Atmospheric nucleation is the dominant source of aerosol particles in the global atmosphere and an important player in aerosol climatic effects. The key steps of this process occur in the sub-2-nanometer (nm) size range, in which direct size-segregated observations have not been possible until very recently. Here, we present detailed observations of atmospheric nanoparticles and clusters down to 1-nm mobility diameter. We identified three separate size regimes below 2-nm diameter that build up a physically, chemically, and dynamically consistent framework on atmospheric nucleation--more specifically, aerosol formation via neutral pathways. Our findings emphasize the important role of organic compounds in atmospheric aerosol formation, subsequent aerosol growth, radiative forcing and associated feedbacks between biogenic emissions, clouds, and climate.

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We present ambient observations of dimeric monoterpene oxidation products (C16–20HyO6–9) in gas and particle phases in the boreal forest in Finland in spring 2013 and 2014, detected with a chemical ionization mass spectrometer with a filter inlet for gases and aerosols employing acetate and iodide as reagent ions. These are among the first online dual‐phase observations of such dimers in the atmosphere. Estimated saturation concentrations of 10−15 to 10−6 µg m−3 (based on observed thermal desorptions and group‐contribution methods) and measured gas‐phase concentrations of 10−3 to 10−2 µg m−3 (~106–107 molecules cm−3) corroborate a gas‐phase formation mechanism. Regular new particle formation (NPF) events allowed insights into the potential role dimers may play for atmospheric NPF and growth. The observationally constrained Model for Acid‐Base chemistry in NAnoparticle Growth indicates a contribution of ~5% to early stage particle growth from the ~60 gaseous dimer compounds.

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Abstract. Concentrations of 5 gases (HCl, HNO 3 , HONO, NH 3 , SO 2 ) and 8 major inorganic ions in particles (Cl − , NO for Na + , 3.0 for Mg 2+ , and 3.0 for Ca 2+ and 0.90 for the MARGA vs. SO 2 monitor. For K + there were not enough data points to calculate a statistically significant linear regression. There were clear seasonal cycles in the concentrations of the nitrogen-containing gases: the median concentrations of HNO 3 , HONO, and NH 3 were 0.09 ppb, 0.37 ppb, and 0.01 ppb in winter, respectively, and 0.15, 0.15, and 0.14 in spring, respectively. The gas-phase fraction of nitrogen decreased roughly with decreasing temperature, so that in the coldest period from January to February the median contribution was 28 % but in April to May was 53 %. There were also large fractionation variations that temperature alone cannot explain. HONO correlated well with NO x but a large fraction of the HONO-to-NO x ratios were larger than published ratios in a road traffic tunnel, suggesting that a large amount of HONO had other sources than vehicle exhaust. Aerosol acidity was estimated by calculating ion equivalent ratios. The sources of acidic aerosols were studied with trajectory statistics that showed that continental aerosol is mainly neutralized and marine aerosol acidic.