GENerator of reduced Organic Aerosol mechanism (GENOA v1.0): An automatic generation tool of semi-explicit mechanisms

This paper describes the GENerator of Reduced Organic Aerosol Mechanisms (GENOA) that produces semi-explicit mechanisms for simulating the formation and evolution of secondary organic aerosol (SOA) in air-quality models. Using a series of predefined reduction strategies and evaluation criteria, GENOA trains and reduces SOA mechanisms from explicit chemical mechanisms (e.g., the master chemical mechanism (MCM)) under representative atmospheric conditions. As a consequence, these trained SOA mechanisms can preserve the accuracy of explicit VOC mechanisms on SOA formation (e.g., molecular structures of crucial compounds, the effect of non-ideality and hydrophilic/hydrophobic partitioning of aerosols), with a size (in terms of reaction and species numbers) that is manageable for three-dimensional aerosol modeling (e.g., regional chemical transport models). Applied to the degradation of a sesquiterpene (&beta;-caryophyllene) from MCM, GENOA builds a concise SOA mechanism (2 % of the MCM size), consisting of 23 reactions and 15 species, six of them being condensable. The generated SOA mechanism has been evaluated for its ability to reproduce SOA concentrations under varying atmospheric conditions encountered over Europe, with an average error lower than 3 %.</p

MICS Asia Phase II - Sensitivity to the aerosol module

International audienceIn the framework of the model inter-comparison study - Asia Phase II (MICS2), where eight models are compared over East Asia, this paper studies the influence of different parameterizations used in the aerosol module on the aerosol concentrations of sulfate and nitrate in PM10. An intracomparison of aerosol concentrations is done for March 2001 using different configurations of the aerosol module of one of the model used for the intercomparison. Single modifications of a reference setup for model configurations are performed and compared to a reference case. These modifications concern the size distribution, i.e. the number of sections, and physical processes, i.e. coagulation, condensation/evaporation, cloud chemistry, heterogeneous reactions and sea-salt emissions. Comparing monthly averaged concentrations at different stations, the importance of each parameterization is first assessed. It is found that sulfate concentrations are little sensitive to sea-salt emissions and to whether condensation is computed dynamically or by assuming thermodynamic equilibrium. Nitrate concentrations are little sensitive to cloud chemistry. However, a very high sensitivity to heterogeneous reactions is observed. Thereafter, the variability of the aerosol concentrations to the use of different chemistry transport models (CTMs) and the variability to the use of different parameterizations in the aerosol module are compared. For sulfate, the variability to the use of different parameterizations in the aerosol module is lower than the variability to the use of different CTMs. However, for nitrate, for monthly averaged concentrations averaged over four stations, these two variabilities have the same order of magnitude

Simulation numérique de la condensation / évaporation et de la coagulation des nanoparticules

National audienceAware of the risks related to nanoparticles (particles which present at least one dimension less than 100 nanometers), INERIS decided in 2009 to create a research program in order to develop a model that would be able to simulate the dynamic of nanoparticles in both confined and free atmospheres. The distinction with usual models is that we need to follow the evolution of the number of particles together with their the mass : in order to simulate the evolution of nanoparticles, the number is much more relevant. A comparative review of algorithms currently used in air quality models and new algorithms adapted to nanoparticles is presented. This first study addresses condensational growth, evaporation and coagulation. The model is to be integrated in chemistry-transport models (CHIMERE) and in CFD models (code_Saturne EdF).Conscient des risques liés aux nanoparticules (particules dont au moins une des dimensions est inférieure à 100 nanomètres), l'INERIS(1) a engagé en 2009 un programme de recherche en collaboration avec le CEREA(2) afin de développer un modèle capable de simuler les transformations des nanoparticules dans les ambiances intérieures (espaces confinés) comme dans l'atmosphère. En effet, les nanoparticules sont notamment susceptibles de coaguler, de grossir par condensation, et de se déposer sur les parois; ce qui modifie leur granulométrie. Une des problématiques liée à la modélisation des nanoparticules est que leur nombre est déterminant devant leur masse, tout au contraire des particules étudiées jusqu'à présent (particules fines ou grossières dont une des dimensions est supérieure à 100 nanomètre). Différents schémas numériques ont été développés pour simuler la condensation/évaporation d'une population de particules, et un noyau de coagulation issu d'algorithmes usuels a été intégré. L'inter-comparaison de ces schémas met en évidence que certains sont plus adaptés que d'autres pour les nanoparticules. Les algorithmes qui sont appropriés pour toutes les tailles de particules sont présentés. A terme, ce modèle de dynamique des nanoparticules a vocation à être intégré dans des modèles de dispersion atmosphérique (CHIMERE) et des modèles CFD (code_Saturne EdF

Influence of emission size distribution and nucleation on number concentrations over Greater Paris

With the growing evidence that high particle number concentrations may impact health, modelling their emissions and understanding formation processes is necessary, especially in cities where many people are exposed. As emission inventories of particle numbers and size distribution over cities are usually not available, a methodology is defined to estimate them from PM2.5 emissions and ratios of PM1 / PM2.5 and PM0.1 / PM2.5 by activity sector. In this methodology, a fitting parameter alpha(em) is used to redistribute the number concentrations in the lowest emission diameter range. This parameter is chosen by comparing measured and simulated number concentrations during non-nucleation days. The emission size distribution is then finely discretised by conserving both mass and number in each of the size ranges where emissions are specified. The methodology is applied over Greater Paris during the MEGAPOLI campaign (July 2009). Three-dimensional simulations are performed using the chemistry transport model Polair3D/Polyphemus coupled to the aerosol module SSH-aerosol to represent the evolution of particles by condensation, evaporation, coagulation, and nucleation, with a sectional approach for the size distribution. The model is first compared to measurements during non-nucleation days, and the influence over the month of July 2009 of three different nucleation parameterisations is assessed, i.e. binary (sulfuric acid, water), ternary (sulfuric acid, ammonia, water), and heteromolecular (extremely low-volatility organic compounds (ELVOCs) from monoterpenes and sulfuric acid). The modelled number concentrations compare very well to measurements, with an average normalised mean error of 42 % for the daily number concentrations of particles larger than 10 nm and 37 % for the number concentrations of particles larger than 100 nm. The influence of the binary nucleation is low, and the ternary nucleation scheme leads to better simulated number concentrations (in terms of bias and error) at only one site out of three, but it systematically reduces the model to measurement correlation, suggesting that ternary nucleation may not be the dominant process in new particle formation. However, the relative bias and error, as well as the correlation at suburban sites, are systematically improved using the heteromolecular nucleation scheme involving sulfuric acid and ELVOCs from monoterpenes. This suggests that heteromolecular nucleation may be important in cities, especially at suburban sites in summer, and that a better characterisation of the emissions of ELVOC precursors from traffic is needed.Peer reviewe

MUNICH v2.0: a street-network model coupled with SSH-aerosol (v1.2) for multi-pollutant modelling

A new version of a street-network model, the Model of Urban Network of Intersecting Canyons and Highways version 2.0 (MUNICH v2.0), is presented. The comprehensive aerosol model SSH-aerosol is implemented in MUNICH v2.0 to simulate the street concentrations of multiple pollutants, including secondary aerosols. The implementation uses the application programming interface (API) technology so that the SSH-aerosol version may be easily updated. New parameterisations are also introduced in MUNICH v2.0, including a non-stationary approach to model reactive pollutants, particle deposition and resuspension, and a parameterisation of the wind at roof level. A test case over a Paris suburb is presented for model evaluation and to illustrate the impact of the new functionalities. The implementation of SSH-aerosol leads to an increase of 11 % in PM10 concentration because of secondary aerosol formation. Using the non-stationary approach rather than the stationary one leads to a decrease in NO2 concentration of 16 %. The impact of particle deposition on built surfaces and road resuspension on pollutant concentrations in the street canyons is low.</p

Population exposure to outdoor NO2, black carbon, particle mass, and number concentrations over Paris with multi-scale modelling down to the street scale

This study focuses on mapping the concentrations of pollutants of health interest (NO2, black carbon (BC), PM2.5, number of particles (PN)) down to the street scale to represent as accurately as possible the population exposure. Simulations are performed over the Greater Paris area with the WRF-CHIMERE/MUNICH/SSH-aerosol chain, using either the top-down inventory EMEP or the bottom-up inventory Airparif with correction of the traffic flow. The concentrations of the pollutants are higher in streets than in the regional-scale urban background, due to the strong influence of road-traffic emissions locally. Model-to-data comparisons were performed at urban background and traffic stations, and evaluated using two performance criteria from the literature. For BC, harmonized equivalent BC (eBC) concentrations were estimated from concomitant mea-surements of eBC and elemental carbon. Using the bottom-up inventory with corrected road-traffic flow, the strictest criteria are met for NO2, eBC, PM2.5, and PN. Using the EMEP top-down inventory, the strictest criteria are also met for NO2, eBC and PM2.5, but errors tend to be larger than with the bottom-up inventory for NO2, eBC and PN. Using the top-down inventory, the concentrations tend to be lower along the streets than those simulated using the bottom-up inventory, especially for NO2 con-centrations, resulting in less urban heterogeneities. The impact of the size-distribution of non-exhaust emissions was analyzed at both regional and local scales, and it is higher in heavy-traffic streets. To assess exposure, a french database detailing the number of inhabitants in each building was used. The population-weighted concentration (PWC) was calculated by weighting populations by the outdoor concentrations to which they are exposed at the precise location of their home. An exposure scaling factor (ESF) was determined for each pollutant to estimate the ratio needed to correct urban background concentrations in order to assess exposure. The average ESF in Paris and Paris Ring Road is higher than 1 for NO2, eBC, PM2.5, PN, because the concentrations simulated at the local scale in streets are higher than those modelled at the regional scale. It indicates that the Parisian population exposure is under-estimated using regional-scale concentrations. Although this underestimation is low for PM2.5, with an ESF of 1.04, it is very high for NO2 (1.26), eBC (between 1.22 and 1.24), and PN (1.12). This shows that urban heterogeneities are important to be considered in order to represent the population exposure to NO2, eBC, and PN, but less so for PM2.5

Modélisation de la qualité de l'air : nombre de particules

National audienceModélisation de la concentration en nombre des particules à l'échelle urbaine avec le logiciel Polyphemus pour la qualité de l'air, et de comparaison des résultats du modèle avec des résultats de campagne de mesures (e.g. megapoli) an d'élucider les principaux facteurs gouvernant le nombre de particules en zone urbaine

Development of a detailed gaseous oxidation scheme of naphthalene for secondary organic aerosol (SOA) formation and speciation

International audienceNaphthalene is the most abundant polycyclic aromatic hydrocarbon (PAH) in vehicle emissions and polluted urban areas. Its atmospheric oxidation products are oxygenated compounds that are potentially harmful for health and/or contribute to secondary organic aerosol (SOA) formation. Despite its impact on air quality, its complex structure and a lack of data mean that no detailed scheme of naphthalene gaseous oxidation for SOA formation and speciation has been established yet. This study presents the construction of the first near-explicit chemical scheme for naphthalene oxidation by OH, including kinetic and mechanistic data. The scheme redundantly represents all the classical steps of atmospheric organic chemistry (i.e., oxidation of stable species, peroxy radical formation and reaction, and alkoxy radical evolution), thus integrating fragmentation or functionalization pathways and the influence of NOx on secondary compound formation. Missing kinetic and mechanistic data were estimated using structure–activity relationships (SARs) or by analogy with existing experimental or theoretical data. The proposed chemical scheme involves 383 species (231 stable species, including 93 % of the major molar masses observed in previous experimental studies) and 484 reactions with products. A first simulation reproducing experimental oxidation in an oxidation flow reactor under high-NOx conditions shows a simulated SOA mass on the same order of magnitude as has been observed experimentally, with an error of −9 %

Modelling the impact of viscosity on Secondary Organic Aerosols formation with the SOAP thermodynamic model inside a 3D air quality model

Numerous studies (e.g. Virtanen et al., 2010) show that the partitioning between the gas and particle phases of Semi-Volatile Organic Compounds (SVOC) can differ significantly from thermodynamic equilibrium due to the high viscosity of the organicphase. However, to our knowledge the impact of organic-phase viscosity on Secondary Organic Aerosol formation has never been investigated in 3D air quality models, which usually assume that organic aerosols are not viscous. The impact of the particle viscosity on secondary organic aerosols (SOA) is studied here by coupling an air quality model to the Secondary Organic Aerosol Processor (SOAP Couvidat and Sartelet, 2015) thermodynamic model. SOAP can compute the partitioning of SVOC concentrations either by assuming thermodynamic equilibrium or by computing the dynamic evolution of concentrations according to the kinetics of condensation/evaporation and particle-phase diffusion (as a function of the organic-phase coefficient diffusion). To compute the evolution of the SVOC partitioning, organic particles are separated into several layers with the external layer at the gas/particle interface and the internal layer at the center of the particle. To accurately solve the equations of diffusion of SVOCs inside particles, a high number of layers would be needed (around 100). However, this discretization would lead to a system that is too complex to be implemented in an air quality model. In SOAP, a simplified representation was designed to represent implicitly the diffusion of SVOC inside the particle with a low number of layers (between 2 and 5). A comparison between the explicit representation and the implicit representation of diffusion is shown in Figure 1. SOAP was implemented in the 3D air quality model Polyphemus and SOA concentrations were simulated over Europe during July 2012. Concentrations were simulated assuming either that particles are not viscous (the condensation/evaporation is not limited by the diffusion inside the particle) or that particles are infinitely viscous (the diffusion inside the particle is very slow and compounds do not diffuse inside the particle). Assuming infinitely viscous particles leads to a slight increase of SOA concentrations (SVOC concentrations in the particle phase). Less volatile compounds appear to not be affected by the viscosity of particles, but the concentrations of more volatile compounds increased for viscous particles and could therefore exceed concentrations at equilibrium. Indeed, for volatile organic compounds, in the infinitely viscous simulation the compounds could condense even if the compounds would evaporate for a non-viscous aerosol