Trace Metals in Fine and Respirable Ambient Air Particulates on Trinidad’s West Coast

The paper analyzed the concentrations of trace metals in fine and respirable particulates (fine-PM 1 and PM 2.5 ; respirable-PM 10 ) to determine baseline concentrations in the ambient air and the factors impacting its distribution such as land use and time of year when levels may be concerning to public health. Measurements of particulates along with meteorological parameters were made at four sites over the heavily populated west coast of Trinidad (10˚32'N, 61˚18'W) during March ’15-May ’16, representing rural, urban, mixed background and industrial land uses. The study found mean levels of trace metals to be highest at the industrial and urban stations. Public health exceedances (referenced to the Canadian AAQ public health standards (Ontario-MoE, 2012)) were measured for beryllium, cadmium, chromium, iron, manganese and nickel (in PM 10 ). Iron, manganese and nickel, most associated with particulates at the industrial station, were in frequent exceedance. Beryllium—concentrated in coarse PM (PM 2.5-10 ) with only a single measured exceedance at the mixed background station likely poses minimal threat to the health of the nearby population. Cadmium—concentrated in fine PM which peaked once only at the rural station was likely due to an irregular event within a narrow timeframe during the time of sampling. Iron and manganese were frequently above the Canadian public health threshold, but predominated in the coarse PM fraction, suggesting localised sources. Nickel, concentrated in the fine PM fraction, was frequently in exceedance particularly at the industrial station. Cadmium and nickel are genotoxic and should be regulated in order to reduce the burden of toxic carcinogens to which the population can be exposed.


Introduction
Ambient air particulates can act as a medium of transport for a myriad of adsorbed organic and inorganic chemicals, trace metals, microscopic biologicals and viruses from various sources. Respirable particulates (PM 10 ) containing contaminants can by-pass the natural defenses of the human respiratory system and enter the body through varied pathways, including inhalation. Trace metals entering the lungs through inhalation can cause harm to human health both from their physical presence inside the lungs, obstructing gas exchange, and leaching of adsorbed metals into the cells and bloodstream (Geiger & Cooper, 2010;DEFRA, 2008;European Commission, 2001).
Pollutants emitted in the air eventually find their way down and can be deposited in the surface water (rivers, ponds, reservoirs, lakes) and marine sediments.
Toxicological and epidemiological studies to establish disease causality and revision of these thresholds for public health protection have been ongoing since international protections under variations of the Clean Air Act were adopted by several countries (Balluz et al., 2007;Bruske et al., 2010;Lepeule, Laden, Dockery, & Schwartz, 2012). As air pollution can effectively be controlled at the point of emission (European Commission, 2001), the regulation of the concentration of respirable particulates and contaminants contained in them (e.g. trace metals) is a common feature in the pollution regulation and legislative arsenal of many states and pollution control agencies, including Trinidad and Tobago.
The concentrations of trace metals measured in this study were analysed to determine baseline concentrations in the ambient air and the factors impacting their distribution e.g. particular land uses, time of year and/or geographical areas where levels pose a threat to public health. Exceedance levels, are defined as the frequency of sampled trace metals levels in PM 10 (measured over24 hrs) exceeding the local standards (Environmental Management Authority of Trinidad and Tobago, 2014) and/or international guidelines (Ontario-MoE, 2012) identified for protection of public health.

Literature Review
Several epidemiological studies have found a causal relationship between levels of air pollution (respirable particulates, toxic levels of trace metals, pollutant gases) and ill health, respiratory and cardiovascular disease, cancer and increased  (Dockery et al., 1993;Grahame & Schlesinger, 2010;Bruske et al., 2010;Pinto, Soares, Couto, & Almeida, 2015). Inhalation exposure to ambient air pollution is usually a chronic, long-term situation that can start very early in life; factors which make its effects so detrimental to public health. Locally, levels of particulates in ambient air have already been linked to increased hospital admissions for asthma and respiratory related illnesses in children (Monteil et al., 2005;Gowrie, Agard, Barclay, & Mohammed, 2016).
Physical assessment of fine and respirable particulates to better estimate the "dose" of trace metals from inhalation exposure have shown that this depends on source contributions and size of PM which carries it. In Detroit, Michigan (Utsunomiya, Jensen, & Keeler, 2004), fine and ultrafine curbside particulates examined using high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) techniques found most trace metals in the 0.01 -1.0 µm PM fraction, which has the longest atmospheric residence time (more than 100 days) in ambient air.Trace metals on particulates in this size range have increased chemical reactivity owing to the size of nanoparticles on which they are adhered/adsorbed. Curbside trace metals in particulates from a busy intersection in Delhi, India were much higher (PM 2.5 daily means-286.23 ± 41.1 μg/m 3 ), mostly originating from diesel exhaust and vehicle "wear and tear" (Khanna, Khare, & Gargava, 2015), i.e. from a mixture of gas-phase fine and coarse particulates. In the Caribbean, a lot of the fine PM comes in from transboundary Sahara dust, which has a similar trace metal composition to crustal dust, but may contain other contaminants (Garrison et al., 2006;Griffin, 2007;Garrison et al., 2010).
The impacts of contamination are manifested both on the environment as well as on human health and biodiversity. Ecologically, trace metal contamination can cause major disruption to the natural biogeochemical cycles by affecting the micro-geochemistry of microbiota and plankton (Mahowald et al., 2018). With regard to its impact on humans, trace metals in the body above toxic thresholds impact at the cellular and metabolic levels. The finer the inhaled particulates, the better its ability to penetrate deeper into the lungs.
According to assessments in China (Zhang, Jin, Johnson, & Gies, 2016) and the European assessment (European Commission, 2001), cadmium and arsenic compounds are enriched in the fine particle mode about or below 1 µm and, consequently, can penetrate deeply into the respiratory system and have long residence times in the atmosphere. Arsenic was found more abundantly in fine particles (PM 1 ); the arsenic found in the coarser PM (5 -10 µm aerodynamic diameter) came from dust and soils and was less prevalent. In the UK, independent evaluation of epidemiological studies containing well explained histological evidence to support "tolerable level" recommendations for trace metals and metalloids (As, Be, Cr and Ni) in ambient air standards (DEFRA, 2008) suggested two threshold tiers; a cancer risk threshold and a non-cancer threshold, with a preference to regulate towards the latter. Journal of Geoscience and Environment Protection The mechanism for cancer induction from insoluble particulate nickel compounds, including the sulphide and sub-sulphide inhalation exposure, is through the release of reactive oxygen species from phagocytic cells (DEFRA, 2008).
These insoluble forms of nickel have been shown to be more carcinogenic than soluble nickel and in vitro studies have demonstrated that they induce greater intracellular free radical production than soluble nickel salts. There is evidence that genotoxic effects of nickel compounds may be indirect, through inhibition of DNA repair systems. This possibly reflects the ability of nickel to form complexes with the amino acid histidine which may then take part in redox reactions.
Similarly, chromium has a projected metabolic pathway for inhaled chromium containing particles or fumes containing carcinogenic hexavalent chromium (Cr-IV). It is reduced to Cr-III by low molecular weight reductants such as ascorbic acid and glutathione in epithelial lining fluid, any remaining Cr-VI is transported into cells via the anion transport channel along a concentration gradient and inside the cell reacts with reductants, which leads to the formation of reactive oxygen species (DEFRA, 2008). Trinidad is the more industrialised island of the twin island state, tourism being the predominant economic activity in Tobago.  The study was designed to assess the potential trace metal contaminants in ambient air that pose a threat to public health (illustrated in Figure 2). Several QA/QC strategies were employed in sample collection and laboratory protocols to minimize cross contamination from handling, storage, labware and reagents for accurate ICP-MS analysis of trace metals (Bruker, 2010;Hassan, 2007;US Environmental Protection Agency, 2005).

Experimental and Numerical Setting
Particulates were collected on three tiers of desiccated (Secador Desiccator) and pre-weighed (Mettler Toledo XP26 microbalance-range 0.001 mg to 22 g) quartz fibre filters (collecting PM 1 , PM 1-2.3 and PM 2.5-10 ) using a combined head Flow rates were recorded in the sampling log and tallied for total volume of air sampled (50 -54 m 3 ) over the sampling timeframe (24 hrs every 6 th day at four sampling sites). Particulate loading was determined by differential weighing of pre-and post-exposed desiccated filters. Filters were stored (folded in plastic sample bags) in the freezer until ready for digestion (Anton Paar Multiwave microwave digester).
Laboratory protocols followed the USEPA standard methods for digestion and All the isotopes for lead (Pb) were measured collectively as a single output.
Mercury (Hg) was measured semi-quantitatively as the ICP-MS was run in standard Journal of Geoscience and Environment Protection mode optimised for most of the measured elements. In this mode, levels of Hg are not measured with the same accuracy. Statistical data analysis using univariate general linear models to identify the significant independent variables affecting the pollutant concentration and exploratory factor analysis used to identify any natural groupings among them to narrow sources and co-emitted pollutants.

Results and Discussion
The mean levels of trace metals were calculated from summing concentrations in the three ambient air PM fractions (thus concentration in PM 10 ) averaged over a specified 2-month period. Means were compared to reference local/international ambient air quality (AAQ) standards where available. Some trace metals did not exceed any of the local/international AAQ standards used (Environmental any of the samples collected during this study. These "non-concerning" metals were caesium, cobalt, copper, mercury, lead, lithium, selenium, strontium, vanadium and zinc. Arsenic levels were below those prescribed in the Canadian guidelines, but this study found that the existent levels tended to predominate in the fine PM fraction. The source of fine PM can be from gas-phase reactions, incineration/combustion processes or from transboundary Sahara dust. However, studies of the trace metals in Sahara dust (collected from a Tobago sampling station) indicated levels that were not significantly different from that contained in the comparative soil samples (Garrison et al., 2010).
The trace metals whose concentrations were found to exceed the prescribed AAQ reference standards for the protection of public health (Ontario-MoE, 2012) at least once during sampling were beryllium, cadmium, chromium, iron, manganese and nickel. The exceedance frequency was tallied for each trace metal, per station, per 2-month period (outlined in Table 3-exceedances highlighted in bold font). The data identified the industrial and urban sites as areas of concern, particularly during June-July '15 and February-March '16 where almost all the trace metals in exceedance occurred. The content of trace metals in each PM fraction is illustrated in Figures 3-8. Public health implications of these trace metals, particularly those found in exceedance AND in the finer particulates (PM 2.5 ) are more serious and require intervention as these are inhaled deeper and take longer to flush out of the lungs, increasing their toxicity.
Beryllium (Be) occurred predominantly in the coarse PM fraction (PM 2.5-10 ) with the highest concentrations measured at the mixed background site ( Figure  3). Likely it originated as a raw material from an industrial process or resuspension from excavation works.
Cadmium (Cd) (notably highest at the rural site only within a limited time) likely reflects an irregular occurrence (Table 3). Its predominance in fine PM ( Figure 4) hints at anthropogenic origin and is concerning as it is a genotoxic carcinogen with an indeterminate source (possibly as a consequence of a bush fire that occurred during the time of sampling), therefore warranting further investigation.
Chromium (Cr), with only two measured exceedances both at the industrial site (Table 3), is of public health concern as it occurs predominantly in the finest PM fraction (PM 1 ) and is carcinogenic ( Figure 5). It is a frequent component of cooling water as well as raw material in industrial steel making, catalyst in industrial processes and rust-proof coating products.
Iron (Fe) recorded exceedances at all four stations (Table 3). Concentrations were consistently high at the Pt. Lisas station for the entire duration of the study, peaking in June-July '15 and February-March '16, falling off in concentration, but still exceeding the limit (4 µg/m 3 ) for safe public health exposure (Ontario-MoE, 2012). Iron is ubiquitous in the environment, however, being a natural crustal element and having multiple sources; including from natural Journal of Geoscience and Environment Protection    Nickel (Ni) is of most concern as a threat to public health. Five exceedances were recorded during sampling, most at the industrial site (Table 3). It is predominantly used in alloy and steel making. Most of the Ni measured during exceedances were recorded with elevated levels of Ni in the fine PM fraction (PM 2.5 ) (Figure 9), which is the dominant form this element was measured in ambient air in this study. It peaked in the same PM fraction at the rural station, indicating a possible common source for both the Cd and Ni contamination at the rural station during Dec '15-Jan '16 period. Nickel and Cr also show similar Journal of Geoscience and Environment Protection

Statistics: Exploratory Factor Analysis
The interpret results since the factors extracted can then be analysed to determine the variables that impact the natural groupings and to make assertions as to why these trace metals group together (e.g. co-emissions, similar source).
Varimax rotation was applied to produce orthogonal factors, with factor loadings of 0.6 or greater considered as substantial. The component matrix for the exploratory factor analysis for the trace metal data yielded a five factor (Varimax with Kaiser normalisation) solution (  Factors 1 and 2 constructs were the most prominent, accounting for 35.2% and 12.9% of the dataset variance respectively. The metal grouping in Factor 1 contains elements which are prominent in coarse PM, includes iron and manganese which occur together naturally in mineral ores and is important input for iron smelting and steel production. These two elements with the highest frequency of exceedances in PM 10 in this study have common sources. Barium is typically used as an additive to cast iron and steel to reduce the size of the carbon grains within the microstructure, as well as an insoluble additive to drilling fluid oils. This factor also includes two trace metals caesium and beryllium which are associated entirely with anthropogenic activities; caesium in drilling muds for oil and gas exploration, and beryllium in the production of alloys used to manufacture springs, electrical contacts, non-sparking tools and spot-welding electrodes locally. Nickel and chromium group together in Factor 2, possibly reflecting the commonality of source from similar industrial use(s). Locally, these elements are used in the steel alloy and coatings manufacture. Factor 3 elements cadmium and vanadium have multiple uses including electroplating on iron and steel prod-

Wind and Precipitation
Particulates dispersion in ambient air is affected by meteorological conditions, particularly wind, temperature and precipitation, as well as topographical features which determine general air flow. With seasonal patterns and local scale meteorological influences superimposed, pollutant dispersion can be complex. The wind fields generated from sampling stations showed a migration of the predominant easterly winds over the study period, significantly different from the predominant winds measured at the national weather station at Piarco International Airport (10.58N, 61.35W).
The industrial station which measured the most frequent exceedances has a variable wind field (illustrated in Figure 9). The direction from which the dominant winds blew varied during the study period and may account for the high levels of some of the trace metals measured in June-July '15 period. In June-July '15 ( Figure 9(a)), the dominant winds came from the northeast across the entire industrial estate (the sampling station was located on the southern periphery of the industrial estate). By December '15-January '16 ( Figure 9(d)), the wind field was much narrower, predominantly southeast, but measured the highest frequency of sluggish winds (9.9% of measurements were <1.54 m/sec) and the lowest frequency of higher speed winds, indicating slower and unidirectional movement of air with pollution loading coming from sources downwind of the industrial estate, mostly residential and agricultural land use areas.

Public Health Impact
Fine PM in ambient air can originate from local gas-phase reactions, fuel incineration/combustion processes, vehicle exhaust, grassfires as well as transboundary Sahara dust. Studies of the trace metals in Sahara dust (collected from a Tobago sampling station) indicated levels that were not significantly different from that contained in the comparative soil samples (Garrison et al., 2010). Nickel and chromium featured prominently in the fine sediment contaminants of the riverine systems in Trinidad indicating that this is an important sink for these metals (Surujdeo-Maharaj, 2010). Studies on nearshore marine and coastal sediments however indicate significant levels of trace metals particularly on Trinidad's west coast (Mohammed et al., 2011;Norville, 2019). The inference is therefore that there are significant streams of waste containing trace metals emanating from Trinidad's west coast. The trace metals can come from burning materials contaminated with the trace metals (such as Cd in the bush fires at Waterloo) or from iron, steel and alloy smelting & recovery plants and cement plants (such as at Pt. Lisas), or traffic emissions and erosion from vehicles (such as in Port-of-Spain and San Fernando).
Several trace elements are genotoxic, meaning there is no "safe exposure level". Arsenic is genotoxic and was predominantly found in the fine PM fraction in the ambient air locally. Levels measured in this study (0.00 -0.015 µg/m 3 ) were relatively on par with typical levels measured in the ambient air in the UK (1 -5 and 5 -10 ng/m 3 in rural and urban locations; and up to 0.002 -2.3 µg/m 3 at industrial sites and non-ferrous smelting facilities) (DEFRA, 2008;European Com-mission, 2001), but were below the Canadian public health standards (Ontario-MoE, 2012) used for comparison in this study, and therefore not regarded as a major public health concern.
This study found that two of these; chromium and nickel; exist at levels that might be harmful. In many cases where it doesn't cause a disease, it often adds to the burden of disease, such that it worsens asthma symptoms in people so prone, exacerbates crises from cardio-vascular complications and COPD increases in occurrence and mortality from lung cancer. Management strategies here have to consider the costs involved in regulation vs the non-regulated scenario and the human health components.
The magnitude of the economic and social benefits to be had (Trejo-Gonzalez et al., 2019) from addressing the air pollution issues and alleviating the burden of disease on children, the workforce and the general population is the current general policy push to get action from regulators (World Health Organisation, Regional Office for Europe, 2019; United Nations Children's Fund, 2017). The elements that exist in concentrations that exceed limits for the protection of public health, the higher the concentrations of the elements in the finer particulates, the greater the detrimental health impact. The findings also suggest that there is avenue for epidemiological work (Gowrie, Agard, Barclay, & Mohammed, 2016;Lipferta & Wyzgab, 2019) in identifying cancer clusters in the non-smoking exposed population due to chronic exposure to air pollutants.
With regard to management, several of the trace metals detected in this study are co-emitted, such as Fe and Mn, therefore, controlling for the particulate emissions (point source and fugitive emissions) at the industrial site can have the adjunct effect of controlling the trace metals contained in them (such as the Factor 1 trace elements). The particulates containing the trace metals also carry other chemicals substances that may pose harm. The maximum benefit would be gained by reducing the levels of overall fine and respirable articulates in the ambient air.

Conclusion
• Iron, manganese and nickel are the trace metals with the highest frequency of exceedance of the Canadian public health protection guidelines regulating contaminants in ambient air; chromium, beryllium and cadmium also exceeded these guidelines but less frequently (once or twice during sampling).
• The data clearly point to nickel as being very "concerning" as it is genotoxic, is prevalent in the finest PM fraction in ambient air and is frequently in exceedance. • Overall reduction of fine particulates in the ambient air would have the most impact on reducing negative health effects on the exposed public.