Simultaneous determination of PM fractions, particle number and particle size distribution in high time resolution applying one and the same optical measurement technique

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Courtesy of Palas GmbH

Abstract

The Fidas fine dust monitoring systems comprise an optical aerosol spectrometer, which is characterized by very good instrument characteristics, and utilize a sophisticated approach to convert the measured information on particle number and particle size into mass concentrations. As monitoring of PM requires reliable and quality-controlled performance of the measuring systems, the described fine dust monitoring systems have a comprehensive concept for quality assurance and have demonstrated its suitability for regulatory monitoring of PM10 and PM2.5 in ambient air in an extensive third party approval process. Besides regulatory monitoring, the Fidas fine dust monitoring systems can be used in many further applications like for portable and flexible measurements indoor or at workplaces and even for measurements in the airspace. Common for all discussed fine dust monitoring systems is the provision of comprehensive, accurate and reliable information on the fine dust content in the air, which can help to understand processes and to make informed decisions in order to reduce the ambient air pollution in future.

Introduction

Air pollution due to particulate matter (abbreviated PM in the following text) and the related negative effects (health problems, economical damages) has become one of the major problems our society is faced with today. According to WHO [1],

  • 23 % of all estimated global deaths are linked to the environment,
  • 4.3 million deaths per year result to exposure to indoor smoke from cooking fuels,
  • 3.7 million deaths every year result to exposure to fine dust.

In order to be able to investigate and to assess the real exposition of the general public in a comprehensive way and to make well-informed decisions, the performance of precise and accurate measurements of PM is an essential part of air pollution control – outdoor as well as indoor. Especially for ambient air monitoring, the majority of applied measuring technologies only allow measurements at the particle collective and the determination of particle mass concentrations for one explicit size fraction only. These methods usually neither offer a high time resolution in the range of minutes nor deliver any information which particle sizes are present at what concentration (number or mass) for any point of time during the measurement. This extra information can be very useful in terms of deeper analysis of the aerosols with respect to health effects or for conclusions /assignments to specific events and sources.

By implementing a counting optical aerosol spectrometer, the fine dust monitoring system Fidas enables to measure simultaneously both particle concentration as well as particle size distribution. In order to be able to determine particle mass concentrations based on the aforementioned measurements in a precise and reliable way and to achieve a good correlation to measured results from gravimetric standard reference methods, the optical aerosol spectrometer has to fulfil various technical requirements. These requirements comprise for example an implemented unique calibration curve, the minimizing of border zone effects and the minimizing of coincidence effects also for high concentration levels. The better these technical requirements are fulfilled, the better technical characteristics like size resolution and classifying accuracy are obtained. Very good technical characteristics are the fundamental basis for a reliable conversion of the number distribution into a mass distribution. Upon this basis, the accurate determination of particle mass concentrations requires especially representative information on particle density, particle shape and refractive indexes. This information is compiled in the so-called evaluation algorithm, which needs to be applied on the number distribution to obtain the intended particle mass concentrations. For ambient air monitoring it is of great importance to cover as much as possible different PM scenarios with only one evaluation algorithm, hence information from many comparison campaigns of the optical aerosol spectrometer with the gravimetric standard reference method need to be collected and evaluated. By this, a solid and representative basis for the conversion calculation is created. Especially for regulatory PM10 and PM2.5-monitoring in ambient air, a high level of confidence in the measured values is mandatory and the calculated particle mass concentration values have to comply with the strict requirements of demonstrating equivalence to the gravimetric standard reference method [2; 3]. The suitability for this measurement task has to be approved explicitly by a third party based a type-approval test and it is confirmed and maintained in a well-defined certification process.

The measurement technology of the Fidas sensor allows the simultaneous measurement at a high time resolution of the relevant PM fractions like e.g. PM10 and PM2.5, the particle number as well as the particle size distribution. The sensor is implemented in various fine dust monitoring systems , e.g. in the fine dust monitoring system Fidas 200, which is approved and certified for regulatory PM monitoring in line with EN Directive 2008/50/EC [4]. It can be successfully used in various applications to investigate and evaluate PM conditions in order to be able to make well-informed decisions.

The following chapter describes the measuring principle and the implemented technology of the implemented sensor, which is the core of all Fidas fine dust monitoring systems.

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