Size-resolved fluxes of sub-100-nm particles over forests

Dry deposition of atmospheric particles is critically dependent on particle size and plays a key role in dictating the mass and number distributions of atmospheric particles. However, modeling dry deposition is constrained by a lack of understanding of controlling dependencies and accurate size-resolved observations. We present size-resolved particle number fluxes for sub-100-nm particle diameters (Dp) over a deciduous forest derived using eddy covariance applied to data from a fast mobility particle sizer. The size-resolved particle number fluxes in 18 diameters between 8 and 100 nm were collected during leaf-on and are statistically robust. Particle deposition velocities normalized by friction velocity (v d +) are approximately four times smaller than comparable values for coniferous forests reported elsewhere. Comparison of the data with output from a new one-dimensional mechanistic particle deposition model designed for broadleaf forest exhibits greater accord with the measurements than two previous analytical models, but modeled v d + underestimate observed values by at least a factor of two for all Dp between 6 and 100 nm. When size-resolved particle deposition velocities for Dp <100 nm are normalized by friction velocity, the key controlling role of particle diffusivity is strongly manifest. On the basis of analyses of these new measurements and recently published size-resolved particle number fluxes from a conifer forest, we present working parameterizations for size-resolved particle deposition velocities over forests that could reasonably be applied in regional and global atmospheric chemistry transport models

Impact of Dust Source Patchiness on the Existence of a Constant Dust Flux Layer During Aeolian Erosion Events

Dust emission fluxes during wind soil erosion are usually estimated using a dust concentration vertical gradient, by assuming a constant dust flux layer between the surface and the dust measurement levels. Here, we investigate the existence of this layer during erosion events recorded in Iceland and Jordan. Size-resolved dust fluxes were estimated at three levels between 2 and 4 m using the eddy-covariance method. Dust fluxes were found mainly constant only between the two upper levels in Iceland, the lower dust flux being often stronger and richer in coarse particles, while dust fluxes in Jordan were nearly constant across all levels. The wind dynamics could not explain the absence of a constant dust flux layer in Iceland. We show that the presence of stationary dust source patches in Iceland, related to surface humidity, created a non-uniform dust layer near the surface, named dust roughness sublayer (DRSL), where individual plumes behind each patch interact but do not fully mix. The lowest dust measurement level was probably located within this sublayer while the upper ones were located above, such that there the emitted dust became spatially well-mixed. This explains near the surface in Iceland, the more intermittent dust concentration, its low correlation with the dust concentrations above, and the richer dust flux in coarse particles due to their lower deposition contribution. Our findings highlight the importance of estimating dust fluxes above a dust blending height whose characteristics depend on the dust source patchiness caused by surface humidity or the presence of sparse non-erosive elements.We acknowledge the European Research Council under the Horizon 2020 research and innovation programme through the ERC Consolidator Grant FRAGMENT (Grant agreement no 773051), and the AXA Research Fund through the AXA Chair on Sand and Dust Storms at BSC for financial support of the field campaigns in Iceland and Jordan. S. Dupont acknowledge the financial support of the Department Agroecosystem of INRAE. K. Kandler and K. Schepanski are funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—416816480; 417012665. For the Icelandic campaign, we thank the staff from the ranger station at Dreki as well as the wardens of the Dreki campsite and the Dreki mountain rescue service for their valuable support and advice. We also thank Vilhjalmur Vernharðsson and his crew from Fjalladýrð for their permanent logistic help. Without all of them, the measurement campaign would not have been successfully feasible. For the Jordanian campaign, we acknowledge efforts by the staff at Wadi Rum Protected Area, Aqaba Special Economic Zone Authority, and Directorate of Environmental Monitoring and Assessment at Ministry of Environment. We also thank the Helmholtz Association's Initiative and Networking Fund (Grant agreement no VH-NG-1533) for financial support of the field campaigns in Iceland and Jordan.Peer reviewe

Assessment of the total, stomatal, cuticular, and soil 2 year ozone budgets of an agricultural field with winter wheat and maize crops

This study evaluates ozone (O-3) deposition to an agricultural field over a period of 2years. A two-layer soil-vegetation-atmosphere-transfer (Surfatm-O-3) model is used to partition the O-3 flux between the soil, the cuticular, and the stomatal pathways. The comparison between measured and modeled O-3 fluxes exhibited a good agreement, independently of the canopy structure and coverage and the climatic conditions, which implicitly validates the O-3 flux partitioning. The total, soil, cuticular, and stomatal O-3 budgets are then established from the modeling. Total ecosystem O-3 deposition over the 2year period was 87.5kgha(-1). Clearly, nonstomatal deposition dominates the deposition budget, especially the soil component which represented up to 50% of the total deposition. Nevertheless, the physiological and phenological differences of maize and winter wheat induced large difference in the stomatal deposition budgets of these two crops. Then, the effect of simplified parameterizations for soil and cuticular resistances currently used in other models on the O-3 budget is tested. Independently, these simplified parameterizations cause an underestimation of the O-3 deposition ranging between 0% and 11.2%. However, the combination of all simplifications resulted in an underestimation of the total O-3 deposition by about 20%. Finally, crop yield loss was estimated to be 1.5-4.2% for the winter wheat, whereas maize was not affected by O-3

Ozone production in a maritime pine forest in water-stressed conditions

International audienceDuring two growing seasons of a maritime pine stand, in 2014 and 2015, ozone (O-3) fluxes have been determined using the eddy covariance (EC) method and compared to the outputs of a big-leaf O-3 deposition model including stomatal, cuticular and soil pathways. The model developed in this study generally allowed to properly reproduce the measured ozone deposition. Ozone fluxes showed a strong reduction during two water stressed periods in September 2014 and July 2015. The model partly explain this fall due to the reduction of stomatal deposition. Despite this stomatal closure, measured O-3 fluxes presented systematically lower negative values than the model outputs, and sometimes even positive values around midday during periods marked by strong water stress. In other words, the difference between observed and modelled O-3 fluxes (hereinafter referred to as the residual O-3 flux) is systematically positive on daytime during these water-stressed periods. This positive residual flux traduced the existence of an O-3 source below the flux measurement level, responsible for positive fluxes that counterbalance deposition fluxes. We developed an O-3 production module based on a terpene emission algorithm and an OH concentration proxy, to try to explain the observed ozone production. As this parametrisation allowed us to reproduce well the daily and inter-daily dynamics of the residual O-3 flux, it confirms that the latter actually resulted from O-3 production processes. This ozone production is here highlighted for the first time using O-3 fluxes measurements by the EC method. The chemical reactions possibly involved in O-3 production processes in this maritime pine forest have been discussed and different mechanisms are proposed, based on peroxy radicals chemistry or stress-induced BVOCs

LANDEX-Episode zero : First observations of secondary organic aerosol formation from the landes forest (France)

National audienc