Observational constraints on size-resolved particle deposition across landscapes

Aerosol

aerosols

Small particles or droplets, such as wildfire smoke, volcanic gases, or salty sea spray, that float in the air. They are emitted by both natural events and human activities.

Source: NASA Global Climate Change Source

particles impact climate through both direct and indirect effects. However, the contribution of aerosols remains the most uncertain component of radiative forcing predicted by global models, and the removal of particles by deposition remains particularly difficult to constrain. This removal rate determines the lifetime of aerosol in the atmosphere – and thus its potential to impact the planet’s radiative balance.

radiative balance

Radiation Balance is the energy balance between incoming energy from the Sun and outgoing energy from the Earth.

Source: National Oceanic and Atmospheric Administration Source

Aerosol dry deposition is parameterized in both regional and global scale models, but these parameterizations rarely reflect observations of size-resolved aerosol deposition accurately. We recently proposed a revision to the parameterizations that included an enhanced role for interception and a diminished role for Brownian diffusion as mechanisms for particle dry deposition. Our work utilized measurements over a coniferous forest, and then tested the revised parameterization against both our and literature data from a variety of land surfaces. However, size-resolved particle flux

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

datasets are limited in availability over the complex array of ecosystems

ecosystem

A natural community of plants, animals, and other living organisms and the physical environment in which they live and interact.

Source: A student's guide to Global Climate Change Source

in the United States. The lack of observational constraints is a key limitation to reducing uncertainty of models, and thus uncertainty in modeled aerosol concentration and impacts.

Properties of the underlying (e.g., ground or ocean) play a substantial role in determining the relative importance of interception, impaction, and Brownian diffusion. Some surfaces, such as water and areas covered in ice and snow are particularly poorly constrained by measurements. Other land surfaces such as broadleaf forests and grasslands change their properties with the seasons – but the role of seasonal changes on particle fluxes is poorly understood because we lack of long-term measurements. Inherent challenges in making particle flux measurements have hindered observational constraints, but developments in online optical particle measurements coupled to sonic anemometers that measure rapid fluctuations in winds have enabled eddy covariance measurements of size-resolved particle flux. Now, newer (and lower-cost) instruments provide us an opportunity to deploy optical particle flux measurements across multiple surfaces. We propose to create the Fluxes of AerosoL Continuous Observing Network (FALCON), a network of particle flux measurements over multiple surface types designed to investigate size-resolved aerosol deposition processes to improve the accuracy of modeled aerosol removal rates and thus aerosol life cycle predictions. This project aims to deploy size-resolved particle flux measurements over five sites across the United States to investigate these key questions:
1. How reproducible are particle flux measurements? What are the measurement uncertainties in particle flux measurements, and to what extent do they influence our ability to evaluate their parameterizations?
2. To what extent do current and revised parameterizations capture dry deposition to different surfaces?
3. To what extent do different surfaces act as particle sources? What drives the often-observed emissions of particles from land surfaces? How well do current models capture emission and deposition of particles over water surfaces?