G. Schebeske

A global database of sea surface dimethylsulfide (DMS) measurements and a procedure to predict sea surface DMS as a function of latitude, longitude, and month

A database of 15,617 point measurements of dimethylsulfide (DMS) in surface waters along with lesser amounts of data for aqueous and particulate dimethylsulfoniopropionate concentration, chlorophyll concentration, sea surface salinity and temperature, and wind speed has been assembled. The database was processed to create a series of climatological annual and monthly 1°×1° latitude-longitude squares of data. The results were compared to published fields of geophysical and biological parameters. No significant correlation was found between DMS and these parameters, and no simple algorithm could be found to create monthly fields of sea surface DMS concentration based on these parameters. Instead, an annual map of sea surface DMS was produced using an algorithm similar to that employed by Conkright et al. [1994]. In this approach, a first-guess field of DMS sea surface concentration measurements is created and then a correction to this field is generated based on actual measurements. Monthly sea surface grids of DMS were obtained using a similar scheme, but the sparsity of DMS measurements made the method difficult to implement. A scheme was used which projected actual data into months of the year where no data were otherwise present.

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.

Biogenic sulfur emissions and aerosols over the tropical South Atlantic: 1. Dimethylsulfide in sea water and in the atmospheric boundary layer

We measured dimethylsulfide in air (DMSa) and the number concentration, size distribution, and chemical composition of atmospheric aerosols, including the concentration of cloud condensation nuclei (CCN), during February-March 1991 over the tropical South Atlantic along 19oS (F/S Meteor, cruise 15/3). Aerosol number/size distributions were determined with a laser-optical particle counter, condensation nuclei (CN) concentrations with a TSI 3020, and cloud condensation nuclei (CCN) with a Hudson-type supersaturation chamber. Aerosol samples were collected on two-stage stacked filters and analyzed by ion chromatography for soluble ion concentrations. Black carbon in aerosols was measured by visible light absorption and used to identify and eliminate periods with anthropogenic pollution from the data set. Meteorological analysis shows that most of the air masses sampled had spent extended periods over remote marine areas in the tropical and subtropical region. DMS a was closely correlated with the sea-to- air DMS flux calculated from DMS concentrations in seawater and meteorological data. Sea salt made the largest contribution to aerosol mass and volume but provided only a small fraction of the aerosol number concentration. The submicron aerosol had a mean composition close to ammonium bisulfate, with the addition of some methanesulfonate. Aerosol (CN and CCN) number and non-sea-salt sulfate concentrations were significantly correlated with DMS concentration and flux. This suggests that DMS oxidation followed by aerosol nucleation and growth in the marine boundary layer is an important, if not dominating, source of CN and possibly CCN. The degree of correlation between DMS and particle concentrations in the marine boundary layer may be strongly influenced by the different time scales of the processes regulating these concentrations. Our results provide strong support for several aspects of the CLAW hypothesis, which proposes the existence of a feedback loop linking DMS emission from marine plankton to sulfate aerosol and global climate.

N-alkane studies in the troposphere—II: Gas and particulate concentrations in Indian Ocean air

Abstract Westerly winds arriving at the Australian Clean Air Baseline Station Cape Grim at the North Western tip of Tasmania have exceedingly long trajectories over the Indian Ocean. In these air masses we found the n-alkanes C 9 -C 28 to be always present in the gas phase and also in the aerosols. In aerosols we also measured total organic matter and its general composition. All these concentrations agree fairly well with our earlier data in marine air over the North Atlantic Ocean. Calculations of the life time of the gas phase n-alkanes due to reaction with OH radicals lead to the conclusion that these n-alkanes must be of oceanic origin in Indian Ocean air and — to a large extent — also in the North Atlantic air. The same conclusion is reached for the organic component in aerosols. Water Analyses show that the n-alkanes C 9 -C 28 seem to be normal constituents of the oceans.

Physical and chemical characteristics of aerosols over the Negev Desert (Israel) during summer 1996

Sde Boker, in the Negev Desert of Israel (30°51′N, 34°47′E; 470 m above sea level (asl), is a long-term station to investigate anthropogenic and natural aerosols in the eastern Mediterranean in the framework of the Aerosol, Radiation and Chemistry Experiment (ARACHNE). Ground-level measurements of physical and chemical properties of aerosols and supporting trace gases were performed during an intensive campaign in summer 1996 (ARACHNE-96). Fine non sea salt (nss)-SO42− averaged 8±3 μg m−3 and fine black carbon averaged 1.4±0.5 μg m−3, comparable to values observed off the east coast of the United States. Optical parameters relevant for radiative forcing calculations were determined. The backscatter ratio for ARACHNE-96 was β = 0.13±0.01. The mass absorption efficiency for fine black carbon (αa,BCEf) was estimated as 8.9±1.3 m2 g−1 at 550 nm, while the mass scattering efficiency for fine nss-SO42− (αs,nss-SO42−f) was 7.4±2.0 m2 g−1. The average dry single-scattering albedo, ω0 characterizing polluted conditions was 0.89, whereas during “clean” periods ω0 was 0.94. The direct radiative effect of the pollution aerosols is estimated to be cooling. At low altitudes (below 800 hPa), the area was generally impacted by polluted air masses traveling over the Balkan region, Greece, and Turkey. Additional pollution was often added to these air masses along the Israeli Mediterranean coast, where population and industrial centers are concentrated. At higher altitudes (700 and 500 hPa), air masses came either from eastern Europe or from North Africa (Algerian or Egyptian deserts). The combination of measurements of SO2, CO, CN (condensation nuclei), and accumulation mode particles allowed to characterize the air masses impacting the site in terms of a mixture of local and long-range transported pollution. In particular, the lack of correlation between SO2 and nss-SO42− indicates that the conversion of regional SO2 into the particulate phase is not an efficient process in summer and that aged pollution dominates the accumulation mode particle concentrations.

Exchange of short‐chain monocarboxylic acids by vegetation at a remote tropical forest site in Amazonia

[1] As part of the project LBA-EUSTACH (European Studies on Trace gases and Atmospheric Chemistry as a contribution to the Large-Scale Biosphere-Atmosphere experiment in Amazonia), the exchange of formic acid and acetic acid between vegetation and the atmosphere was investigated in the wet-to-dry season transition and the dry-to-wet season transition periods in 1999 in Rondonia, Brazil. Direct exchange measurements on the branch level mainly exhibited uptake of formic acid and acetic acid for all plant species in both seasons, although diel, seasonal, and interspecies variations were observed. Even though other physiological and physico-chemical parameters may have contributed, the uptake of organic acids was found to be primarily a function of the ambient atmospheric mixing ratios. The linear dependence suggests a bidirectional exchange behavior of the plants and calculated deposition velocities (0.17-0.23 cm s -1 ), compensation point mixing ratios (0.16-0.30 ppb), and potential emission rates under purified air conditions (0.013-0.031 nmol m -2 s -1 ) are discussed. Vertical profile measurements in and above the primary forest canopy further strengthened the assumption that the forest is rather a sink than a source for organic acids. The generally lower mixing ratios observed within the canopy were indicative of an uptake by vegetation and/or the soil surface. Continuous measurements of the ambient atmospheric mixing ratios at the canopy top revealed strong diel variations in both seasons and a marked seasonality with higher mixing ratios during the dry season, both being mirrored in the variation of observed uptake rates of the plants. High atmospheric concentrations during the dry season were attributed to biomass burning. During the wet season, when biomass burning activity was low, indirect emission by the vegetation, i.e., photochemical oxidation of primarily emitted biogenic reactive hydrocarbons, was assumed to dominantly contribute to the atmospheric burden of the organic acids. The high degree of correlation between atmospheric formic acid and acetic acid indicated that similar atmospheric processes were affecting their mixing ratios.

Simultaneous field measurements of terpene and isoprene emissions from two dominant mediterranean oak species in relation to a North American species

We investigated the emission of monoterpenes and isoprene from tree species growing at a site near Montpellier (South France). We compared the emission pattern and behaviour of two important oak species representative of the Mediterranean ecosystem, namely, Quercus ilex (Holm oak), a strong monoterpene emitter, and Quercus pubescens (White oak), a strong isoprene emitter by measuring the two species growing side by side simultaneously. Additionally, we included a Californian oak species Quercus agrifolia (Coast Live Oak), which is anatomically and morphologically comparable with Quercus ilex. The data show that Live Oak and Holm oak though they appear identical, differ as far as the emission of terpenoids is concerned, emitting isoprene or terpenes, respectively. The isoprene emitting White oak and the monoterpene emitting Holm oak react identically to the same environmental conditions, fluctuating light and temperature, indicating that close metabolic pathways are involved in the actual emission of the C5 and C10 compounds from these two species, as seen under field conditions.

EXCHANGE OF SHORT-CHAIN ALDEHYDES BETWEEN AMAZONIAN VEGETATION AND THE ATMOSPHERE

As a part of the LBA-EUSTACH (EUropean Studies on Trace Gases and Atmospheric CHemistry as a contribution of the Large-scale Biosphere-Atmosphere ex- periment in Amazonia) project, the exchanges of formaldehyde (HCHO) and acetaldehyde (CH3CHO) between Amazonian vegetation and the atmosphere were investigated by branch enclosures and compared with gradient measurements during the wet-to-dry transition and dry-to-wet-transition periods at a remote forest site in Brazil, 1999. Branch enclosure measurements of several tree species showed emission as well as deposition of short-chain aldehydes, but fluxes were clearly dominated by deposition during both seasons. This bidirectional exchange was found to depend mainly on the actual ambient concentrations of these compounds and to exhibit a compensation point below 0.6 ppb of the compound in air with deposition velocities between 0.16 and 0.21 cm/s during the wet-to-dry season. During the dry-to-wet season, the deposition velocities and the compensation point in- creased. Under the clean air conditions of the wet-to-dry season, the major pathway for the aldehyde uptake was via leaf stomata. For HCHO, a mesophyll resistance of the same order of magnitude as the stomatal resistance contributed to the total leaf surface resistance, whereas the mesophyll resistance for CH3CHO was small, allowing a rapid uptake. This finding indicates a major contribution of metabolic consumption processes in addition to physical and chemical processes to the overall resistance. During the dry-to-wet period, when ambient air concentrations substantially increased, we found indications for an ad- ditional deposition to the leaf cuticle. Vertical gradient measurements of ambient air con- centrations in and above the canopy closely agreed with the branch enclosure studies and confirmed that the forest acts rather as a sink than as a direct source for HCHO and CH3CHO. Diel courses of ambient air concentrations and ratios of HCHO and CH3CHO above the canopy suggest photochemical oxidation of biogenically or pyrogenically emitted precursor compounds as the major sources for short-chain aldehydes in the tropical atmosphere.

Distribution and air-sea gas exchange of dimethyl sulphide at the European western continental margin

Abstract The concentrations of dissolved dimethyl sulphide (DMS) were measured on cruises aboard the R/V Meteor from Las Palmas, Gran Canaria, to Bremen, Germany (7–22 September 1994) and aboard the R/V Valdivia from Cork, Ireland, to Hamburg, Germany, via Cascais, Portugal (14 July–1 August 1995). The arithmetic means (ranges) of the DMS concentrations were 2.8 nM (0.6–33.4 nM, n =208) in September 1994 and 7.2 nM (2.9–38.5 nM, n =393) in July 1995. The distribution of dissolved DMS was characterised by higher mesoscale variability in July than in September. Patches of elevated DMS concentration were consistently associated with frontal regions along the Celtic Sea margin and on the shelf as well as with wind-driven upwelling along the Iberian margin. The pronounced difference between July and September concentrations is attributed to the seasonal pattern of dissolved DMS in the study area. DMS concentrations showed little correlation with chlorophyll a (chl a ), 19′-hexanoyloxy-fucoxanthin, or other selected chemotaxonomic pigment biomarkers. On the contrary, the chlorophyll-normalised DMS concentrations in July 1995 (45 nmol DMS/μg chl a ) were about a factor of 5.5 higher than those in September 1994 (8 nmol DMS/μg chl a ). Our results and a compilation of presently available data suggest that the DMS-to-chlorophyll ratios follow a sinusoidal pattern with peak values around July. The implications of these findings are discussed in relation to seasonal changes in DMS cycling and sea-to-air fluxes along the western European continental margin.