Characteristics of fine particle growth events observed above a forested ecosystem in the Sierra Nevada Mountains of California

Atmospheric aerosols from natural and anthropogenic processes have both primary and secondary origins, and can influence human health, visibility, and climate. One key process affecting atmospheric concentrations of aerosols is the formation of new particles and their subsequent growth to larger particle sizes. A field study was conducted at the Blodgett Forest Research Station in the Sierra Nevada Mountains of California from May through September of 2002 to examine the effect of biogenic volatile organic compounds on aerosol formation and processing. The study included in-situ measurements of concentration and biosphere-atmosphere flux of VOCs, ozone, aerosol size distribution, aerosol physical and optical properties, and meteorological variables. Fine particle growth events were observed on approximately 30 percent of the 107 days with complete size distribution data. Average particle growth rates measured during these events were 3.8 ± 1.9 nm hr⁻¹. Correlations between aerosol properties, trace gas concentrations, and meteorological measurements were analyzed to determine conditions conducive to fine particle growth events. Growth events were typically observed on days with a lesser degree of anthropogenic influence, as indicated by lower concentrations of black carbon, carbon monoxide, and total aerosol volume. Days with growth events also had lower temperatures, increased wind speeds, and larger momentum flux. Measurements of ozone concentrations and ozone flux indicate that gas phase oxidation of biogenic volatile organic compounds occur in the canopy, strongly suggesting that a significant portion of the material responsible for the observed particle growth are oxidation products of naturally emitted very reactive organic compounds.