The vertical structure of aerosol and its relationship to boundary-layer thermodynamics over the rural UK

The vertical structure of aerosol, covering sizes from 0.05-4 μm radius, was examined under conditions of subsidence during winter and summer over the rural UK. Under well-mixed boundary layer conditions, dry accumulation mode aerosol was found to be well mixed with height. During the winter campaigns, nocturnal cooling resulted in the development of a stable surface layer, typically 100 m in depth, within which the surface emitted pollutants became trapped leading to concentrations significantly greater than that observed in the mixed boundary layer above it. Under stable boundary-layer conditions, the aerosol and water vapour vertical profiles exhibited strong negative gradients with height and were indicative of suppressed turbulence associated with stable boundary-layer conditions. During summer, the boundary layer was normally decoupled and possessed two cloud layers: cumuli forming just below, and penetrating the surface-layer inversion; and stratocumulus occupying the region under the boundary-layer capping inversion. Aerosol profiles under decoupled conditions exhibited considerable variability with peak concentrations being observed in the vicinity of cloud edges. Average aerosol concentrations in the main boundary-layer ranged from 209-651 cm^-3 and 0.89-4.3 μm^-3 cm^-3 for dry number and volume respectively, whilst concentrations and volumetric loadings of 239-2430 cm^-3 and 1.1-13.5 μm^-3 cm^-3 were encountered in surface layers. The majority of the aerosol number and mass concentrations were almost exclusively derived from the size range 0.05-0.2 μm radius with mode radii often occurring at 0.1 μm or larger. By comparison, free tropospheric aerosol possessed typically an order of magnitude lower concentrations and mass with an associated mode radius of 0.05-0.06 μm or less.