Atmospheric pollution supposes a risk factor for cancer and cardiovascular and respiratory diseases. However, atmospheric aerosol particles are poorly understood components of the atmosphere. They have a large diversity of sources, formation and transformation processes, which imply a large number of different aerosol species and, therefore, different properties. Hence, and taking into account their large variability, it is necessary to characterize them to know how they behave in the atmosphere in order to assess, for example, their effects on health and climate [1
], and even in the degradation of building materials [2
Black carbon (BC) is the fraction of the carbonaceous aerosol in the atmosphere that is characterized by its strong absorption of visible light [3
] and also by its resistance to chemical transformation. BC is a good indicator of primary emissions and it is often used as an indicator for the efficiency of abatement initiatives [4
]. The importance of BC determination also relies on the fact that BC and organic carbon are the constituents of particulate matter that are most likely to cause adverse health effects [6
]. In this manner, the identification of its sources, which rely on the different optical properties of carbonaceous aerosols from different sources [7
], is certainly very relevant. Particularly, it is important to distinguish between the contribution of fossil fuel (ff) and biomass burning (bb). This is possible by a multi-wavelength determination of the absorption coefficient [8
] using an aethalometer, manufactured by Magee Scientific [9
]. This instrument forms an important tool for source apportionment of carbonaceous aerosols since it is robust and easy to operate. In fact, it is widespread across Europe [11
]. In this sense, when optical absorption methods are used for BC measurement, the term “equivalent black carbon” (eBC) should be used instead of BC [12
]. Moreover, it is necessary to use a suitable MAC (mass absorption cross-section) value [13
] for the conversion of the light absorption coefficient into mass concentration.
Querol et al. [14
] concluded that continuous monitoring of eBC by absorption photometers is an adequate strategy for air quality monitoring, mainly at urban sites. The reason is that this parameter can be considered a good tracer of exposure to anthropogenic emissions since it can be emitted by local sources, or transported regionally. In fact, a technical report for the European Environmental Agency concluded that eBC monitoring would be viable in current European air quality networks, where these types of instruments are already present.
Additionally, numerous studies have revealed that exposure to road traffic emissions is best assessed by combining measurements of ultrafine particles (UFP) and eBC concentrations, since these parameters need to be controlled by air quality limit values [15
]. One of the recommendations of the CARE (Carbonaceous Aerosol in Rome and Environs) experiment [19
] is to update the air quality standards by including measurements of particle composition (at least BC) and particle number (and size) with a shorter data averaging period. This is because several studies about physical aerosol parameters show that elevated aerosol particle concentrations may result in an increased risk of health hazards. This may be due to UFP and BC having higher surface areas per mass to absorb toxic materials rather than larger particles. The reason is their high concentrations and small diameters [20
With regard to UFP, their properties are greatly dependent on their sources. They vary geographically depending on the land use and the atmospheric processing and transport. Thus, the size distribution of the aerosols in the atmosphere at different spatial and temporal scales, as well as their associated effects, can be very different. In situ measurements of aerosol size distributions are therefore needed. Many kinds of instruments can obtain these measurements based on two principles of operation: light scattering and electric field. The differential mobility analyzer (DMA) used in conjunction with a particle counting system (condensation particle counter (CPC) or electrometers), called the Scanning Mobility Particle Sizer (SMPS), is the most common technique employed for long-term characterization of the atmospheric sub-micrometric aerosol fraction [1
]. With this equipment, the total number of particles (Nt) in the size range measured is obtained, the daily averages ranging between a few hundred and over 50,000 cm−3
In order to assess the source apportionment of eBC, the absorption Ångström exponent (AAE) can be used as a source specific parameter to distinguish between wood smoke and diesel exhaust aerosols. Wood smoke contains aerosolized substances that strongly absorb in the blue and ultraviolet (UV) part of the light spectrum and may not absorb in the infrared (IR) parts of the spectrum; that is, brown carbon (BrC) [22
]. High aerosol absorption at low wavelengths leads to high AAE values [7
]. For wood smoke or smoke resulting from the combustion of biomass, an AAE of around 2 is expected [8
], although much higher AAEs have been observed, for example, for the smoldering combustion of peat [23
]. Fresh diesel exhaust has an AAE close to 1 [24
The west of Spain lacks studies on eBC, on eBC in combination with Nt, and on source apportionment of eBC. However, there is a traditional use of wood in this region as an energy source for domestic heating in single family homes outside urban areas. In fact, the Galician Autonomous Government (in the northwest (NW) of Spain) has encouraged the use of biomass for this end.
Taking all the previous information into account, the aim of this study was (i) to evaluate eBC levels and their temporal variability in a residential area in the NW of Spain, (ii) to assess the previous information with data related to Nt, and (iii) to identify eBC primary local and regional sources of emissions from fossil fuel combustion and biomass burning. For this purpose, (i) we carried out a sampling campaign from January 2015 to December 2016, and (ii) we developed new software features/functionalities for the SCALA© software application, which was the atmospheric aerosol data management software used to carry out this work.
2. Materials and Methods
2.1. Measurement Site
The study here presented was carried out at the University Institute of Research in Environmental Studies of the University of A Coruña (43°20′11″ north (N,) 08°21′7″ west (W); Galicia, Spain). Specifically, the Institute is located in Oleiros (Figure 1
), in the northwest part of the autonomous region of Galicia (NW of Spain). It is a commuter town with a low population density characterized by (i) medium traffic density, especially in summer due to tourism caused by its beach area [25
], and (ii) by single family homes with heating systems based on wood combustion. The nearest populations are two small villages at 2 and 4 km distance. The closest large population is the city of A Coruña (around 250,000 inhabitants) at a distance of 10 km. There are industries close to the sampling site that can influence air quality: three power plants (10 km W, 25 km southwest (SW) and 60 km northeast (NE)), one factory for the production and transformation of metals (9 km W), an industrial area (petrochemical refinery, primary aluminum manufacturing and graphite electrode manufacturing) (10 km W), one solid waste incinerator (25 km SW), several port areas (2–11 km NW) and one airport (4 km SW). There are also other significant activities in a radius of about 15 km: paper manufacturing and processing, food and beverage plants, ports, hospitals, funerary homes, printers and laundries.
The meteorological parameters here used were measured at Alvedro Airport (43°18′00″ N, 08°22′59″ W; NOAA code: 080020-99999), and related data was accessed through the worldmet (v0.9.2; David Carslaw, 2020) package for R.
eBC mass concentrations were measured using one multi-wavelength aethalometer (Magee Scientific aethalometer model AE33, Aerosol d.o.o, Slovenia), with a BGI, MiniPM®
Inlet at a flow rate of 5 L min−1
. Data were recorded with a 1 min time resolution. Filter-based light attenuation by the deposited aerosol particles was measured at 7 wavelengths (370, 470, 520, 590, 660, 880 and 950 nm). The eBC mass concentration was calculated using the measurement at the 950 nm wavelength with a MAC of 7.19 m2
The system used to carry out the measurements of UFP was one SMPS, operated in the scanning mode. The SMPS consists of an Electrostatic Classifier (Model 3080, TSI United Kingdom) and a DMA (Model 3081, TSI United Kingdom) connected to a Water Condensation Particle Counter (Model 3785, TSI United Kingdom). Only the data of Nt were used in this study. The sampled aerosol was dried using a Nafion® dryer. Additionally, a pre-impactor with 0.0514 cm nozzle was used. The SMPS was set with a sheath and a polydisperse aerosol flow rate of 10 and 11pm, respectively, to scan the size range between 10 and 289 nm (although this range is not properly UFP (<100 nm); it is called this way in this paper since the range is closer to the ultrafine range than to the fine range (<1 µm)). Every 5 min, the system sampled two 120 s scans per sample. Data were compensated for losses by diffusion and multiple charges with the Aerosol Instrument Manager® software (version 9.0.0., TSI, Inc., St Paul, MN, USA).
Quality control protocols were considered for both instruments, including a weekly check for verifying a correct operation and data acquisition. Sampling flows were checked on a monthly basis for SMPS, and on an annual basis for aethalometer. Moreover, several tests were performed annually for this last instrument (e.g., leakage tests). Finally, SMPS national intercomparison exercises were performed in the framework of the Spanish Network of Environmental DMAs (REDMAAS) [26
] in order to evaluate the accuracy, since there are no standards to verify the correct measurement of Nt.
The sampling campaign considered in this paper was performed during the years 2015 and 2016, with data coverage close to 75% for the aethalometer and close to 55% for the SMPS.
The source apportionment of eBC was performed using the two-component model described by Sandradewi et al. [8
], using light absorption measurements at 470 nm and 950 nm [27
], since eBC from fossil fuel has a weak dependence on wavelength (i.e., AAE ~ 1), whereas eBC from biomass burning features a stronger absorption spectral dependence and shows enhanced absorption at a shorter wavelength (i.e., AAE > 1) [29
]. The absorption at 470 nm was used, instead of the UV channel at 370 nm, to minimize the interferences introduced by types of organic compounds, based on the sensitivity of the aethalometer model due to different wavelength combinations carried out by Zotter et al. [31
]. Even though 880 nm is considered the standard channel for eBC measurement by aethalometers, the 950 nm wavelength was used in this research according to the results obtained in the sensitivity of the aethalometer model using different pairs of wavelengths carried out by Zotter et al. [31
]. Therefore, the analysis here conducted was based on the wavelength range 470–950 nm.
2.3. SCALA© (Version 1.1, UDC-CIEMAT, Spain)
For the purpose of this research, the software features of the first complete functional version of the web based software application called WEP-PROACLIM, which is an atmospheric aerosol data management program presented by Andrade-Garda et al. [32
] and later known as SCALA©
(Sampling Campaigns for Aerosols in the Low Atmosphere) (https://proaclim.udc.es
], had to be extended. That is to say, new functionalities (i.e., a new software increment following the software development process presented by Andrade-Garda et al. in [33
]) were developed. Next, we will present the first version of SCALA©
, then the new increment will be addressed.
SCALA© is a web-based software application that was incrementally developed in a multidisciplinary way to integrally support the documentation and the management and analysis of atmospheric aerosol data from sampling campaigns. Therefore, this software application allows the comprehensive management of the sampling campaigns’ life cycle (i.e., management of the profiles and processes involved in the start-up, beginning, development and ending of a campaign) and provides support for both intra- and inter-campaign data analysis. Thus, for example, the campaigns involving the different groups belonging to the Spanish REDMAAS are currently managed through SCALA©.
The need for SCALA©
arises from the fact that such campaign management was traditionally approached in an eminently manual way. Thus, for example, date(s) and place(s) for a campaign were agreed upon after a huge exchange of mails and, or, telephone calls (or using Doodle in the best case), and data processing and analysis were performed by integrating and managing (through Dropbox or Google Drive in the best case) a hand-made spreadsheet created by the environment technicians. Obviously, a great effort of format standardization was previously required for this integration. After that, to proceed with the data analysis, it was also necessary to have a skilled (error-prone) handling of, for example, a spreadsheet like Excel and/or a programming language like R [35
]. In brief, there was no software application supporting, in a holistic way, the work related to all the sampling campaign activities. Accordingly, SCALA©
was developed as a holistic solution, avoiding arbitrary errors in the manual handling of data (thus achieving efficacy) and optimizing the effort (thus achieving efficiency). As an example of the latter, it should be pointed out that the effort required to prepare the data files for uploading to the ACTRIS (Aerosols, Clouds and Trace gases Research InfraStructure network; the reference network in Europe [36
]) website could be up to a month of work the first time; due to its complex format and, mainly, to the ad hoc work needed depending on each measuring instrument considered. The standardization of the data files for SCALA©
takes less than thirty minutes, and much less even if, for example, macros are used. Moreover, the call for campaigns, the work during the campaigns and the data analysis have been fully integrated into a unique software application. Thus, once the data has been uploaded, the data analysis of both intra- and inter-campaigns is available immediately in SCALA©
integrates (i) a public section (in Spanish), to promote and inform about related research activities or projects in the Spanish R&D Plan; and (ii) a private functional section (in English or Spanish, depending on the registered user’s preferences), for managing and analyzing data and documentation from campaigns. There are five user profiles: administrator (technical administrator of the software application), group manager, campaign manager, technical user and external/guest/anonymous user (i.e., a non-registered user, who can request access). Next, the three main profiles are briefly described along with the most relevant features of SCALA©
. More details are described by Andrade-Garda et al. [33
Group manager: Responsible for a group of environmental technicians. She/he manages the members and the measuring instruments of her/his group, votes in surveys about the campaigns the group has been invited to, and sets the configuration for the group equipment in campaigns.
Campaign manager: The group manager that registers a campaign in SCALA© and, therefore, is responsible for managing the campaign life cycle (i.e., invitations to groups for participation in the campaign, campaign surveys for date and place agreement, campaign opening and campaign closing).
Technical user: A member belonging to a group of environmental technicians. Its main features are the following: Data files, documentation and incident uploads, and intra- and inter-campaign analysis (data files selection, charts selection, and exportation options selection).
With regards to the above-mentioned new software increment for SCALA©
, required to meet the new management needs arising from the new research presented in this paper, it was developed by a subset of the original multidisciplinary team following the same software development model used for the first version. This new increment mainly considered the possibility of adding aethalometers into the measurement instruments set managed by a group manager, along with the corresponding new functionalities regarding data file uploads, graphical representations and statistical analysis. Basically, and for what concerns here, this new software increment focused on the technical integration of SCALA©
with R in order to (i) embed new plots (percentile rose plots, conditional probability function plots, and bivariate polar plots) into SCALA©
and (ii) represent eBC concentrations from the aethalometers jointly with data already in SCALA©
(i.e., particle number size distributions from SMPSs). The statistical elaborations here presented were embedded into SCALA©
thanks to the mentioned technical integration with R; specifically, with R’s openair [37
] package. Briefly, SCALA©
server actually invokes (i.e., “exec” in Computing) R features (in the SCALA©
server), and the results coming from R are embedded into the web pages served to the user (web browser). In this manner, the R programming and testing activities are completely hidden from the user (environmental technician). Moreover, the user does not need to have R installed in her/his computer (computationally speaking, the client). All this (i) avoids computing and testing activities by non-technical users, (ii) avoids handling errors, (iii) automates and integrates the analysis into a single environment and (iv) reduces the required client’s computational power.