W. H. Pollock

Biomass Burning as a Source of Atmospheric Gases CO, H2, N2O, NO, CH3Cl and COS

The potential importance of deforestation and biomass burning for the atmospheric CO2 cycle has received much attention and caused some controversy. Biomass burning can contribute extensively to the budgets of several gases which are important in atmospheric chemistry. In several cases the emission is comparable to the technological source. Most burning takes place in the tropics in the dry season and is caused by man’s activities. The potential importance of deforestation and biomass burning for the atmospheric CO2 cycle has received much attention and caused some controversy. In this article we will show the probable importance of biomass burning as a trace gas source, which is caused by man’s activities in the tropics. We used the results of our global biomass burning analysis to derive some rough estimates of the sources of the important atmospheric trace gases CO, H2, CH4, N2O, NOx (NO and NO2), COS and CH3Cl from the worldwide burning of biomass.

Tropospheric chemical composition measurements in Brazil during the dry season

Field measurement programs in Brazil during the dry seasons in August and September 1979 and 1980 have demonstrated the large importance of the continental tropics in global air chemistry. Many important trace gases are produced in large amounts over the continents. During the dry season, much biomass burning takes place, especially in the cerrado regions, leading to a substantial emission of air pollutants, such as CO, NOx, N2O, CH4 and other hydrocarbons. Ozone concentrations are enhanced due to photochemical reactions. The large biogenic organic emissions from tropical forests play an important role in the photochemistry of the atmosphere and explain why CO is present in such high concentrations in the boundary layer of the tropical forest. Carbon monoxide production may represent more than 3% of the net primary productivity of the tropical forests. Ozone concentrations in the boundary layer of the tropical forests indicate strong removal processes. Due to atmospheric supply of NOx by lightning, there is probably a large production of O3 in the free troposphere over the Amazon tropical forests. This is transported to the marine-free troposphere and to the forest boundary layer.

Bacteria produce the volatile hydrocarbon isoprene

Various bacterial species, both Gram-negative and Gram-positive, were found to produce the volatile hydrocarbon isoprene (2-methyl-1,3-butadiene). Out of the tested cultures, Bacillus produced the most isoprene. The production of isoprene from bacteria was confirmed by gas chromatography-mass spectrometry. Media and growth effects on isoprene production were investigated: growth in rich media led to higher levels of isoprene than growth in minimal media, and highest isoprene emission rates were seen in log-phase cultures. Temperature profiles for bacterial isoprene production showed an optimum of 45°C and were suggestive of an enzymatic mechanism for isoprene formation.

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.

On the evaluation of ozone depletion potentials

Observations of methane, CFC-11, and ozone losses are used along with insights from models and observations regarding interrelationships between tracers to develop a semi-empirical framework for evaluating global ozone depletion potentials. Direct measurements of some hydrochlorofluorocarbons including HCFC-22 in the Arctic lower stratosphere are also used to evaluate the local ozone depletion potentials there. This approach assumes that all of the observed ozone destruction in the contemporary atmosphere is due to chlorine and that the depletion is proportional to the local relative chlorine release. It is shown that the global ozone depletion potentials for compounds with relatively long stratospheric lifetimes such as HCFC-22 and HCFC-142b are likely to be larger than those generally predicted by gas phase chemical models, due largely to the importance of lower stratospheric ozone losses that are not simulated in gas phase studies. The analysis presented suggests that the globally averaged efficiency for ozone depletion by HCFC-22 is as much as a factor of 2 larger than some gas phase model estimates. For compounds with short stratospheric lifetimes such as (CCl4). and (CH3CCl3), on the other hand, gas phase models likely overestimate the ozone depletion potentials for the present-day stratosphere. Observations of polar ozone loss and reactive halogen radical abundances also imply that the globally averaged ozone depletion potentials for brominated species for the contemporary stratosphere could be as much as 1.5–3 times greater than some gas phase model predictions, depending upon lower stratospheric loss processes.

Measurements of Halogenated Organic Compounds near the Tropical Tropopause

The amount of organic chlorine and bromine entering the stratosphere have a direct influence on the magnitude of chlorine and bromine catalyzed ozone losses. Twelve organic chlorine species and five organic bromine species were measured from 12 samples collected near the tropopause between 23.8°N and 25.3°N during AASE II. The average mixing ratios of total organic chlorine and total organic bromine were 3.50 ± 0.06 ppbv and 21.1 ± 0.8 pptv, respectively. CH3Cl represented 15.1% of the total organic chlorine, with CFC 11 (CCl3F) and CFC 12 (CCl2F2) accounting for 22.6% and 28.2%, respectively, with the remaining 34.1% primarily from CCl4, CH3CCl3, and CFC 113 (CCl2FCClF2). CH3Br represented 54% of the total organic bromine. The 95% confidence intervals of the mixing ratios of all but four of the individual compounds were within the range observed in low and mid-latitude mid-troposphere samples. The four compounds with significantly lower mixing ratios at the tropopause were CHCl3, CH2Cl2, CH2Br2, and CH3CCl3. The lower mixing ratios may be due to entrainment of southern hemisphere air during vertical transport in the tropical region and/or to exchange of air across the tropopause between the lower stratosphere and upper troposphere.

Photochemical partitioning of the reactive nitrogen and chlorine reservoirs in the high‐latitude stratosphere

Partitioning of the major components of the reactive nitrogen and inorganic chlorine reservoirs is derived from aircraft measurements in the lower stratosphere during the winter season in both hemispheres at latitudes of about 60° to 80°. The goal of this work is to exercise the power of the correlated set of measurements from polar missions of the NASA ER-2 to extend what can be learned from looking at the measurements individually. The results provide a consistent method for comparing distributions, and hence the controlling processes, between different areas of the near-polar regions. The analysis provides clear evidence of the effects of heterogeneous processes in the atmosphere. Values for NO2, ClONO2, N2O5, and Cl2O2 are derived in a simplified steady state model based on in situ NO, ClO, O3, temperature, and pressure measurements; laboratory-measured reaction rates; and modeled photodissociation rates. Values for the reservoir totals are independently derived from measurements of N2O, organic chlorine, and total reactive nitrogen. The relative abundances of the measured and derived species within the reservoirs are calculated, and the longer-lived species HCl and HNO3 are estimated as the residuals of their respective reservoirs. The resulting latitude distributions in the Arctic outside the vortex agree reasonably well with predictions of a two-dimensional photochemical model, indicating that partitioning in this region is largely controlled by standard homogeneous gas phase chemistry. Inside the Arctic vortex a large fraction of the HCl has been converted to reactive chlorine species ClO and Cl2O2, consistent with the extensive action of known heterogeneous reactions, presumably occurring on the surfaces of polar stratospheric clouds formed in the cold temperatures of the vortex. The partitioning in the Antarctic suggests that nearly the entire range of latitudes sampled by the ER-2 is affected by heterogeneous processes in situ, including that portion of the “collar” region equatorward of the nominal chemically perturbed region (CPR). Consideration of heterogeneous processing in the region outside the CPR is important in predicting the possible expansion of Antarctic ozone depletion and the transport of chemically perturbed air to lower latitudes.

Gas chromatography mass spectrometry analysis of volatile organic trace gases at Mauna Loa Observatory, Hawaii

Volatile organic trace gases in the remote troposphere at the Mauna Loa Observatory were identified in July and August 1992 during the Mauna Loa Observatory Photochemical Experiment (MLOPEX 2) using an in situ fully automated gas chromatography mass spectrometry (GCMS) instrument. Identification was obtained for 65 organic compounds, 7 additional compounds were identified tentatively. Four target parts per trillion (ppt)-level species were detected in the selected ion monitoring mode (SIM). The experimental data show typical background air characteristics; for example, abundance of long-lived compounds such as halogenated hydrocarbons during all measurement periods. Volatile organic compounds (VOCs) from local sources from the island were also seen during daytime upslope flow conditions. These included short-lived biogenic hydrocarbons such as isoprene and monoterpenes and typical emissions from combustion processes such as alkylated aromatics.

On the age of stratospheric air and ozone depletion potentials in polar regions

Observations of the nearly inert, man-made chlorofluorocarbon CFC-115 obtained during January 1989 are used to infer the age of air in the lower stratosphere. These observations together with estimated release rates suggest an average age of high-latitude air at pressure altitudes near 17-21 km of about 3 to 5 years. This information is used together with direct measurements of HCFC-22, HCFC-142b, CH{sub 3}Br, H-1301, H-1211, and H-2402 to examine the fractional dissociation of these species within the Arctic polar lower stratosphere compared to that of CFC-11 and hence to estimate their local ozone depletion potentials in this region. It is shown that these HCFCs are much less efficiently dissociated within the stratosphere than CFC-11, lowering their ozone depletion potentials to only about 30-40% of their chlorine loading potentials. In contrast, the observations of CH{sub 3}Br and the Halons considered here confirm that they are rapidly dissociated within the stratosphere, with important implications for their ozone depletion potentials. 20 refs., 4 figs., 3 tabs.

The vertical distribution of trace gases in the stratosphere

SummaryNew measurements on the stratospheric distribution of H2, CH4, CO and N2O are presented and used to demonstrate the natural variability of the trace gas concentrations. The present CH4 and H2 measurements and data from older balloon flights are combined to give average vertical profiles. These profiles are compared with water vapor data from various authors to see if the vertical decrease in CH4 is matched by a corresponding increase in H2O. By comparing the average measured profiles to those predicted by a one-dimensional chemical model, profiles of the vertical eddy diffusion coefficientkz are deduced. Generally, a barrier in the low stratosphere and increasing transport in middle and upper stratosphere seem required to match theoretical and experimental profiles. The limitations of the calculatedkz are discussed.