Evidence for Diverse Biogeochemical Drivers of Boreal Forest New Particle Formation

New particle formation (NPF) is an important contributor to particle number in many locations, but the chemical drivers for this process are not well understood. Daytime NPF events occur regularly in the springtime Finnish boreal forest and strongly impact aerosol abundance. In April 2014 size‐resolved chemical measurements of ambient nanoparticles were made using the Time‐of‐Flight Thermal Desorption Chemical ionization Mass Spectrometer and we report results from two NPF events. While growth overall was dominated by terpene oxidation products, newly formed 20–70 nm particles showed enhancement in apparent alkanoic acids. The events occurred on days with rapid transport of marine air, which correlated with low background aerosol loading and higher gas phase methanesulfonic acid levels. These results are broadly consistent with previous studies on Nordic NPF but indicate that further attention should be given to the sources and role of non‐terpenoid organics and the possible contribution of transported marine compounds in this process. Plain Language Summary Clouds are an enormously important part of the climate system because they control the radiation entering and leaving the Earth. Clouds form as water condenses onto small particles called cloud condensation nuclei. These particles can be directly emitted from the Earth's surface, like sea spray, for example, or they can form in the atmosphere out of precursor gases. We have measured the composition of these atmosphere‐formed particles to understand better how this process works in the Nordic boreal forest. We found that a diverse mix of processes and molecules are likely involved, possibly including the transport of materials from the ocean. While these results will ultimately lead to a better understanding of ocean‐land‐cloud interactions, they currently indicate that more work is needed to learn the processes involved. Key Points The roles of organic acids, amines, inorganic acids, and highly oxidized organics are analyzed for two new particle formation events using in situ observations Saturated linear organic acids are the particulate species most associated with the appearance of new 20–50 nm particles The event days were associated with rapid transport of marine air to the site