Evaluation of an aerosol optical scheme in the chemistry-transport model CHIMERE

This paper presents an aerosol optical scheme developed in the chemistry-transport model CHIMERE dedicated to calculate optical properties of particles. Such developments are very helpful as they complement the usual validation with PM (Particulate Matter) ground-based measurements by using surface (AERONET/PHOTONS network) and satellite (MODIS) remote sensing observations. To reach this goal, Aerosol Optical Thickness (AOT), column-averaged Single Scattering Albedo (SSA) and asymmetry parameter (g) are calculated at 440 nm, 675 nm, 870 nm and 1020 nm (AERONET wavelengths) under three hypotheses on the particle mixing state (external, internally homogeneous and core-shell). Furthermore and in addition to optical calculations, an original development has been made to estimate column volume size distributions in CHIMERE, directly comparable with AERONET retrievals. Comparisons between simulations and observations are made over Western Europe for the year 2003 but also for one specific case focused on ammonium nitrate aerosols. Observed AOT display a seasonal cycle (with highest values during summer) rather well reproduced by the model but biases with observational data have been found depending on seasons. In fall, winter and early spring, modeled AOT values agree well with AERONET retrievals with small negative biases. Focus on a pollution episode of ammonium nitrate origin during March 2003 reveals that CHIMERE is able to well reproduce the fine mode volume size distribution retrieved by AERONET, leading to good agreements between modeled and observed AOT. In late spring and summer, AERONET AOT values are underpredicted by the model, which could be due to uncertainties in modeling secondary species.     

Assessment of the detection abilities of monitoring networks for passive tracers at local and regional scales

We propose a method to evaluate the detection abilities of networks used for protection purposes. Such networks are designed for the detection of nuclear, biological or gaseous emissions, without constraint on the source location. Their assigned goal is to have the best chance to detect a threatening emission located anywhere in the vicinity of a domain to protect. Two sensors siting applications are addressed: sensors placed in the surroundings of a facility to protect, or sensors carried by people scattered within a small area. A network protection ability is related both to its detection scope, and to its response time. To assess the performance of such networks, two statistical indicators are therefore designed: the detection probability, computed on a large number of possible source locations, and the saturation time, which is the time when the maximum detection probability has been reached.Simulations are then carried out with the Polyphemus air quality modeling system for many emission scenarios, including 961 possible source locations, various emitted species, and a few representative meteorological situations. This allows to assess the performance of single sensors as well as full networks, and their sensitivity to parameters like meteorological conditions and source characteristics. The emitted quantity and meteorological dispersion are found to be important parameters, whereas the species type does not significantly influence the results. Two network design methods are considered: (1) networks composed of a given number of the “best” sensors according to an indicator, and (2) sensors placed in circles around the protected domain. The networks built with respect to the detection probability show good results with a limited number of sensors, while the saturation time is not reliable enough to build networks. The networks based on circles also show a good performance in the studied cases, provided there is a sufficient number of sensors.     

Measurements of dissolved methylmercury in natural waters using diffusive gradients in thin film (DGT)

A diffusive gradient in thin films (DGT) technique for measuring methylmercury (MeHg) concentrations in natural waters was developed using 3-mercaptopropyl-functionalized silica gel to preconcentrate the methylmercury. The new resin was characterized and calibrated. Methylmercury is efficiently accumulated at a pH range of 3-9. Basic performance tests of the new DGT device confirmed the applicability of Fick's first law for such DGT measurements. The diffusion coefficient of methylmercury in polyacrylamide gel was 5 x 10(-6) cm(2) s(-1). Methylmercury concentrations determined by DGT deployed for different time periods in the field are statistically not different from results obtained through direct measurements. The DGT technique represents therefore an alternative in situ sampling method for methylmercury. The detection limit of the overall method is 1 pg of MeHg, which correspond to approximately 30 pg L(-1) of MeHg in a water sample, when deploying a typical DGT device for 24 hours. Lower MeHg concentrations are measurable using longer deployment times or thinner diffusive gel layers.     

使用沃特世的On-line SPE/LC/MS/MS技术分析饮用水源地水中的痕量优先污染物

"优先污染物"(EC)的分析已经成为环境研究领域中一个倍受关注的话题.EC包括许多种化学品,如药物、兽药产品、三纳米材料、个人护理产品和化工产品等.在90年代早期,欧洲科学家就在地表水和地下水中测定出了痕量药物.这促使欧美各国展开了全国性的调查.结果发现了超过100种这样的污染物,浓度达到了ppb(十亿分之一)水平.EC的定义为目前尚未在常规监测项目中登记的污染物.     

Simulation of aerosol radiative properties with the ORISAM-RAD model during a pollution event (ESCOMPTE 2001)

The aim of this study is to present the organic and inorganic spectral aerosol module-radiative (ORISAM-RAD) module, allowing the 3D distribution of aerosol radiative properties (aerosol optical depth, single scattering albedo and asymmetry parameter) from the ORISAM module. In this work, we test ORISAM-RAD for one selected day (24th June) during the ESCOMPTE (expérience sur site pour contraindre les modèles de pollution atmosphérique et de transport d’emissions) experiment for an urban/industrial aerosol type. The particle radiative properties obtained from in situ and AERONET observations are used to validate our simulations. In a first time, simulations obtained from ORISAM-RAD indicate high aerosol optical depth (AOD)0.50–0.70±0.02 (at 440 nm) in the aerosol pollution plume, slightly lower (10–20%) than AERONET retrievals. In a second time, simulations of the single scattering albedo (ω_o) have been found to well reproduce the high spatial heterogeneities observed over this domain. Concerning the asymmetry parameter (g), ORISAM-RAD simulations reveal quite uniform values over the whole ESCOMPTE domain, comprised between 0.61±0.01 and 0.65±0.01 (at 440 nm), in excellent agreement with ground based in situ measurements and AERONET retrievals. Finally, the outputs of ORISAM-RAD have been used in a radiative transfer model in order to simulate the diurnal direct radiative forcing at different locations (urban, industrial and rural). We show that anthropogenic aerosols strongly decrease surface solar radiation, with diurnal mean surface forcings comprised between −29.0±2.9 and −38.6±3.9 W m⁻², depending on the sites. This decrease is due to the reflection of solar radiations back to space (−7.3±0.8<ΔF_TOA<−12.3±1.2 W m⁻²) and to its absorption into the aerosol layer (21.1±2.1<ΔF_ATM<26.3±2.6 W m⁻²). These values are found to be consistent with those measured at local scale.