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Browsing Research publications by Author "Andreae, M.O."
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Item Open Access Airborne studies of emissions from savanna fires in southern Africa. 1. Aerosol emissions measured with a laser optical particle counter(AGU, 1996) LeCanut, P.; Andreae, M.O.; Harris, G.W.; Wienhold, F.G.; Zenker, T.During the SAFARI-92 experiment (Southern Africa Fire Atmosphere Research Initiative, September–October 1992), we flew an instrumented DC-3 aircraft through plumes from fires in various southern African savanna ecosystems. Some fires had been managed purposely for scientific study (e.g., those in Kruger National Park, South Africa), while the others were “fires of opportunity” which are abundant during the burning season in southern Africa. We obtained the aerosol (0.1–3.0 μm diameter) number and mass emission ratios relative to carbon monoxide and carbon dioxide from 21 individual fires. The average particle number emission ratio ΔN/ΔCO (Δ: concentrations in plume minus background concentrations) varied between 14±2 cm−3 ppb−1 for grasslands and 23±7 cm−3 ppb−1 for savannas. An exceptionally high value of 43±4 cm−3 ppb−1 was measured for a sugarcane fire. Similarly, the mass emission ratio ΔM/ΔCO varied from 36±6 ng m−3 ppb−1 to 83±45 ng m−3 ppb−1, respectively, with again an exceptionally high value of 124±14 ng m−3 ppb−1 for the sugarcane fire. The number and mass emission ratios relative to CO depended strongly upon the fire intensity. Whereas the emission ratios varied greatly from one fire to the other, the aerosol number and volume distributions as a function of particle size were very consistent. The average background aerosol size distribution was characterized by three mass modes (0.2–0.4 μm, ≈1.0 μm, and ≈2.0 μm diameter). On the other hand, the aerosol size distribution in the smoke plumes showed only two mass modes, one centered in the interval 0.2–0.3 μm and the other above 2 μm diameter. From our mean emission factor (4±1 g kg−1 dm) we estimate that savanna fires release some 11–18 Tg aerosol particles in the size range 0.1–3.0 μm annually, a somewhat lower amount than emitted from tropical forest fires. Worldwide, savanna fires emit some 3–8 × 1027 particles (in the same size range) annually, which is expected to make a substantial contribution to the cloud condensation nuclei population in the tropics.Item Open Access Airborne studies of emissions from savanna fires in southern Africa. 2. Aerosol chemical composition(AGU, 1998) Andreae, M.O.; Andreae, T.W.; Annegarn, H.; Beer, F.; Cachier, H.; Elbert, W.; Harris, G.W.; Maenhaut, W.; Salma, I.; Swap, R.; Wienhold, F.G.; Zenker, T.We investigated smoke emissions from fires in savanna, forest, and agricultural ecosystems by airborne sampling of plumes close to prescribed burns and incidental fires in southern Africa. Aerosol samples were collected on glass fiber filters and on stacked filter units, consisting of a Nuclepore prefilter for particles larger than ∼1–2 μm and a Teflon second filter stage for the submicron fraction. The samples were analyzed for soluble ionic components, organic carbon, and black carbon. Onboard the research aircraft, particle number and volume distributions as a function of size were determined with a laser‐optical particle counter and the black carbon content of the aerosol with an aethalometer. We determined the emission ratios (relative to CO2 and CO) and emission factors (relative to the amount of biomass burnt) for the various aerosol constituents. The smoke aerosols were rich in organic and black carbon, the latter representing 10–30% of the aerosol mass. K+ and NH4+ were the dominant cationic species in the smoke of most fires, while Cl− and SO42- were the most important anions. The aerosols were unusually rich in Cl−, probably due to the high Cl content of the semiarid vegetation. Comparison of the element budget of the fuel before and after the fires shows that the fraction of the elements released during combustion is highly variable between elements. In the case of the halogen elements, almost the entire amount released during the fire is present in the aerosol phase, while in the case of C, N, and S, only a small proportion ends up as particulate matter. This suggests that the latter elements are present predominantly as gaseous species in the fresh fire plumes studied here.Item Open Access Chemistry and aerosols in the marine boundary layer: 1-D modelling of the three ACE-2 Lagrangian experiments(Elsevier, 2000) Suhre, K.; Crassier, V.; Mari, C.; Rosset, R.; Johnson, D.W.; Osborne, S.; Wood, R.; Andreae, M.O.; Bandy, B.; Bates, T.S.; Businger, S.; Gerbig, C.; Raes, F.; Rudolph, J.Item Open Access Evolution of the aerosol, cloud and boundary-layer dynamic and thermodynamic characteristics during the 2nd Lagrangian experiment of ACE-2(Wiley-Blackwell, 2000) Osborne, S.R.; Johnson, D.W.; Wood, R.; Bandy, B.J.; Andreae, M.O.; O'Dowd, C.D.; Glantz, P.; Noone, K.J.; Gerbig, C.; Rudolph, J.; Bates, T.S.; Quinn, P.We present observations from the 2nd Aerosol Characterisation Experiment where over a 29‐h period between 16–18 July 1997 a tagged column of air was followed by a fully instrumented aircraft. The Lagrangian framework this offered made it possible to measure the evolution of the aerosol size distribution, the cloud structure and microphysics, and the dynamic and thermodynamic structure of the marine boundary layer within a polluted airmass advecting off northwest Europe over the sub‐tropical North Atlantic Ocean. The salient observations are presented and analysed. Processes responsible for the evolution are suggested, but quantification of their respective rates must be taken up by future modelling studies. Stratocumulus capped the boundary layer throughout the period that produced negligible washout of aerosol. This implies that the conversion of a continental to a maritime airmass within the cloud‐capped sub‐tropical marine boundary layer is not controlled by the drizzle process but by entrainment from the free troposphere. We find evidence of processing of aerosol particles by stratocumulus cloud, in particular by aqueous‐phase reactions. The processing of the aerosol, realised by modification of the aerosol size distribution in the particle diameter range 0.1–0.5 μm, was complicated by rapid changes in boundary layer height and structure, and also by entrainment of both polluted and relatively clean aerosol from the free troposphere. The cloud microphysics was affected by these changes in the boundary layer aerosol through changes in the cloud condensation nuclei activation spectra. The cloud microphysics was also strongly affected by changes in the dynamics of the boundary layer which included variations (e.g., diurnal) in cloud thickness and an increase in vertical wind speed. Thermodynamic changes within the boundary layer included decoupling due to an increasing sea‐surface temperature and a change in the subsidence rate in the free troposphere superimposed on diurnal decoupling. Hypotheses have been devised so that future modellers can focus their efforts to either validate or invalidate potentially important processes.Item Open Access The IGBP/IGAC SAFARI-92 field experiment: Background and overview(AGU, 1996) Lindesay, J.A.; Andreae, M.O.; Goldammer, J.G.; Harris, G.W.; Annegarn, H.J.; Garstang, M.; Scholes, R.J.; van Wilgen, B.W.The International Geosphere-Biosphere Programme/International Global Atmospheric Chemistry (IGBP/IGAC) Southern Africa Fire-Atmosphere Research Initiative (SAFARI-92) field experiment was conducted in the 1992 dry season in southern Africa. The objective of the experiment was a comprehensive investigation of the role of vegetation fires, particularly savanna fires, in atmospheric chemistry, climate, and ecology. During SAFARI-92 experimental fires were conducted in Kruger National Park, South Africa, and at some sites in Zambia, in order to study fire behavior and trace gas and aerosol emissions. Regional studies on atmospheric chemistry and meteorology showed that vegetation fires account for a substantial amount of photochemical oxidants and haze over the subcontinent, and that the export of smoke-laden air masses contributed strongly to the ozone burden of the remote atmosphere in the southern tropical Atlantic region. The relationships between fire, soil moisture status, and soil trace gas emissions were investigated for several climatically and chemically important gases. Remote sensing studies showed that advanced very high resolution radiometer/local area coverage (AVHRR/LAC) imagery was valuable for fire monitoring in the region and in combination with biomass models could be used for the estimation of pyrogenic emissions.Item Open Access Methyl Halide Emissions from Savanna Fires in Southern Africa(AGU, 1996) Andreae, M.O.; Atlas, E.; Harris, G.W.; Helas, G.; deKock, A.; Koppmann, R.; Mano, S.; Pollock, W.H.; Rudolph, J.; Scharffe, D.; Schebeske, G.; Welling, M.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.