Wet and dry deposition remove aerosols from the atmosphere, and these processes control aerosol lifetime and thus impact climate and air quality. Dry deposition is a significant source of aerosol uncertainty in global chemical transport and climate models. Dry deposition parameterizations in most global models were developed when few particle deposition measurements were available. However, new measurement techniques have enabled more size-resolved particle flux observations. We combined literature measurements with data that we collected over a grassland in Oklahoma and a pine forest in Colorado to develop a dry deposition parameterization. We find that relative to observations, previous parameterizations overestimated deposition of the accumulation and Aitken mode particles, and underestimated in the coarse mode. These systematic differences in observed and modeled accumulation mode particle deposition velocities are as large as an order of magnitude over terrestrial ecosystems. As accumulation mode particles form most of the cloud condensation nuclei (CCN) that influence the indirect radiative effect, this model-measurement discrepancy in dry deposition alters modeled CCN and radiative forcing. We present a revised observationally driven parameterization for regional and global aerosol models. Using this revised dry deposition scheme in the Goddard Earth Observing System (GEOS)-Chem chemical transport model, we find that global surface accumulation-mode number concentrations increase by 62% and enhance the global combined anthropogenic and natural aerosol indirect effect by −0.63 W m−2. Our observationally constrained approach should reduce the uncertainty of particle dry deposition in global chemical transport models.

Black carbon (BC) aerosol plays an important role in the Earth’s climate system because it absorbs solar radiation and therefore potentially warms the climate; however, BC can also act as a seed for cloud particles, which may offset much of its warming potential. If BC acts as an ice nucleating particle (INP), BC could affect the lifetime, albedo, and radiative properties of clouds containing both supercooled liquid water droplets and ice particles (mixed-phase clouds). Over 40% of global BC emissions are from biomass burning; however, the ability of biomass burning BC to act as an INP in mixed-phase cloud conditions is almost entirely unconstrained. To provide these observational constraints, we measured the contribution of BC to INP concentrations ([INP]) in real-world prescribed burns and wildfires. We found that BC contributes, at most, 10% to [INP] during these burns. From this, we developed a parameterization for biomass burning BC and combined it with a BC parameterization previously used for fossil fuel emissions. Applying these parameterizations to global model output, we find that the contribution of BC to potential [INP] relevant to mixed-phase clouds is ∼5% on a global average.

The atmospheric lifetime of black carbon (BC) is controlled by wet and dry deposition, which are poorly constrained by observations. We show that the single-particle soot photometer can measure surface-atmosphere exchange fluxes of refractory BC (rBC) particle mass (m rBC ) and number (N rBC ) by eddy covariance. We report field measurements of rBC dry and wet deposition rates during summer 2017 at the Southern Great Plains site in Oklahoma. On average, dry deposition of rBC is 0.3 ± 0.2 mm/s. We estimate a wet deposition flux of 2,600 ng·m À2 ·hr À1 over the 148.5 mm of rainfall observed. These data indicate a composite lifetime of 7-11 days. Key Points:• First direct measurements of black carbon by SP2 and eddy covariance over grassland suggest a dry deposition velocity of 0.3 ± 0.2 mm/s • Wet deposition is the dominant process of black carbon loss, but dry deposition can significantly impact black carbon lifetime • Measurements suggest that current atmospheric model parameterizations capture the atmospheric lifetime of black carbon reasonably wellSupporting Information:• Supporting Information S1

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We report on significant spatial gradients of accumulation mode particles emitted during the HOMEChem campaign and assess particle loss pathways.

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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.