Differential and total cross sections for the quasifree reactions γp → ηp and γn → ηn have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes η → 2γ and η → 3π 0 → 6γ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The η-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the S11(1535) resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy W = (1670 ± 5) MeV with an intrinsic width of Γ ≈ 30 MeV.

Abstract. The Glasgow photon tagging spectrometer at Mainz has been upgraded so that it can be used with the 1500 MeV electron beam now available from the Mainz microtron MAMI-C. The changes made and the resulting properties of the spectrometer are discussed.

Abstract. Aerosol size distribution measurements have been performed in the free troposphere during the Second Aerosol Characterization Experiment (ACE 2) near Tenerife, Canary Islands, in July 1997. During one measurement flight, on July 8, a uniform aerosol layer was encountered between 2.5 and 5.5 km altitude, characterized by a relatively low Aitken mode particle number concentration and high concentrations of accumulation and coarse mode particles, resulting in a relatively large aerosol surface area and mass, which is estimated to be about 400/xg m -3. Five-day backward trajectories indicate that the aerosol in this layer was mineral dust originating from arid regions on the North African continent. The dust layer was associated with reduced ozone mixing ratios. Model simulations have been performed with a photochemical box model including different heterogeneous removal reactions to study the interaction between gas phase chemistry and mineral aerosol. The best agreement between the observed and modeled ozone concentrations was obtained when heterogeneous removal of ozone and precursor gases on dust aerosol were taken into account. Heterogeneous 0 3 loss is estimated at 4 ppbv 0 3 per day. Although uncertainties concerning heterogeneous ozone removal remain, in particular related to the reactive uptake coefficient of 0 3 , it is likely that the loss of 0 3 and precursor gases on mineral dust aerosol significantly reduces the 0 3 abundance in large-scale dust plumes.

Abstract. This work will present aerosol size distributions measured in a Saharan dust plume between 0.9 and 12 km altitude during the ACE-2 campaign 1997. The distributions contain a significant fraction of large particles of diameters from 4 to 30 µm. Radiative transfer calculations have been performed using these data as input. Shortwave, longwave as well as total atmospheric radiative effects (AREs) of the dust plume are investigated over ocean and desert within the scope of sensitivity studies considering varied input parameters like solar zenith angle, scaled total dust optical depth, tropospheric standard aerosol profiles and particle complex refractive index. The results indicate that the large particle fraction has a predominant impact on the optical properties of the dust. A single scattering albedo of ω o =0.75−0.96 at 550 nm was simulated in the entire dust column as well as 0.76 within the Saharan dust layer at ∼4 km altitude indicating enhanced absorption. The measured dust leads to cooling over the ocean but warming over the desert due to differences in their spectral surface albedo and surface temperature. The large particles absorb strongly and they contribute at least 20% to the ARE in the dusty atmosphere.From the measured size distributions modal parameters of a bimodal lognormal column volume size distribution were deduced, resulting in a coarse median diameter of ∼9 µm and a column single scattering albedo of 0.78 at 550 nm. A sensitivity study demonstrates that variabilities in the modal parameters can cause completely different AREs and emphasises the warming effect of the large mineral dust particles.

The photoproduction of eta-mesic 3He has been investigated using the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI. The total inclusive cross section for the reaction gamma3He-->etaX has been measured for photon energies from threshold to 820 MeV. The total and angular differential coherent eta cross sections have been extracted up to energies of 745 MeV. A resonancelike structure just above the eta production threshold with an isotropic angular distribution suggests the existence of a resonant quasibound state. This is supported by studies of a competing decay channel of such a quasibound eta-mesic nucleus into pi(0)pX. A binding energy of (-4.4+/-4.2) MeV and a width of (25.6+/-6.1) MeV is deduced for the quasibound eta-mesic state in 3He.