Mid-latitude North-Atlantic aerosol characteristics in clean and polluted air

Abstract

Aerosol number concentrations ( r>1.5 nm) and size distributions (5 nm–150 μm radius) are reported for air masses typical of clean and polluted air masses over a North–East Atlantic coastal site (Mace Head). In clean marine air, total particle concentration is typically 400–600 cm −3, rising to between 600 and 1500 cm −3 for modified maritime air, and in the most polluted air, concentrations increase to ≈7000 cm −3. In all air masses, regular bursts of ultra-fine (1.5–5 nm) particles (UFP) were observed over several hours duration and coincided with low tide occurrence in the presence of solar radiation. During these bursts, measured UFP concentrations exceeded 180,000 cm −3. Typically, the sub-micron marine size distribution possessed a bi-modal shape, typical of cloud-induced aerosol growth, with an accumulation mode centred at 0.1–0.2 μm (radius) and a fine mode ≈0.02 μm, while super-micron sea-spray particles possessed a mode at ≈1–5 μm, extending to ≈10 μm under moderately low wind speeds of 5–6 m s −1. Under higher wind speed conditions (10–11 m s −1), a spume drop mode, extending to sizes of 150 μm, was observed. In anthropogenically influenced air masses, the sub-micron size distribution remained bi-modal, although to a lesser extent. Number concentration is dominated by sub-micron aerosol in all air masses, in marine air masses, surface area is generally dominated by super-micron particles, and volume is overwhelmingly dominated by the mode centred on 10 μm under moderate conditions, and 100 μm under higher winds. By comparison, in polluted air, surface area is dominated by sub-micron sizes and volume dominated by super-micron sizes. After nucleation bursts, growth of new, ultra-fine, particles into sizes in the accumulation mode can be observed under some conditions with growth rates of the order of 10 nm/h. Calculating the pre-existing aerosol condensation sink and the growth rate of newly formed particles allowed estimation of the condensing vapour concentration producing the observed growth along with the source rate of the condensing vapour. The concentration of condensable vapour and its source strength were 10–14×10 7 molecules cm −3 and 3–14×10 5 molecules cm −3 s −1, respectively.