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/m². 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/m². Hence, while HOM SOA contributes to climate cooling, NPF can result in climate warming over the Boreal forest.

This work assesses the binding energies of atmospherically relevant clusters containing H₂SO₄, H₂O, NH₃ and (CH₃)₂NH using density functional theory. The performance of seven DFT functionals (B3LYP, CAM-B3LYP, M06-2X, PW91, LC-PW91, PBE0 and ωB97X-D) is evaluated against high level explicitly correlated coupled cluster methods using a test set of 107 atmospherically relevant clusters. Our studies show that all the tested functionals correlate well with the coupled cluster results, but with highly varying mean absolute errors. The PBE0, CAM-B3LYP, PW91 and M06-2X functionals are found to perform similarly with errors in the range of 2.53–3.46 kcal mol⁻¹, while the B3LYP and LC-PW91 functionals yield higher errors of 6.95 kcal mol⁻¹ and 10.66 kcal mol⁻¹, respectively. The ωB97X-D functional gives the best estimate of the binding energies with a mean absolute error as low as 2.12 kcal mol⁻¹ over the large test set of clusters.

Over oceans and in coastal regions, methane sulfonic acid (MSA) is present in substantial concentrations in aerosols and in the gas phase. We present an investigation into the effect of MSA on sulfuric acid- and dimethyl amine (DMA)-based cluster formation rates. From systematic conformational scans and well-tested ab initio methods, we optimise the structures of all MSAx (H2SO4)yDMAz clusters where x + y ≤ 3 and z ≤ 2. The resulting thermodynamic data are used in the Atmospheric Cluster Dynamics Code, and the effect of MSA is evaluated by comparing ternary MSA–H2SO4–DMA cluster formation rates to binary H2SO4–DMA cluster formation rates. Within the range of atmospherically relevant MSA concentrations, we find that MSA may increase cluster formation rates by up to 1 order of magnitude, although typically, the increase will be less than 300 % at 258 K, less than 100 % at 278 K and less than 15 % at 298 K. The results are rationalised by a detailed analysis of the main growth paths of the clusters. We find that MSA-enhanced clustering involves clusters containing one MSA molecule, while clusters containing more than one MSA molecule do not contribute significantly to the growth.