Elsevier

Atmospheric Research

Volume 235, 1 May 2020, 104799
Atmospheric Research

Classifying aerosol particles through the combination of optical and physical-chemical properties: Results from a wintertime campaign in Rome (Italy)

https://doi.org/10.1016/j.atmosres.2019.104799Get rights and content

Highlights

  • Combination of intensive optical parameters discriminates aerosol types.

  • High-time resolved composition and size distribution support the identification.

  • The methodology produces look-up tables useful for real-time implementation.

  • High PM events origin can be identified by optical parameters only.

Abstract

The “Carbonaceous Aerosol in Rome and Environs” (CARE) experiment took place at a Mediterranean urban background site in Rome (Italy) deploying a variety of instrumentation to assess aerosol physical-chemical and optical properties with high-time resolution (from 1 min to 2 h). In this study, aerosol optical properties, chemical composition, and size distribution data were examined with a focus on the analysis of several intensive optical properties obtained from multi-wavelength measurements of aerosol scattering and absorption coefficients. The spectral behaviour of several quantities related to both aerosol composition and size was explored, analysing their high-time resolved temporal patterns and combining them in order to extract the maximum information from all the available data.

A methodology to separate aerosol types using optical data only is here proposed and applied to an urban area characterised by a complex mixture of particles. A key is given to correctly disentangle cases that could not be distinguished observing only one or few parameters, but that can be clearly separated using a suitable ensemble of optical properties.

The SSCAAE, i.e. the wavelength dependence of the Single Scattering co-albedo 1-SSA (where SSA is the Single Scattering Albedo) - that efficiently responds to both aerosol size and chemical composition – resulted to be the best optical intensive parameter to look at for the discrimination between episodes characterised by specific aerosol types (e.g. sea salt, Saharan dust) and more mixed conditions dominated by local emissions. However, this study also highlighted that it is necessary to combine temporal patterns of different optical parameters to robustly associate SSCAAE features to specific aerosol types. In addition, the complete chemical speciation and the high-time resolved size distribution were used to confirm the aerosol types identified via a combination of aerosol optical properties. Look-up tables with most suitable ranges of values for optical variables were produced; therefore, these pieces of information can be used at the same site or at locations with similar features to quickly identify the occurrence of aerosol episodes. Graphical frameworks (both from the literature and newly designed) are also proposed; for each scheme features, advantages, and limitations are discussed.

Introduction

Atmospheric aerosol is a complex mixture of suspended particles characterised by a huge variability in terms of chemical composition, size and shape. Aerosols can have a direct (by direct scattering and absorption) and indirect (by impacting on cloud formation and albedo) effect on atmospheric radiation (IPCC, 2013). Moreover, atmospheric aerosol is known to have detrimental effects on human health (Dockery et al., 1993; Pope et al., 2002).

Estimates of the climate impacts of atmospheric aerosol and their optical properties are affected by very large uncertainties; nevertheless, direct measurements of aerosol optical properties are not usually performed by air quality (AQ) monitoring networks, in contrast to particle concentration and chemical composition, often performed for different size fractions. Aerosol optical properties are related to the size and composition of the particles, as well as to their mixing state (e.g. Bond and Bergstrom, 2006). Spectral scattering and absorption properties depend on the considered aerosol type; therefore, simultaneous measurements of multi-wavelength aerosol optical properties, chemical composition, and size distribution can improve our knowledge about atmospheric particles impact on the radiative forcing and air quality.

Several classification schemes have been proposed in the literature to distinguish aerosol types and mixtures. Most of these methods make use of column-integrated properties usually retrieved from remote-sensing data, such as those provided by the global network of ground-based sun and sky radiometers AERONET (Aerosol Robotic Network) or obtained by Sun photometers (e.g. Dubovik et al., 2002; Gobbi et al., 2007; Kalapureddy et al., 2009; Russell et al., 2010; Giles et al., 2011, Giles et al., 2012; Cazorla et al., 2013; Rupakheti et al., 2019). There are also few studies dealing with in-situ measurements of optical properties, both ground-based and airborne (e.g. Yang et al., 2009; Lee et al., 2012; Costabile et al., 2013; Cappa et al., 2016; Donateo et al., 2018; Romano et al., 2019). As pointed out by Schmeisser et al. (2017), the majority of the existing classification schemes work well at sites where the aerosol characteristics are fairly homogeneous, while their performance is worse in areas that experience a heterogeneity of particle sources and/or episodes characterised by aerosol transported from e.g. deserts or oceans. The methods proposed to distinguish PM types are sometimes supported by chemical composition, size distribution data, or back trajectory analyses; however, these pieces of information are not usually included in the classifying approaches themselves.

Data analysed in this paper were collected in the frame of the CARE Experiment (Carbonaceous Aerosol in Rome and Environs), which was carried out in Rome (Italy) using a variety of instruments and techniques to obtain a comprehensive and highly time-resolved picture of the aerosol properties at a Mediterranean urban background site. Indeed, several studies in recent literature (e.g. Timonen et al., 2010; Lucarelli et al., 2015; Costabile et al., 2017) pointed out the importance of shorter time scale (<1 h) to study atmospheric processes and source variability. The CARE campaign was carried out at a site impacted by different local emission sources and sometimes affected by medium-long range transport of e.g. sea salt or Saharan dust. An overview of measurements performed and methodologies applied during the CARE experiment is given by Costabile et al. (2017).

A phenomenology of specific episodes characterised by aerosol with different properties is given exploiting all the available information about high-time resolved optical properties, chemical composition, and size distribution of atmospheric aerosol. The main objective here is to find out one or more possible combinations of intensive optical parameters that can be used as a tool to identify aerosols with different origin. In addition, graphical classification schemes reported in the literature were applied and some were newly developed to visually distinguish specific episodes and aerosol types via 2D plots of optical parameters. These representations appear useful to have a first hint on the typologies of particles observed during a campaign, even though they are not able to clearly disentangle different contributions, especially when atmospheric aerosol is dominated by mixtures of particles emitted by a variety of sources. In these cases, it is shown here that, exploiting multi-wavelength optical properties measured with high-time resolution, only the combined analysis of their temporal patterns allows to identify the dominant contributions.

Section snippets

Site description

The CARE experiment took place from January 27th to February 28th 2017 at an urban background site in downtown Rome. Due to its geographical position (in the middle of the Mediterranean Sea) and its meteorological conditions, this site can experience the advection of air masses transported from the Sahara Desert (Barnaba and Gobbi, 2004; Gobbi et al., 2007; Barnaba et al., 2017; Gobbi et al., 2019) or from the sea (Perrino et al., 2009). The CARE measurement site is also affected by local urban

Aethalometer data corrections

The multi-wavelength aerosol absorption coefficient σa(λ) was obtained by an AE33 Aethalometer, which gives eBC concentration using instrument-specific MAC values at seven wavelengths (e.g. 10.35 m2/g at 660 nm). As described in Drinovec et al. (2015), the instrument internal software retrieves σa(λ) from attenuation measurements and corrects them for loading (k parameter) and multiple scattering (C factor) effects. It is noteworthy that literature studies (e.g. Collaud Coen et al., 2010;

Results and discussion

In the following, a detailed analysis of temporal patterns for intensive optical properties taken into account for aerosol classification is reported.

Conclusions

Aerosol physical-chemical and optical properties measured at high time resolution during the CARE experiment were analysed to classify different aerosol types, both from advection episodes and by local source emissions.

In particular, this study was focused on multi-wavelength optical properties measured by widespread on-line instrumentation (i.e. Nephelometer, Aethalometer, and MAAP). It has to be noted that, when analysing such kind of data, attention has to be paid to their interpretation and

Author contributions

SV drafted the paper and performed the data analysis; RV proposed the topic of this paper and contributed to the the synthesis of the results; VB, ACF, and GV collaborated to data analysis and interpretation; FB, FC, LDL, and GPG took care of the whole campaign, performed optical data measurements, data reduction and contributed to data interpretation; GC, FL, and SN carried out streaker sampling, PIXE analysis, EC/OC measurements and data analysis; MG and EP provided ACSM data and performed

Data availability

The data in the study are available from the authors upon request ([email protected]).

Declaration of Competing Interest

The authors declare no conflict of interest.

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