Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to climatically important cloud condensation nuclei (CCN). Autoxidation of monoterpenes leads to rapid formation of Highly Oxygenated organic Molecules (HOM). We have developed the first model with near-explicit representation of atmospheric new particle formation (NPF) and HOM formation. The model can reproduce the observed NPF, HOM gas-phase composition and SOA formation over the Boreal forest. During the spring, HOM SOA formation increases the CCN concentration by~10 % and causes a direct aerosol radiative forcing of −0.10 W/m 2. In contrast, NPF reduces the number of CCN at updraft velocities < 0.2 m/s, and causes a direct aerosol radiative forcing of +0.15 W/m 2. Hence, while HOM SOA contributes to climate cooling, NPF can result in climate warming over the Boreal forest.
Abstract. In the present-day atmosphere, sulfuric acid is the most
important vapour for aerosol particle formation and initial growth. However,
the growth rates of nanoparticles (<10 nm) from sulfuric acid
remain poorly measured. Therefore, the effect of stabilizing bases, the
contribution of ions and the impact of attractive forces on molecular
collisions are under debate. Here, we present precise growth rate
measurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performed
under atmospheric conditions in the CERN (European
Organization for Nuclear Research) CLOUD chamber. Our results show
that the evaporation of sulfuric acid particles above 2 nm is negligible,
and growth proceeds kinetically even at low ammonia concentrations. The
experimental growth rates exceed the hard-sphere kinetic limit for the
condensation of sulfuric acid. We demonstrate that this results from
van der Waals forces between the vapour molecules and particles and
disentangle it from charge–dipole interactions. The magnitude of the
enhancement depends on the assumed particle hydration and collision
kinetics but is increasingly important at smaller sizes, resulting in a
steep rise in the observed growth rates with decreasing size. Including the
experimental results in a global model, we find that the enhanced growth rate of
sulfuric acid particles increases the predicted particle number concentrations
in the upper free troposphere by more than 50 %.
Abstract. Atmospheric new particle formation occurs frequently in the global atmosphere and may play a crucial role in climate by affecting cloud properties. The relevance of newly formed nanoparticles depends largely on the dynamics governing their initial formation and growth to sizes where they become important for cloud microphysics. One key to the proper understanding of nanoparticle effects on climate is therefore hidden in the growth mechanisms. In this study we have developed and successfully tested two independent methods based on the aerosol general dynamics equation, allowing detailed retrieval of time-and size-dependent nanoparticle growth rates. Both methods were used to analyze particle formation from two different biogenic precursor vapors in controlled chamber experiments. Our results suggest that growth rates below 10 nm show much more variation than is currently thought and pin down the decisive size range of growth at around 5 nm where in-depth studies of physical and chemical particle properties are needed.
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