Solar-induced chlorophyll fluorescence (SIF) has been increasingly used as a proxy for terrestrial gross primary productivity (GPP). Previous work mainly evaluated the relationship between satellite-observed SIF and gridded GPP products both based on coarse spatial resolutions. Finer resolution SIF (1.3 km × 2.25 km) measured from the Orbiting Carbon Observatory-2 (OCO-2) provides the first opportunity to examine the SIF-GPP relationship at the ecosystem scale using flux tower GPP data. However, it remains unclear how strong the relationship is for each biome and whether a robust, universal relationship exists across a variety of biomes. Here we conducted the first global analysis of the relationship between OCO-2 SIF and tower GPP for a total of 64 flux sites across the globe encompassing eight major biomes. OCO-2 SIF showed strong correlations with tower GPP at both midday and daily timescales, with the strongest relationship observed for daily SIF at the 757 nm (R = 0.72, p < 0.0001). Strong linear relationships between SIF and GPP were consistently found for all biomes (R = 0.57-0.79, p < 0.0001) except evergreen broadleaf forests (R = 0.16, p < 0.05) at the daily timescale. A higher slope was found for C grasslands and croplands than for C ecosystems. The generally consistent slope of the relationship among biomes suggests a nearly universal rather than biome-specific SIF-GPP relationship, and this finding is an important distinction and simplification compared to previous results. SIF was mainly driven by absorbed photosynthetically active radiation and was also influenced by environmental stresses (temperature and water stresses) that determine photosynthetic light use efficiency. OCO-2 SIF generally had a better performance for predicting GPP than satellite-derived vegetation indices and a light use efficiency model. The universal SIF-GPP relationship can potentially lead to more accurate GPP estimates regionally or globally. Our findings revealed the remarkable ability of finer resolution SIF observations from OCO-2 and other new or future missions (e.g., TROPOMI, FLEX) for estimating terrestrial photosynthesis across a wide variety of biomes and identified their potential and limitations for ecosystem functioning and carbon cycle studies.

A B S T R A C TA large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in January and February 2008. The aerosol at Praia is a superposition of mineral dust, sea-salt, sulphates and soot. Particles smaller than 500 nm are mainly mineral dust, mineral dust-sulphate mixtures, sulphates and soot-sulphate mixtures. Particles larger then 2.5 µm consist of mineral dust, sea-salt and few mineral dust-sulphate mixtures. A transition range exists in between. The major internal mixtures are mineral dust-sulphate and soot-sulphate. Mineral dust-sea-salt mixtures occur occasionally, mineral dust-soot mixtures were not observed. The aspect ratio was 1.3-1.4 for dry particles smaller than 500 nm and 1.6-1.7 for larger ones. Parameterizations are given for dry and humid state. Although the real part of the refractive index showed low variation (1.55-1.58 at 532 nm), a multi-modal imaginary part was detected as function of particle size, reflecting the complex composition. Soot mainly influences the absorption for wavelengths longer than the haematite absorption edge, whereas for shorter wavelengths dust is dominating. The refractive index of the aerosol depends on the source region of the mineral dust and on the presence/absence of a marine component.

Evaporation from wet canopies (E) can return up to half of incident rainfall back into the atmosphere and is a major cause of the difference in water use between forests and short vegetation. Canopy water budget measurements often suggest values of E during rainfall that are several times greater than those predicted from Penman-Monteith theory. Our literature review identified potential issues with both estimation approaches, producing several hypotheses that were tested using micrometeorological observations from 128 FLUXNET sites worldwide. The analysis shows that FLUXNET eddycovariance measurements tend to provide unreliable measurements of E during rainfall. However, the other micrometeorological FLUXNET observations do provide clues as to why conventional Penman-Monteith applications underestimate E. Aerodynamic exchange rather than radiation often drives E during rainfall, and hence errors in air humidity measurement and aerodynamic conductance calculation have considerable impact. Furthermore, evaporative cooling promotes a downwards heat flux from the air aloft as well as from the biomass and soil; energy sources that are not always considered. Accounting for these factors leads to E estimates and modelled interception losses that are considerably higher. On the other hand, canopy water budget measurements can lead to overestimates of E due to spatial sampling errors in throughfall and stemflow, underestimation of canopy rainfall storage capacity, and incorrect calculation of rainfall duration. There are remaining questions relating to horizontal advection from nearby dry areas, infrequent large-scale turbulence under stable atmospheric conditions, and the possible mechanical removal of splash droplets by such eddies. These questions have implications for catchment hydrology, rainfall recycling, land surface modelling, and the interpretation of eddy-covariance measurements.

A B S T R A C T During the second Saharan Mineral Dust Experiment (SAMUM-2) field campaign, particles with geometric diameters (d) between ∼0.1 and 25 μm were collected on board of the Deutsches Zentrum für Luft-und Raumfahrt (German Aerospace Center, DLR) Falcon aircraft. Size, chemical composition and mixing state of aerosols sampled (spatially and vertically resolved) along the West African coastline and in the Cape Verde Islands region were determined by electron microscopy. A pronounced layer structure of biomass-burning aerosol and desert dust was present for all days during the sampling period from 23 January to 6 February. The aerosol composition of the small particles (d < 0.5 μm) was highly variable and in cases of biomass burning strongly dominated by soot with up to 90% relative number abundance. Internal mixtures of soot particles with mineral dust were not detected. Soot was only observed to mix with secondary sulphate. The coarse particles (d > 0.5 μm) were dominated by silicates. In the Cape Verde Islands region mineral dust is well mixed. The determination of source regions by elemental or mineralogical composition was generally not possible, except for air masses which were transported over the Gulf of Guinea. The real part of the refractive index showed little variation. In contrast, the imaginary part strongly depended on the abundance of soot (biomass-burning aerosol) and haematite (mineral dust).

[1] Density currents fed by evaporationally cooled air are an important dust stormgenerating feature and can constitute a source of moisture in arid regions. Recently, the existence of such systems has been demonstrated for the area between the High Atlas Mountains and the Sahara desert in southern Morocco on the basis of case studies. Here, a climatological analysis is presented that uses data from the dense climate station network of the IMPETUS project (An Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa) for the 5 year period 2002-2006. Objective criteria mainly based upon abrupt changes in wind and dew point temperature are defined to identify possible density current situations. The preselected events are then subjectively evaluated with the help of satellite imagery and surface observations to exclude causes for air mass changes other than moist convective cold pool formation. On average, 11 ± 4 density currents per year are detected with the main season lasting from April to September. Density currents occur mainly in the afternoon and evening due to the diurnal cycle of moist convection. Mean changes at the leading edge are increases in 2 m dew point temperature and wind speed by 5.4°C and 8.2 m s −1 , respectively, and a decrease in 2 m air temperature of 2.3°C. The High Atlas and Jebel Saghro are found to be the most important source regions, while only a few systems originate over the Saharan lowlands. Labilization of the atmosphere due to upper-level troughs over northwest Africa and an enhanced moisture content favor density current formation. In addition, detailed case studies representative of different density current types are presented.