Evaluation of a three-dimensional chemical transport model (PMCAMx) in the European domain during the EUCAARI May 2008 campaign

PMCAMx-2008, a detailed three-dimensional chemical transport model (CTM), was applied to Europe to simulate the mass concentration and chemical composition of particulate matter (PM) during May 2008. The model includes a state-of-the-art organic aerosol module which is based on the volatility basis set framework treating both primary and secondary organic components as semivolatile and photochemically reactive. The model performance is evaluated against high time resolution aerosol mass spectrometer (AMS) ground and airborne measurements. Overall, organic aerosol is predicted to account for 32% of total PM<sub>1</sub> at ground level during May 2008, followed by sulfate (30%), crustal material and sea-salt (14%), ammonium (13%), nitrate (7%), and elemental carbon (4%). The model predicts that fresh primary OA (POA) is a small contributor to organic PM concentrations in Europe during late spring, and that oxygenated species (oxidized primary and biogenic secondary) dominate the ambient OA. The Mediterranean region is the only area in Europe where sulfate concentrations are predicted to be much higher than the OA, while organic matter is predicted to be the dominant PM<sub>1</sub> species in central and northern Europe. The comparison of the model predictions with the ground measurements in four measurement stations is encouraging. The model reproduces more than 94% of the daily averaged data and more than 87% of the hourly data within a factor of 2 for PM<sub>1</sub> OA. The model tends to predict relatively flat diurnal profiles for PM<sub>1</sub> OA in many areas, both rural and urban in agreement with the available measurements. The model performance against the high time resolution airborne measurements at multiple altitudes and locations is as good as its performance against the ground level hourly measurements. There is no evidence of missing sources of OA aloft over Europe during this period

Organic aerosol concentration and composition over Europe: insights from comparison of regional model predictions with aerosol mass spectrometer factor analysis

A detailed three-dimensional regional chemical transport model (Particulate Matter Comprehensive Air Quality Model with Extensions, PMCAMx) was applied over Europe, focusing on the formation and chemical transformation of organic matter. Three periods representative of different seasons were simulated, corresponding to intensive field campaigns. An extensive set of AMS measurements was used to evaluate the model and, using factor-analysis results, gain more insight into the sources and transformations of organic aerosol (OA). Overall, the agreement between predictions and measurements for OA concentration is encouraging, with the model reproducing two-thirds of the data (daily average mass concentrations) within a factor of 2. Oxygenated OA (OOA) is predicted to contribute 93% to total OA during May, 87% during winter and 96% during autumn, with the rest consisting of fresh primary OA (POA). Predicted OOA concentrations compare well with the observed OOA values for all periods, with an average fractional error of 0.53 and a bias equal to −0.07 (mean error = 0.9 μg m−3, mean bias = −0.2 μg m−3). The model systematically underpredicts fresh POA at most sites during late spring and autumn (mean bias up to −0.8 μg m−3). Based on results from a source apportionment algorithm running in parallel with PMCAMx, most of the POA originates from biomass burning (fires and residential wood combustion), and therefore biomass burning OA is most likely underestimated in the emission inventory. The sensitivity of POA predictions to the corresponding emissions' volatility distribution is discussed. The model performs well at all sites when the Positive Matrix Factorization (PMF)-estimated low-volatility OOA is compared against the OA with saturation concentrations of the OA surrogate species C* ≤ 0.1 μg m−3 and semivolatile OOA against the OA with C* > 0.1 μg m−3

The Finokalia Aerosol Measurement Experiment – 2008 (FAME-08): an overview

A month (4 May to 8 June 2008) of ambient aerosol, air ion and gas phase sampling (Finokalia Aerosol Measurement Experiment 2008, FAME-08) was conducted at Finokalia, on the island of Crete, Greece. The purpose of the study was to characterize the physical and chemical properties of aged aerosol and to investigate new particle formation. Measurements included aerosol and air ion size distributions, size-resolved chemical composition, organic aerosol thermal volatility, water uptake and particle optical properties (light scattering and absorption). Statistical analysis of the aerosol mass concentration variations revealed the absence of diurnal patterns suggesting the lack of strong local sources. Sulfates accounted for approximately half of the particulate matter less than 1 micrometer in diameter (PM<sub>1</sub>) and organics for 28%. The PM<sub>1</sub> organic aerosol fraction was highly oxidized with 80% water soluble. The supermicrometer particles were dominated by crustal components (50%), sea salt (24%) and nitrates (16%). The organic carbon to elemental carbon (OC/EC) ratio correlated with ozone measurements but with a one-day lag. The average OC/EC ratio for the study period was equal to 5.4. For three days air masses from North Africa resulted in a 6-fold increase of particulate matter less than 10 micrometers in diameter (PM<sub>10</sub>) and a decrease of the OC/EC ratio by a factor of 2. Back trajectory analysis, based on FLEXPART footprint plots, identified five source regions (Athens, Greece, Africa, other continental and marine), each of which influenced the PM<sub>1</sub> aerosol composition and properties. Marine air masses had the lowest PM<sub>1</sub> concentrations and air masses from the Balkans, Turkey and Eastern Europe the highest

Influence of water uptake on the aerosol particle light scattering coefficients of the Central European aerosol

The influence of aerosol water uptake on the aerosol particle light scattering was examined at the regional continental research site Melpitz, Germany. The scattering enhancement factor f(RH), defined as the aerosol particle scattering coefficient at a certain relative humidity (RH) divided by its dry value, was measured using a humidified nephelometer. The chemical composition and other microphysical properties were measured in parallel. f(RH) showed a strong variation, e.g. with values between 1.2 and 3.6 at RH=85% and λ=550 nm. The chemical composition was found to be the main factor determining the magnitude of f(RH), since the magnitude of f(RH) clearly correlated with the inorganic mass fraction measured by an aerosol mass spectrometer (AMS). Hysteresis within the recorded humidograms was observed and explained by long-range transported sea salt. A closure study using Mie theory showed the consistency of the measured parameters

Modeling of photochemical pollution in Athens, Greece. Application of the RAMS-CALGRID modeling system

The causes of the poor air quality in Athens, Greece during the severe episode of 25-26 May 1990 has been studied, using a prognostic model (RAMS) and a three-dimensional Eulerian air quality model (CALGRID). The modeling effort indicates that the main urban area of Athens exhibited high concentrations of nitrogen oxides, the main sources of which are automobiles, while the NNE suburban area exhibited high ozone concentrations, the product of photochemical activity of the primary pollutants that were transported by the sea-breeze. The application of the models also demonstrated the need for an accurate emission inventory for improved predictions of the pollutant concentrations. It was also found that a 50% reduction of the nitrogen oxide emissions will increase the ozone levels in the downtown area substantially. © 1994

Modeling of aerosol properties related to direct climate forcing

A long-term local experiment was designed with the purpose to accurately quantify aerosol parameters needed in order to estimate aerosol climate forcing at an anthropogenically perturbed continental site. Total light-scattering ωλ,bsp and backscattering ωλ,sp coefficients at wavelength λ, the hygroscopic growth factors with respect to scattering, f(RH)λ,s, and the backscatter ratio bλ are the parameters considered in the paper. Reference and controlled relative humidity nephelometry measurements were taken at a ground level field sampling station, located near Bondville Illinois (40°03′12″ N, W 88°22′19″ W). Aerosol particle chemical composition and mass particle size distributions were also measured. The target parameters were also estimated from models. The modeling approach involved a two-step process. In the first step, aerosol properties were parameterized with an approach that made use of a modified thermodynamic equilibrium model, published laboratory measurements of single hygroscopic particle properties, and empirical mixing rules. In the second step, the parameterized aerosol properties were used as inputs into a code that calculate ωλ,sp and ωλ,bsp as functions of λ, RH, particle size, and composition. Comparison between the measured and the modeled results showed that depending on the assumptions, the differences between the modeled and observed results were within 5 to 28% for f(RH)λ,s and within 22-35% for bλ at low RH and 0-20% for bλ at high RH. The temporal variation of the particle size distribution, the equilibrium state of the particles, and the hygroscopicity of the material characterized as residual were the major factors limiting the predictive ability of the models. Copyright 1998 by the American Geophysical Union

Marginal direct climate forcing by atmospheric aerosols

Previous research on the direct effect of atmospheric aerosols on climate has estimated the average radiative forcing per unit sulfate mass, and has used this average to calculate the magnitude and spatial distribution of sulfate forcing. In this paper, we posit that radiative forcing is often a nonlinear function of sulfate mass concentration. In contrast to measures of average forcing, we introduce the concept of 'marginal forcing', which is defined as the change in radiative forcing for an incremental change in sulfate concentration. A multi-component, size-resolved aerosol box model is used, which couples an aerosol chemical equilibrium model with a model for calculating radiative forcing based on Mie theory. The results for a typical nonurban continental aerosol show that total aerosol mass and radiative forcing are nonlinear functions of sulfate concentration. This nonlinearity is mainly due to the chemical interaction of sulfate with volatile inorganic components of the aerosol (ammonium, nitrate, and water). As a result, the marginal forcing varies significantly as a function of sulfate concentration; from - 550 to + 20 W (g SO4 2-)-1 at a relative humidity (RH) of 80%. Estimates of marginal forcing are strongly sensitive to RH. Absolute marginal forcing also decreases significantly with total nitrate concentration, increases with total ammonia concentration, and generally increases with temperature. We estimate that the bias in assuming a constant average forcing may cause overestimates in local continental aerosol radiative forcing by up to 50%, and in the marginal forcing by a factor of two or more. This bias is greatest at intermediate sulfate concentration, high RH, high total nitrate concentration, low total ammonia concentration( ≤ 2 μg m-3), and low temperature. Previous research on the direct effect of atmospheric aerosols on climate has estimated the average radiative forcing per unit sulfate mass, and has used this average to calculate the magnitude and spatial distribution of sulfate forcing. In this paper, we posit that radiative forcing is often a nonlinear function of sulfate mass concentration. In contrast to measures of average forcing, we introduce the concept of `marginal forcing', which is defined as the change in radiative forcing for an incremental change in sulfate concentration. A multi-component, size-resolved aerosol box model is used, which couples an aerosol chemical equilibrium model with a model for calculating radiative forcing based on Mie theory. The results for a typical nonurban continental aerosol show that total aerosol mass and radiative forcing are nonlinear functions of sulfate concentration. This nonlinearity is mainly due to the chemical interaction of sulfate with volatile inorganic components of the aerosol (ammonium, nitrate, and water). As a result, the marginal forcing varies significantly as a function of sulfate concentration, from -550 to +20 W(g SO4 2-)-1 at a relative humidity (RH) of 80%. Estimates of marginal forcing are strongly sensitive to RH. Absolute marginal forcing also decreases significantly with total nitrate concentration, increases with total ammonia concentration, and generally increases with temperature. We estimate that the bias in assuming a constant average forcing may cause overestimates in local continental aerosol radiative forcing by up to 50%, and in the marginal forcing by a factor of two or more. This bias is greatest at intermediate sulfate concentration, high RH, high total nitrate concentration, low total ammonia concentration (≥2 μg m-3), and low temperature