Abstract
Abstract. Optical particle counters (OPCs) are common tools for the in situ measurement of aerosol particle number size distributions. As the actual quantity measured by OPCs is the intensity of light scattered by individual particles, it is necessary to translate the distribution of detected scattering signals into the desired information, i.e., the distribution of particle sizes. A crucial part in this challenge is the modeling of OPC response and the calibration of the instrument – in other words, establishing the relation between instrument-specific particle scattering cross-section and measured signal amplitude. To date, existing methods lack a comprehensive parametrization of OPC response, particularly regarding the instrument-induced broadening of signal amplitude distributions. This deficiency can lead to significant size distribution biases. We introduce an advanced OPC response model including a simple parametrization of the broadening effect and a self-consistent way to evaluate calibration measurements using a Markov chain Monte Carlo (MCMC) method. We further outline how to consistently derive particle number size distributions with realistic uncertainty estimates within this new framework. Based on measurements of particle standards for two OPCs, the Grimm model 1.129 (SkyOPC) and the DMT Passive Cavity Aerosol Spectrometer Probe (PCASP), we demonstrate that residuals between measured and modeled response can be substantially reduced when using the new approach instead of existing methods. More importantly, for the investigated set of measurements only the new approach yields results that conform with the true size distributions within the range of model uncertainty. The presented innovations will help improving the accuracy of OPC-derived size distributions and the assessment of their precision.
Highlights
The size distribution of aerosol particles is a key property to understand the impact of aerosols on human health and Earth’s climate
In the following paper we focus on the central aspect of optical particle counters (OPCs) response modeling and calibration and present a new approach that
In Sect. 2.5.1 we present a new approach that includes the instrument-specific part of signal broadening within the basic parametrization of OPC response
Summary
The size distribution of aerosol particles is a key property to understand the impact of aerosols on human health and Earth’s climate. Several studies directly comparing size distributions from different OPC instruments (e.g., Belosi et al, 2013; Renard et al, 2016) and OPCs with other sizing methods (e.g., Reid et al, 2003; Müller et al, 2012) find significant disagreements and in some cases OPCs show systematic mis-sizing and artificial broadening of size spectra This highlights that, OPCs allow for a fast assessment of qualitative size information, the task to gain proper particle number size distributions can be challenging. One reason for this is the measurement principle itself, as particle size is only indirectly inferred from scattered light intensity. For particle sizes that are comparable or larger than the wavelength of the incident light, the size dependency of scattered intensity tends to be flat and occasionally ambiguous, so that uncertainties in the particle’s
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