Upward particle number fluxes over forest: Where, when, why?

Of the 60% of particle number fluxes over two forests that exceed the associated uncertainty bounds, approximately one-third are upward. These 'apparent emission' fluxes are not solely observed during periods when other micrometeorological fluxes are ill-defined, which implies they derive from a/multiple physical cause/s. Upward fluxes are slightly more frequent at night over the Danish beech forest but do not depend on wind direction or speed. Data from the pine forest in Finland indicate no diurnal cycle in the frequency with which upward fluxes are observed, although as in data from the beech forest the magnitude of upward fluxes is higher during the day. At the pine forest local emissions may account for some of the upward fluxes but other mechanisms appear also to play a role. Entrainment of particle depleted air from above the boundary layer, analysed via use of quadrant analysis and scalar correlations, appears to be important in the occurrence of upward fluxes at both sites. The rate of upward fluxes scales with prevailing geometric mean diameter (GMD) and consistent with the hypothesis of entrainment of relatively particle-depleted air upward fluxes appear to be associated with particle ensembles characterized by larger prevailing GMD.