Sensitivity of Cloud Microphysics to BVOC-Induced Aerosol Growth Over Boreal Forests

This study explores how the evolution of aerosol size distribution and chemical composition, driven by exposure to biogenic volatile organic compounds (BVOCs), influences cloud microphysics over the boreal forests of Southern Finland. Aerosol properties were derived from eight years of particle number size distribution (PNSD) and chemical composition measurements collected at the SMEAR II station. These data were categorized based on the time air masses spent traversing forested regions (Time Over Land, ToL), calculated using 97-hour HYSPLIT back trajectories. ToL was divided into 5-hour bins, and the median PNSD and aerosol composition for each bin were used to drive simulations with the PseudoAdiabatic bin-micRophySics University of Exeter Cloud parcel model (PARSEC).

The boreal forest emits biogenic volatile organic compounds (BVOCs) into the atmosphere, where these compounds undergo various oxidation processes. These reactions influence the growth and composition of atmospheric particles, ultimately contributing to the formation of secondary organic aerosols (SOA). Our simulation results show that with longer ToL, aerosols exhibit increased particle size and higher organic mass fractions. These changes significantly affect simulated cloud droplet activation and subsequent microphysical processes. PARSEC simulations revealed that the fraction of activated particles—cloud droplets relative to total aerosols—increases with both ToL and updraft velocity. However, for high ToL conditions (>3 days), the maximum supersaturation plateaus, particularly at stronger updraft velocities (>1 m/s), even as the activated fraction continues to increase. Moreover, once ToL exceeds one day, the albedo of clouds stabilizes rapidly, underscoring the importance of the initial 30-hour period in modulating the local climate.

While these observations provide insights into the coupling of aerosols and cloud properties, additional complexities remain. For instance, the impact of cloud droplet collisions, coalescence, and entrainment on cloud microphysics along ToL trajectories will be further discussed, highlighting their role in shaping cloud lifetime and albedo feedbacks.

By focusing on clean-sector air masses to minimize anthropogenic influences, this work underscores the critical interplay between BVOC-driven aerosol evolution and cloud microphysics. These findings emphasize the need to account for dynamic aerosol changes over boreal forests in climate models, particularly under conditions where BVOCs drive efficient SOA formation. Expanding our understanding of these interactions is essential for accurately representing the contribution of boreal forest ecosystems to local and regional climate systems.