Determination of isoprene and α-/β-pinene oxidation products in boreal forest aerosols from Hyytiälä, Finland: diel variations and possible link with particle formation events

Biogenic volatile organic compounds (VOCs), such as isoprene and α‐/β‐pinene, are photo‐oxidized in the atmosphere to non‐volatile species resulting in secondary organic aerosol (SOA). The goal of this study was to examine time trends and diel variations of oxidation products of isoprene and α‐/β‐pinene in order to investigate whether they are linked with meteorological parameters or trace gases. Separate day‐night aerosol samples (PM1) were collected in a Scots pine dominated forest in southern Finland during 28 July–11 August 2005 and analyzed with gas chromatography/mass spectrometry (GC/MS). In addition, inorganic trace gases (SO2, CO, NOx, and O3), meteorological parameters, and the particle number concentration were monitored. The median total concentration of terpenoic acids (i.e., pinic acid, norpinic acid, and two novel compounds, 3‐hydroxyglutaric acid and 2‐hydroxy‐4‐isopropyladipic acid) was 65 ng m−3, while that of isoprene oxidation products (i.e., 2‐methyltetrols and C5 alkene triols) was 17.2 ng m−3. The 2‐methyltetrols exhibited day/night variations with maxima during day‐time, while α‐/β‐pinene oxidation products did not show any diel variation. The sampling period was marked by a relatively high condensation sink, caused by pre‐existing aerosol particles, and no nucleation events. In general, the concentration trends of the SOA compounds reflected those of the inorganic trace gases, meteorological parameters, and condensation sink. Both the isoprene and α‐/β‐pinene SOA products were strongly influenced by SO2, which is consistent with earlier reports that acidity plays a role in SOA formation. The results support previous proposals that oxygenated VOCs contribute to particle growth processes above boreal forest.