Large scale modeling of the transport, chemical transformation and mass budget of the sulfur emitted during the April 2007 eruption of Piton de la Fournaise

In April 2007, the Piton de la Fournaise volcano (Réunion island) entered into its biggest eruption recorded in the last century. Due to the absence of a sensors network in the vicinity of the volcano, an estimation of degassing during the paroxysmal phase of the event has not been performed. Nevertheless, the SO<sub>2</sub> plume and aerosols have been observed by the OMI and CALIOP space sensors, respectively. The mesoscale chemical model MesoNH-C simulates the observed bulk mass of SO<sub>2</sub> and the general shape of the SO<sub>2</sub> plume spreading over the Indian Ocean. Moreover, an analysis of the SO<sub>2</sub> plume budget estimates a total SO<sub>2</sub> release of 230 kt, among of which 60 kt have been transformed into H<sub>2</sub>SO<sub>4</sub>. 27 kt of SO<sub>2</sub> and 21 kt of H<sub>2</sub>SO<sub>4</sub> have been deposited at the surface by dry deposition. With this top down approach, the temporal evolution of the SO<sub>2</sub> emission has been estimated during the most active period of the eruption. The peak of degassing was estimated at 1800 kg s<sup>−1</sup> in the morning of 6~April. The temporal evolution of SO<sub>2</sub> emission presented here can also be used for local studies

Numerical study of tracers transport by a mesoscale convective system over West Africa

A three-dimensional cloud-resolving model is used to investigate the vertical transport from the lower to the upper troposphere in a mesoscale convective system (MCS) that occurred over Niger on 15 August 2004. The redistribution of five passive tracers initially confined in horizontally homogeneous layers is analyzed. The monsoon layer tracer (0–1.5 km) is the most efficiently transported in the upper troposphere with concentrations 3 to 4 times higher than the other tracers in the anvil. On the contrary the African Easterly Jet tracer (~3 km) has the lowest contribution above 5 km. The vertical profiles of the mid-troposphere tracers (4.5–10 km) in the MCS exhibit two peaks: one in their initial layers, and the second one at 13–14 km altitude, underlying the importance of mid-tropospheric air in feeding the upper troposphere. Mid-tropospheric tracers also experience efficient transport by convective downdrafts with a consequent increase of their concentrations at the surface. The concentration of the upper troposphere–lower stratosphere tracer exhibits strong gradients at the edge of the cloud, meaning almost no entrainment of this tracer into the cloud. No downward transport from the upper troposphere is simulated below 5 km. A proxy for lightning produced NO_x is transported preferentially in the forward anvil in the upper troposphere. Additionally, lateral inflows significantly contribute to the updraft and downdraft airflows emphasizing the three-dimensional structure of the West African MCSs

3D direct impacts of urgan aerosols on dynamics during the CAPITOUL field experiment

International audienceEvaluating the radiative impacts of aerosol particles is of great interest for understanding atmospheric physics and processes feedbacks. To respond to such objectives, the online fully coupled model Meso-NH is applied to a real case during a two-day Intensive Observation Period (IOP) of the CAPITOUL campaign. The aerosol optical properties are computed from the chemical composition and the size distribution of the particle population, and are compared to observations and analysed at local and regional scales. The differences between two simulations are then studied in order to isolate the direct radiative impacts of aerosols on dynamics. Results show that the aerosol particles generate a forcing on shortwave flux by a decrease of the amount reaching the surface up to 30 Wm−2. The resulting feedbacks lead to a cooling up to 0.6 K on the 2-meter temperature over the city of Toulouse and over the larger 125 km by 125 km area around Toulouse. This cooling is also modeled along the whole boundary layer, leading to a decrease of the boundary layer height up to −50 m during the afternoon and a decrease of the vertical velocities with an average of −3 %

Modeling secondary organic aerosol formation through cloud processing of organic compounds

Interest in the potential formation of secondary organic aerosol (SOA) through reactions of organic compounds in condensed aqueous phases is growing. In this study, the potential formation of SOA from irreversible aqueous-phase reactions of organic species in clouds was investigated. A new proposed aqueous-phase chemistry mechanism (AqChem) is coupled with the existing gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics (MPMPO) that simulate SOA formation. AqChem treats irreversible organic reactions that lead mainly to the formation of carboxylic acids, which are usually less volatile than the corresponding aldehydic compounds. Zero-dimensional model simulations were performed for tropospheric conditions with clouds present for three consecutive hours per day. Zero-dimensional model simulations show that 48-h average SOA formation is increased by 27% for a rural scenario with strong monoterpene emissions and 7% for an urban scenario with strong emissions of aromatic compounds, respectively, when irreversible organic reactions in clouds are considered. AqChem was also incorporated into the Community Multiscale Air Quality Model (CMAQ) version 4.4 with CACM/MPMPO and applied to a previously studied photochemical episode (3–4 August 2004) focusing on the eastern United States. The CMAQ study indicates that the maximum contribution of SOA formation from irreversible reactions of organics in clouds is 0.28 μg m<sup>−3</sup> for 24-h average concentrations and 0.60 μg m<sup>−3</sup> for one-hour average concentrations at certain locations. On average, domain-wide surface SOA predictions for the episode are increased by 9% when irreversible, in-cloud processing of organics is considered. Because aldehydes of carbon number greater than four are assumed to convert fully to the corresponding carboxylic acids upon reaction with OH in cloud droplets and this assumption may overestimate carboxylic acid formation from this reaction route, the present study provides an upper bound estimate of SOA formation via this pathway

High resolution modelling of aerosol dispersion regimes during the CAPITOUL field experiment: from regional to local scale interactions

High resolution simulation of complex aerosol particle evolution and gaseous chemistry over an atmospheric urban area is of great interest for understanding air quality and processes. In this context, the CAPITOUL (Canopy and Aerosol Particle Interactions in the Toulouse Urban Layer) field experiment aims at a better understanding of the interactions between the urban dynamics and the aerosol plumes. During a two-day Intensive Observational Period, a numerical model experiment was set up to reproduce the spatial distribution of specific particle pollutants, from the regional scales and the interactions between different cities, to the local scales with specific turbulent structures. Observations show that local dynamics depends on the day-regime, and may lead to different mesoscale dynamical structures. This study focuses on reproducing these fine scale dynamical structures, and investigate the impact on the aerosol plume dispersion. The 500-m resolution simulation manages to reproduce convective rolls at local scale, which concentrate most of the aerosol particles and can locally affect the pollutant dispersion and air quality

3D direct impacts of urban aerosols on dynamics during the CAPITOUL field experiment

. [1] Evaluating the radiative impacts of aerosol particles is of great interest for understanding atmospheric physics and processes feedbacks. To respond to such objectives, the online fully coupled model Meso-NH is applied to a real case during a two-day Intensive Observation Period (IOP) of the CAPITOUL campaign. The aerosol optical properties are computed from the chemical composition and the size distribution of the particle population, and are compared to observations and analysed at local and regional scales. The differences between two simulations are then studied in order to isolate the direct radiative impacts of aerosols on dynamics. Results show that the aerosol particles generate a forcing on shortwave flux by a decrease of the amount reaching the surface up to 30 Wm À2 . The resulting feedbacks lead to a cooling up to 0.6 K on the 2-meter temperature over the city of Toulouse and over the larger 125 km by 125 km area around Toulouse. This cooling is also modeled along the whole boundary layer, leading to a decrease of the boundary layer height up to À50 m during the afternoon and a decrease of the vertical velocities with an average of À3 %. Citation: Aouizerats, B., P. Tulet, and L. Gomes (2012), 3D direct impacts of urban aerosols on dynamics during the CAPITOUL field experiment, Geophys. Res. Lett., 39

Characterization of dust emission from alluvial sources using aircraft observations and high-resolution modeling

International audienceWe investigate mineral dust emission from alluvial sediments within the upland region in northern Mauritania in the vicinity of a decaying nocturnal low-level jet (LLJ). For the first time, the impact of valleys that are embedded in a rather homogeneous surrounding is investigated with regard to their role as dust source. Measures for local atmospheric dust burden were retrieved from airborne observations, satellite observations, and model simulations and analyzed in order to provide complementary information at different horizontal scales. Observations by the LEANDRE Nouvelle Generation backscatter lidar system flying aboard the French Falcon 20 aircraft were taken along five parallel flight legs perpendicular to the orientation of the main valley system dominating the topography of the study area. Results from a comparison of lidar-derived extinction coefficients with topography and aerial photographs confirm the relevance of (1) alluvial sediments at the valley bottoms as a dust source, and (2) the break-down of the nocturnal LLJ as a trigger for dust emission in this region. An evaluation of the AROME regional model, forecasting dust at high resolution (5 km grid), points toward an under-representation of alluvial dust sources in this region. This is also evident from simulations by the MesoNH research model. Although MesoNH simulations show higher dust loadings than AROME, which are more comparable to the observations, both models underestimate the dust concentrations within the boundary layer compared to lidar observations. A sensitivity study on the impact of horizontal grid spacing (5 km versus 1 km) highlights the importance of spatial resolution on simulated dust loadings

High occurrence of new particle formation events at the Maïdo high-altitude observatory (2150 m), Réunion (Indian Ocean)

This study aims to report and characterise the frequent new particle formation (NPF) events observed at the Maïdo observatory, Réunion, a Southern Hemisphere site located at 2150 m (a.s.l.) and surrounded by the Indian Ocean. From May 2014 to December 2015, continuous aerosol measurements were made using both a differential mobility particle sizer (DMPS) and an air ion spectrometer (AIS) to characterise the NPF events down to the lowest particle-size scale. Carbon monoxide (CO) and black carbon (BC) concentrations were monitored, as well as meteorological parameters, in order to identify the conditions that were favourable to the occurrence of nucleation in this specific environment. We point out that the annual NPF frequency average (65 %) is one of the highest reported so far. Monthly averages show a bimodal variation in the NPF frequency, with a maximum observed during transition periods (autumn and spring). A high yearly median particle growth rate (GR) of 15.16 nm h−1 is also measured showing a bimodal seasonal variation with maxima observed in July and November. Yearly medians of 2 and 12 nm particle formation rates (J2 and J12) are 0.858 and 0.508 cm−3 s−1, respectively, with a seasonal variation showing a maximum during winter, that correspond to low temperature and RH typical of the dry season, but also to high BC concentrations. We show that the condensation sink exceeds a threshold value (1.04×10−3 s−1) with a similar seasonal variation than the one of the NPF event frequency, suggesting that the occurrence of the NPF process might be determined by the availability of condensable vapours, which are likely to be transported together with pre-existing particles from lower altitudes.</p

Maïdo observatory: a new high-altitude station facility at Reunion Island (21° S, 55° E) for long-term atmospheric remote sensing and in situ measurements

Since the nineties, atmospheric measurement systems have been deployed at Reunion Island, mainly for monitoring the atmospheric composition in the framework of NDSC/NDACC (Network for the Detection of <i>Stratospheric</i> Change/Network for the Detection of Atmospheric Composition Change). The location of Reunion Island presents a great interest because there are very few multi-instrumented stations in the tropics and particularly in the southern hemisphere. In 2012, a new observatory was commissioned in Maïdo at 2200 m above sea level: it hosts various instruments for atmospheric measurements, including lidar systems, spectro-radiometers and in situ gas and aerosol measurements. <br><br> This new high-altitude Maïdo station provides an opportunity:<br> 1. to improve the performance of the optical instruments above the marine boundary layer, and to open new perspectives on upper troposphere and lower stratosphere studies;<br> 2. to develop in situ measurements of the atmospheric composition for climate change surveys, in a reference site in the tropical/subtropical region of the southern hemisphere;<br> 3. to offer trans-national access to host experiments or measurement campaigns for focused process studies

The SURFEXv7.2 land and ocean surface platform for coupled or offline simulation of Earth surface variables and fluxes

CC Attribution 3.0 License.Final revised paper also available at http://www.geosci-model-dev.net/6/929/2013/gmd-6-929-2013.pdfInternational audienceSURFEX is a new externalized land and ocean surface platform that describes the surface fluxes and the evolution of four types of surface: nature, town, inland water and ocean. It can be run either coupled or in offline mode. It is mostly based on pre-existing, well validated scientific models. It can be used in offline mode (from point scale to global runs) or fully coupled with an atmospheric model. SURFEX is able to simulate fluxes of carbon dioxide, chemical species, continental aerosols, sea salt and snow particles. It also includes a data assimilation module. The main principles of the organization of the surface are described first. Then, a survey is made of the scientific module (including the coupling strategy). Finally the main applications of the code are summarized. The current applications are extremely diverse, ranging from surface monitoring and hydrology to numerical weather prediction and global climate simulations. The validation work undertaken shows that replacing the pre-existing surface models by SURFEX in these applications is usually associated with improved skill, as the numerous scientific developments contained in this community code are used to good advantage