R. Koppmann

A review of biomass burning emissions part III: intensive optical properties of biomass burning particles

Because of its wide coverage over much of the globe, biomass burning has been widely studied in the context of direct radiative forcing. Such study is warranted as smoke particles scatter and at times absorb solar radiation efficiently. Further, as much of what is known about smoke transport and impacts is based on remote sensing measurements, the optical properties of smoke particles have far reaching effects into numerous aspects of biomass burning studies. Global estimates of direct forcing have been widely varying, ranging from near zero to −1 W m−2. A significant part of this difference can be traced to varying assumptions on the optical properties of smoke. This manuscript is the third part of four examining biomass-burning emissions. Here we review and discuss the literature concerning measurement and modeling of optical properties of biomassburning particles. These include available data from published sensitivity studies, field campaigns, and inversions from the Aerosol Robotic Network (AERONET) of Sun photometer sites. As a whole, optical properties reported in the literature are varied, reflecting both the dynamic nature of fires, variations in smoke aging processes and differences in measurement technique. We find that forward modeling or “internal closure” studies ultimately are of little help in resolving outstanding measurement issues due to the high degree of degeneracy in solutions when using “reasonable” input parameters. This is particularly notable with respect to index of refraction and the treatment of black carbon. Consequently, previous claims of column closure may in fact be more ambiguous. Differences between in situ and retrieved ωo values have implications for estimates of mass scattering and mass absorption efficiencies. In this manuscript we Correspondence to: J. S. Reid (reidj@nrlmry.navy.mil) review and discuss this community dataset. Strengths and lapses are pointed out, future research topics are prioritized, and best estimates and uncertainties of key smoke particle parameters are provided.

Methyl halide emissions from savanna fires in southern Africa

The methyl halides, methyl chloride (CH3Cl), methyl bromide (CH3Br), and methyl iodide (CH3I), were measured in regional air samples and smoke from savanna fires in southern Africa during the Southern Africa Fire-Atmosphere Research Initiative-92 (SAFARI-92) experiment (August–October 1992). All three species were significantly enhanced in the smoke plumes relative to the regional background. Good correlations were found between the methyl halides and carbon monoxide, suggesting that emission was predominantly associated with the smoldering phase of the fires. About 90% of the halogen content of the fuel burned was released to the atmosphere, mostly as halide species, but a significant fraction (3–38%) was emitted in methylated form. On the basis of comparison with the composition of the regional background atmosphere, emission ratios to carbon dioxide and carbon monoxide were determined for the methyl halide species. The emission ratios decreased in the sequence CH3Cl > CH3Br > CH3I. Extrapolation of these results in combination with data from other types of biomass burning, e.g. forest fires, suggests that vegetation fires make a significant contribution to the atmospheric budget of CH3Cl and CH3Br. For tropospheric CH3I, on the other hand, fires appear to be a minor source. Our results suggest that pyrogenic emissions of CH3Cl and CH3Br need to be considered as significant contributors to stratospheric ozone destruction.

Volatile organic compounds in the exhaled breath of young patients with cystic fibrosis

Inflammatory mediators in the exhaled breath are receiving growing medical interest as noninvasive disease markers. Volatile organic compounds have been investigated in this context, but clinical information and methodological standards are limited. The levels of ethane, propane, n-pentane, methanol, ethanol, 2-propanol, acetone, isoprene, benzene, toluene, dimethyl sulphide (DMS) and limonene were measured in repeated breath samples from 20 cystic fibrosis patients and 20 healthy controls (aged 8–29 yrs). Three end-exhaled and one ambient air sample were collected per person and analysed on a customised gas chromatography system. Intra-subject coefficients of variation ranged between 9 and 34%, and hydrocarbon breath levels were influenced by their inspired concentrations. The alveolar gradient for pentane was higher in cystic fibrosis patients than in healthy controls (0.36 versus 0.21 ppb) and inversely proportional to forced expiratory volume in one second; highest values were observed in patients with pulmonary exacerbations (0.73 versus 0.24 ppb). Cystic fibrosis patients also exhibited a lower output of DMS (3.9 versus 7.6 ppb). Group differences were not significant for ethane and the remaining substances. It was concluded that chemical breath analysis for volatile organic compounds is feasible and may hold potential for the noninvasive diagnosis and follow-up of inflammatory processes in cystic fibrosis lung disease.

The budgets of ethane and tetrachloroethene: Is there evidence for an impact of reactions with chlorine atoms in the troposphere?

Abstract In a number of recent papers evidence has been presented that reactions with Cl atoms substantially contribute to the turnover of organic compounds in certain regions of the marine atmosphere. However, the impact of this possible sink mechanism for organic compounds on global or hemispheric scales is still unknown. Based on the budgets of organic substances which react with Cl-atoms much faster than with OH-radicals it is possible to derive upper limits for the average tropospheric Cl-atom concentration. The rate: constants for the ethane and tetrachloroethene reaction with Cl are by a factor of 200–300 higher than those with OH-radicals, this reaction being their only significant established sink in the troposphere. From several series of measurements in the remote troposphere we derived an estimate of mean tropospheric distributions and seasonal cycles of tetrachloroethene and ethane. Together with OH-fields from model calculations we calculated the removal of ethane and tetrachloroethene by OH-radicals. Within the uncertainties the calculated removal agrees with the known emissions of these substances. In spite of the substantial uncertainties of these budgets, the relatively high reactivity of these substances towards Cl-atoms allows to estimate useful upper limits of the removal rates by Cl-atoms and thus of the average tropospheric Cl-atom concentration. For the Northern Hemisphere a plausible upper limit of less than 1000 Cl-atoms cm−3 can be derived. Due to higher uncertainties in the budgets for the Southern Hemisphere, the upper limit of the Cl-atom concentration in the Southern Hemisphere is nearly 2000 Cl-atoms cm−3. Nevertheless, these results show that on a global scale the Cl-atom-induced reactions for most organic trace gases are of minor importance.

Hydrocarbon measurements during tropospheric ozone depletion events : Evidence for halogen atom chemistry

During the Arctic Tropospheric Ozone Chemistry 1996 (ARCTOC 96) field campaign (March 29 to May 15, 1996), in situ measurements of C2-C8 hydrocarbons, selected C1-C2 halocarbons, and carbon monoxide were carried out at Ny Alesund, Svalbard (78°55′N, 11°56′E). Two major tropospheric ozone depletions were observed during this period. In each case, concurrent depletion of alkanes and ethyne but no significant changes in benzene, chloromethane, or CO mixing ratios were detected. The change in the propane/benzene ratio can be used as evidence for the presence of chlorine radicals. Time integrated chlorine and bromine atom concentrations were calculated from the concentration changes of light alkanes and ethyne, respectively. At background ozone mixing ratios (O3 > 30 ppbv) our calculations yielded no significant integrated halogen atom concentrations (Cl: 5 ± 14 × 108 s cm−3, Br: 9 ± 42 × 1010 s cm−3). During major ozone depletion events, these values increase by more than a factor of 10 to values of about 1010 s cm−3 (Cl) and 5 × 1012 s cm−3 (Br). For such events the observed ozone losses can be explained quantitatively with these data. Our results show that free bromine atoms appear to be the major cause for ozone depletion (more than 92%). The contribution of chlorine atoms to the ozone loss is of the order of 1% or less. Highest integrated chlorine and bromine atom concentrations were found at lowest ozone mixing ratios and reached up to 1.4 × 1010 and 1.4 × 1013 s cm−3, respectively. A closer analysis reveals that during each ozone depletion event the integrated chlorine atom concentration increases earlier than the integrated bromine atom concentration and remains at high levels for a longer period of time. The bromine atom concentration starts to increase when ozone mixing ratios are below 15–20 ppbv and reaches very high levels for ozone <5 ppbv. The integrated chlorine concentration appears to be anticorrelated to the ozone mixing ratio (r2 = 0.811), whereas the integrated bromine concentration was found to be anticorrelated to the logarithm of the ozone mixing ratio (r2 = 0.895).

Volatile organic compound emissions from Scots pine: Mechanisms and description by algorithms

The mechanisms of volatile organic compound (VOC) emissions from Scots pine (Pinus sylvestris L.) were investigated in laboratory experiments. The plants emitted mainly monoterpenes and acetone. Isoprene was emitted only in small amounts, but the mechanisms of its emissions were similar to those of the other compounds. Isoprene, acetone, and monoterpene emissions from Scots pine could be well described by an algorithm that considers emissions caused by evaporation of VOCs out of pools and emissions in parallel with their biosynthetic production. Monoterpene emissions were mainly affected by temperature. In some cases, monoterpene emissions were also influenced by photosynthetic active radiation implying that monoterpene emissions from Pinus sylvestris occur from storage pools as well as from processes that are linked to monoterpene biosynthesis. The coupling of monoterpene emissions with photosynthesis was confirmed by results of experiments with 13CO2. The 13CO2 exposure resulted in emission of 13C labeled monoterpenes during 13CO2 exposure as well as during the night following the exposure. Similar results were also obtained for isoprene emissions. Scots pine emitted isoprene during illumination as well as in darkness. The emitted isoprene was labeled during 13CO2 exposure and in the night following the exposure. The results obtained for monoterpene emissions in the laboratory were compared to those of outdoor measurements with Scots pine. While the temperature dependencies of emission rates were comparable to those obtained from laboratory experiments, a PAR dependence was not detectable. Temperature variations during outdoor measurements prevented a detection of this dependence.

Field study of the emissions of methyl chloride and other halocarbons from biomass burning in Western Africa

A field study of trace gas emissions from biomass burning in Equatorial Africa gave methyl chloride emission ratios of 4.3×10−5±0.8×10−5 mol CH3Cl/mol CO2. Based on the global emission rates for CO2 from biomass burning we estimate a range of 226−904×109 g/y as global emission rate with a best estimate of 515×109 g/y. This is somewhat lower than a previous estimate which has been based on laboratory studies. Nevertheless, our emission rate estimates correspond to 10–40% of the global turnover of methyl chloride and thus support the importance of biomass burning as methyl chloride source. The emission ratios for other halocarbons (CH2Cl2, CHCl3, CCl4, CH3CCl3, C2HCl3, C2Cl4, F-113) are lower. In general there seems to be a substantial decrease with increasing complexity of the compounds and number of halogen atoms. For dichloromethane biomass burning still contributes significantly to the total global budget and in the Southern Hemisphere biomass burning is probably the most important source for atmospheric dichloromethane. For the global budgets of other halocarbons biomass burning is of very limited relevance.

Distribution of methylchloride, dichloromethane, trichloroethene and tetrachloroethene over the north and south Atlantic

During the cruise ANT VIII/1 of the German R/V Polarstern in August/September 1989 the latitudinal distributions of the atmospheric concentrations of methylchloride, dichloromethane, trichloroethene, and tetrachloroethene were measured over the Atlantic between 45°N and 30°S by in situ gas chromatography. With the exception of trichloroethene they showed mixing ratios well above the lower limit of detection. The methylchloride distribution was uniform with average mixing ratios of 532 ± 8 and 550 ± 12 ppt in the northern and southern Hemispheres, respectively. Dichloromethane increased linearly between the Intertropical Convergence Zone and 45°N with average mixing ratios of 36 ± 6 ppt and was almost constant in the southern hemisphere with an average of 18 ± 1 ppt. Tetrachloroethene mixing ratios were between less than 1 and 10 ppt in the northern hemisphere and always below 3 ppt in the southern hemisphere. Similar to dichloromethane, tetrachloroethene was nearly constant in the southern hemisphere and increased linearly toward northern latitudes. This is compatible with the predominantly industrial origin of these compounds. Trichloroethene varied between 0.3 ppt and about 15 ppt in the northern hemisphere with an average of 3 ± 1 ppt and was generally lower than 1 ppt in the southern hemisphere with mixing ratios often near or below the detection limit of 0.1 ppt. For CH3Cl we estimate a global turnover of 3.5 × 1012 g/yr which is compatible with previous results. Using a simple model calculation our measurements imply a global turnover for CH2Cl2 and C2Cl4 of 0.9 × 1012 g/yr and 0.6 × 1012 g/yr, respectively.

Toluene emissions from plants

The emission of toluene from different plants was observed in continuously stirred tank reactors and in field measurements. For plants growing without stress, emission rates were low and ranged from the detection limit up to 2·10−16 mol·cm−2·s−1. Under conditions of stress, the emission rates exceeded 10−14 mol·cm−2·s−1. Exposure of sunflower (Helianthus annuus L. cv. Gigantheus) to 13CO2 resulted in 13C-labeling of the emitted toluene on a time scale of hours. Although no biochemical pathway for the production of toluene is known, these results indicate that toluene is synthesized by the plants. The emission rates of toluene from sunflower are dependent on nutrient supply and wounding. Since α-pinene emission rates are also influenced by these factors, toluene and α-pinene emissions show a high correlation. During pathogen attack on Scots pines (Pinus sylvestris L.) significant toluene emissions were observed. In this case emissions of toluene and α-pinene also show a good correlation. Toluene emissions were also found in field experiments with pines using branch enclosures.

Comparison of measured OH concentrations with model calculations

The influence of chemical precursors and sunlight on the atmospheric OH abundance is investigated by a comparison of locally measured tropospheric OH with model calculations. The latter are based on the gas phase reaction mechanism of the regional acid deposition model (RADM2) which incorporates an explicit inorganic and a comprehensive organic chemistry. The experimental data were obtained in the planetary boundary layer during two sets of campaigns. In Deuselbach (1983) and Schauinsland (1984), rural conditions were encountered with NOx concentrations on the average of 2.2 and 0.9 ppb, respectively. This data set was already compared with model calculations based upon an older and less detailed chemical reaction scheme (Perner et al., 1987). Since then the experimental data were reanalyzed leading to modified measured OH concentrations and also to modified precursor concentrations. For a consistent comparison with the more recent campaigns in Julich (1987 and 1988) we have redone the calculations. The modeled and measured OH concentrations of the campaigns in 1983 and 1984 correlate well with a coefficient of correlation of r = 0.73. The model overpredicts OH by about 20%. Under more polluted conditions in Julich with average NOx concentrations of 4 ppb the correlation coefficient between experimental and modeled data are significantly smaller (r = 0.61). Possible reasons are the influence of not measured precursors, for example isoprene, and the inapplicability of a quasi-steady state model under the spatially inhomogeneous conditions in Julich. Again the model overpredicts the OH concentration by about 15%, which is somewhat smaller than for the rural case. The precision of the comparison is limited by the uncertainties of the chemical reaction rate constants.