The Hohenpeissenberg aerosol formation experiment (HAFEX): a long-term study including size-resolved aerosol, H2SO4, OH, and monoterpenes measurements

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Abstract
Ambient aerosol size distributions (> 3 nm) and OH, H2SO4, and terpene concentrations were measured from April 1998 to August 2000 at a rural continental site in southern Germany. New particle formation (NPF) events were detected on 18% of all days, typically during midday hours under sunny and dry conditions. Surprisingly, most NPF events occurred during spring and winter, whereas the concentrations of aerosol precursors (H2SO4, monoterpenes) clearly peaked in summer. The number of newly formed particles correlated significantly with solar irradiance and ambient levels of H2SO4 and anti-correlated, especially in the cold season, with relative humidity and the condensational sink provided by pre-existing particles. The particle formation rates were experimentally estimated to be on order of 1 cm-3 s-1. Binary homogeneous H2SO4-H2O nucleation rates calculated from measured H2SO4 were substantially lower than this, even if assuming particle formation under the thermodynamic conditions on top of the boundary layer. The nucleation mode particle growth rates derived from the evolution of the size distribution were 2.6 nm h-1 on average, with a fraction of 0.7 nm h-1 attributed to the co-condensation of H2SO4/H2O/NH3. Turn-over rate calculations of measured monoterpenes and aromatic hydrocarbons suggest that especially the oxidation products of monoterpenes may contribute to the observed particle growth, although no indications were found that the reaction products of organic compounds would generally control the occurrence of NPF events.
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