F. X. Meixner

HONO Emissions from Soil Bacteria as a Major Source of Atmospheric Reactive Nitrogen

From Soil to Sky Trace gases emitted either through the activity of microbial communities or from abiotic reactions in the soil influence atmospheric chemistry. In laboratory column experiments using several soil types, Oswald et al. (p. 1233) showed that soils from arid regions and farmlands can produce substantial quantities of nitric oxide (NO) and nitrous acid (HONO). Ammonia-oxidizing bacteria are the primary source of HONO at comparable levels to NO, thus serving as an important source of reactive nitrogen to the atmosphere. HONO emissions from soil are comparable to those of NO in arid and arable regions. Abiotic release of nitrous acid (HONO) in equilibrium with soil nitrite (NO2–) was suggested as an important contributor to the missing source of atmospheric HONO and hydroxyl radicals (OH). The role of total soil-derived HONO in the biogeochemical and atmospheric nitrogen cycles, however, has remained unknown. In laboratory experiments, we found that for nonacidic soils from arid and arable areas, reactive nitrogen emitted as HONO is comparable with emissions of nitric oxide (NO). We show that ammonia-oxidizing bacteria can directly release HONO in quantities larger than expected from the acid-base and Henry’s law equilibria of the aqueous phase in soil. This component of the nitrogen cycle constitutes an additional loss term for fixed nitrogen in soils and a source for reactive nitrogen in the atmosphere.

The NH4+‐NO3−‐Cl−‐SO42−‐H2O aerosol system and its gas phase precursors at a pasture site in the Amazon Basin: How relevant are mineral cations and soluble organic acids?

Real-time measurements of ammonia, nitric acid, hydrochloric acid, sulfur dioxide and the water-soluble inorganic aerosol species, ammonium, nitrate, chloride, and sulfate were performed at a pasture site in the Amazon Basin (Rondonia, Brazil). The measurements were made during the late dry season (biomass burning), the transition period, and the onset of the wet season (clean conditions) using a wet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector (SJAC). Measurements were conducted from 12 September to 14 November 2002 within the framework of LBA-SMOCC (Large-Scale Biosphere Atmosphere Experiment in Amazonia - Smoke Aerosols, Clouds, Rainfall, and Climate: Aerosols From Biomass Burning Perturb Global and Regional Climate). Real-time data were combined with measurements of sodium, potassium, calcium, magnesium, and low-molecular weight (LMW) polar organic acids determined on 12-, 24-, and 48-hours integrated filter samples. The contribution of inorganic species to the fine particulate mass (Dp = 2.5 µm) was frequently below 20% by mass, indicating the preponderance of organic matter. The measured concentration products of NH3 × HNO3 and NH3 × HCl persistently remained below the theoretical equilibrium dissociation constants of the NH3/HNO3/NH4NO3 and NH3/HCl/NH4Cl systems during daytime (RH 90%) fine-mode NH4NO3 and NH4Cl are predicted to be formed in the aqueous aerosol phase. Probably, Cl- was driven out of the aerosol phase largely by reaction of pyrogenic KCl with HNO3 and H2SO4. As shown by an updated version of the equilibrium simplified aerosol model (EQSAM2), which incorporates mineral aerosol species and lumped LMW polar organic acids, daytime aerosol NH4 + was mainly balanced by organic compounds.

Carbonyl sulfide exchange on an ecosystem scale: soil represents a dominant sink for atmospheric COS

The soil/plant/atmosphere exchange of carbonyl sulfide (COS) was investigated in an open oak woodland ecosystem at a rural site in northern California. Measurements of atmospheric concentrations of COS were made in June and in December 1994. We found a significant diel cycle with a drop of COS levels by approximately 150 ppt during the night in both seasons. The mean COS daytime background mixing ratios showed a distinct seasonal di⁄erence with 465

Combined application of INAA and PIXE for studying the regional aerosol composition in Southern Africa

77 ppt in summer and 375

Ammonia Exchange between Terrestrial Plants and the Atmosphere Controlled by Plant Physiology: Compensation Point and CO2 Exchange

56 ppt in winter. The nighttime bulk COS flux into the ecosystem was estimated using a micrometeorological model. To address the observed depletion of COS during stable nocturnal boundary layer conditions, the potential of various ecosystem compartments to act as a sink for COS was investigated. Studies using dynamic enclosures flushed with ambient air excluded vegetation as an important sink during nighttime due to high stomatal resistance. Results from soil chamber measurements indicate that the soil can act as a dominant sink for atmospheric COS. ( 1999 Elsevier Science Ltd. All rights reserved.

Exchange of Short Chained Organic Acids between the Biosphere and the Atmosphere

As part of the SAFARI-92 biomass buming experiment, aerosol collections were carried out with several size-fractionating sampling devices at a number of sites in Southern Africa. One of the samplers used at all ground-based sites was a stacked filter unit (SFU). The SFU samples were analyzed by both INAA and PIXE analysis. The present paper gives an intercomparison of the analytical results obtained in order to assess the accuracy and to check the quality assurance of the analytical procedures. Twenty-one common elements were determined by both INAA and PIXE. Concentrations of 13 elements (i.e., Na, Mg, Al, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Zn and Br) were generally measured with sufficient precision in both techniques for performing the intercomparison. The INAA and PIXE data were compared in terms of PIXE/INAA concentration ratios on a sample by sample basis for the coarse and fine size fraction separately. the atmospheric concentrations for K, Mn and Fe agreed within 5–10%, the agreement between the data for the other common elements was typically better than 15%. Possible explanations for lower than 1.0 ratios for Cl, Br and Na are presented. The common elements were classified into groups according to their detectability and sensitivity in each technique.

Hydroxylamine released by nitrifying microorganisms is a precursor for HONO emission from drying soils

Based on forest experiments, we found a compensation point of 0.4 ppb for the exchange of ammonia with spruces at the Schachtenau site in June 1991. Thus, emission of ammonia from spruces was observed, when the atmospheric concentration was below 0.4 ppb, and uptake was found under conditions with higher mixing ratios. In general, a spruce site like the Schachtenau site acts as a sink for ammonia emitted from other ecosystems (e.g. agricultural sites). This is further supported by gradient measurements in correlation with cuvette data under higher atmospheric mixing ratios in September 1990. For a wheat field site at Manndorf we report a strong diurnal variation of net NH3 fluxes showing emission over the day and deposition during the night in close correlation to CO2 fluxes induced by the assimilation and respiration of the wheat field. This behaviour points to the plant physiological control of the NH3 exchange.

Exchange of Reduced Sulfur Compounds between the Biosphere and the Atmosphere

We have investigated the exchange of short chain organic acids between the biosphere and the atmosphere. Emission from different tree species under laboratory as well as field conditions was studied in order to get an estimate of the contribution of the release of organic acids into the atmosphere by the vegetation. Additionally we gathered information about the correlation of the emission with several plant physiological parameters such as photosynthesis, respiration, transpiration and stomatal conductance. Furthermore, a one year study (1994) supported by micrometeorological measurements over an agricultural site will give some insight into the fluctuations of acids in the air as well as the emission/deposition processes. Investigations on the role of leaf litter were done to discuss the role of litter layers in forests, and first experiments on the determination of the apoplastic concentration of acids and ions will help to discuss production, transport and release phenomena within and from plant tissues.

Isoprene and monoterpene fluxes from Central Amazonian rainforest inferred from tower-based and airborne measurements, and implications on the atmospheric chemistry and the local carbon budget

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH), the atmosphere´s primary oxidant. An unknown strong daytime source of HONO is required to explain measurements in ambient air. Emissions from soils are one of the potential sources. Ammonia-oxidizing bacteria (AOB) have been identified as possible producers of these HONO soil emissions. However, the mechanisms for production and release of HONO in soils are not fully understood. In this study, we used a dynamic soil-chamber system to provide direct evidence that gaseous emissions from nitrifying pure cultures contain hydroxylamine (NH2OH), which is subsequently converted to HONO in a heterogeneous reaction with water vapor on glass bead surfaces. In addition to different AOB species, we found release of HONO also in ammonia-oxidizing archaea (AOA), suggesting that these globally abundant microbes may also contribute to the formation of atmospheric HONO and consequently OH. Since biogenic NH2OH is formed by diverse organisms, such as AOB, AOA, methane-oxidizing bacteria, heterotrophic nitrifiers, and fungi, we argue that HONO emission from soil is not restricted to the nitrifying bacteria, but is also promoted by nitrifying members of the domains Archaea and Eukarya.

Impact of Manaus City on the Amazon Green Ocean atmosphere: ozone production, precursor sensitivity and aerosol load

In the course of our studies we developed a new sampling method for reduced sulfur compounds and used this method for investigations on the role of trees, crops, algae, lichens and leaf litter within the sulfur cycle. These studies were done under field conditions in conjunction with micrometeorological measurements as well as leaf or branch cuvettes. Laboratory studies gave insight into some basic physiological processes and regulation mechanisms.