An investigation of processes controlling the evolution of the boundary layer aerosol size distribution properties at the Swedish background station Aspvreten

摘要

Aerosol size distributions have been measured at the Swedish backgroundstation Aspvreten (58.8° N, 17.4° E). Different states of theaerosol were determined using a novel application of cluster analysis. Theanalysis resulted in eight different clusters capturing different stages ofthe aerosol lifecycle. The atmospheric aerosol size distributions wereinterpreted as belonging to fresh, intermediate and aged types of sizedistribution. With aid of back trajectory analysis we present statisticsconcerning the relation of source area and different meteorologicalparameters using a non-Lagrangian approach. Source area is argued to beimportant although not sufficient to describe the observed aerosolproperties. Especially processing by clouds and precipitation is shown to becrucial for the evolution of the aerosol size distribution. As much as60% of the observed size distributions present features that are likelyto be related to cloud processes or wet deposition. The lifetime propertiesof different sized aerosols are discussed by means of measured variabilityof the aerosol size distribution. Processing by clouds and precipitation isshown to be especially crucial in the size range 100 nm and larger. Thisindicates an approximate limit for activation in clouds to 100 nm in thistype of environment. The aerosol lifecycle is discussed. Size distributionsindicating signs of recent new particle formation (~30% of theobserved size distributions) represent the first stage in the lifecycle.Aging of the aerosol size distribution may follow two branches: eithergrowth by condensation and coagulation or processing by non-precipitatingclouds. In both cases mass is accumulated. Wet removal is the main processcapable of removing aerosol mass. Wet deposition is argued to be animportant mechanism in reaching a state where nucleation may occur (i.e.sufficiently low aerosol surface area) in environments similar to the onestudied.