Author's personal copy Atmospheric Environment 44 (2010) 1152e1161 Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Aerosol load study in urban area by Lidar and numerical model A. Miffre a, M. Abou Chacra a, S. Geffroy a, P. Rairoux a,*, L. Soulhac b, R.J. Perkins b, E. Frejafon c a Université Lyon 1, LASIM, CNRS UMR 5579, F-69622 Villeurbanne cedex, France b Laboratoire de Mécanique des Fluides et d'Acoustique, CNRS UMR 5509 Université de Lyon, Ecole Centrale de Lyon, F-69134 Ecully cedex, France c Institut National de l'Environnement Industriel et des Risques, F-60550 Verneuil en Halatte, France article info abstract Article history: Vertical profiles of particle mass concentration in the urban canopy above the city of Lyon have been Received 1 August 2009 obtained from Lidar measurements of atmospheric backscattering, over a period of three days. The Received in revised form concentrations measured at 50 m above the ground have been compared with the mass concentration of 18 December 2009 PM10 measured by a ground-based sampler located near the Lidar site. At certain times during the Accepted 23 December 2009 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 Keywords: are so great that they cannot be explained by possible uncertainties in the data processing. Even when Urban aerosol fi UBL the Lidar and ground-based measurements coincide, there are signi cant differences between the two PM10 signals. To explain these differences we have computed the trajectories of the air parcels that pass over Lidar the Lidar, using a numerical model for the wind field that takes into account surface features such as Numerical model 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 discrim- inating between different sources of pollution, which could be useful in enforcing an urban air quality policy. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Boundary Layer (PBL) has become a key issue for atmospheric chemistry, atmospheric surveys and for climate change modelling. Atmospheric aerosols are one of the greatest sources of uncer- Regional scale studies of atmospheric chemistry such as INDOEX tainty in climate change modelling, responsible for direct and (Satheeshl and Ramanathan, 2000), ESCOMPTE (Dobrinski et al., indirect radiative forcing (IPCC, 2007). They play an important role 2007), ESQUIF (Vautard et al., 2003), CHABLAIS (Beniston et al., in the complex physical and chemical processes involved in the 1990) have all highlighted the importance of the spatial distribu- photochemical reactions that affect air quality in polluted areas tion of atmospheric aerosols in explaining the chemical trans- (Ravishankara, 2005). Recent studies have shown that aggregation formations in the atmosphere. In this context, remote sensing by and nucleation processes can occur everywhere (Kulmala and Lidar can provide valuable information; vertical profiles of the Kerminen, 2008) and especially in urban areas, where they atmosphere with high temporal and spatial resolution provide real- contribute to the formation of primary and secondary organic time dynamic visualization of the PBL height and vertical profiles of aerosols in both condensation and accumulation modes (Baltens- aerosol density changes, which can be compared with model berger et al., 2005; Imhof et al., 2005). Human health is also predictions. However, in order to be useful for pollution monitoring sensitive to aerosols; ultra fine urban particles are thought to cause and to provide the data required for the development of emission health problems due to their small size and their capacity to absorb reduction strategies, it is necessary to develop methods for making Poly Aromatic Hydrocarbons (Kwamena et al., 2006). For all these quantitative estimates of aerosol concentration in the urban reasons, the study of the emission, transport and transformation of canopy. aerosols in the troposphere and especially in the Planetary This paper describes a method to explain aerosol time and space concentration variations in an urban canopy of a major city (Lyon, France). It is based on associating UV-Lidar measurements per- * Corresponding author. Tel.: þ33 (0) 472 448 176; fax: þ33 (0) 472 431 507. formed during a winter smog episode with measurements from E-mail address: [email protected] (P. Rairoux). a ground-based sampler close to the Lidar; the differences between 1352-2310/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.12.031 Author's personal copy A. Miffre et al. / Atmospheric Environment 44 (2010) 1152e1161 1153 the two data sets are investigated using vertical profiles of air which supplied hourly-averaged traffic flow data. The Lidar was temperature, data for traffic flows in the agglomeration and located on the campus of the Lyon University, to the North East of a numerical model to compute the trajectories of the air masses the city. Meteorological data (wind speed and vertical profiles of passing over the measurement site. Although the measurements temperature and humidity) were supplied by Météo France from that form the basis of this paper were made in 2002, we believe that measurements made at Bron. Measurements at the Lidar site the results remain relevant, and the combination of techniques that should not be too different from those measured at Bron, because of we have employed here should be useful in future studies. the relatively flat and homogeneous terrain between these two The paper is organized as follows: we first describe our meth- sites. The simulations of the trajectories of the air masses passing odology, based on combining several standard instruments and over the Lidar site were made using hourly measurements of wind a commercial numerical dispersion model. We then analyze the velocity at Solaize, to the South of the city. Lidar data and explain how we obtain the aerosol mass concen- tration distribution. The paper ends with a combined analysis of the 2.2. Meteorological conditions Lidar data, the trajectory calculations, and the ground-based PM10 measurements which demonstrates how this combined approach During the whole measurement period, the weather conditions can be used to identify and discriminate between different sources remained relatively settled with a light wind (wind speeds in the À of pollution. The thermal stability of the atmosphere is shown to range 0.5e1ms 1) blowing from the SouthWest (wind direction play an important role in determining the differences between generally around 210); the data are shown in Fig. 2. The very rapid what is measured at the ground and what is measured higher up, and short-lived changes in wind direction on January 11th and by the Lidar. January 13th are not significant, since they occurred at times of very light winds, but on the 12th January the wind changed direction, À1 2. Methodology and blew steadily from the North East, at a speed of about 0.4 m s , for a period of about 7 h, before reverting to its original direction. Lidar vertical profiles of aerosol mass concentration show strong Vertical profiles of temperature and humidity were measured diurnal variations that are not directly linked to variations in four times a day by Météo France at the airport at Bron; these meteorological conditions. To explain these variations, we have profiles are shown in Fig. 3. The profiles show similar behaviour computed the trajectories of the air masses that pass through the over the three days of this study; the lower part of the atmosphere measuring volume of the Lidar at each instant, and we have used is very stable at night and in the early morning, with a temperature ground-based measurements of traffic flows and PM10 concen- inversion around 400 m, and a nearly adiabatic profile above 400 m. trations to provide a qualitative indicator of the temporal variation By around midday each day, heating from the ground has inverted in the aerosol mass load of the air passing through the measuring the temperature gradient, creating a slightly convective layer which volume. extends up to about 400 m, capped by a thin layer of stable air. The Relative Humidity (RH) varied in a similar way over the 2.1. Geographical situation period of the study. Close to the ground, it rises to 90e100% during the night and early morning, falling to about 55e70% during the Lyon is located at the upper end of the Rhône valley where the day; these diurnal variations in RH extend up to about 100 m from wind blows mainly from the SouthWest. Substantial hills to the the ground, and it is only above this height that the values tend to North (Croix-Rousse) and West (Fourvière) of the city centre become relatively steady over the day. For most of the time, the RH influence air flow over the city; the terrain to the East and the South in the PBL remained below 70% and this has a significant impact on is relatively flat and homogeneous. The main sources of aerosols are the hygroscopic growth of aerosol particles. the chemical plants located to the South of the city, domestic heating and vehicle emissions. Air quality in the city is measured by 2.3. Ground-based measurements of traffic flows and PM10 the COPARLY1, and we have used data from two of their samplers to supplement and explain the profiles measured by the Lidar: Fig.