Seasonal variation in size-resolved particle deposition and the effect of environmental conditions on dry deposition in a pine forest

Abstract. Dry deposition is a fundamental process that removes particles from the atmosphere, and therefore directly controls their lifetime and total impact on air quality and radiative forcing. The processes influencing dry deposition are poorly constrained in models. Seasonal changes in dry deposition remain uncertain due to the lack of observations over multiple seasons. We present measurements of size-resolved sub-micron particle deposition from a flux study that surveyed all four major seasons. Particle concentrations and therefore fluxes were highest in the summer and lowest in the winter. Size-dependent deposition velocities in all seasons were consistent with previously observed trends, however, our observations show a 130 ± 60 % increase in wintertime deposition velocity compared to the summer, which is not currently captured in size-resolved deposition models. We explore the influence of scalar gradients and changes in environmental conditions as possible drivers of this increase. We find that phoretic effects, such as thermophoresis, and the addition of snow to the canopy had negligible impacts on our canopy level measurements. While turbophoresis impacted the observed seasonal changes in size-resolved particle deposition velocity, it did not fully explain the observed differences between the summer and winter. We suggest that the increase in deposition velocity is instead caused by changes to the leaf-level conditions and physiology during the wintertime, which increase interception of particles.