Maritime-continental air-mass exchanges of biogenic marine volatile compounds boost inorganic new particle formation and production of cloud condensation nuclei over the boreal forest

Biogenic volatile organic compounds (BVOCs) and their oxidation products are known to facilitate the formation and growth of secondary aerosol particles in the ambient atmosphere. Some originate from the ocean, emitted by plankton or bacteria, while others stem from vegetation over the continents. Both marine and continental processes leading to aerosol formation or growth has been researched extensively and described in numerous publications. Their interactions however are yet to be examined. We seek to understand how marine precursors such as dimethyl sulfide (DMS) affects new particle formation over the boreal forest, and how highly oxygenated organic molecules (HOM) originating from monoterpenes grows said particles into the cloud condensation nuclei (CCN) size range.

We have utilised the Lagrangian chemistry transport model ADCHEM in reproducing observations from the SMEARII station (61°51' N, 24°17' E) located
in Hyytiala, Finland, during the year of 2018. The model operates along trajectories generated by HYSPLIT, using meteorology data from GDAS and incorporating emission inputs from CAMS. The atmospheric cluster dynamics code (ACDC) is coupled to the model in order to consider ion mediated new particle formation from sulphuric acid and ammonia.

We demonstrate how ADCHEM captures the gas-phase concentrations of key species including sulfuric acid (SA), HOM monomers and HOM dimers along with new particle formation and growth as observed by CI-APi-TOF and SMPS instrumentation, respectively. By running the model without anthropogenic influence, we show how DMS-derived SA and ammonia emitted from the ocean is transported inland in quantities sufficient to initiate
NPF over the boreal forest. The newly formed particles grow by condensation of HOM to reach the CCN size range. Our model results also indicate gas-phase concentrations of iodic acid (IA) at approximately 10⁵ molecules cm^-3, originating from marine emissions of methyl iodide (MeI). While ADCHEM does not consider the effect of IA on NPF, studies have claimed that IA may be able to cluster is the presence of SA and ammonia. This could increase the marine impact on continental NPF and will become the focus of future work with ADCHEM.

We theorise that the interactive marine and continental aerosol formation may act as a key element in the hydrological cycle. Marine air masses not only transport water vapour inland from the ocean but also SA, ammonia and halogens that under the influence of HOM formed over the boreal forest initiates the formation and growth of aerosol particles (without anthropogenic influence) ultimately reaching the CCN size range. These particles in turn can influence the formation, lifetime and precipitation patterns of clouds.

Figure (1). Measured and modelled aerosol particle number concentrations at the SMEARII station. The bottom panel
illustrates ADCHEM model results without the influence of DMS.