The determination of the depth of daytime and nighttime mixing layers must be known very accurately to relate boundary-layer concentrations of gases or particles to upstream fluxes. The mixing-height ...is parametrized in numerical weather prediction models, so improving the determination of the mixing height will improve the quality of the estimated gas and particle budgets. Datasets of mixing-height diurnal cycles with high temporal and spatial resolutions are sought by various end users. Lidars and ceilometers provide vertical profiles of backscatter from aerosol particles. As aerosols are predominantly concentrated in the mixing layer, lidar backscatter profiles can be used to trace the depth of the mixing layer. Large numbers of automatic profiling lidars and ceilometers are deployed by meteorological services and other agencies in several European countries providing systems to monitor the mixing height on temporal and spatial scales of unprecedented density. We investigate limitations and capabilities of existing mixing height retrieval algorithms by applying five different retrieval techniques to three different lidars and ceilometers deployed during two 1-month campaigns. We studied three important steps in the mixing height retrieval process, namely the lidar/ceilometer pre-processing to reach sufficient signal-to-noise ratio, gradient detection techniques to find the significant aerosol gradients, and finally quality control and layer attribution to identify the actual mixing height from multiple possible layer detections. We found that layer attribution is by far the most uncertain step. We tested different gradient detection techniques, and found no evidence that the first derivative, wavelet transform, and two-dimensional derivative techniques have different skills to detect one or multiple significant aerosol gradients from lidar and ceilometer attenuated backscatter. However, our study shows that, when mixing height retrievals from a ultraviolet lidar and a near-infrared ceilometer agreed, they were 25–40% more likely to agree with an independent radiosonde mixing height retrieval than when each lidar or ceilometer was used alone. Furthermore, we point to directions that may assist the layer attribution step, for instance using commonly available surface measurements of radiation and temperature to derive surface sensible heat fluxes as a proxy for the intensity of convective mixing. It is a worthwhile effort to pursue such studies so that within a few years automatic profiling lidar and ceilometer networks can be utilized efficiently to monitor mixing heights at the European scale.
Within the framework of a French nationally funded project (CO2-MEGAPARIS) for quantifying the CO2 emissions of the Paris area, a lidar-based experimental investigation of the variability of the ...atmospheric boundary layer (ABL) depths was performed over four days in March 2011 under clear sky conditions. The prevailing synoptic settings were mainly characterized by anti-cyclonic situations with low wind. The key aim of this paper is to assess the impact of the urban heat island intensity (UHII) on the spatio-temporal variability of the ABL depths over the Paris megacity. A network of fixed aerosol lidars was deployed inside the city and in the vicinity of sub-urban and rural areas. Additionally, the spatial heterogeneity of the nocturnal boundary layer (NBL) depths over greater Paris area is addressed, thanks in particular, to the deployment of a 355-nm elastic lidar in a mobile van to measure the aerosol distributions. Radiosonde-derived profiles (twice a day) of thermodynamic variables over the sub-urban site helped investigate the temperature inversion above ground and hence to compare the lidar-derived ABL depths. Comparing these two results, an excellent concordance was found with a correlation coefficient of 0.994.
Five important factors closely related to the ABL circulation, namely, spatio-temporal variability of the ABL depths, growth rate of the ABL depths, entrainment zone thickness, and near-surface temperature fields including resultant UHII were considered to infer the urban–rural contrasts. The mean NBL depth over the urban area was on average 63 m (45%) higher than its adjacent sub-urban area which was, on occasion, as much as (74 m) 58% higher mainly due to the effect of UHII. Daytime well-mixed convective boundary layer and associated strong turbulent mixing near its top over the urban area showed higher entrainment zone thickness (326 m) than over sub-urban (234 m) and rural (200 m) areas. Temperature growth rates during sunrise increased up to more than 3 °C h−1 over the sub-urban area while over the urban region it was 2.5 °C h−1 or even less. The ABL depths over the urban site decayed more slowly (500 m h−1) than over the sub-urban area (600 m h−1) during the late afternoon transition period suggesting an impact of the UHII on the ABL dynamics over the urban area.
► Multi-lidar investigation of spatio-temporal variability of the ABL depth around Paris. ► First mobile lidar-based study of the spatial variability of NBL depth around a megacity. ► Assessment of impact of urban heat island (UHI) intensity and urban–rural contrast on ABL depth evolution. ► Higher entrainment zone thickness over urban area than over rural and sub-urban areas.
Today several lidar networks around the world provide large datasets that are extremely valuable for aerosol and cloud research. Retrieval of atmospheric constituent properties from lidar profiles ...requires detailed analysis of spatial and temporal variations of the signal. This paper presents an algorithm called Structure of the Atmosphere (STRAT), which is designed to retrieve the vertical distribution of cloud and aerosol layers in the boundary layer and through the free troposphere and to identify near-particle-free regions of the vertical profile and the range at which the lidar signal becomes too attenuated for exploitation, from a single lidar channel. The paper describes each detection method used in the STRAT algorithm and its application to a tropospheric backscatter lidar operated at the SIRTA observatory, in Palaiseau, 20 km south of Paris, France. STRAT retrievals are compared to other means of layer detection and classification; retrieval performances and uncertainties are discussed. PUBLICATION ABSTRACT
High uncertainties affect black carbon (BC) emissions, and, despite its important impact on air pollution and climate, very few BC emissions evaluations are found in the literature. This paper ...presents a novel approach, based on airborne measurements across the Paris, France, plume, developed in order to evaluate BC and NOx emissions at the scale of a whole agglomeration. The methodology consists in integrating, for each transect, across the plume observed and simulated concentrations above background. This allows for several error sources (e.g., representativeness, chemistry, plume lateral dispersion) to be minimized in the model used. The procedure is applied with the CHIMERE chemistry-transport model to three inventories - the EMEP inventory and the so-called TNO and TNO-MP inventories - over the month of July 2009. Various systematic uncertainty sources both in the model (e.g., boundary layer height, vertical mixing, deposition) and in observations (e.g., BC nature) are discussed and quantified, notably through sensitivity tests. Large uncertainty values are determined in our results, which limits the usefulness of the method to rather strongly erroneous emission inventories. A statistically significant (but moderate) overestimation is obtained for the TNO BC emissions and the EMEP and TNO-MP NOx emissions, as well as for the BC / NOx emission ratio in TNO-MP. The benefit of the airborne approach is discussed through a comparison with the BC / NOx ratio at a ground site in Paris, which additionally suggests a spatially heterogeneous error in BC emissions over the agglomeration.
During July 2009, a one-month measurement campaign was performed in the megacity of Paris. Amongst other measurement platforms, three stationary sites distributed over an area of 40 km in diameter in ...the greater Paris region enabled a detailed characterization of the aerosol particle and gas phase. Simulation results from the FLEXPART dispersion model were used to distinguish between different types of air masses sampled. It was found that the origin of air masses had a large influence on measured mass concentrations of the secondary species particulate sulphate, nitrate, ammonium, and oxygenated organic aerosol measured with the Aerodyne aerosol mass spectrometer in the submicron particle size range: particularly high concentrations of these species (about 4 mu g m super(-3), 2 mu g m super(-3), 2 mu g m super(-3), and 7 mu g m super(-3), respectively) were measured when aged material was advected from continental Europe, while for air masses originating from the Atlantic, much lower mass concentrations of these species were observed (about 1 mu g m super(-3), 0.2 mu g m super(-3), 0.4 mu g m super(-3), and 1-3 mu g m super(-3), respectively). For the primary emission tracers hydrocarbon-like organic aerosol, black carbon, and NO sub(x) it was found that apart from diurnal source strength variations and proximity to emission sources, local meteorology had the largest influence on measured concentrations, with higher wind speeds leading to larger dilution and therefore smaller measured concentrations. Also the shape of particle size distributions was affected by wind speed and air mass origin. Quasi-Lagrangian measurements performed under connected flow conditions between the three stationary sites were used to estimate the influence of the Paris emission plume onto its surroundings, which was found to be rather small. Rough estimates for the impact of the Paris emission plume on the suburban areas can be inferred from these measurements: Volume mixing ratios of 1-14 ppb of NO sub(x), and upper limits for mass concentrations of about 1.5 mu g m super(-3) of black carbon and of about 3 mu g m super(-3) of hydrocarbon-like organic aerosol can be deduced which originate from both, local emissions and the overall Paris emission plume. The secondary aerosol particle phase species were found to be not significantly influenced by the Paris megacity, indicating their regional origin. The submicron aerosol mass concentrations of particulate sulphate, nitrate, and ammonium measured during time periods when air masses were advected from eastern central Europe were found to be similar to what has been found from other measurement campaigns in Paris and south-central France for this type of air mass origin, indicating that the results presented here are also more generally valid.
Ground‐based lidar and Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) data sets gathered over four midlatitude sites, two U.S. and two French sites, are used to evaluate the consistency of ...cloud macrophysical and optical property climatologies that can be derived by such data sets. The consistency in average cloud height (both base and top height) between the CALIOP and ground data sets ranges from −0.4 km to +0.5 km. The cloud geometrical thickness distributions vary significantly between the different data sets, due in part to the original vertical resolutions of the lidar profiles. Average cloud geometrical thicknesses vary from 1.2 to 1.9 km, i.e., by more than 50%. Cloud optical thickness distributions in subvisible, semitransparent, and moderate intervals differ by more than 50% between ground‐ and space‐based data sets. The cirrus clouds with optical thickness below 0.1 (not included in historical cloud climatologies) represent 30–50% of the nonopaque cirrus class. An important part of this work consists in quantifying the different possible causes of discrepancies between CALIOP and surface lidar. The differences in average cloud base altitude between ground and CALIOP data sets can be attributed to (1) irregular sampling of seasonal variations in the ground‐based data, (2) day‐night differences in detection capabilities by CALIOP, and (3) the restriction to situations without low‐level clouds in ground‐based data. Cloud geometrical thicknesses are not affected by irregular sampling of seasonal variations in the ground‐based data but by the day‐night differences in detection capabilities of CALIOP and by the restriction to situations without low‐level clouds in ground‐based data.
Clouds represent the largest uncertainty in future climate projections. As a result, unbiased long‐term vertically‐resolved cloud observations must be collected and analyzed in order to produce ...regional cloud climatologies. In the present study, we use model outputs to evaluate the impact of conditional temporal sampling and instrumental effects on the 2‐year statistics of frequency of cloud occurrence and cloud fraction. We then quantify the radiative significance of the ice clouds undetected by cloud radars. We find that in order to evaluate the representation of all types of clouds in operational models both a cloud radar and a lidar must be used. The cloud radar alone can do a reasonable job at describing cloud properties up to 8–9 km, however the lidar is mandatory to detect most of the high‐altitude clouds above 9 km. The sampling should be regular but not necessarily continuous, and should not be driven by meteorological conditions. This result applies to all sites having a lidar without a radome. It is finally suggested that a cloud radar of around −60 dBZ sensitivity at 1 km range would be required to detect almost all radiatively‐significant ice clouds.
The ability of the aerosol chemistry transport model CHIMERE to simulate the vertical aerosol concentration profiles at a site near the city of Paris is evaluated using routine elastic backscatter ...lidar and Sun photometer measurements. The comparisons of model aerosols with measurements are carried out over a full year time period between October 2002 and September 2003. The methodology we propose here is new: From the model concentration outputs (optical properties varying with chemical composition and mass vertical distribution) we simulate the lidar backscattering profiles and compare them with the observed ones. The comparisons demonstrate the ability of the model to reproduce correctly the aerosol vertical distributions and their temporal variability. However, the aerosol load within the boundary layer is generally underestimated by the model, in particular during the afternoon hours and the summertime period. Several sensitivity tests indicate that this underestimation may have two origins: the lack of secondary organic and, to a lesser extent, mineral aerosols inside the model. The second deficiency is due to the absence of erosion/resuspension of soil material in the primary aerosol sources considered here; the first deficiency is probably due to incomplete knowledge about the formation of organic species in a photochemically active atmosphere. The results also show that the particles ranging from 0.08 to 1.25 μm in radius represent more than 89% of the volume backscattering at 532 nm, while the coarse particles are not efficient in terms of optical properties. The missing aerosol mass must therefore be found within the accumulation mode.
The ability of the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) to simulate midlatitude ice clouds is evaluated. Model outputs are compared to long-term meteorological ...measurements by active (radar and lidar) and passive (infrared and visible fluxes) remote sensing collected at an atmospheric observatory near Paris, France. The goal is to understand which of four microphysical schemes is best suited to simulate midlatitude ice clouds. The methodology consists of simulating instrument observables from the model outputs without any profile inversion, which allows the authors to use fewer assumptions on microphysical and optical properties of ice particles. Among the four schemes compared in the current study, the best observation-to-simulations scores are obtained with Reisner et al. provided that the particles' sedimentation velocity from Heymsfield and Donner is used instead of that originally proposed. For this last scheme, the model gives results close to the measurements for clouds with medium optical depth of typically 1 to 3, whatever the season. In this configuration, MM5 simulates the presence of midlatitude ice clouds in more than 65% of the authors' selection of observed cloud cases. In 35% of the cases, the simulated clouds are too persistent whatever the microphysical scheme and tend to produce too much solid water (ice and snow) and not enough liquid water.
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