Abstract
Abstract. It has been claimed for more than a century that atmospheric new particle formation is primarily influenced by the presence of sulfuric acid. However, the activation process of sulfuric acid related clusters into detectable particles is still an unresolved topic. In this study we focus on the PARADE campaign measurements conducted during August/September 2011 at Mt Kleiner Feldberg in central Germany. During this campaign a set of radicals, organic and inorganic compounds and oxidants and aerosol properties were measured or calculated. We compared a range of organic and inorganic nucleation theories, evaluating their ability to simulate measured particle formation rates at 3 nm in diameter (J3) for a variety of different conditions. Nucleation mechanisms involving only sulfuric acid tentatively captured the observed noon-time daily maximum in J3, but displayed an increasing difference to J3 measurements during the rest of the diurnal cycle. Including large organic radicals, i.e. organic peroxy radicals (RO2) deriving from monoterpenes and their oxidation products, in the nucleation mechanism improved the correlation between observed and simulated J3. This supports a recently proposed empirical relationship for new particle formation that has been used in global models. However, the best match between theory and measurements for the site of interest was found for an activation process based on large organic peroxy radicals and stabilised Criegee intermediates (sCI). This novel laboratory-derived algorithm simulated the daily pattern and intensity of J3 observed in the ambient data. In this algorithm organic derived radicals are involved in activation and growth and link the formation rate of smallest aerosol particles with OH during daytime and NO3 during night-time. Because the RO2 lifetime is controlled by HO2 and NO we conclude that peroxy radicals and NO seem to play an important role for ambient radical chemistry not only with respect to oxidation capacity but also for the activation process of new particle formation. This is supposed to have significant impact of atmospheric radical species on aerosol chemistry and should be taken into account when studying the impact of new particles in climate feedback cycles.
Highlights
The formation of new particles by gaseous molecules has been observed in and above a variety of different forest ecosystems (Kulmala et al, 2004d)
While the simplified theories based only on concentrations of gas phase sulfuric acid were able to reproduce the maximum formation rates observed around local noon they were unable to reproduce the observed daily variability
Nucleation theories that involved the concentrations of organic radical species better matched the observed daily cycle in particle formation rate
Summary
The formation of new particles by gaseous molecules has been observed in and above a variety of different forest ecosystems (Kulmala et al, 2004d). It has been suggested that biogenic emissions from forests are involved in new particle formation and growth, meaning that forests could impact climate through aerosol–cloud interactions. A new climate feedback process linking forest emissions, aerosol and climate effects has been proposed (Kulmala et al, 2004d; Bonn et al, 2008, 2009; Carslaw et al, 2010; Paasonen et al, 2013). For both steps involved in new particle formation, i.e. nucleation and first steps of growth, knowledge of the chemical composition and the controlling parameters for the initial size range (particle diameter Dp < 10 nm) are essential but so far unidentified
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