Observations of ultrafine aerosol particle formation and growth in boreal forest

Abstract

We have measured submicron particles rather continuously since January 31st 1996 at a forest site in Southern Finland. Number size distribution data from the size range of 3-500 nm (particle diameter) are obtained every 10 minutes. This article attemps to give an overall view over diurnal pattern of submicron size distribution. We will show the typical ambient submicron particle size distribution as it is monitored at the site and, furthermore, we want to highlight some certain events that have been observed during the nine month period. Since the measurements are carried out inside forest, and there are no local sources of pollution nearby of the site, we have also to include the possibility to observe the formation of aerosol particles of biogenic origin.

Precipitation removal of ultrafine aerosol particles from the atmospheric boundary layer

“…Using instruments that can detect particles with diameters as low as 3 nm, UFP have been documented in the free troposphere [Clarke et al, 1998;Raes et al, 1997;Schröder and Ström, 1997] and in the vicinity of clouds [Hegg et al, 1991]. Significant evidence of nucleation of new particles was found at continental boundary layer sites [Birmili and Wiedensohler, 2000;Verheggen and Mozurkewich, 2002;Birmili et al, 2003], including boreal forests [Mäkelä et al, 1997;Kulmala et al, 1998Kulmala et al, , 2001aKulmala et al, , 2001bKomppula et al, 2003] and Arctic regions [Wiedensohler et al, 1996;Pirjola et al, 1998]. The marine boundary layer (BL) provided evidence of homogeneous nucleation both in remote sites and near-coastal regions [Covert et al, 1992;Hoppel et al, 1994;Weber et al, 1998;Allen et al, 1999;O'Dowd et al, 1999].…”

“…[13] Newly formed particles by homogeneous nucleation of water and sulfuric acid, in presence of ammonia and other organic species, tend to have very small sizes (d p $ 2 nm) [Covert et al, 1992;Mäkelä et al, 1997;Marti et al, 1997;Weber et al, 1997Weber et al, , 1998Kulmala et al, 1998;O'Dowd et al, 1999;Clarke et al, 1998]. It has been shown that the nucleation rate can be enhanced by the presence of organics and ions [Yu et al, 1999;Yu and Turco, 2000].…”

“…Thus particles that are not scavenged by coagulation continue to grow, and eventually they reach sizes of d p $ 100 nm. It has been shown that UFP can reach Aitken sizes during one day under favorable growing conditions [Mäkelä et al, 1997]. In some atmospheric environments the growth rate is small, and UFP need more than 3 days to reach a diameter d p $ 100 nm and to be able to be activated as cloud condensation nuclei (CCN) when supersaturation condition occurs.…”

Deep convection as a source of new particles in the midlatitude upper troposphere

“…[7] Two types of nucleation events have been observed in which the main condensing species are not sulfuric acid and ammonia: in Finnish forests [Mäkelä et al, 1997] and off the Irish coast [O'Dowd et al, 1999]. Their characteristics, covering many events, have been explored in large cooperative European campaigns, Biogenic Aerosol Formation in the Boreal Forest (BIOFOR) and New Particle Formation and Fate in the Coastal Environment (PARFORCE), whose results have recently been reported [Kulmala and Hämeri, 2000;O'Dowd and Hämeri, 2000].…”

First observations of increased ultrafine particle number concentrations near the inversion of a continental planetary boundary layer and its relation to ground‐based measurements

“…[2] Significantly increased concentrations of ultrafine particles in the size range of a few nanometers in diameter have been observed in the atmosphere at various locations worldwide [e.g., Weber et al, 1997;Covert et al, 1992;Wiedensohler et al, 1996;Mäkelä et al, 1997;O'Dowd et al, 1999]. Although these ultrafine particles appear to be caused by nucleation, the causes of these nucleation events are unclear.…”