Items

Advanced search

GEM/POPs: a global 3-D dynamic model for semi-volatile persistent organic pollutants – Part 1: Model description and evaluations of air concentrations

GEM/POPs was developed to simulate the transport, deposition and partitioning of semi-volatile persistent organic pollutants (POPs) in the atmosphere within the framework of Canadian weather forecasting model GEM. In addition to the general processes such as anthropogenic emissions, atmosphere/water and atmosphere/soil exchanges, GEM/POPs incorporates a dynamic aerosol module to provide the aerosol surface areas for the semi-volatile POPs to partition between gaseous and particle phases and a mechanism for particle-bound POPs to be removed. Simulation results of three PCBs (28, 153 and 180) for the year 2000 indicate that the model captured the main features of global atmospheric PCBs when compared with observations from EMEP, IADN and Alert stations. The annual averaged concentrations and the fractionation of the three PCBs as a function of latitudes agreed reasonably well with observations. The impacts of atmospheric aerosols on the transports and partitioning of the three PCBs are reasonably simulated. The ratio of particulate to gaseous PCBs in the atmospheric column ranges from less than 0.1 for PCB28 to as high as 100 for PCB180, increasing from the warm lower latitudes to the cold high latitudes. Application of GEM/POPs in a study of the global transports and budgets of various PCBs accompanies this paper.
Atmospheric effects of volcanic eruptions as seen by famous artists and depicted in their paintings

Paintings created by famous artists, representing sunsets throughout the period 1500–1900, provide proxy information on the aerosol optical depth following major volcanic eruptions. This is supported by a statistically significant correlation coefficient (0.8) between the measured red-to-green ratios of a few hundred paintings and the dust veil index. A radiative transfer model was used to compile an independent time series of aerosol optical depth at 550 nm corresponding to Northern Hemisphere middle latitudes during the period 1500–1900. The estimated aerosol optical depths range from 0.05 for background aerosol conditions, to about 0.6 following the Tambora and Krakatau eruptions and cover a period practically outside of the instrumentation era.
Predicting diurnal variability of fine inorganic aerosols and their gas-phase precursors near downtown Mexico City

Partitioning of semi-volatile nitrate and ammonium between the gas and particulate phases is studied combining two thermodynamic models that explicitly include crustal elements and simulate both branches (deliquescence, efflorescence) of aerosol behavior and measurements taken near downtown Mexico City during a field campaign conducted in February–March, 2005. Overall, no significant differences between model predictions (within 30% of error) are observed for particulate ammonium (PM2.5, PM1). In cases of moderate to high RH (40–70%), mostly occurring during the 1st and 2nd daily sampling periods (06:00–10:00 h, 10:00–14:00 h, LST), 4 h PM2.5 nitrate measurements are predicted within 30%. When RH drops below 30%, characteristic of the afternoon sampling periods (14:00-18:00 h), the efflorescence branch is most consistent with observed PM nitrate. Residual error analysis of these low RH cases suggest that aerosol nitrate loading or sulfate-to-nitrate molar ratio control phase behavior, hence the partitioning of semi-volatile PM2.5 nitrate in gas and particulate phases. Finally, inclusion of crustal elements in the modeling framework reduces the error in predicted PM2.5 ammonium by 25%. These findings, if generally applicable, can help improve air quality modeling in nitrate deficient environments.
Wildfire particulate matter in Europe during summer 2003: meso-scale modeling of smoke emissions, transport and radiative effects

The present study investigates effects of wildfire emissions on air quality in Europe during an intense fire season that occurred in summer 2003. A meso-scale chemistry transport model CHIMERE is used, together with ground based and satellite aerosol optical measurements, to assess the dispersion of fire emissions and to quantify the associated radiative effects. The model has been improved to take into account a MODIS-derived daily smoke emission inventory as well as the injection altitude of smoke particles. The simulated aerosol optical properties are put into a radiative transfer model to estimate (off-line) the effects of smoke particles on photolysis rates and atmospheric radiative forcing. We have found that the simulated wildfires generated comparable amounts of primary aerosol pollutants (130 kTons of PM2.5, fine particles) to anthropogenic sources during August 2003, and caused significant changes in aerosol optical properties not only close to the fire source regions, but also over a large part of Europe as a result of the long-range transport of the smoke. Including these emissions into the model significantly improved its performance in simulating observed aerosol concentrations and optical properties. Quantitative comparison with MODIS and POLDER data during the major fire event (3–8 August 2003) showed the ability of the model to reproduce high aerosol optical thickness (AOT) over Northern Europe caused by the advection of the smoke plume from the Portugal source region. Although there was a fairly good spatial agreement with satellite data (correlation coefficients ranging from 0.4 to 0.9), the temporal variability of AOT data at specific AERONET locations was not well captured by the model. Statistical analyses of model-simulated AOT data at AERONET ground stations showed a significant decrease in the model biases suggesting that wildfire emissions are responsible for a 30% enhancement in mean AOT values during the heat-wave episode. The implications for air quality over a large part of Europe are significant during this episode. First, directly, the modeled wildfire emissions caused an increase in average PM2.5 ground concentrations from 20 to 200%. The largest enhancement in PM2.5 concentrations stayed, however, confined within a 200 km area around the fire source locations and reached up to 40 μg/m³. Second, indirectly, the presence of elevated smoke layers over Europe significantly altered atmospheric radiative properties: the model results imply a 10 to 30% decrease in photolysis rates and an increase in atmospheric radiative forcing of 10–35 W m−2 during the period of strong fire influence throughout a large part of Europe. These results suggest that sporadic wildfire events may have significant effects on regional photochemistry and atmospheric stability, and need to be considered in current chemistry-transport models.
Evaluation and modeling of the size fractionated aerosol particle number concentration measurements nearby a major road in Helsinki – Part II: Aerosol measurements within the SAPPHIRE project

This study presents an evaluation and modeling exercise of the size fractionated aerosol particle number concentrations measured nearby a major road in Helsinki during 23 August–19 September 2003 and 14 January–11 February 2004. The available information also included electronic traffic counts, on-site meteorological measurements, and urban background particle number size distribution measurement. The ultrafine particle (UFP, diameter<100 nm) number concentrations at the roadside site were approximately an order of magnitude higher than those at the urban background site during daytime and downwind conditions. Both the modal structure analysis of the particle number size distributions and the statistical correlation between the traffic density and the UFP number concentrations indicate that the UFP were evidently from traffic related emissions. The modeling exercise included the evolution of the particle number size distribution nearby the road during downwind conditions. The model simulation results revealed that the evaluation of the emission factors of aerosol particles might not be valid for the same site during different time.
Evaluation and modelling of the size fractionated aerosol particle number concentration measurements nearby a major road in Helsinki – Part I: Modelling results within the LIPIKA project

A field measurement campaign was conducted near a major road "Itäväylä" in an urban area in Helsinki in 17–20 February 2003. Aerosol measurements were conducted using a mobile laboratory "Sniffer" at various distances from the road, and at an urban background location. Measurements included particle size distribution in the size range of 7 nm–10 μm (aerodynamic diameter) by the Electrical Low Pressure Impactor (ELPI) and in the size range of 3–50 nm (mobility diameter) by Scanning Mobility Particle Sizer (SMPS), total number concentration of particles larger than 3 nm detected by an ultrafine condensation particle counter (UCPC), temperature, relative humidity, wind speed and direction, driving route of the mobile laboratory, and traffic density on the studied road. In this study, we have compared measured concentration data with the predictions of the road network dispersion model CAR-FMI used in combination with an aerosol process model MONO32. For model comparison purposes, one of the cases was additionally computed using the aerosol process model UHMA, combined with the CAR-FMI model. The vehicular exhaust emissions, and atmospheric dispersion and transformation of fine and ultrafine particles was evaluated within the distance scale of 200 m (corresponding to a time scale of a couple of minutes). We computed the temporal evolution of the number concentrations, size distributions and chemical compositions of various particle size classes. The atmospheric dilution rate of particles is obtained from the roadside dispersion model CAR-FMI. Considering the evolution of total number concentration, dilution was shown to be the most important process. The influence of coagulation and condensation on the number concentrations of particle size modes was found to be negligible on this distance scale. Condensation was found to affect the evolution of particle diameter in the two smallest particle modes. The assumed value of the concentration of condensable organic vapour of 1012 molecules cm−3 was shown to be in a disagreement with the measured particle size evolution, while the modelling runs with the concentration of condensable organic vapour of 109–1010 molecules cm−3 resulted in particle sizes that were closest to the measured values.
Isoprene and monoterpene measurements in a secondary forest in northern Benin

The biogenic volatile organic compound (BVOC) composition of ambient air at a rural field site near Djougou, Benin has been studied as part of the AMMA (African Monsoon Multidisciplinary Analysis) project. Ambient air was sampled during day and night during the period 2 June 2006 to 13 June 2006. Gas samples from within the forest canopy and from branch and cuvette enclosure systems for four vegetation species were also obtained and emissions flux estimates made. All samples were analysed for the presence of isoprene, monoterpenes and sesquiterpenes by either gas chromatography-time of flight mass spectrometry (GC-TOF/MS) or comprehensive gas chromatography-time of flight mass spectrometry (GCxGC-TOF/MS). Concentrations of isoprene ranged from a few tens of pptV to in excess of 3000 pptV. Similar concentration ranges for certain monoterpenes were also observed. Limonene was seen at a maximum concentration in ambient air of 5000 pptV. The combination of leaf-level observations and direct analysis of dried vegetation samples suggests that emissions of terpene species from indigenous species are unlikely to account for the unexpectedly high ambient concentrations of monoterpenes. Leaf scale emission measurements and biological sample analysis indicated that extit{Anacardium occidentale}, a non-native crop species found throughout the tropics, was the dominant source of monoterpenes at this location. These preliminary findings suggest that activities involving species replacement have potential implications for the chemistry of the African troposphere that have not been widely considered previously.
Satellite measurements of the global mesospheric sodium layer

Optimal estimation theory is used to retrieve the absolute Na density profiles in the mesosphere/lower thermosphere from limb-scanning measurements of the Na radiance at 589 nm in the dayglow. Two years of observations (2003 and 2004), recorded by the OSIRIS spectrometer on the Odin satellite, have been analysed to yield the seasonal and latitudinal variation of the Na layer column abundance, peak height, and peak width. The layer shows little seasonal variation at low latitudes, but the winter/summer ratio increases from a factor of ~3 at mid-latitudes to ~10 in the polar regions. Comparison of the measurements made at about 06:00 and 18:00 LT shows little diurnal variation in the layer, apart from the equatorial region where, during the equinoxes, there is a two-fold increase in Na density below 94 km between morning and evening. This is most likely caused by the strong downward wind produced by the diurnal tide between ~02:00 and 10:00 LT. The dramatic removal of Na below 85 km at latitudes above 50° during summer is explained by the uptake of sodium species on the ice surfaces of polar mesospheric clouds, which were simultaneously observed by the Odin satellite.
Comparison and synergy of stratospheric ozone measurements by satellite limb sounders and the ground-based microwave radiometer SOMORA

Stratospheric O3 profiles obtained by the satellite limb sounders Aura/MLS, ENVISAT/MIPAS, ENVISAT/GOMOS, SAGE-II, SAGE-III, UARS/HALOE are compared to coincident O3 profiles of the ground-based microwave radiometer SOMORA in Switzerland. Data from the various measurement techniques are within 10% at altitudes below 45 km. At altitudes 45–60 km, the relative O3 differences are within a range of 50%. Larger deviations at upper altitudes are attributed to larger relative measurement errors caused by lower O3 concentrations. The spatiotemporal characteristics of the O3 differences (satellite – ground station) are investigated by analyzing about 2300 coincident profile pairs of Aura/MLS (retrieval version 1.5) and SOMORA. The probability density function of the O3 differences is represented by a Gaussian normal distribution. The dependence of the O3 differences on the horizontal distance between the sounding volumes of Aura/MLS and SOMORA is derived. While the mean bias (Aura/MLS – SOMORA) is constant with increasing horizontal distance (up to 800 km), the standard deviation of the O3 differences increases from around 8 to 11% in the mid-stratosphere. Geographical maps yield azimuthal dependences and horizontal gradients of the O3 difference field around the SOMORA ground station. Coherent oscillations of O3 are present in the time series of Aura/MLS and SOMORA (e.g., due to traveling planetary waves). Ground- and space-based measurements often complement one another. We discuss the double differencing technique which allows both the cross-validation of two satellites by means of a ground station and the cross-validation of distant ground stations by means of one satellite. Temporal atmospheric noise in the geographical ozone map over Payerne is significantly reduced by combination of the data from SOMORA and Aura/MLS. These analyses illustrate the synergy of ground-based and space-based measurements.
Kinetic modeling of secondary organic aerosol formation: effects of particle- and gas-phase reactions of semivolatile products

The distinguishing mechanism of formation of secondary organic aerosol (SOA) is the partitioning of semivolatile hydrocarbon oxidation products between the gas and aerosol phases. While SOA formation is typically described in terms of partitioning only, the rate of formation and ultimate yield of SOA can also depend on the kinetics of both gas- and aerosol-phase processes. We present a general equilibrium/kinetic model of SOA formation that provides a framework for evaluating the extent to which the controlling mechanisms of SOA formation can be inferred from laboratory chamber data. With this model we examine the effect on SOA formation of gas-phase oxidation of first-generation products to either more or less volatile species, of particle-phase reaction (both first- and second-order kinetics), of the rate of parent hydrocarbon oxidation, and of the extent of reaction of the parent hydrocarbon. The effect of pre-existing organic aerosol mass on SOA yield, an issue of direct relevance to the translation of laboratory data to atmospheric applications, is examined. The importance of direct chemical measurements of gas- and particle-phase species is underscored in identifying SOA formation mechanisms.