1. Introduction
Black carbon (BC) is one of the components of fine respirable particle matter (PM
2.5); it comes from the incomplete combustion of fossil fuels and biomass. Exposure to BC has been linked to short-term [
1,
2] and long-term [
3,
4,
5,
6,
7] health effects, but its regulation is indirect through the regulation of ambient PM
2.5. Recently, the World Health Organization has updated its air quality guidelines [
8], setting an annual average of PM
2.5 of 5 μg/m
3, which means that long-term BC is implicitly recommended to be well below that guideline since BC is usually below 20% of the total PM
2.5.
BC has been traditionally measured offline using thermal-optical methods applied to filter samples [
9,
10]; these results are reported as total BC in PM
2.5 [
11]. More recently, continuous instruments based on optical absorption at several wavelengths (from UV to IR) have been developed. These instruments (aethalometers) can apportion BC coming from fossil fuel (BC
ff) and wood burning (BC
wb) combustion because the BC emitted from those sources has a different wavelength dependence for that absorption [
12]. This technological development has led to many studies worldwide that report that BC source apportionment in urban [
13,
14,
15,
16,
17,
18,
19,
20] and rural areas [
21,
22,
23]. Despite this improvement, those apportionment results—on any given receptor site—report the total BC
ff (or BC
wb) coming from local and non-local sources; for instance, regional wildfires may contribute to BC
wb as much as local sources, BC
ff may come from local and regional traffic sources, etc. Additional tools, like air quality models, have been used to resolve those local and non-local BC contributions [
24]. Recently, aethalometers have been used to assess the changes in ambient BC
ff and BC
wb associated with urban lockdowns worldwide [
25,
26,
27,
28,
29,
30,
31]. All these studies report significant decreases in ambient BC concentrations under those exceptional circumstances, with traffic sources being the largest contributors to those decreases, while BC
wb sometimes has not changed [
29] or has even increased [
32].
The purpose of this work is twofold: (a) report ambient BCff and BCwb concentrations for the very first time in Santiago, Chile, and estimate the reductions in ambient BC concentrations brought by lockdowns during SARS-CoV-2 pandemics, (b) apply a new methodology of spatiotemporal pattern recognition for estimating local and non-local contributions to ambient BCff and BCwb.
The new methodology is based on a fuzzy clustering algorithm applied to ambient BC
ff and BC
wb concentrations along with surface meteorological variables (wind speed and direction, air temperature). This methodology—named FUSTA (Fuzzy SpatioTemporal Apportionment)—splits ambient concentrations into several spatiotemporal patterns, each one corresponding to a contribution from one of the major emission sources [
32]. This novel method generates a source apportionment for local and non-local BC sources without the need for air quality modeling applied to the city. The latter would require (a) an accurate emission inventory for BC
ff and BC
wb, (b) the meteorological input fields should be accurate and capture the strong mixing layer seasonality over Santiago, and (c) the air quality model used should not have significant biases.
We find a reduction in total BC in Santiago during the lockdowns in 2020, from 40% to 80%, as compared with previous measurements in 2015; we also find that the FUSTA approach is a useful tool to resolve local and non-local sources of BCff and BCwb.
4. Discussion
This is the first report of BC source apportionment conducted in Santiago, Chile that includes the effects of lockdowns brought on by the SARS-CoV-2 pandemics. Hence, the results reported here may be considered a baseline for future studies.
The total BC reductions associated with Santiago 2020 lockdowns—from 40% to 80%—are like the ones estimated for other cities worldwide: Delhi, India [
25], up to 78%; Kigali, Rwanda [
29], 59%; Sommerville, MA, USA [
28], 22–56%; Wuhan, China [
31], 39%. One limitation of our estimated reduction is that the baseline is not 2019 but 2015; since ambient PM
2.5 has been steadily decreasing in Santiago for the period 2015–2020 [
48], this means our estimates are upper bounds (in magnitude) of 2019–2020 BC reductions.
Regarding BC source apportionment during lockdown conditions, BC
ff is dominant all year long, between 82 and 86% of total BC in Santiago. This is higher than in other cities during lockdowns: 70% in Ahmedabad, India [
30], 60–86% in Wuhan, China [
31], 51–69% in Delhi, India [
25], 50% in Kiwali, Rwanda [
29]. We ascribe this to the mild, Mediterranean climate of Santiago, the large fleet of motor vehicles therein and the lower proportion of wood-burning emissions as compared to the above cities.
The non-local contributions coming from the greater Santiago metropolitan area are associated with the development of anabatic winds during daylight hours, so these contributions are zero overnight; the spatial and temporal plots (
Figure 5,
Figure 6,
Figure 7 and
Figure 8) show that FUSTA methodology separates this contribution from the local sources. This novel approach circumvents the use of an air quality model to estimate how much BC
ff (or BC
wb) originates locally or is transported from upwind urban sources. In addition, the noisy fuzzy cluster concept handles intermittent sources arriving at the monitoring site, which are local emissions from residential cooking and heating; these are resolved from the other local sources because their spatiotemporal patterns are different. This split of local, non-local and intermittent contributions to ambient BC
ff and BC
wb concentrations will facilitate further air quality modeling studies for these ambient combustion tracer particles.
5. Conclusions
A 2020 baseline of ambient BCff and BCwb concentrations has been compiled for Santiago, Chile, during the SARS-CoV-2 lockdown periods, at an urban site located on the east border of the city. BCff is the dominant contribution all year long, accounting for more than 80% of total BC. During the more restrictive lockdowns, total BC decreased by ~80% compared with a 2015 ambient BC campaign in the same part of the city; likewise, when lockdowns were relaxed, the decrease in total BC reached ~40% on the same comparison basis.
A new methodology to resolve local and non-local BC sources has been developed. This new methodology is based on a fuzzy clustering of ambient observations of BC and four meteorological variables: wind speed and direction, temperature, and pressure. This new methodology (named FUSTA) can resolve different spatiotemporal patterns (i.e., fuzzy clusters) of ambient BC, which arise from different BC sources contributing to ambient BC concentrations at the monitoring site. The methodology resolves, for instance, the arrival of Santiago’s urban plume to the monitoring site due to the daylight anabatic wind regime in Santiago’s basin. Besides, the methodology also handles intermittent sources like residential heating and cooking, especially in the winter season.
The application of FUSTA methodology to ambient BCff and BCwb concentrations has led to the result that local sources are dominant in both BC fractions: traffic and wood burning sources, respectively, with 66% and 75%, respectively. The contributions from Santiago’s urban plume arriving at the monitoring site increased towards the end of the year when lockdowns were relaxed; on average, this contribution reached 14% of BCff and BCwb concentrations. Intermittent residential heating and cooking sources contribute to 7% and 11% of BCff and BCwb concentrations, respectively. When these intermittent contributions are added to the regular spatiotemporal patterns (clusters), the total contribution of local residential heating and cooking sources reaches up to 20% and 86% for BCff and BCwb concentrations, respectively.