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Abstract. The production mechanism of light alkenes, alkanes, and isoprene was investigated in laboratory experiments by measuring their concentrations in natural seawater as a function of spectral range, exposure time and origin, and concentration of dissolved organic carbon (DOC). The production mechanism of alkanes and of isoprene could not be clarified. Ethene and propene are produced photochemically from DOC. The relevant spectral range is UV and short-wavelength visible light. Initial production rates (up to day 10 of exposure) were in the range of several pmol L -• h -• (mg DOC)-i; the corresponding mean quantum yields for the spectral range of 300-420 nm were about 10 -s. Generally, the production rates and the quantum yields for ethene were about 2 times that of propene. The key factors in the total column integrated oceanic alkene production are the solar photon flux at sea surface, the penetration depth of the light into the ocean (especially the relation between different light absorbers, i.e., the extinction due to absorption of DOC), and the wavelength-and DOC-dependent quantum yields. As a result of the high variability of these parameters, actual local alkene production rates for a specific oceanic region may differ considerably from the globally averaged oceanic alkene production rates. The latter were estimated to be at most 5 Mt yr -•. We carried out several experiments in the laboratory and studied the formation of NMHC, including isoprene, in natural seawater. We investigated several parameters affecting the NMHC production, especially the wavelength dependence and the role of DOC. We will focus on C2-to C3-hydrocarbons, which represent more than 80% of total NMHC emissions from the oceans [Plass-Dalmer et al., 1995].

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In late winter/early spring, sunrise after the polar night, followed by photodissociation oI HNO3 leads to formation o• Nt) x and, via reaction (1), of C1ONO 2. This is expected to shift the partitioning within NOy towards NOx and C1ONO 2 and to a decrease of the HNO3 fraction within NOy. This paper reports on measurements of HNO 3, NOy, N20 , and 03 during two aircraft campaigns in winter 1997 in the Arctic lower stratosphere. We focus on a strong reduction of the ratio HNO3/NOy observed from January to March.

[1] Measurements of temperature, water vapor, total water, ozone, and cloud properties were made above the western equatorial Indian Ocean in February and March 1999. The cold-point tropopause was at a mean pressure-altitude of 17 km, equivalent to a potential temperature of 380 K, and had a mean temperature of 190 K. Total water mixing ratios at the hygropause varied between 1.4 and 4.1 ppmv. The mean saturation water vapor mixing ratio at the cold point was 3.0 ppmv. This does not accurately represent the mean of the measured total water mixing ratios because the air was unsaturated at the cold point for about 40% of the measurements. As well as unsaturation at the cold point, saturation was observed above the cold point on almost 30% of the profiles. In such profiles the air was saturated with respect to water ice but was free of clouds (i.e., backscatter ratio <2) at potential temperatures more than 5 K above the tropopause and hygropause. Individual profiles show a great deal of variability in the potential temperatures of the cold point and hygropause. We attribute this to short timescale and space-scale perturbations superimposed on the seasonal cycle. There is neither a clear and consistent ''setting'' of the tropopause and hygropause to the same altitude by dehydration processes nor a clear and consistent separation of tropopause and hygropause by the Brewer-Dobson circulation. Similarly, neither the tropopause nor the hygropause provides a location where conditions consistently approach those implied by a simple ''tropopause freeze drying'' or ''stratospheric fountain'' hypothesis.