Mixing layer height measurements determine influence of meteorology on air pollutant concentrations in urban area Klaus Schäfer*a, Thomas Blumenstockb, Boris Bonnc, Holger Gerwigd, Frank Haseb; Christoph Münkele, Rainer Nothardf, Erika von Schneidemesserc a Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research, Department of Atmospheric Environmental Research (IMK-IFU), Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany; bKarlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research, Department Atmospheric Gases and Remote Sensing (IMKASF), H.-v.-Helmholtz-Platz 1, 76344 Leopoldshafen, Germany; cInstitute for Advanced Sustainability Studies (IASS), Berliner Str. 130 14467 Potsdam, Germany; dFederal Environment Agency (UBA), FG II, 4.4 Air Quality Standards and Monitoring Methods, Paul-Ehrlich-Str. 29, 63225 Langen, Germany; eVaisala GmbH, Weather Instruments, Notkestr. 11, 22607 Hamburg, Germany; fSenate Department for Urban Development and the Environment, Brückenstr. 6, 10179 Berlin, Germany; ABSTRACT Mixing layer height (MLH) is a key parameter to determine the influence of meteorological parameters upon air pollutants such as trace gas species and particulate concentrations near the surface. Meteorology, and MLH as a key parameter, affect the budget of emission source strengths, deposition, and accumulation. However, greater possibilities for the application of MLH data have been identified in recent years. Here, the results of measurements in Berlin in 2014 are shown and discussed. The concentrations of NO, NO2, O3, CO, PM1, PM2.5, PM10 and about 70 volatile organic compounds (anthropogenic and biogenic of origin) as well as particle size distributions and contributions of SOA and soot species to PM were measured at the urban background station of the Berlin air quality network (BLUME) in Nansenstr./Framstr., Berlin-Neukölln. A Vaisala ceilometer CL51, which is a commercial mini-lidar system, was applied at that site to detect the layers of the lower atmosphere in real time. Special software for these ceilometers with MATLAB provided routine retrievals of MLH from vertical profiles of laser backscatter data. Five portable Bruker EM27/SUN FTIR spectrometers were set up around Berlin to detect column averaged abundances of CO2 and CH4 by solar absorption spectrometry. Correlation analyses were used to show the coupling of temporal variations of trace gas compounds and PM with MLH. Significant influences of MLH upon NO, NO2, PM10, PM2.5, PM1 and toluene (marker for traffic emissions) concentrations as well as particle number concentrations in the size modes 70 – 100 nm, 100 – 200 nm and 200 – 500 nm on the basis of averaged diurnal courses were found. Further, MLH was taken as important auxiliary information about the development of the boundary layer during each day of observations, which was required for the proper estimation of CO2 and CH4 source strengths from Berlin on the basis of atmospheric column density measurements. Keywords: Air pollution, remote sensing, ceilometer, solar absorption spectrometry, trace gases, particle size distribution, mixing layer height
1. INTRODUCTION Urban regions are frequently influenced by enhanced air pollution, mainly due to strong emissions and chemical transformation processes but also due to meteorological influences. Wind speeds and directions as well as mixing layer height (MLH) are important factors which influence exchange processes of ground level emissions (Schäfer et al., 2006 1, 20112; Alföldi et al., 20073; Tai et al., 20104; Tandon et al., 20105; El-Metwally and Alfaro, 20136). Wind speed and
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wind direction determine the transport conditions of air pollutants so that distant emissions can influence urban air quality and by secondary air pollutants which can be formed during transport. Mainly during winter the MLH determines near-surface concentrations of air pollutants by about 50 % in areas which are not influenced by strong emissions and during time periods without effective vertical mixing and advection. Under these weather conditions air pollutants, which are emitted from the surface, will accumulate within the mixing layer resulting in limit value exceedances (Directive 2008/50/EC7). If the MLH is located near to the ground, air pollution concentrations can be high due to a strongly limited air mass dilution. In this sense MLH can be taken as a key parameter to determine influence of meteorology on air pollutant concentrations. Therefore a continuous determination of the MLH is required (Emeis and Schäfer, 20068). Because MLH is a consequence of vertical temperature (inversion), wind (vertical shear) and moisture profiles (strong decrease) in the lower atmosphere, remote sensing is a suitable tool to monitor MLH (Emeis et al., 20049): lidar or mini-lidar like ceilometer, Sound Detection and Ranging (SODAR) and Radio Acoustic Sounding System (RASS) (for comparisons see Emeis et al. (2009)10, Emeis et al. (2012)11, Schäfer et al. (2012)12). In the absence of low clouds and precipitation and during scattered clouds ceilometers can estimate the MLH fairly well. These instruments are easy to handle, frequently available today and not influencing the surroundings by sound or light. The investigations presented here focus on the continuous monitoring of the MLH with ceilometers as during earlier air pollution measurement campaigns in urban and sub-urban areas (Hannover, Munich, Budapest; see Schäfer et al., 2006 1, 20112; Alföldy et al., 20073) and at airports (e.g. Athens International Airport; see Helmis et al., 201113). Significant correlations were found there from the investigation of the original continuous hourly-mean or daily-mean values of concentration of air pollutants near the surface and meteorological values. The topics of the investigations, which are presented here, are the following. In Berlin, Germany, at an urban background site the influences of MLH upon gaseous air pollutants, volatile organic compounds (VOC) as well as number and mass concentrations of particles of different sizes are studied. MLH data were applied for the proper estimation of CO2 and CH4 source strengths from Berlin on the basis of atmospheric column density measurements. At first the measurements campaigns and the applied materials and methodologies will be presented. A description of the results of measurements follows and will be discussed. Finally, conclusions are drawn from these analyses.
2. MEASUREMENT CAMPAIGN Berlin is the capital of Germany with about 3,500,000 inhabitants. It is characterized by the location of different industries and transport routes, mainly between east and west. About 33% of the land surface is covered by vegetation which is 57% forests and 43% park areas. Wide streets cross the city so that good air ventilation exists. Further, agricultural areas are 4.4% as well as rivers and lakes 6.7% so that the building area is 56.1% of the land surface (http://www.berlin.de/berlin-im-ueberblick/zahlenfakten/index.de.html). The land surface is flat with altitude differences up to about 25 m and some small hills up to about 85 m. The surrounding of Berlin has similar land surface characteristics. During winter the influence of continental air (caused by eastern European highs accompanied by east to southeast winds and temperature inversions) is more frequent than in summer (westerly to northwesterly winds with oceanic influence dominate). That means that air pollution transport from eastern European countries can contribute to air pollution in Berlin significantly. The BÄRLIN2014 (Berlin Air quality and Ecosytem Research: Local and long-range Impact of anthropogenic and Natural hydrocarbons 2014) campaign was performed from beginning of June till end of August 2014 in Berlin, Germany (Bonn et al., 201514). Fine and ultrafine particles (UFP, diameter
