Exploratory analysis of the atmospheric levels of BTEX, criteria air pollutants and meteorological parameters in a tropical urban area in Northeastern Brazil

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EXPLORATORY ANALYSIS OF THE ATMOSPHERIC LEVELS OF BTEX, CRITERIA AIR POLLUTANTS AND METEOROLOGICAL PARAMETERS IN A TROPICAL URBAN AREA IN NORTHEASTERN BRAZIL L´ıcia P.S. Cruz , Daniela F. Santos , Ivanice F. dos Santos , ´Icaro V.S. Gomes , Akacia ´ V.S. Santos , Keliane S.P.P. Souza PII: DOI: Reference:

S0026-265X(19)31554-1 https://doi.org/10.1016/j.microc.2019.104265 MICROC 104265

To appear in:

Microchemical Journal

Received date: Revised date: Accepted date:

28 June 2019 12 September 2019 14 September 2019

Please cite this article as: L´ıcia P.S. Cruz , Daniela F. Santos , Ivanice F. dos Santos , ´Icaro V.S. Gomes , Akacia ´ V.S. Santos , Keliane S.P.P. Souza , EXPLORATORY ANALYSIS OF THE ATMOSPHERIC LEVELS OF BTEX, CRITERIA AIR POLLUTANTS AND METEOROLOGICAL PARAMETERS IN A TROPICAL URBAN AREA IN NORTHEASTERN BRAZIL, Microchemical Journal (2019), doi: https://doi.org/10.1016/j.microc.2019.104265

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Highlights 

BTEX compounds were measured in the urban atmosphere of Salvador in rainy and dry periods, and toluene was the most abundant pollutant.



Vehicular emissions were identified as the main emission source of the BTEX compounds in the study area.



PCA and HCA contributed to a better interpretation of the correlations between BTEX compounds, criteria air pollutants and meteorological parameters.



BTEX levels were influenced by seasonality, as solar radiation and high temperatures contributed to a decrease of these levels in the dry period.

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EXPLORATORY ANALYSIS OF THE ATMOSPHERIC LEVELS OF BTEX, CRITERIA AIR POLLUTANTS AND METEOROLOGICAL PARAMETERS IN A TROPICAL URBAN AREA IN NORTHEASTERN BRAZIL

Lícia P. S. Cruz1*, Daniela F. Santos1, Ivanice F. dos Santos2, Ícaro V. S. Gomes1, Akácia V. S. Santos1, Keliane S. P. P. Souza1 1

Analytical Chemistry Department, Chemistry Institute, Federal University of Bahia. 40170-270, Salvador, Bahia, Brazil 2 Department of Exact Science, State University of Feira de Santana, 44036-900, Bahia, Brazil

*Corresponding author. Campus Universitário de Ondina, 147, Salvador – Bahia - Brazil. 40.170-115. E-mail address: [email protected] (L. P. S. Cruz)

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ABSTRACT Air pollution is responsible for serious damage to the environment and to the health of the population. The objective of this study was to apply chemometric methods to determine and evaluate the relationships between the atmospheric concentrations of BTEX (benzene, toluene, ethylbenzene and xylene) compounds, criteria air pollutants and meteorological parameters, as well as to identify the type of emission source associated with these pollutants at seven sites in the city of Salvador, Bahia, Brazil during rainy and dry periods. BTEX compounds were monitored using passive samplers with a solvent desorbable adsorbent (activated charcoal) for 14 days and determined by gas chromatography with flame ionization detection. Criteria air pollutants were determined using automatic continuous analyzers and the meteorological parameters by means of multi-parameters sensors. The average data obtained were submitted to multivariate analysis, using principal component analysis (PCA) with varimax rotation and hierarchical cluster analysis (HCA). Some diagnostic ratios between the BTEX species, besides correlation analysis between the pollutants and meteorological parameters were also used to identify the origin of the emissions. The sum of average concentrations of BTEX compounds was lower in the dry period (5.90 ± 3.28 μg m-3) than in the rainy period (7.95 ± 2.95 μg m-3), probably due to higher values of temperature and solar radiation which favor photochemical reactions in the dry period, thereby increasing the rate of removal of the BTEX compounds from the atmosphere. The first three principal components together (PC1, PC2 and PC3) explain 80.1% of the total data variance, with a tendency to separate the samples into two groups, depending on the seasonal period. According to HCA, four main groups were formed with high degrees of similarity. Strong correlations were found among BTEX species, and between these compounds with CO, NO, and NO2, thus indicating a common emission source for these compounds, the vehicular fleet. Toluene/benzene (T/B), m,p-xylene/benzene (m,p-X/B) and o-xylene/benzene (o-X/B) ratios suggested that vehicular emissions constituted the main source of BTEX compounds. PCA and HCA results also confirmed these observations.

Keywords: BTEX; PCA; HCA; Criteria air pollutants; Interspecies ratios; Urban air quality

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1. Introduction

The atmosphere absorbs a large amount of solid, liquid or gaseous compounds from both natural and anthropogenic sources. According to local characteristics, they can be dispersed, transformed or transported and subsequently deposited by wet or dry processes. The dynamics of this system depend mainly on atmospheric reactions, removal mechanisms, and meteorological and topographic conditions [1]. Urban expansion with the growing fleet of vehicles in circulation, and the increase of industrial activities in urban areas are associated with fuel consumption demand, resulting in air quality degradation. Urban air pollution has significant regional-scale impacts on human health, ecosystems productivity, and reduced visibility.

In

addition, it can promote global-scale changes in climate, ozone depletion and increased oxidative capacity of the atmosphere [2,3]. Several pollutants can be emitted into the atmosphere in urban areas, such as: particulate matter (PM), classified according to particle diameter as PM10 (<10 μm), PM2.5 (<2.5 μm) and ultra-fine particles (<0.1 μm), resulting from incomplete burning of fuels and their additives, as well as

brake and tyre wear [4]; and gaseous

pollutants such as sulfur dioxide (SO2), from oxidation of sulfur present as impurity in some fuels; carbon monoxide (CO), produced by partial oxidation of hydrocarbons; nitrogen oxides (NOx = NO + NO2), obtained by internal combustion in engines, under conditions of high temperatures and pressures [5,6]; volatile organic compounds (VOCs) constituting an important group of compounds with boiling point below 250 °C at ambient atmospheric pressure including aromatics, aliphatics, aldehydes, ketones, ethers, carboxylic groups and alcohols [7]; and tropospheric ozone (O3), a secondary pollutant formed through reactions involving VOCs and

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NOx, in the presence of sunlight [8]. Various studies have demonstrated scientific evidence of the relationship between these air pollutants and the negative impacts on human health [9-11]. It is estimated that approximately 35% of all VOCs emissions into the atmosphere come from vehicular sources [12]. Among these compounds, the most abundant in urban environments are BTEX compounds (benzene, toluene, ethylbenzene and m,p,o-xylenes). These compounds have a harmful impact on human health, because they are toxic [13] and, in the case of benzene, it is classified as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer (IARC), associated with myeloid leukemia, besides causing neurological, endocrine and immune changes [14,15]. BTEX compounds in the gas phase are degraded mainly by photolysis and/or chemical reactions with reactive species such as hydroxyl radical - OH (diurnal reactions) and nitrate radical - NO3 (nocturnal reactions). In these reactions there is the formation of free radicals such as organic peroxy (RO2) and hydroperoxy (HO2) which favor the transformation of NO into NO2, contributing to the increase of tropospheric ozone. Therefore, BTEX compounds have a great influence on atmospheric chemistry, since they are considered important precursors in forming oxidizing substances such as O3

and

peroxyacetyl nitrate (PAN), as well as

secondary organic aerosols (SOA) [16, 17]. BTEX emissions in urban areas can be used as indicators of organic air pollutants from vehicular emissions. Benzene originates predominantly from these sources and is considered a marker of vehicular exhaust and evaporative emissions [18]. Particularly, the toluene/benzene (T/B) ratio is widely used as an indicator of these emission sources and some studies report T/B values lower than 3 as a

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characteristic of vehicular emissions in various locations around the world [19,20]. In addition, other ratios are considered as vehicle emissions indicators such as m, pxylene / benzene (m, p-X /B) and o-xylene / benzene (o-X/B) [19]. Chemometrics is extensively used to perform classification, calibration and exploratory analysis, and can be divided in supervised and unsupervised methods. Unsupervised statistical methods are used to study the data structure, to determine similarities between the samples and to check outliers in the data set. Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) are the most widely used tools to explore similarities and patterns between samples, allowing to obtain a greater amount of information [21,22]. In recent years, these tools have also been used in atmospheric chemistry to evaluate air quality by identifying possible relationships between different air pollutants and meteorological parameters as well as to indicate emission sources [12,23-28]. The aim of this study was to apply multivariate techniques to determine and evaluate the relationships between the concentrations of the BTEX compounds, criteria air pollutants (CO, NO2, NO, NOx, O3 and PM10) and the values of meteorological parameters (wind speed, relative humidity, precipitation, temperature and solar radiation), as well as to identify the type of emission source associated with these pollutants, during two different periods of the year (rainy and dry), at seven sites impacted by the vehicular fleet in the city of Salvador, Bahia, in northeastern Brazil, contributing to the assessment of the air quality.

2. Material and Methods

2.1. Description of the urban area

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Salvador, the capital of Bahia state, is located in a peninsular area of 361 km2 on the northeast coast of Brazil (12°58'16" S and 38°30'39" W). It has a humid tropical climate and predominant winds in the east quadrant (SE, E and NE), receiving air masses from the Atlantic Ocean most of the time and little influence from industrial emissions. The average annual rainfall is above 1700 mm, with higher precipitation occurring between the months of April and August (rainy period) and lower in the months of September to March (dry period) [27]. This city has around 3.0 million inhabitants and an expressive vehicular fleet causing constant traffic jams, totaling 996,931 vehicles until May 2019 [29], which corresponds to about one vehicle for every three inhabitants. Seven sampling points S1-S7 (Fig. 1) were selected considering traffic density and proximity to the air quality monitoring network stations in the city of Salvador. The S1 site is a low traffic region impacted mainly by the flow of light vehicles, S2 and S3 sites are intermediate traffic regions affected mostly by intense flow of public transport (buses), S4 and S6 sites are high traffic regions impacted by an intense flow of public transport and light vehicles, and finally the S5 and S7 sites are mainly affected by an intense flow of heavy vehicles.

Fig. 1. Sampling sites in the city of Salvador, Bahia, Brazil

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2.2. Sampling and analytical methods for BTEX determination

BTEX compounds were sampled simultaneously at seven different urban sites in Salvador for 14 days during two different periods, 2014/07/14-28 (rainy season) and 2014/11/28 - 2014/12/12 (dry season), using Radiello® passive samplers manufactured by Fondazione Salvatore Maugeri (Padova, Italy). These samplers consist of a cylindrical adsorbing cartridge inserted in a cylindrical diffusive body of microporous polyethylene. The cartridge used was a stainless steel cylinder, with 100 mesh grid opening and 5.8 mm diameter, packed with 530 ± 30 mg of 35–50 mesh activated charcoal. Other studies have also used Radiello® passive samplers (chemically or thermally desorbable) for monitoring air quality [12,24,30,31]. The analytes, chemically desorbed with carbon disulfide (CS2), were analyzed in a gas chromatograph (7820A GC System, Agilent) equipped with a flame ionization detector (FID). Chromatographic conditions used for the quantification of the BTEX compounds were described in a previous study [32].

2.3. Measurements of criteria air pollutants and meteorological parameters

In the city of Salvador, the Environmental Protection Company (CETREL) is responsible for the operation of the air quality monitoring network. Automatic monitoring stations are located in urban areas; however, BTEX compounds have been not monitored, because the Brazilian environmental legislation [33], which establishes national air quality standards in outdoor environments, does not define standards for these compounds.

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On-line automatic analyzers manufactured by Environnement, frequently calibrated, were employed to monitor the following criteria air pollutants at these stations: nitrogen oxides (NOx = NO + NO2) and O3 were measured by using chemiluminescence method, while CO and PM10 by infrared and by optical properties, respectively. Meteorological parameters (wind direction and speed, relative humidity, temperature, precipitation and solar radiation) were also monitored using Met One's multiparameter sensors. The average concentrations of criteria air pollutants and the average values of meteorological parameters in each sampling period were provided by CETREL. Information concerning criteria air pollutants concentrations and values of air temperature, wind speed, relative humidity (average, and minimum and maximum) during the sampling periods are presented in the Supplementary File (Supplementary Information, Table S1).

2.4. Statistical analysis

Principal component analysis (PCA) and hierarchical cluster analysis (HCA) belong to exploratory data analysis. PCA is an important method in multivariate data analysis and has two main applications: visualization of multivariate data and data reduction and transformation. Two- or three-dimensional projection of samples is usually constructed using the axes as factors for visual analysis. Each PC is a linear combination of the original responses and the PCs are orthogonal to each other. HCA is a method which allows grouping of samples based on their similarities or differences. The HCA result is usually presented in a dendrogram, a tree-like graph that shows the organization of samples and their relationships [21,22].

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The correlations between the average concentrations of the measured BTEX compounds and data provided by the air quality monitoring network of the city of Salvador (mean concentrations of the criteria air pollutants and mean values of meteorological parameters) were evaluated in seven sampled points. The data were submitted to multivariate analysis, applying PCA with Varimax rotation and HCA, using the Statistica software 7.0 [34] with data pre-processed by autoscaling. The letters R and D were used to define the rainy and dry periods, respectively. Pearson's correlation coefficients were also used to quantify associations between two variables.

3. Results and discussion

3.1 Concentrations of the BTEX compounds

A statistical summary of BTEX concentrations measured in the city of Salvador during dry and rainy periods is presented in Table 1. The results presented herein demonstrate that toluene had the highest concentration, followed by benzene, m,pxylene, ethylbenzene and o-xylene compounds in both periods.These observations are consistent with previous studies where toluene was the most abundant pollutant among BTEX compounds in the atmosphere of urban areas [24, 35-38].

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Table 1 - Statistical summary (average, minimum and maximum) of the BTEX concentrations (μg m-3) in the city of Salvador during dry and rainy periods Dry Period

BTEX Average ± SD

Median

Rainy Period Min

Max

Average ± SD

Median

Min

Max

Benzene

1.55 ± 0.99

1.27

0.46

2.73

2.05 ± 0.82

1.76

1.06

3.38

Toluene

2.47 ± 1.49

2.52

0.49

4.37

3.18 ± 0.97

3.06

1.70

4.44

Ethylbenzene

0.64 ± 0.25

0.53

0.42

1.11

1.00 ± 0.42

0.95

0.46

1.64

m,p-Xylene

0.72 ± 0.41

0.72

0.34

1.45

1.04 ± 0.43

0.93

0.52

1.79

o-Xylene

0.52 ± 0.14

0.47

0.44

0.82

0.68 ± 0.31

0.49

0.45

1.13

ΣBTEX

5.90 ± 3.28

7.95 ± 2.95

The main process of degradation of BTEX compounds in the atmosphere is through photochemical reactions with OH radicals. Benzene, toluene and ethylbenzene have atmospheric lifetimes of 9.4, 1.9 and 1.6 days, respectively, assuming [OH] = 106 molecules cm-3. Benzene is the most stable species in the atmosphere because it has a longer atmospheric lifetime and therefore a lower reactivity, followed by toluene. Xylenes are considered the most reactive species and generally remain in the atmosphere for a short time, having atmospheric lifetimes of 20.3 h (o-xylene), 19.4 h (p-xylene) and 11.8 h (m-xylene) [1, 39]. Benzene and toluene with lower reactivities remain in the atmosphere longer and can be transported over long distances, up to tens of kilometers without degradation. Thus, these compounds can also have been from other regions and transported to the sampling sites, resulting in higher concentrations. Studies indicate strong evidence that toluene is also emitted by plants [40-42], which may also justify the higher concentrations found for this compound when compared to the concentrations of the other compounds in the city of Salvador, because the toluene, besides being emitted from the vehicular fleet, can also have been emitted from biogenic sources from neighboring regions and transported to the sampling sites. The low

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concentrations of the xylenes can be attributed to the higher reactivity of these compounds in the atmosphere. As shown in Table 1, benzene levels were 0.46–2.73 and 1.06-3.38 μg m-3; toluene levels were 0.49–4.37 and 1.70-4.44 μg m-3; ethylbenzene levels were 0.42– 1.11 and 0.46-1.64 μg m-3; m,p-xylene levels were 0.34–1.45 and 0.52-1.79 μg m-3; and o-xylene levels were 0.44-0.82 and 0.45-1.13 μg m-3, in the city of Salvador in the dry and rainy periods, respectively. For benzene, which is carcinogenic to humans, the concentrations found in this city are below ambient air quality and cleaner air for Europe limit of 5 μg m-3 [43]. However, the maximum concentrations of benzene in both periods exceed a more restricted value (2.30 μg m-3) according to Ambient Air Quality Criteria (AAQCs) developed by the Ontario Ministry

of the

Environment (OME) [44]. The concentrations of BTEX compounds found in this study are lower when compared to other studies carried out elsewhere in the world (India, China, Greece, Thailand) [37,45-47], but are comparable to studies conducted in other Brazilian cities in recent years [25,41,48-50]. The justification for this fact should be associated with the regulations on vehicle emissions adopted in Brazil through the Program for Control of Air Pollution by Motor Vehicles (PROCONVE) and other specific environmental laws, emission control technologies, use of alternative fuels and improvements in fuel quality [51]. In addition, the city of Salvador has a privileged geographical position because it is located on the Atlantic Ocean coast, allowing the penetration of trade winds from east to the interior of the city, favoring a good dispersion of the air pollutants. The distribution of BTEX fractions in the atmosphere was similar during the two sampling periods, being 26%, 42%, 11% and 21% in the dry period, and 25%, 40%,

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13% and 24% in the rainy season, considering the compounds benzene, toluene, ethylbenzene and sum of xylenes, respectively. Similar results were obtained in the city of Rio de Janeiro, in southeastern Brazil, with values of 11% for benzene and ethylbenzene, 48% for toluene and 21% for xylenes [25]. Different fuels are commonly used in Brazil, such as: diesel with 10% biodiesel, hydrated ethanol, compressed natural gas and gasohol – gasoline blends with 25-27% v/v of anhydrous ethyl alcohol as antiknock additive, containing a maximum of 1.0% v/v of benzene and 35% v/v of aromatic hydrocarbons [27,52]. Such similarities in these two Brazilian cities are possibly related to the type of emission source of these pollutants, predominantly from light vehicles fleet (‘‘flex-fuel’’ vehicles) that use mainly gasohol in addition to hydrated ethanol. Table 1 also shows that the sum of the average concentrations of BTEX compounds determined in the dry period (5.90 ± 3.28 μg m -3) is lower than in the rainy period (7.95 ± 2.95 μg m-3). In cities with temperate climate, variations in BTEX concentrations can also be attributed to increased emissions during the winter due to combustion processes for indoor heating. However, in cities with tropical climate such as Salvador, indoor heating is not required, and the evaluation of meteorological parameters helps to better understand the seasonal variations, since these have influence on the BTEX compound concentrations in urban areas, at different times of the year [53,54]. The city of Salvador is surrounded by a great water volume, as it is bathed by the All Saints Bay and the Atlantic Ocean (Fig. 1), thereby making relative humidity values are high throughout the year. Moreover, according to data obtained from the air quality monitoring network, the wind speed values had little variation in both sampled periods, with a small increase during the dry period (Table S1).

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However, average solar radiation values of 1107 KJ m-2 and 2216 KJ m-2 in the rainy and dry periods, respectively, indicate that there was a significant variation in this parameter, with an increase of more than 100% in the dry period. Therefore, the trend of decreasing BTEX concentrations observed in the urban areas of the city of Salvador during the dry period is probably related to the increase of BTEX degradation through photochemical reactions, increasing the removal rate of these compounds from the atmosphere, and thus reducing BTEX concentrations in this period. These observations are also confirmed by other studies [35,36]. Furthermore, although average temperatures in this city are high in both periods (24 ºC and 27 ºC in the rainy and dry periods, respectively), the maximum temperature values can reach 32 ºC in the dry period (Table S1), favoring photochemical reactions [54-56].

3.2. Multivariate Analysis

In order to carry out the exploratory analysis, a data matrix was generated (14x16) in which results of of sampling at the seven sites in the rainy (RS1-RS7) and dry (DS1-DS2) periods were arranged in lines, while the informations about BTEX variables (benzene, toluene, ethylbenzene, m,p-xylene and o-xylene), criteria air pollutants (NO, NO2, NOx, CO, O3 and PM10) and meteorological parameters (wind speed, temperature, relative humidity, precipitation and solar radiation) were disposed in columns. Autoscaling was chosen as pre-processing due to the different orders of magnitude of these variables. The PCA was performed with Varimax rotation, because it allowed a better data interpretation.

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The first three principal components (PC1, PC2 and PC3) were selected for data characterization, as they presented eigenvalues greater than 1 and together explain 80.1% of total data variance, as shown in Table 2. PC1 and PC2 can explain 67.3% of the data variance, i. e., more than half of the variance is described. PC1 has accumulated 45.8% of the data variance. The variables that most contributed to data characterization were the criteria air pollutants (NO, NOx, NO2, O3 and CO), because these are the variables which had the largest absolute loadings for this PC (Table 2, Fig. 2). Nitrogen oxides and CO are negatively correlated with O3, meaning the higher the ozone concentrations, the lower the nitrogen oxides and CO concentrations. This was expected as these pollutants are precursors of tropospheric ozone formation [8,25]. Table 2. Loadings of 16 variables on the first three principal components Benzene Toluene Ethylbenzene m,p-Xylene o-Xylene CO O3 PM10 NO NOx NO2 Wind Speed Temperature Humidity Preciptation Radiation Variance% Cumulative variance % Eigenvalue

PC1 0.175 0.182 0.047 0.280 0.135 0.757 -0.759 0.354 0.831 0.877 0.769 -0.277 -0.119 -0.174 0.114 -0.082 45.8 7.32

PC2 0.120 0.135 0.392 0.177 0.205 -0.139 0.362 0.138 0.057 0.142 0.384 -0.081 -0.848 0.946 0.936 -0.957 21.5 67.3 3.44

PC3 0.887 0.832 0.827 0.932 0.846 0.365 0.269 0.627 0.388 0.356 0.139 0.107 -0.406 -0.021 0.239 -0.175 12.8 80.1 2.05

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On the other hand, PC2 has accumulated 21.5% of the data variance and presented only the influence of meteorological parameters - temperature, solar radiation, relative humidity and precipitation - since they are the variables with the highest absolute weights on this PC, according to Table 2. The following correlations were verified: i) temperature and solar radiation had positive correlation; ii) relative humidity and precipitation had positive correlation; iii) temperature/solar radiation and relative humidity/precipitation had negative correlation (Table 2, Fig. 2). Additionally, the wind speed had a low influence on this PC, because it showed an absolute weight close to zero. The direct relationship between temperature and solar radiation was due to the fact that solar radiation is one of the factors influencing the temperature of a site, so that the higher solar radiation intensity, the higher the temperature. Moreover, relative humidity is related to the amount of water in the atmosphere, which explains the direct relationship between precipitation and relative humidity. It is also possible to observe in PC2 that BTEX compounds and criteria air pollutants (NO, NOx, NO2, PM10 and CO) presented negative correlations between temperature and solar radiation, whereas positive correlations between relative humidity and precipitation were verified (Fig. 2). The increase in temperature and solar radiation favor photochemical reactions, mainly involving hydroxyl radical, resulting in increased degradation of BTEX, nitrogenous compounds and CO in the atmosphere, leading consequently to a decrease in their concentrations and an increase of tropospheric ozone concentration, which is one of the pollution problems in urban areas [36, 55, 56]. Regarding PC3, 12.8% of data variance was accumulated. It was observed the BTEX compounds (benzene, toluene, ethylbenzene, m,p-xylene and o-xylene) were

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the variables that most contributed to the discrimination of the sample set, because they had the highest absolute weights on this PC. These compounds were positively correlated (Table 2), thereby indicating a common source for these compounds, predominantly the vehicle fleet of the city of Salvador.

Fig. 2. Plot of loadings (PC1 versus PC2) of the 16 variables

The graph of scores referring to the first two main components (PC1 and PC2) is shown in Fig 3. According to this figure and in relation to PC2, it was possible to observe a tendency towards separation of the samples into two groups, depending on the seasonal period.

Samples collected during rainy period (RS1-RS7) were

displaced to the region of positive scores, while those collected during the dry period (DS1-DS7) were displaced to the region of negative scores. A comparison between the graph scores (Fig 3) and the loading graph (Fig. 2) shows that samples from the rainy period presented higher values both for precipitation and relative humidity, whereas samples from the dry period demonstrated higher values of temperature and solar radiation. These four variables

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were those which most contributed to the discrimination of samples in PC2, corresponding to average values for temperature and solar radiation of 24 °C and 1107 W m -2, 27 °C and 2216 W m-2; and for precipitation and relative humidity of 119 mm and 76%, 35 mm and 70% in the rainy and dry periods, respectively. Besides, it can also be noted in PC2 that the BTEX compounds exhibited higher average concentrations during the rainy period than the dry period, as it is also shown in Table 1.

Fig. 3. Plot of scores (PC1 versus PC2) for sampling sites of the city of Salvador during the rainy (RSx) and dry (DSx) periods

A very similar behavior of the variables under study could also be verified through the application of HCA. The grouping of data was performed acoording to the Ward method, and the metric used to calculate the degree of similarity was 1Pearson’s r distance. The result is presented in the form of a horizontal dendrogram, where the vertical lines, or the y-axis, represent the groups united in descending

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order of similarity, and the position of the line on the x-axis indicates the distances between the groups that were formed. Thus, the dendrogram should be read from top to bottom (Fig. 4).

Fig. 4. Dendrograms for BTEX compounds, six criteria air pollutants and five meteorological parameters obtained during rainy and dry periods in the city of Salvador

According to the dendrogram data (Fig. 4), it was possible to observe at a distance of 1.5 there was formation of 4 main groups. The first group was formed by BTEX and PM10. The PM10 constituted an isolated subgroup, while BTEX compounds represented the variables which presented well established linear correlations (high degrees of similarity), since they had the shortest 1-Pearson’s r distance, signaling that they came from the same emission source. The second large group was formed by criteria air pollutants (CO, NO, NOx and NO2) also with a high degree of similarity. These compounds are the precursors of tropospheric ozone formation and are also directly related to vehicular emissions in urban areas. There was also formation of a third group consisting of ozone, relative humidity and precipitation, including a

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subgroup due to the high similarity between relative humidity and precipitation. Wind velocity, temperature and solar radiation represented the fourth group, where temperature and solar radiation were considered as a subgroup due to the high similarity of these meteorological parameters, corroborating with the data observed in the PCs.

3.3. Pearson’ correlation of BTEX, criteria air pollutants and meteorological parameters

A linear correlation matrix (Pearson) between data generated by BTEX compounds, six criteria air pollutants and five meteorological parameters is shown in Table 3. Results from the correlation analysis show that BTEX compounds presented positive correlations ranging from strong to very strong, thereby reaffirming that these compounds probably come from the same source. In addition, BTEX compounds were exposed to the same influence of meteorological parameters during the sampling periods. They showed negative correlations with temperature and solar radiation, indicating that these meteorological parameters affect photochemical reactions involving these compounds and contribute to reduce their concentrations in the atmosphere, corroborating the data obtained in the PCs. Nitrogen compounds (NO, NO2 and NOx), mainly from vehicular emissions in urban areas, showed strong positive correlations with BTEX compounds, suggesting these compounds were probably emitted from the same source and presented similarities in the degradation process in the atmosphere, again in accordance with data obtained in the PCs.

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Carbon monoxide (CO) is generated by incomplete combustion processes of nearby vehicle engines and it has been used as an indicator for vehicular emissions in urban areas [57]. Thus, as can be seen in Table 3, CO presents a linear correlation ranging from strong to very strong with BTEX compounds, hence confirming a common source for these compounds in urban areas of the city of Salvador, predominantly the vehicular fleet.

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Table 3. Linear correlation matrix for BTEX compounds, six criteria air pollutants and five meteorological parameters 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

1-B

1

2-T

0.98

1

3-E

0.85

0.79

1

4-m,p-X

0.89

0.95

0.82

1

5-o-X

0.80

0.84

0.98

0.86

1

6-CO

0.67

0.75

0.69

0.85

0.69

1

7-O3

0.02

0.48

0.34

0.51

0.45

-0.43

1

8-PM10

0.51

0.51

0.51

0.73

0.65

0.48

-0.01

1

9-NO

0.5

0.56

0.66

0.63

0.58

0.81

-0.43

0.65

1

10-NOx

0.60

0.59

0.5

0.61

0.64

0.8

-0.44

0.62

0.98

1

11-NO2

0.58

0.60

0.62

0.67

0.56

0.74

-0.31

0.32

0.62

0.76

1

12-WS

-0.18

-0.16

-0.12

-0.22

-0.15

-0.29

0.11

-0.23

-0.32

-0.32

-0.24

1

13-T

-0.62

-0.61

-0.67

-0.77

-0.69

-0.18

-0.33

-0.41

-0.34

-0.39

-0.45

0.54

1

14-RH

0.07

0.05

0.36

0.08

0.17

-0.29

0.4

0.04

-0.11

-0.04

0.19

0.14

-0.75

1

15-PPT

0.70

0.75

0.50

0.68

0.72

-0.04

0.4

0.3

0.13

0.2

0.37

-0.05

-0.86

0.87

1

-0.48

-0.52

-0.57

-0.65

-0.60

-0.01

-0.32

-0.36

-0.22

-0.28

-0.38

0.15

0.89

-0.87

-0.98

16-SR

3.4. Ratios between BTEX compounds

The ratios of toluene/benzene (T/B), (m,p)-xylene/benzene (m,p-X/B) and oxylene/benzene (o-X/B) are commonly used for the determination and evaluation of the type of emission source associated with these pollutants [58]. Figure 5 shows these ratios obtained for each sampling point in the rainy and dry period. The ratios of T/B <2.7, m, p-X/B <1.8 and o-X/B <0.9 are indicative of vehicular emissions sources [20,58] and were the rations adopted in this study. All data achieved were below these limit values, which confirms that BTEX compounds were emitted from the same source, predominantly vehicular emissions. The ratios between toluene and benzene (T/B) found in the city of Salvador were similar to those found in other cities, such as: Cairo (1.29 - 2.45), Gdansk (1.43)

16

1

23

and Caracas (2.04) [20, 56, 59], indicating that the main source of benzene and toluene in these cities was the vehicular fleet. Possible differences in these ratios may reflect the differences between type and age of vehicles, composition, type and quality of the fuels, influence of meteorological parameters, industrial activities and gas stations near monitoring areas [54,60].

Fig. 5. Concentrations ratios of toluene/benzene (T/B) versus m,p-xylene/benzene (m,p-X/B) and versus o-xylene/benzene (o-X/B)

The ratios between m,p-xylene and ethylbenzene (m, p-X/ E) are used to investigate the degree of photochemical reactivity of these compounds in the atmosphere, because the photochemical reactivity of the m,p-xylene is larger than of ethylbenzene [39,58]. The m,p-X/E ratios have a tendency to increase with increased temperature and solar radiation due to enhanced photochemical activity [53]. In this study, it was observed that during dry period, in some sites, there was an

24

increase in the m,p-X/E ratios (Table 4), contributing to the formation of tropospheric ozone. The m, p-X/E ratios showed in general low variability between the different sites for each sampling period, indicating the existence of a common emission source throughout the region, which is also reinforced by the strong correlation between these compounds, as demonstrated by the Pearson correlation (Table 3).

Table 4. Ratios of m,p-xylene/ethylbenzene (m,p-X/E), average concentrations of ozone and temperatures during rainy and dry periods at seven sampling sites in the city of Salvador Rainy Sites

Dry Period

Period m,p- X/E

-3

O3 (µg m )

T (°C)

m,p- X/E

-3

O3 (µg m )

T(°C)

S1

1.48

20.02

24.50

0.89

16.49

27.10

S2

0.66

9.23

24.70

0.75

9.82

27.30

S3

1.52

12.37

25.30

1.76

8.83

27.50

S4

0.94

22.18

24.20

0.60

13.55

26.30

S5

1.35

11.39

23.60

1.37

11.58

25.80

S6

0.94

16.49

25.10

1.29

14.14

27.60

S7

0.80

14.90

23.90

1.03

15.10

26.50

4. Conclusions

In this study, concentrations of BTEX compounds were measured for the first time in the city of Salvador. The relationships between the concentrations of these compounds, criteria air pollutants and meteorological parameters were determined in two different periods (rainy and dry) using PCA and HCA analyses. Toluene was the most abundant amongst the BTEX compounds, followed by benzene, m,p-xylene, ethylbenzene and o-xylene. BTEX concentrations showed a

25

seasonal variation, with lower concentrations in the dry period, probably due to higher chemical removal reaction rates of BTEX compounds. The values obtained from the T/B, m, p-X/B and o-X/B ratios indicate the combustion of fuel in engine vehicles as the main source of BTEX. The significantly positive correlations between BTEX and the CO, NO, and NO2 compounds confirm that the emission sources for all these pollutants are similar, predominantly the vehicle fleet. This fact, associated with strong correlations between solar radiation and temperature also confirm the contribution of these compounds to the formation of tropospheric ozone in the atmosphere of Salvador city, especially in the dry period. PCA and HCA results corroborate the relationships obtained in this study, demonstrating the importance of these chemometric methods for the air quality assessment. This study shows the need for the implementation of effective actions for controlling vehicular emissions in the city of Salvador, and for a revision of the Brazilian legislation to introduce air quality standards, especially for benzene, which is considered carcinogenic to humans.

Acknowledgements

The authors acknowledge the Foundation for Research Support of the State of Bahia (FAPESB) for financial support, the National Council for Scientific and Technological Development (CNPq) and the Coordination for the Improvement of Higher Education Personnel (CAPES) of Brazil (Finance Code 001) for fellowships, and CETREL company for providing data on criteria air pollutants and meteorological parameters.

26

Conflict of Interest

The authors declare no conflict of interest.

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