Aerosol load study in urban area by Lidar and numerical model

Vertical profiles of particle mass concentration in the urban canopy above the city of Lyon have been obtained from Lidar measurements of atmospheric backscattering, over a period of three days. The concentrations measured at 50 m above the ground have been compared with the mass concentration of PM10 measured by a ground-based sampler located near the Lidar site. At certain times during the measurement campaign, the Lidar concentration measurements at 50 m agree reasonably well with the concentrations at ground level but at other times the differences between the two sets of measurements are so great that they cannot be explained by possible uncertainties in the data processing. Even when the Lidar and ground-based measurements coincide, there are significant differences between the two signals. To explain these differences we have computed the trajectories of the air parcels that pass over the Lidar, using a numerical model for the wind field that takes into account surface features such as relief and changes in roughness. This analysis showed that the differences can be explained by the meteorological conditions (wind speed and direction, vertical profiles of temperature) and the positions of the different sources of particulate matter relative to the measurement site. The combination of Lidar, ground-based sampler and air mass trajectory calculations is shown to be a powerful tool for discriminating between different sources of pollution, which could be useful in enforcing an urban air quality policy.     

On the influence of meteorological input on photochemical modelling of a severe episode over a coastal area

The modelling reconstruction of the processes determining the transport and mixing of ozone and its precursors in complex terrain areas is a challenging task, particularly when local-scale circulations, such as sea breeze, take place. Within this frame, the ESCOMPTE European campaign took place in the vicinity of Marseille (south-east of France) in summer 2001. The main objectives of the field campaign were to document several photochemical episodes, as well as to constitute a detailed database for chemistry transport models intercomparison.CAMx model has been applied on the largest intense observation periods (IOP) (June 21–26, 2001) in order to evaluate the impacts of two state-of-the-art meteorological models, RAMS and MM5, on chemical model outputs. The meteorological models have been used as best as possible in analysis mode, thus allowing to identify the spread arising in pollutant concentrations as an indication of the intrinsic uncertainty associated to the meteorological input.Simulations have been deeply investigated and compared with a considerable subset of observations both at ground level and along vertical profiles. The analysis has shown that both models were able to reproduce the main circulation features of the IOP. The strongest discrepancies are confined to the Planetary Boundary Layer, consisting of a clear tendency to underestimate or overestimate wind speed over the whole domain.The photochemical simulations showed that variability in circulation intensity was crucial mainly for the representation of the ozone peaks and of the shape of ozone plumes at the ground that have been affected in the same way over the whole domain and all along the simulated period. As a consequence, such differences can be thought of as a possible indicator for the uncertainty related to the definition of meteorological fields in a complex terrain area.     

Application of ground-based Lidar for studies of the dynamics of ozone in a mountainous basin

A ground-based Differential Absorption Lidar was employed to study the dynamics of atmospheric O3 within the planetary boundary layer of a basin in the 'Fichtelgebirge' mountains, NE Bavaria. In particular, the night-time dynamics of O3 linked to the ground were investigated. The Lidar system measured vertical profiles of O3 up to 1 km above ground. For detailed analysis of the night-time dynamics of ozone, supplementary data from three ground-based stations (measuring mixing ratios of O3 and NO(x), as well as meteorological parameters) are essential. The Lidar results could be evaluated with these data from various altitudes above the basin floor. For the station with the largest (vertical) distance to the ground-based Lidar, the agreement was very good at all times. The Lidar method proved to be useful for examining the spatial distribution of O3. The observed night-time decrease of O3 at the bottom of the basin was due to deposition and to advection of air masses containing less O3 from the mountain slopes.     

Summertime photochemical ozone formation in Santiago,Chile

The city of Santiago, Chile experiences frequent high pollution episodes and as a consequence very high ozone concentrations, which are associated with health problems including increasing daily mortality and hospital admissions for respiratory illnesses. The development of ozone abatement strategies requires the determination of the potential of each pollutant to produce ozone, taking into account known mechanisms and chemical kinetics in addition to ambient atmospheric conditions. In this study, the photochemical formation of ozone during a summer campaign carried out from March 8–20, 2005 has been investigated using an urban photochemical box model based on the Master Chemical Mechanism (MCMv3.1). The MCM box model has been constrained with 10 min averages of simultaneous measurements of HONO, HCHO, CO, NO, j(O¹D), j(NO₂), 31 volatile organic compounds (VOCs) and meteorological parameters. The O₃–NO_x–VOC sensitivities have been determined by simulating ozone formation at different VOC and NO_x concentrations. Ozone sensitivity analyses showed that photochemical ozone formation is VOC-limited under average summertime conditions in Santiago. The results of the model simulations have been compared with a set of potential empirical indicator relationships including H₂O₂/HNO₃, HCHO/NO_y and O₃/NO_z. The ozone forming potential of each measured VOC has been determined using the MCM box model. The impacts of the above study on possible summertime ozone control strategies in Santiago are discussed.     

Modelling of stomatal conductance and ozone flux of Norway spruce: comparison with field data

It has been proposed that stomatal flux of ozone would provide a more reliable basis than ozone exposure indices for the assessment of the risk of ozone damage to vegetation across Europe. However, implementation of this approach requires the development of appropriate models which need to be rigorously tested against actual data collected under field conditions. This paper describes such an assessment of the stomatal component of the model described by Emberson et al. (2000. Modelling stomatal ozone flux across Europe. Environmental Pollution 110). Model predictions are compared with field measurements of both stomatal conductance (g(s)) and calculated ozone flux for shoots of mature Norway spruce (Picea abies) growing in the Tyrol Mountains in Austria. The model has been developed to calculate g(s) as a function of leaf phenology and four environmental variables: photosynthetic flux density (PFD), temperature, vapour pressure deficit (VPD) and soil moisture deficit (SMD). The model was run using climate data measured on site, although the SMD component was omitted since the necessary data were not available. The model parameterisation for Norway spruce had previously been collected from the scientific literature and therefore established independently from the measurement study. Overall, strong associations were found between model predictions and measured values of stomatal conductance to ozone (GO(3)) and calculated stomatal ozone flux (FO(3)). Average diurnal profiles of GO(3) and FO(3) showed good agreement between the field data and modelled values except during the morning period of 1990. The diurnal pattern of ozone flux was determined primarily by PFD and VPD, as there was little diurnal variation in ozone concentration. In general, the model predicted instances of high ozone flux satisfactorily, indicating its potential applicability in identifying areas of high ozone risk for this species.     

Daytime resolved analysis of polycyclic aromatic hydrocarbons in urban aerosol samples - impact of sources and meteorological conditions

Urban aerosol was collected in a summer and a winter campaign for 7 and 3 days, respectively. Low volume samples were taken with a time resolution of 160 min using a filter/sorption cartridge system extended by an ozone scrubber. Concentrations of mainly particle associated polycyclic aromatic hydrocarbons (PAH) and oxidised PAH (O-PAH) were determined by gas chromatography/high resolution mass spectrometry. The sampling site was located in the city centre of Augsburg, Germany, near major roads with high traffic volume. The daily concentrations and profiles were mainly governed by local emissions from traffic and domestic heating, as well as by the meteorological conditions. During the winter campaign, concentrations were more than 10 fold higher than during the summer campaign. Highest concentrations were found concurrent with low boundary layer heights and low wind speeds. Significant diurnal variation of the PAH profiles was observed. Enhanced influences of traffic related PAH on the PAH profiles were evident during daytime in summer, whereas emissions from hot water generation and domestic heating were obvious during the night time of both seasons. A general idea about the global meteorological situation was acquired using back trajectory calculations (NOAA ARL HYSPLIT4). Due to high local emissions in combination with low air exchange during the two sampling campaigns, effects of mesoscale transport were not clearly observable.     

Nocturnal boundary layer characteristics and land breeze development in Houston,Texas during TexAQS II

The nocturnal boundary layer in Houston, Texas was studied using a high temporal and vertical resolution tethersonde system on four nights during the Texas Air Quality Study II (TexAQS II) in August and September 2006. The launch site was on the University of Houston campus located approximately 4 km from downtown Houston. Of particular interest was the evolution of the nocturnal surface inversion and the wind flows within the boundary layer. The land–sea breeze oscillation in Houston has important implications for air quality as the cycle can impact ozone concentrations through pollutant advection and recirculation. The results showed that a weakly stable surface inversion averaging in depth between 145 and 200 m AGL formed on each of the experiment nights, typically within 2–3 h after sunset. Tethersonde vertical winds were compared with two other Houston data sets (High Resolution Doppler Lidar and radar wind profiler) from locations near the coastline and good agreement was found, albeit with a temporal lag at the tethersonde site. This comparison revealed development of a land breeze on three nights which began near the coastline and propagated inland both horizontally and vertically with time. The vertical temperature structure was significantly modified on one night at the tethersonde site after the land breeze wind shift, exhibiting near-adiabatic profiles below 100 m AGL.     

Study of atmospheric ozone formation by means of a neural network-based model

The occurrence of high ozone levels in the atmosphere of urban areas has become a serious pollution problem in a number of large cities in the world. Although mathematical models have been proposed for predicting ozone concentrations as a function of a number of gas components, sometimes there are uncertainties due to lack of the combined effects of meteorological factors and the complex chemical reaction system involved. The application of neural network models, based on measured values of air pollutants and meteorological factors at different locations within the S?o Paulo Metropolitan Area, combine chemical and meteorological information. This has shown to be a promising tool for predicting ozone concentration. Simulations carried out with the model indicate the sensitivity of ozone in relation to different air pollution and weather conditions. Predictions using this model have shown good agreement with measured values of ozone concentrations.     

Empirical Evaluation of Ambient Ozone Interpolation Procedures to Support Exposure Models

ABSTRACT

An intercomparison study has been performed with six empirical ozone interpolation procedures to predict hourly concentrations in ambient air between monitoring stations. The objective of the study is to use monitoring network data to empirically identify an improved procedure to estimate ozone concentrations at subject exposure points. Four of the procedures in the study are currently used in human exposure models (nearest monitors daily mean and maximum, regression estimate used in the U.S. Environmental Protection Agency's (EPA) pNEM, and inverse distance weighting), and two are being evaluated for this purpose (kriging in space and kriging in space and time). The study focused on spatial estimation during June 1-June 5, 1996, with relatively high observed ozone levels over Houston, Texas. The study evaluated these procedures at three types of locations with monitors of varying proximity. Results from the empirical evaluation indicate that kriging in space and time provides excellent estimates of ozone concentrations within a monitoring network, while the more often used techniques failed to capture observed pollutant concentrations. Improved estimation of pollutant concentrations within the region, and thus at subject locations, should result in improved exposure modeling.     

Ground-based remote sensing measurements of aerosol and ozone in an urban area: A case study of mixing height evolution and its effect on ground-level ozone concentrations

We have estimated the mixing height (MH) and investigated the relationship between vertical mixing and ground-level ozone concentrations in Seoul, Korea, by using three ground-based active remote sensing instruments operating side by side: micro-pulse lidar (MPL), differential absorption lidar (DIAL), and differential optical absorption spectroscopy (DOAS). The MH is estimated from MPL measurements of aerosol extinction profiles by the gradient method under convective conditions. Comparisons of the MHs estimated from MPL and radiosonde measurements show a good agreement (r²=0.99). Continuous MPL measurements with high temporal and vertical resolution reveal the diurnal variations of the MH under convective conditions and the presence of a residual layer during the nighttime. Comprehensive measurements of ozone and aerosol by MPL, DIAL and DOAS during an high ozone episode (24–26 May 2000) in Seoul, Korea, reveal that (1) photochemical ozone production and advection from upwind regions (the western part of Seoul) contribute two peaks of ozone concentrations at the ground around 14:00 and 18:00 local time on 25 May 2000, respectively, and (2) the entrainment and the fumigation processes of ozone aloft in the nighttime residual layer into the ground is a major contributor of high concentrations of ground-level ozone observed on the following day (26 May 2000).     

Modelling stomatal ozone flux across Europe

A model has been developed to estimate stomatal ozone flux across Europe for a number of important species. An initial application of this model is illustrated for two species, wheat and beech. The model calculates ozone flux using European Monitoring and Evaluation Programme (EMEP) model ozone concentrations in combination with estimates of the atmospheric, boundary layer and stomatal resistances to ozone transfer. The model simulates the effect of phenology, irradiance, temperature, vapour pressure deficit and soil moisture deficit on stomatal conductance. These species-specific microclimatic parameters are derived from meteorological data provided by the Norwegian Meteorological Institute (DNMI), together with detailed land-use and soil type maps assembled at the Stockholm Environment Institute (SEI). Modelled fluxes are presented as mean monthly flux maps and compared with maps describing equivalent values of AOT40 (accumulated exposure over threshold of 40 ppb or nl l(-1)), highlighting the spatial differences between these two indices. In many cases high ozone fluxes were modelled in association with only moderate AOT40 values. The factors most important in limiting ozone uptake under the model assumptions were vapour pressure deficit (VPD), soil moisture deficit (for Mediterranean regions in particular) and phenology. The limiting effect of VPD on ozone uptake was especially apparent, since high VPDs resulting in stomatal closure tended to co-occur with high ozone concentrations. Although further work is needed to link the ozone uptake and deposition model components, and to validate the model with field measurements, the present results give a clear indication of the possible implications of adopting a flux-based approach for future policy evaluation.     

Issues with ozone attainment methodology for Houston, TX

To comply with the federal 8-hr ozone standard, the state of Texas is creating a plan for Houston that strictly follows the U.S. Environmental Protection Agency's (EPA) guidance for demonstrating attainment. EPA's attainment guidance methodology has several key assumptions that are demonstrated to not be completely appropriate for the unique observed ozone conditions found in Houston. Houston's ozone violations at monitoring sites are realized as gradual hour-to-hour increases in ozone concentrations, or by large hourly ozone increases that exceed up to 100 parts per billion/hr. Given the time profiles at the violating monitors and those of nearby monitors, these large increases appear to be associated with small parcels of spatially limited plumes of high ozone in a lower background of urban ozone. Some of these high ozone parcels and plumes have been linked to a combination of unique wind conditions and episodic hydrocarbon emission events from the Houston Ship Channel. However, the regulatory air quality model (AQM) does not predict these sharp ozone gradients. Instead, the AQM predicts gradual hourly increases with broad regions of high ozone covering the entire Houston urban core. The AQM model performance can be partly attributed to EPA attainment guidance that prescribes the removal in the baseline model simulation of any episodic hydrocarbon emissions, thereby potentially removing any nontypical causes of ozone exceedances. This paper shows that attainment of all monitors is achieved when days with observed large hourly variability in ozone concentrations are filtered from attainment metrics. Thus, the modeling and observational data support a second unique cause for how ozone is formed in Houston, and the current EPA methodology addresses only one of these two causes.     

OH and HO2 radical chemistry in a forested region of north-western Greece

This paper explores several aspects of the chemistry of a forested region in north-western Greece, from data collected during the AEROBIC97 campaign. An observationally constrained box model has been constructed to enable comparisons between modelled concentrations of OH and HO₂ and those determined by the fluorescence assay by gas expansion (FAGE) technique. These results represent the first comparison of measured and modelled OH concentrations in such an environment. The modelled OH concentrations are, on average,∼50% of those measured (range of 16–61%) over 4 days of model and measurement comparison. Possible reasons for the model-measurement discrepancy are discussed. A rate of production analysis illustrates the dominance of isoprene and the monoterpenes on OH loss, as well as the significance of the ozonolysis of biogenic species as an OH source. The measured and modelled [HO₂]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than has been found previously for similar NO_x concentrations,∼75 and 340, respectively, cf. 10–20. The high ratio reflects the rapid recycling through the OH–HO₂ oxidation chain, involving biogenic species. The high biogenic concentrations result in a midday OH lifetime of∼0.15 s. Finally, for the conditions encountered during the campaign, there is high net photochemical ozone production, peaking at∼20 ppbv h⁻¹ around 09:00 h.     

Development and application of a regional-scale atmospheric mercury model based on WRF/Chem: a Mediterranean area investigation

The emission, transport, deposition and eventual fate of mercury (Hg) in the Mediterranean area has been studied using a modified version of the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem). This model version has been developed specifically with the aim to simulate the atmospheric processes determining atmospheric Hg emissions, concentrations and deposition online at high spatial resolution. For this purpose, the gas phase chemistry of Hg and a parametrised representation of atmospheric Hg aqueous chemistry have been added to the regional acid deposition model version 2 chemical mechanism in WRF/Chem. Anthropogenic mercury emissions from the Arctic Monitoring and Assessment Programme included in the emissions preprocessor, mercury evasion from the sea surface and Hg released from biomass burning have also been included. Dry and wet deposition processes for Hg have been implemented. The model has been tested for the whole of 2009 using measurements of total gaseous mercury from the European Monitoring and Evaluation Programme monitoring network. Speciated measurement data of atmospheric elemental Hg, gaseous oxidised Hg and Hg associated with particulate matter, from a Mediterranean oceanographic campaign (June 2009), has permitted the model’s ability to simulate the atmospheric redox chemistry of Hg to be assessed. The model results highlight the importance of both the boundary conditions employed and the accuracy of the mercury speciation in the emission database. The model has permitted the reevaluation of the deposition to, and the emission from, the Mediterranean Sea. In light of the well-known high concentrations of methylmercury in a number of Mediterranean fish species, this information is important in establishing the mass balance of Hg for the Mediterranean Sea. The model results support the idea that the Mediterranean Sea is a net source of Hg to the atmosphere and suggest that the net flux is ≈30 Mg year^?1 of elemental Hg.     

Predicting ozone levels

This paper presents a statistical model that is capable of predicting ozone levels from precursor concentrations and meteorological conditions during daylight hours in the Shuaiba Industrial Area (SIA) of Kuwait. The model has been developed from ambient air quality data that was recorded for one year starting from December 1994 using an air pollution mobile monitoring station. The functional relationship between ozone level and the various independent variables has been determined by using a stepwise multiple regression modelling procedure. The model contains two terms that describe the dependence of ozone on nitrogen oxides (NOx) and nonmethane hydrocarbon precursor concentrations, and other terms that relate to wind direction, wind speed, sulphur dioxide (SO₂) and solar energy. In the model, the levels of the precursors are inversely related to ozone concentration, whereas SO₂ concentration, wind speed and solar radiation are positively correlated. Typically, 63 % of the variation in ozone levels can be explained by the levels of NOx. The model is shown to be statistically significant and model predictions and experimental observations are shown to be consistent. A detailed analysis of the ozone-temperature relationship is also presented; at temperatures less than 27 °C there is a positive correlation between temperature and ozone concentration whereas at temperatures greater than 27 °C a negative correlation is seen. This is the first time a non-monotonic relationship between ozone levels and temperature has been reported and discussed.     

Deposition of ozone to a mature spruce forest: measurements and comparison to models

Removal of ozone at terrestrial surfaces provides a major sink for tropospheric ozone and, therefore, a constraint on the peak concentrations achieved during photochemical episodes. This study reports results from 5 years of almost continuous measurements of vertical profiles of ozone and related meteorological variables over a mature spruce forest in Bavaria. Deposition velocities calculated from flux/gradient and eddy correlation flux measurements have been compared with estimates based on a resistance model and yield satisfactory agreement during fine weather conditions. The results also suggest that biogenic emissions of reactive hydrocarbons from the forest influence the vertical profile of ozone.     

Using a reduced Common Representative Intermediates (CRIv2-R5) mechanism to simulate tropospheric ozone in a 3-D Lagrangian chemistry transport model

A reduced chemical scheme (CRIv2-R5) which describes ozone formation from the tropospheric degradation of methane and 22 emitted non-methane hydrocarbons and oxygenated volatile organic compounds has been applied in a global-3D chemistry transport model (STOCHEM). The scheme, which contains 220 species in 609 reactions, has been used to simulate ozone and its precursors for the meteorological year of 1998 and the results have been compared with those from STOCHEM runs with its original chemistry. Compared with the original chemistry scheme, the degradation of a larger number of more reactive VOCs in the CRI scheme results in the formation (and their consequent transportation) of more NO_x active reservoirs thus leading to formation of more ozone away from land-based sources. Conversely, the more reactive VOCs also lead to greater removal of OH in continental areas and greater formation of OH in marine environments. STOCHEM run with the CRI scheme simulates more ozone (by up to 10 ppb), which results in better agreement with observed vertical ozone profiles. The CRI scheme transforms the globally and annually integrated ozone budget for the considered year in STOCHEM from a net loss of ?55 Tg yr^?1 to a net gain of +50 Tg yr^?1.     

Boundary layer photochemistry simulated with a two-stream convection scheme

We explore the sensitivity of the simulation of photochemical smog to the turbulent mixing scheme, using two diffusion schemes and an original two-stream model (TSM) scheme, assuming in the column an updraft and a downdraft. In this latter scheme both updraft and downdraft concentrations are prognostic variables, unlike in previously proposed schemes. The comparisons are made using a one-dimensional column model, in a Eulerian or a Lagrangian mode. The diffusion schemes produce tilted concentration profiles for primary species, with higher concentrations near the surface and lower values at the top of the boundary layer, while TSM profiles yield more homogeneous concentrations in the planetary boundary layer (PBL). Ozone concentrations are also more homogeneous in the TSM PBL than in the diffusive PBL. Only deposition makes ozone concentrations slightly lower near the surface, while in diffusive case ozone is lower also due to titration by higher nitrogen oxide concentrations. The overall differences between the schemes remain small for ozone.Also, the development time and amplitude of an ozone city plume is not very sensitive to the choice of the mixing scheme. In the urban framework ozone build-up is slightly delayed by higher nitrogen oxide concentrations near the surface in the diffusive cases, but the plume development is similar to that of the TSM once the plume travels away from the emission area. Results also show that the sensitivity of ozone to nitrogen oxide and non-methane volatile organic compounds is itself not very sensitive to the mixing scheme.     

Lidar and satellite retrieval of dust aerosols over the Azores during SOFIA/ASTEX

The synergy between active (airborne lidar) and passive (Meteosat) sensors is achieved with the help of a numerical transport model (TM2z) to derive optical properties of Saharan dust during a long range transport over the Azores. Measurements were taken in June 1992 during the surface of the ocean, fluxes and interaction with the atmosphere campaign, which took place during the Atlantic stratocumulus transition experiment. The dust source is identified to be in north Morocco from a TM2z back-trajectory analysis. Lidar observations over the Azores show that the dust is maintained in multiple thin layers (few hundred meters) up to 5 km altitude after a 4-day transport. Horizontal gradients are less marked, with a typical scale of variation of about 5 km. Lidar inversions yield dust optical thicknesses from 0.1 to 0.16 mainly due to two layers centered at 1.3 and 3.7 km. Since the weather was extremely cloudy over this region, the dust plume was not observable on the coincident Meteosat image. We thus processed the image taken two days earlier that clearly shows a dust plume between Azores and Spain. The Meteosat inversion was constrained by using the airborne lidar measurements in the marine boundary layer. The retrieved dust optical thicknesses are in good agreement with that retrieved from the lidar. Coherence of both lidar and radiometry measurements suggests that such a combined analysis is promising for retrieving the optical thickness of elevated dust layers as well as their spatial extent outside the source region even under cloudy conditions.