Florian Couvidat

Comparison of different gas-phase mechanisms and aerosol modules for simulating particulate matter formation.

ABSTRACT The effects of two gas-phase chemical kinetic mechanisms, Regional Atmospheric Chemistry Mechanism version 2 (RACM2) and Carbon-Bond 05 (CB05), and two secondary organic aerosol (SOA) modules, the Secondary Organic Aerosoi Model (SORGAM) and AER/EPRI/Caltech model (AEC), on fine (aerodynamic diameter ≤2.5 μm) particulate matter (PM2.5) formation is studied. The major sources of uncertainty in the chemistry of SOA formation are investigated. The use of all major SOA precursors and the treatment of SOA oligomerization are found to be the most important factors for SOA formation, leading to 66% and 60% more SOA, respectively. The explicit representation of high-NOx and low-NOx gas-phase chemical regimes is also important with increases in SOA of 30–120% depending on the approach used to implement the distinct SOA yields within the gas-phase chemical kinetic mechanism; further work is needed to develop gas-phase mechanisms that are fully compatible with SOA formation algorithms. The treatment of isoprene SOA as hydrophobic or hydrophilic leads to a significant difference, with more SOA being formed in the latter case. The activity coefficients may also be a major source of uncertainty, as they may differ significantly between atmospheric particles, which contain a myriad of SOA, primary organic aerosol (POA), and inorganic aerosol species, and particles formed in a smog chamber from a single precursor under dry conditions. Significant interactions exist between the uncertainties of the gas-phase chemistry and those of the SOA module. IMPLICATIONS The current state of the science is more advanced for the gas-phase chemistry of ozone formation than for the chemistry and gas/particle partitioning of particulate matter (PM) formation. As a result, there are larger uncertainties associated with aerosol modules than with gas-phase chemical kinetic mechanisms. Nevertheless, the uncertainties associated with those modules are not additive in an air quality model and there are close interactions between the gas-phase chemical mechanism and the secondary aerosol formation. In particular, the effect of the NOx regime on SOA formation should be explicitly treated in air quality models.

Limited formation of isoprene epoxydiols‐derived secondary organic aerosol under NOx‐rich environments in Eastern China

Secondary organic aerosol (SOA) derived from isoprene epoxydiols (IEPOX) has potential impacts on regional air quality and climate yet is poorly characterized under NOx-rich ambient environments. We report the first real-time characterization of IEPOX-derived SOA (IEPOX-SOA) in Eastern China in summer 2013 using comprehensive ambient measurements, along with model analysis. The ratio of IEPOX-SOA to isoprene high-NOx SOA precursors, e.g., methyl vinyl ketone and methacrolein, and the reactive uptake potential of IEPOX was lower than those generally observed in regions with prevailing biogenic emissions, low NOx levels, and high particle acidity, elucidating the suppression of IEPOX-SOA formation under NOx-rich environments. IEPOX-SOA showed high potential source regions to the south with large biogenic emissions, illustrating that the interactions between biogenic and anthropogenic emissions might have played an important role in affecting the formation of IEPOX-SOA in polluted environments in Eastern China.

Investigating the Impact of Aqueous-Phase Chemistry and Wet Deposition on Organic Aerosol Formation Using a Molecular Surrogate Modeling Approach

A molecular surrogate representation of secondary organic aerosol (SOA) formation is used to investigate the effect of aqueous-phase (in clouds and particles) chemical processing and wet deposition on SOA atmospheric concentrations. To that end, the hydrophilic/hydrophobic organic (H(2)O) model was augmented to account for several gas/aqueous-phase equilibria and aqueous-phase processes, including the formation of oxalic, glyoxilic and pyruvic acids, the oxidation of methyl vinyl ketone (MVK) and methacrolein (MACR), the formation of tetrols and organosulfates from epoxydiols (IEPOX), and further oxidation of water-soluble SOA (aging). Among those processes, SOA chemical aging and IEPOX reactions led to the most significant increases (up to 1 μg m(-3) in some areas) in SOA concentrations in a one-month summer simulation over Europe. However, large uncertainties remain in the gas/aqueous-phase partitioning of oxalic acid, MVK, and MACR. Below-cloud scavenging of SOA precursor gases and of gas-phase SVOC was found to affect SOA concentrations by up to 20%, which suggests that it should be taken into account in air quality models.

Modeled deposition of nitrogen and sulfur in Europe estimated by 14 air quality model systems: evaluation, effects of changes in emissions and implications for habitat protection

The evaluation and intercomparison of air quality models is key to reducing model errors and uncertainty. The projects AQMEII3 and EURODELTA-Trends, in the framework of the Task Force on Hemispheric Transport of Air Pollutants and the Task Force on Measurements and Modelling, respectively (both task forces under the UNECE Convention on the Long Range Transport of Air Pollution, LTRAP), have brought together various regional air quality models to analyze their performance in terms of air concentrations and wet deposition, as well as to address other specific objectives. This paper jointly examines the results from both project communities by intercomparing and evaluating the deposition estimates of reduced and oxidized nitrogen (N) and sulfur (S) in Europe simulated by 14 air quality model systems for the year 2010. An accurate estimate of deposition is key to an accurate simulation of atmospheric concentrations. In addition, deposition fluxes are increasingly being used to estimate ecological impacts. It is therefore important to know by how much model results differ and how well they agree with observed values, at least when comparison with observations is possible, such as in the case of wet deposition. This study reveals a large variability between the wet deposition estimates of the models, with some performing acceptably (according to previously defined criteria) and others underestimating wet deposition rates. For dry deposition, there are also considerable differences between the model estimates. An ensemble of the models with the best performance for N wet deposition was made and used to explore the implications of N deposition in the conservation of protected European habitats. Exceedances of empirical critical loads were calculated for the most common habitats at a resolution of 100 × 100 m2 within the Natura 2000 network, and the habitats with the largest areas showing exceedances are determined. Moreover, simulations with reduced emissions in selected source areas indicated a fairly linear relationship between reductions in emissions and changes in the deposition rates of N and S. An approximate 20 % reduction in N and S deposition in Europe is found when emissions at a global scale are reduced by the same amount. European emissions are by far the main contributor to deposition in Europe, whereas the reduction in deposition due to a decrease in emissions in North America is very small and confined to the western part of the domain. Reductions in European emissions led to substantial decreases in the protected habitat areas with critical load exceedances (halving the exceeded area for certain habitats), whereas no change was found, on average, when reducing North American emissions in terms of average values per habitat.

Precursors and formation of secondary organic aerosols fromwildfires in the Euro-Mediterranean region

This work aims at quantifying the relative contribution of secondary organic aerosol (SOA) precursors emitted by wildfires to organic aerosol (OA) formation, during summer 2007 over the Euro-Mediterranean region, where intense wildfires occurred. A new SOA formation mechanism, HOaro, including recently identified aromatic volatile organic compounds (VOCs) emitted from wildfires is developed based on smog chamber experiment measurements, under low and high-NOx regimes. The aromatic VOCs included in the mechanism are toluene, xylene, benzene, phenol, cresol, catechol, furan, naph5 thalene, methylnaphthalene, syringol, guaiacol and structurally assigned and unassigned compounds with at least 6 carbon atoms per molecule (USC>6). This mechanism HOaro is an extension of the HO (Hydrophilic/Hydrophobic organic) aerosol mechanism: the oxidation of the precursor forms surrogate species with specific thermodynamic properties (volatility, oxidation degree, affinity to water). The SOA concentrations over the Euro-Mediterranean region in summer 2007 are simulated using the chemistry transport model (CTM) Polair3D of the air-quality plateform Polyphemus, where the mechanism HOaro 10 was implemented. To estimate the relative contribution of the aromatic VOCs, intermediate, semi and low volatile organic compounds (I/S/L-VOCs) to wildfires OA concentrations, different estimations of the gaseous I/S/L-VOC emissions (from primary organic aerosol (POA) using a factor of 1.5 or from non-methanic organic gas (NMOG) using a factor of 0.36) and their ageing (one-step oxidation vs multi-generational oxidation), are also tested in the CTM. Most of the particle organic aerosol (OA) concentrations are formed from I/S/L-VOCs. In average during the summer 2007 15 and over the Euro-Mediterranean domain, they are about 10 times higher than the OA concentrations formed from VOCs. However, locally, the OA concentrations formed from VOCs can represent up to 30% of the OA concentrations from biomass burning. Amongst the VOCs, the main contributors to SOA formation are phenol, benzene and catechol (47%), USC>6 compounds (23%), and toluene and xylene (12%). Sensitivity studies of the influence of the VOCs and the I/S/L-VOCs emissions and chemical ageing mechanisms on PM2.5 concentrations show that surface PM2.5 concentrations are more sensitive to the 20 parameterization used for gaseous I/S/L-VOCs emissions than for ageing. Estimating the gaseous I/S/L-VOCs emissions from POA or from NMOG has a high impact on local surface PM2.5 concentrations (reaching -30% in Balkans, -8 to -16% in the fire plume and +8 to +16% in Greece). Considering the VOC emissions results in a moderate increase of PM2.5 concentrations 1 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1065 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 22 October 2018 c

Simulating secondary organic aerosol from anthropogenic and biogenic precursors: comparison to outdoor chamber experiments, effect of oligomerization on SOA formation and reactive uptake of aldehydes

Couvidat et al. develop updates to a secondary organic aerosol (SOA) model and report their findings on how those updates perform against chamber SOA measurements made with mixtures of biogenic and anthropogenic precursors. They find that, in general, the updates help improve the model-measurement comparison but offer nuanced insights on the role of NO, oligomerization, vapor wall losses, reactive uptake of pinonaldehyde, and particle phase on SOA formation.

Evaluation of some SOA formation schemes for the oxidation of anthropogenic gases against experiments in two outdoor chambers

Secondary organic aerosol (SOA) formation remains a challenge for the air quality modelling community. Gaps in the current knowledge of certain processes involved in SOA formation and evolution, as well as the large number of uncertainties in many parameters involved, affect the accuracy of the simulation of atmospheric SOA concentrations. We present here an evaluation of several published SOA schemes by simulating experiments conducted in two outdoor chambers, EUPHORE (Ceam, Valencia, Spain) and UF (University of Florida, Gainesville, USA). The experiments focused on the oxidation of selected anthropogenic volatile organic compounds. We compare model estimates for one-step, two-step and multi-step gas-phase oxidation schemes with SOA concentrations measured in the chambers. For all schemes, some modifications were needed to obtain a better agreement between model simulations and observations.