1. Introduction
It is well known that air pollution has many adverse effects on health [
1,
2], and recent research suggests substantially higher health impacts than previously assumed, with a near doubling of the global mortality rate attributable to ambient air pollution to 8.8 million [
3]. Air pollution has been classified as the biggest environmental risk to health [
4], and a recent review highlights how children are uniquely vulnerable to air pollution due to physiological, behavioural, and environmental factors and how exposure to air pollution in early years can lead to a life-long health burden [
5]. The World Health Organisation (WHO) sets Air-Quality Guidelines (AQGs) for particulate matter (PM
and PM
), sulphur dioxide (SO
), nitrogen dioxide (NO
), and ozone (O
) [
6]. Globally, 91% of the world’s population and 93% of children live in environments with air-pollution levels exceeding the WHO guidelines [
4,
7].
Desert dust outbreaks contribute directly to air pollution by increasing particulate matter concentrations, often by orders of magnitude [
8,
9,
10]. Airborne desert dust impacts air quality on both local and global scales as dust can be transported thousands of km in the atmosphere [
11]; airborne desert dust also has impacts on climate and biogeochemistry [
8]. The most active global dust source regions are located in a broad band or “dust belt” extending from the west coast of North Africa through the Middle East to central Asia [
9]. North Africa alone accounts for 55% of global dust emissions while North Africa, the Middle East, and Asia together account for ∼87% of global emissions [
9].
The health effects of desert dust are more uncertain than those of, for example, carbonaceous particulate matter, as most of the epidemiological studies of health effects of air pollution have been conducted in urban areas dominated by anthropogenic particles [
11,
12]. However, various studies have found associations between mortality and coarse particles and/or PM
during dust outbreaks in urban areas [
13,
14,
15,
16]. Recent studies in Tenerife found that exposure to Saharan dust may be a precipitating factor of hospital admision due to heart failure [
17], while a study in the capital cities of both Tenerife and Gran Canaria (Canary Islands) found that fine and coarse particles and NO
were associated with an increased risk of emergency hospital admission for respiratory diseases [
18]. The processes involved in the adverse health effects of desert dust outbreaks are still being investigated. Desert dust has been shown to be mixed with particulate pollutants as the dust plume encounters polluted air masses, for example, on its way from North Africa to the Western Mediterranean [
19] and as it travels westward from North Africa to the North Atlantic [
20]. The results of a study in two regions of contrasting African influence found that the mechanism of pollution scavenging by dust was more effective in the wet mode at sites further away from the desert dust source [
19].
In addition to direct health effects of desert dust and the increasing particulate concentrations from transport of desert dust, recent studies have indicated there are also indirect health effects of dust outbreaks. For example, the adverse health effects or toxicity of the locally produced particulate matter was also found to increase during dust outbreaks [
21]. Studies in southern Europe (Barcelona and Madrid, Spain) have found that the mixing layer height significantly decreased with increasing intensity of Saharan dust events [
22,
23] and that an increased risk of mortality was observed with the reduction in mixing layer height [
22]. This thinning of the mixing layer leads to an accumulation of local anthropogenic pollutants and may favour new particle formation or new species formed from condensation of gaseous precursors on the surface of dust particles [
22,
24]. A study of PM
chemical composition in Seville (Spain) reported that, as well as the expected natural PM
load increase during Saharan dust outbreaks, the anthropogenic load also increased to twice the concentration observed in non-dust outbreak days [
25].
One of the challenges of investigating the health effect of dust outbreaks is having adequate air-quality observations in dust outbreak regions [
11,
16,
26]. In this study, we use air-quality observations from Santa Cruz de Tenerife (Canary Islands, Spain) as an illustrative case of a city located close to Saharan dust sources. The proximity of Santa Cruz de Tenerife to the west coast of North Africa (∼300 km) in the desert dust export pathway to the North Atlantic (
Figure 1) causes it to be frequently subjected to dust outbreaks, which increase the particulate concentrations and affect the air quality. This desert dust export from western Africa to the North Atlantic establishes a dry, warm, and dust-laden air layer that is referred to as the Saharan Air Layer (SAL), which can expand westward to the Americas [
27,
28,
29,
30]. In winter and early spring, the Saharan Air Layer intrusions over the North Atlantic occur at low altitude (0–3 km) [
29,
31,
32], while in summer, the SAL and dust transport occurs at higher altitudes (1–5 km) [
29,
33]. However, although the dust transport largely take place at altitude in the summer, the impact on concentrations at lower altitudes can occur through gravitational settlement of the dust [
34].
Santa Cruz de Tenerife is a medium-size coastal city, and in addition to the impact of desert dust particles, it has a mixture of anthropogenic sources of pollutants. Here, we combine air-quality observations and data of the vertical structure of the atmosphere from 2012 to 2017, obtained from both observations and a numerical weather prediction model, to investigate the direct and indirect influence of desert dust outbreaks on air quality in urban areas.
4. Conclusions
We combined air-quality observations and marine boundary layer height data for Santa Cruz de Tenerife as an illustrative case of a city located close to and frequently impacted by desert dust sources to address the question of both the direct and indirect influences of desert dust outbreaks on air quality in urban areas. On average, 38% of days during 2012 to 2017 were affected by dust outbreaks, with a maximum of 55% of days in 2017; this led to exceedances above the WHO PM and PM 24-h mean AQGs.
Thinning of the marine boundary layer occurred during dust outbreaks, with >45% reduction in median marine boundary layer height in the more intense dust outbreaks during summer and ∼25% reduction in median marine boundary layer height in the more intense dust outbreaks during winter. Local anthropogenic pollution was shown to increase during dust outbreaks. We observed more than a doubling of NO and NO mean concentrations and even larger relative increases in black carbon during the more intense summer dust outbreaks. Increases in NO, NO. and BC mean concentrations by factors of 1.7, 2.1, and 2.7, respectively, were also observed during the winter outbreaks, but these increases were less than in summer, consistent with the stronger reduction of the marine boundary layer height in summer.
Desert dust outbreaks, through the effect of the reduction in the marine boundary layer height can increase anthropogenic air pollution in urban areas, and it appears from recent health studies that the resulting cocktail of desert dust and local anthropogenic pollutants has a greater effect than solely local pollution or dust. This has public health implications; local anthropogenic emissions need to be reduced even further in areas that are impacted by desert dust outbreaks if we are to achieve the health benefits we expect from emission reductions and to reduce adverse health effects. This is contrary to the current air-quality policy in the European Union, where, from a legislative point of view, the PM concentrations can be discounted during these “natural” episodes [
45]. In fact, the health outcomes during desert dust outbreaks cannot be discounted [
54] and, as shown here, the adverse health effects can be exacerbated during these events.
Consequently, anthropogenic emissions need to be controlled more strongly in areas that are impacted by desert dust outbreaks to counteract this detrimental coupling of dust outbreaks and anthropogenic air-quality episodes. This is of particular importance for urban areas strongly and frequently impacted by dust outbreaks, and these conclusions need to be taken account of in other geographical areas strongly impacted by desert dust outbreaks such as the Mediterranean, North Africa, the Middle East and Asia.