We study the influence of the large-scale atmospheric contribution to the dynamics of the convective boundary layer (CBL) in a situation observed during the Boundary Layer Late Afternoon and Sunset ...Turbulence (BLLAST) field campaign. We employ two modeling approaches, the mixed-layer theory and large-eddy simulation (LES), with a complete data set of surface and upper-air atmospheric observations, to quantify the contributions of the advection of heat and moisture, and subsidence. We find that by only taking surface and entrainment fluxes into account, the boundary-layer height is overestimated by 70%. Constrained by surface and upper-air observations, we infer the large-scale vertical motions and horizontal advection of heat and moisture. Our findings show that subsidence has a clear diurnal pattern. Supported by the presence of a nearby mountain range, this pattern suggests that not only synoptic scales exert their influence on the boundary layer, but also mesoscale circulations. LES results show a satisfactory correspondence of the vertical structure of turbulent variables with observations. We also find that when large-scale advection and subsidence are included in the simulation, the values for turbulent kinetic energy are lower than without these large-scale forcings. We conclude that the prototypical CBL is a valid representation of the boundary-layer dynamics near regions characterized by complex topography and small-scale surface heterogeneity, provided that surface- and large-scale forcings are representative for the local boundary layer.
High altitude stations are the only platforms allowing for continuous measurements of the free-troposphere composition, and monitoring of trends away from pollution sources. However, they are ...influenced by mountain breezes and convection that bring air from the lowland boundary layer up to the summits. In summer 2005, a field campaign involving in situ measurements and ozone lidars was organized in the Pyrenees to investigate the impact of such processes on in situ measurements at the Pic du Midi (PDM) high altitude station (2875ma.s.l.). On June 17 and 19, a plain-to-mountain thermal circulation developed during the day. Observations show that direct transport of lowland air masses to PDM cannot account for ozone measurements at the station. Also, according to measurements, the PDM station did not directly sample the free troposphere. These two days were further investigated using a Lagrangian box model combining transport, photochemistry and mixing with the background troposphere. It was possible to reproduce and analyze ozone time series recorded at PDM, and quantify the partial mixing with free tropospheric air during the transport. A large fraction (43 to 86%) of air from the lower free troposphere was found to contribute to the gas melange sampled at PDM, with the best agreement found for fractions 57% (resp. 74%) on June 17 (resp. June 19).
•Reproduction and analysis of ozone time series recorded at Pic du Midi (PDM).•Ozone concentrations in the PBL few hours earlier influence PDM measurements.•Photochemistry during transport to PDM contributes to a few ppb of ozone.•PDM sampled a large fraction (43 to 86%) of air from the lower free troposphere.
11 ultra‐high‐frequency (UHF) and very‐high‐frequency (VHF) wind‐profiler radars were in operation for 13–18 months during the international Hydrological Cycle in Mediterranean Experiment (HyMeX) ...field campaign, devoted to the study of the atmospheric and marine water cycle in the western Mediterranean basin. These profilers provided vertical profiles of wind vector, turbulence, precipitation and the height of the atmospheric boundary layer and tropopause. The inland three VHF profilers aimed to document the upstream or downstream synoptic flow conditions. Five UHF profilers for lower atmosphere description were deployed along the French Mediterranean coast and Corsica Island. They were used to retrieve the 3D atmospheric wind fields over the basin, by assuming linearity of the fields inside a limited spatial and temporal domain. The objective of this article is to establish to what extent the 3D wind fields derived from the coastline profiler network are representative of the offshore kinematics. This assessment is performed by comparing more than one year of continuous profiler observations during different weather conditions with balloon radiosoundings and in situ aircraft or boundary‐layer pressurized balloon measurements.
Vegetation fires emit large amount of gases and aerosols which are detrimental to human health. Smoke exposure near and downwind of fires depends on the fire propagation, the atmospheric circulations ...and the burnt vegetation. A better knowledge of the interaction between wildfire and atmosphere is a primary requirement to investigate fire smoke and particle transport. The purpose of this paper is to highlight the usefulness of an UV scanning lidar to characterise the fire smoke plume and consequently validate fire-atmosphere model simulations. An instrumented burn was conducted in a Mediterranean area typical of ones frequently subject to wildfire with low dense shrubs. Using lidar measurements positioned near the experimental site, fire smoke plume was thoroughly characterised by its optical properties, edge and dynamics. These parameters were obtained by combining methods based on lidar inversion technique, wavelet edge detection and a backscatter barycentre technique. The smoke plume displacement was determined using a digital video camera coupled with the lidar. The simulation was performed using a mesoscale atmospheric model in a large eddy simulation configuration (Meso-NH) coupled to a fire propagation physical model (ForeFire), taking into account the effect of wind, slope and fuel properties. A passive numerical scalar tracer was injected in the model at fire location to mimic the smoke plume. The simulated fire smoke plume width remained within the edge smoke plume obtained from lidar measurements. The maximum smoke injection derived from lidar backscatter coefficients and the simulated passive tracer was around 200 m. The vertical position of the simulated plume barycentre was systematically below the barycentre derived from the lidar backscatter coefficients due to the oversimplified properties of the passive tracer compared to real aerosol particles. Simulated speed and horizontal location of the plume compared well with the observations derived from the videography and lidar method, suggesting that fire convection and advection were correctly taken into account.
The Pic 2005 field campaign took place from 13 June to 7 July 2005 close to the high-altitude permanent atmospheric observatory Pic-du-Midi (PDM), situated at 2875
m
asl in the French Pyrenees. The ...experimental set-up combined in situ ground-based observations at PDM with ozone lidar measurements at two lower sites in close vicinity (600
m
asl/28
km away, and 2380
m
asl/500
m away). Such an experimental configuration is appropriate to address the question of the vertical layering of the chemical atmosphere in a mountain area and above the plain nearby, and how this influences measurements conducted on a mountain summit under the influence of horizontal transport at regional scale, and vertical transport at local scale. Forecast tools made it possible to plan and carry out 6 one-day Intensive Observation Periods (IOPs), mostly in anticyclonic conditions favoring local thermally induced circulations, with and without local pollution in the lower troposphere.
It was thus possible to document i) ozone diurnal variations at PDM; ii) correlation between ozone measurements at PDM and their counterparts at the same altitude in the free troposphere; iii) ozone variability in the vicinity of PDM.
The field campaign provided direct experimental evidence that at daytime in the encountered conditions (mostly anticyclonic), PDM failed in a large extent to be representative of the troposphere above the surrounding flat areas at similar altitude. First, ozone daily averages at PDM were found lower than their free-tropospheric counterpart. Thermally induced circulations and convection pumping clean air from the rural boundary layer can account qualitatively for ozone depletion observed at PDM during daytime. However the surface measurements do not support the hypothesis of direct lifting of near-surface air masses up to PDM. Thus, mixing with free-tropospheric air, photochemistry and surface deposition in the valleys appear to be needed ingredients to account quantitatively for the observed variations (in proportions that further studies should determine). Second, ozone variability was found to be much lower at PDM than in the free troposphere—again an indication of atmospheric mixing. In particular at daytime, the PDM observatory did not allow for detection of ozone-rich layers simultaneously visible above the plain. Beyond these first results, the data set presented here paves way to detailed studies of the IOPs.
► Surface monitors and profilers deployed around a mountain-top station (2.9
km
asl). ► Ozone very stratified in the area due to deposition and complex transport patterns. ► Evidence of daytime transport of ozone-depleted air from the valleys to the station. ► Station representative at daytime of lower layers above the plain (1–2
km
asl). ► Mixing made ozone less variable at the station than in the regional free troposphere.
Medium-to-large fluctuations and coherent structures (mlf-cs's) can be observed using horizontal scans from single Doppler lidar or radar
systems. Despite the ability to detect the structures ...visually on the images, this method would be time-consuming on large datasets, thus limiting
the possibilities to perform studies of the structures properties over more than a few days. In order to overcome this problem, an automated
classification method was developed, based on the observations recorded by a scanning Doppler lidar (Leosphere WLS100) installed atop a 75 m tower
in Paris's city centre (France) during a 2-month campaign (September–October 2014). The mlf-cs's of the radial wind speed are estimated using the
velocity–azimuth display method over 4577 quasi-horizontal scans. Three structure types were identified by visual examination of the wind fields:
unaligned thermals, rolls and streaks. A learning ensemble of 150 mlf-cs patterns was classified manually relying on in situ and satellite
data. The differences between the three types of structures were highlighted by enhancing the contrast of the images and computing four texture
parameters (correlation, contrast, homogeneity and energy) that were provided to the supervised machine-learning algorithm, namely the quadratic
discriminant analysis. The algorithm was able to classify successfully about 91 % of the cases based solely on the texture analysis
parameters. The algorithm performed best for the streak structures with a classification error equivalent to 3.3 %. The trained algorithm
applied to the whole scan ensemble detected structures on 54 % of the scans, among which 34 % were coherent structures (rolls and streaks).
We propose an experimental method for detecting molecules in the UV–visible range using ultrashort laser pulses. Two types of sources are used: a continuum generated by 200 kHz Ti:sapphire ...regenerative amplifier system extending from 320 to 1100 nm, and a near-gaussian femtosecond pulse (100 fs) generated by an optical parametric amplifier. Both broadband sources allow the real-time detection of the oxygen, the nitrogen dioxide NO
2 and the water vapor bands. Moreover, the concentration of NO
2 can be determined within the 10 ppb sensitivity range by using a specific nonlinear fit technique.
Volcanic sulfate aerosols play a key role in air quality and climate. However, the rate of oxidation of sulfur dioxide (SO2) precursor gas to sulfate aerosols (SO42-) in volcanic plumes is poorly ...known, especially in the troposphere. Here we determine the chemical speciation as well as the intensity and temporal persistence of the impact on air quality of sulfate aerosols from the 2014–2015 Holuhraun flood lava eruption of Icelandic volcano Bárðarbunga. To do so, we jointly analyse a set of SO2 observations from satellite (OMPS and IASI) and ground-level measurements from air quality monitoring stations together with high temporal resolution mass spectrometry measurements of an Aerosol Chemical Speciation Monitor (ACSM) performed far from the volcanic source. We explore month/year long ACSM data in France from stations in contrasting environments, close and far from industrial sulfur-rich activities. We demonstrate that volcanic sulfate aerosols exhibit a distinct chemical signature in urban/rural conditions, with NO3:SO4 mass concentration ratios lower than for non-volcanic background aerosols. These results are supported by thermodynamic simulations of aerosol composition, using the ISORROPIA II model, which show that ammonium sulfate aerosols are preferentially formed at a high concentration of sulfate, leading to a decrease in the production of particulate ammonium nitrate. Such a chemical signature is however more difficult to identify at heavily polluted industrial sites due to a high level of background noise in sulfur. Nevertheless, aged volcanic sulfates can be distinguished from freshly emitted industrial sulfates according to their contrasting degree of anion neutralization. Combining AERONET (AErosol RObotic NETwork) sunphotometric data with ACSM observations, we also show a long persistence over weeks of pollution in volcanic sulfate aerosols, while SO2 pollution disappears in a few days at most. Finally, gathering 6-month long datasets from 27 sulfur monitoring stations of the EMEP (European Monitoring and Evaluation Programme) network allows us to demonstrate a much broader large-scale European pollution, in both SO2 and SO4, associated with the Holuhraun eruption, from Scandinavia to France. While widespread SO2 anomalies, with ground-level mass concentrations far exceeding background values, almost entirely result from the volcanic source, the origin of sulfate aerosols is more complex. Using a multi-site concentration-weighted trajectory analysis, emissions from the Holuhraun eruption are shown to be one of the main sources of SO4 at all EMEP sites across Europe and can be distinguished from anthropogenic emissions from eastern Europe but also from Great Britain. A wide variability in SO2:SO4 mass concentration ratios, ranging from 0.8 to 8.0, is shown at several stations geographically dispersed at thousands of kilometres from the eruption site. Despite this apparent spatial complexity, we demonstrate that these mass oxidation ratios can be explained by a simple linear dependency on the age of the plume, with a SO2-to-SO4 oxidation rate of 0.23 h−1. Most current studies generally focus on SO2, an unambiguous and more readily measured marker of the volcanic plume. However, the long persistence of the chemical fingerprint of volcanic sulfate aerosols at continental scale, as shown for the Holuhraun eruption here, casts light on the impact of tropospheric eruptions and passive degassing activities on air quality, health, atmospheric chemistry and climate.
The structure of the lower troposphere has been studied during the sea-breeze and post sea-breeze events in an industrialized coastal area of the North Sea. Atmospheric dynamics and dispersion of ...pollutants in the lower troposphere have been analyzed by the experimental results of the 3D nonhydrostatic Meso-NH model in Dunkerque area (51°N, 2.20°E), in the north of France. The simulations were verified and extended by data of the measurement campaign. Ground-based remote sensing systems (lidar and sodar), surface meteorology and air quality network stations data have been employed. We illustrate the different pollution scenarios and breeze structure by the analysis of Lagrangian tracers and back trajectories.
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