Observation of nighttime new particle formation over the French Landes forest.

Improving the understanding of processes related to atmospheric particle sources is essential to better assess future climate. Especially, how biogenic volatile organic compounds (BVOCs) are involved in new particle formation (NPF) is still unclear, highlighting the need for field studies in sites that have not yet been explored. Weakly anthropised, mostly composed of maritime pines (known as strong monoterpene emitters), vast and under the influence of sea spray inputs, the Landes forest (located in the southwestern part of France) is a suitable ecosystem to explore these questions. The aim of the present work was to investigate for the first time NPF in the Landes forest, and to identify the conditions for NPF. During a field campaign conducted in July 2015, clear NPF was observed during nighttime, at a high frequency rate (37.5%), whereas only two daytime episodes were observed. Growth rates during NPF events were in the range 9.0-15.7nmh-1, and nucleation rates (J10) in the range 0.8-8 particles cm3s-1, typically in the range of reported values from rural sites. Nocturnal NPF started at sunset, lagging the reductions of temperature and ozone concentration as well as the increase of relative humidity, atmospheric stability and monoterpene concentration. We established that NPF occurred during more stratified atmosphere episodes, reflecting that NPF is more influenced by local processes at the Landes forest site (Bilos). Concentration of the sum of monoterpenes, here mainly α- and ß-pinene, was observed to be maximal during NPF episodes. On the contrary, ozone concentration was lower, which may indicate a larger consumption during nights where NPF episodes occur. Results strongly suggest the contribution of BVOC oxidation to nocturnal NPF, in both nucleation and the growth stages.

Fruit development, not GPP, drives seasonal variation in NPP in a tropical palm plantation.

We monitored seasonal variations in net primary production (NPP), estimated by allometric equations from organ dimensions, gross primary production (GPP), estimated by the eddy covariance method, autotrophic respiration (R(a)), estimated by a model, and fruit production in a coconut (Cocos nucifera L.) plantation located in the sub-tropical South Pacific archipelago of Vanuatu. Net primary production of the vegetative compartments of the trees accumulated steadily throughout the year. Fruits accounted for 46% of tree NPP and showed large seasonal variations. On an annual basis, the sum of estimated NPP (16.1 Mg C ha(-1) year(-1)) and R(a) (24.0 Mg C ha(-1) year(-1)) for the ecosystem (coconut trees and herbaceous understory) closely matched GPP (39.0 Mg C ha(-1) year(-1)), suggesting adequate cross-validation of annual C budget methods. However, seasonal variations in NPP + R(a) were smaller than the seasonal variations in GPP, and maximum tree NPP occurred 6 months after the midsummer peak in GPP and solar radiation. We propose that this discrepancy reflects seasonal variation in the allocation of dry mass to carbon reserves and new plant tissue, thus affecting the allometric relationships used for estimating NPP.

Transpiration of a 64-year old maritime pine stand in Portugal : 2. Evapotranspiration and canopy stomatal conductance measured by an eddy covariance technique.

The sensible and latent heat losses of a maritime pine stand (Pinus pinaster Aiton) and of its understorey were measured in Portugal, in conditions of mild water stress, using an eddy covariance technique (monodimensional sonic anemometer coupled with a thin thermocouple and a fast-response hygrometer) at two levels above and under the canopy: canopy exchanges were estimated by difference. This paper first discusses the corrections to be made to the raw fluxes, then shows that, over a 24-h interval, the energy balance closure was very satisfactory, as well as the comparison between estimations of canopy evaporation by eddy covariance and sap flow. Moreover, the phase shift between the two methods was small when sap flow was measured just at the base of the canopy. Canopy stomatal conductance, estimated with a flux-gradient model, increased very sharply at dawn, then decreased progressively in compensation for the increase in air saturation deficit.