Ultrafine particle number fluxes over and in a deciduous forest

Abstract

Ultrafine particles (UFP, particles with diameters (Dp) < 100 nm) play a key role in climate forcing; thus, there is interest in improved understanding of atmosphere‐surface exchange of these particles. Long‐term flux measurements from a deciduous forest in the Midwestern USA (taken during December 2012 to May 2014) show that although a substantial fraction of the data period indicates upward fluxes of UFP, on average, the forest is a net sink for UFP during both leaf‐active and leaf‐off periods. The overall mean above‐canopy UFP number flux computed from this large data set is −4.90 × 106 m−2 s−1 which re‐emphasizes the importance of these ecosystems to UFP removal from the atmosphere. Although there remain major challenges to accurate estimation of the UFP number flux and in drawing inferences regarding the actual surface exchange from measurements taken above the canopy, the above the canopy mean flux is shown to be downward throughout the day (except at 23.00) with largest‐magnitude fluxes during the middle of the day. On average, nearly three quarters of the total UFP capture by this ecosystem occurs at the canopy. This fraction increases to 78% during the leaf‐active period, but the over‐storey remains dominant over the subcanopy even during the leaf‐off period.

Seasonal variation of size-resolved aerosol fluxes in a Peri-urban deciduous broadleaved forest

“…Concerning the forest-atmosphere aerosol exchange, the prevailing deposition of aerosol particles observed by several experimental studies (Lamaud et al, 1994;Pryor et al, 2017;R. J. Vong et al, 2010) led to consider forests mainly as particle sinks.…”

“…Up to now, only a few studies performed above broadleaf forests lasted long enough to account for a seasonal change in the aerosol behaviour, as well as for an influence of different phenological conditions of the forest on the fluxes. Among them, Pryor et al (2017) focused on ultrafine particles only, while Petroff et al (2018) and Chiesa et al (2019) included in their analysis both ultrafine and fine aerosols. However, their studies were limited to the autumn season and did not provide information about the variation of the diurnal exchange pattern through the year.…”

Dry Deposition of Atmospheric Aerosols: Approaches, Observations, and Mechanisms

“…With the exception of urban, ocean, and chemically resolved particle flux studies, most work to date assumes that observed downward fluxes represent purely depositional processes. However, upward fluxes are frequently observed, whether over forests or grasslands (45,95,103). While such observations are reasonable over urban or marine areas where primary emission sources may be substantial, upward fluxes over remote regions have proven to be more puzzling and have often been attributed to particle nucleation, entrainment, or vertical gradients in gas-particle partitioning.…”

“…The role of these changes in surface structure is poorly understood but may have substantive effects on aerosol dry deposition and thus atmospheric lifetime. For example, Pryor et al (95) observed enhanced deposition rates for ultrafine (<100 nm in diameter) particles over a midwestern US forest during leaf-out relative to bare trees and were able to attribute the bulk of ultrafine particle deposition to canopy uptake (rather than to the ground). In contrast, Rannik et al (96) investigated long-term integrated (i.e., not size-segregated) fluxes over a boreal pine forest and found stronger particle deposition fluxes in the winter than in the summer; however, the researchers attributed this seasonal variability to shifts in size distribution rather than to surface collectors (97).…”