, methyl vinyl ketone (MVK), methacrolein (MAC) and formaldehyde (HCHO) were measured in a Portuguese eucalyptus forest. All compounds were analyzed by two or three different techniques. Isoprene mixing ratios were highest on sunny days (8 to 12 ppbv) correlating with temperature, and lowest at night (<0.02 ppbv). MVK and MAC reached on daytime ∼1 and ∼0.5 ppbv, respectively, and minimum values at night, for both species, comprised between 0.1 and 0.2 ppbv. Formaldehyde varied between 1 ppbv (night) and 9 ppbv (sunny day). Ambient mixing ratios of all compounds were highly variable due to changeable meteorological conditions. The MVK/MAC ratio between 1.5 and 2.5 is indicative of OH-oxidation as the primary isoprene degradation process during the day.

The aim of the EU FIELDVOC'94 project (field study on the tropospheric degradation mechanisms of biogenic VOCs, isoprene and dimethylsulfide) was to understand the contribution of the isoprene chemistry to the budget of ozone and the formation of peroxy radicals (RO₂) under various conditions, particularly high and low nitrogen oxides concentrations (NO_x). An additional objective was to evaluate the involvement of nitrate radicals (NO₃) in the production of RO₂ during night-time. An experiment was undertaken in June–July 1994, at a forest site in central Portugal. During this campaign most of the compounds involved in the ozone/isoprene chemistry were measured, i.e., nitrogen oxides, carbon monoxide, degradation and secondary products (aldehydes, ketones, organic acids and peroxides), as well as free radicals (RO₂ and NO₃). Several intensive measurement periods were carried out under different conditions of high and low photochemical activity. Isoprene was the most abundant hydrocarbon measured at the site, with mixing ratios as high as 10–12 ppbv during periods of high photochemical activity and elevated temperatures. Terpenes were also produced in the ppbv level with maximum concentrations observed at night. Ozone exhibited a diurnal variability, with maximum levels in the afternoon of 80–100 ppbv attributed to the interaction of polluted air masses and the isoprene chemistry. NO₃ radicals never exceeded a maximum of 6 pptv. RO₂ radicals displayed a diurnal maximum of 200–250 pptv and in some case reached night-time levels of 10–20 pptv. Radicals and ozone budgets deduced from photostationary models or chemical box models showed that hydroxyl radical (OH) initiated isoprene chemistry contributed to about 45% of the observed NO₃ levels during daytime and that ozone initiated chemistry accounted for approximately half of the RO₂ observed. The chemistry of isoprene strongly enhanced the ozone net production and reduced by a factor 2–3 the OH concentration.

As part of the FIELDVOC'94 study, ambient concentrations of peroxy radicals (RO₂, R=H, CH₃, CH₃CO, etc.) were continuously measured at a forested site in Portugal, using the chemical amplification technique. The amount of RO₂ varied between 0 and 260 pptv with maximum values being obtained under clear sky conditions. The RO₂ mixing ratio correlates with that of isoprene. The rate of production of ozone, O₃, indicates that the noon-time maximum values lay between 5 and 17 ppbv h⁻¹. On several days, the maximum O₃ mixing ratio at this rural site was similar to that found in urban areas, which experience smog episodes (O₃ mixing ratio > 90 ppbv). The RO₂ estimated assuming a photostationary state RO₂,(PSS) of NO₂, NO and O₃, was not found to have a significant statistical correlation with the measured RO₂. The latter is best explained by the errors associated with the calculation of RO₂,(PSS) at low NO_x (NO + NO₂) mixing ratios. The diurnal behavior of O₃ in the boundary layer was estimated using a relatively simple source/sink parameterization. On days having high O₃ mixing ratios, both predicted and observed O₃ behavior agree well. For days having O₃ mixing ratios < 75 ppbv, it was found that the simple parameterization needed to be extended to include entrainment of O₃ after dawn from above the night-time inversion layer.

Emission to the atmosphere of volatile organic compounds (VOCs) by Eucalyptus globulus was studied in the laboratory with young specimen, and in the field with adult trees. Eucalyptus emits both monoterpenes and isoprene. The leaves of young trees emit at higher rates than the leaves of adult trees. The emission of isoprene is highly predominant during the day. The emission of isoprene is dependent on temperature and solar radiation. The emission rate follows the Guenther algorithm if a based emission factor of $\text{32}\text{μg}\text{g}{}_{\mathrm{<mtext>dw</mtext>}}{}^{\mathrm{-1}}\text{h}{}^{\mathrm{-1}}$ is used, increasing with temperature, to a maximum at 40°C. At higher temperatures there is a decrease in the emission rate. The main C₁₀ emitted compound is 1,8-cineol. Cineol emissions increase exponentially with temperature, and are also seasonally dependent. Application of the emission algorithm to the Portuguese eucalyptus forests shows that during summer isoprene and monoterpene emissions by eucalyptus are of the same order of anthropogenic VOC production. Furthermore, in certain regions, in the center-north of Portugal, where eucalyptus forests are predominant, isoprene emissions can reach an order of magnitude higher than anthropogenic production of VOCs during daytime periods in July and August.

Gas-phase H₂O₂, organic peroxides and organic acids were measured from mid-June to mid-July 1994 during the second FIELDVOC'94 campaign in a forested area near Tábua (Portugal). The site was located in a plantation of eucalyptus trees. Methylhydroperoxide (MHP, CH₃OOH) and H₂O₂ were quantified in the air samples. Hydroxymethylhydroperoxide (HMHP, HOCH₂OOH) was observed occasionally. Formic and acetic acids were the organic acids determined in the ambient air. The hydroperoxides and the organic acids showed pronounced diurnal variations with peak mixing ratios in the early afternoon (12:00–16:00 UT). Sometimes a second maximum was observed in the late afternoon (16:00–20:00 UT). In comparison with the peroxides the maximum organic acid mixing ratios were shifted by 1–2 h. The maximum mixing ratios were 1.38 ppbv (H₂O₂), 0.64 ppbv (MHP), 0.22 ppbv (HMHP), 2.8 ppbv (formic acid), and 1.5 ppbv (acetic acid). The ratio of formic acid over acetic acid mixing ratios was 1.5. The H₂O₂ mixing ratio was strongly dependent on O₃,NO_x, isoprene, RO_x and relative humidity. Mixing ratios of all peroxides were below detection limit (d.l.) (<15 pptv) during night-time. Deposition processes were found to determine the mixing ratios of the hydroperoxides and organic acids during the nights.

The impact of the oxidation of natural non-methane hydrocarbon, particularly of isoprene, on the free radical formation and on ozone budgets has been evaluated on the basis of the results of the FIELDVOC'94 campaign. Four reduced chemical mechanisms for hydrocarbon oxidation and a detailed chemical scheme for C₁–C₅ hydrocarbon oxidation incorporated into a box model suitable for the integration of stiff chemical reactions have been used for this study. The observed peroxy radical concentrations are well simulated by the models. Hydroperoxy radicals contribute by about 40–60% to the daytime peak of peroxy radicals. Terpenes and isoprene chemistry could account for about 10% and 45% of the observed levels of daytime peroxy radical concentrations. Ozone reactions contribute up to 50% to the organic peroxy radicals produced at night by isoprene oxidation. Isoprene chemistry reduces by a factor of two to three the computed radical concentrations and contributes by about 50–100% to the net ozone daytime photochemical production in this forested area.

Vertical profiles of carbonyl sulfide (COS) have been obtained in a eucalyptus forest in Portugal within the first 20 m above ground. Significantly lower mixing ratios were observed below the canopy level than above, indicating a net absorption of COS from the eucalyptus trees. The ratio between mean COS concentrations below and above the canopy was 0.86 in mid-afternoon, whereas in the morning and at midnight this ratio increased up to 0.97. COS mixing ratios were by more than 100 pptv higher above the canopy than below it in mid-afternoon, whereas in the morning and at midnight the difference never exceeded 25 pptv. At all levels COS presented a clear diurnal cycle with highest values around mid-afternoon. Experiments conducted in parallel with the enclosure chamber technique confirm the COS absorption from the eucalyptus trees. COS uptake reached its highest values in mid-afternoon, when photosynthetic active radiation (PAR) was maximum.