Determination of PM2.5 concentrations is critical for urbanized areas to evaluate the air quality and to prevent adverse effects on human health. Additionally, the chemical composition of particles has been suggested as finger print of the emission sources, and atmospheric chemical processes.
The PM2.5 concentration and composition has been studied in many countries. There are factors that influence PM2.5 concentration, such as weather and topography. In our study area, as in others, topography plays a key role in atmospheric dynamics, impacting air quality of urbanized and industrialized areas; additionally, the predominant weather determines the atmospheric dynamics (Querol et al., 2007a). The season and climate parameters such as temperature, wind direction, relative humidity, rainfall, cloudiness (Kulshrestha et al., 2009), and some atmospheric phenomena such as thermal inversion, can modify the half-life, and concentration of pollutants in the air. Because of its diameter, the PM2.5, remains longer in the atmosphere, and is efficiently transported; mountain systems can influence their transport and local deposit (Cheng Miao-Ching et al., 2012). Additionally, wind movement, its force, and direction must be considered in the displacement, distribution and final fate of air pollutants. Wind speed is not constant along each day, week, months, seasons, and also between years, because it undergoes variations due to the topographic and thermal features of a given area.
In our study area, we observed discrete differences between seasons, with approximately an increment of 4ºC in the average daily temperature between cold and hot seasons; average daily RH was approximately 10% higher in the dry-cold season compared to the dry-hot season. For AWS, differences were around 0.5 m/s; these discrete changes could be associated with the high altitude of TVMA (2660 m.a.s.l.).
The influence of air flow, direction, and calm winds was observed in our study. The AWS was quite different among sites between seasons. However, it seems that direction of air flow in OX displace the atmospheric particles and move the air pollutants to other site. In the rest of the sites, the influence of east winds flows from AP and SM sites to the SC site, where AP and SM can be considered industrial zones and SC the receptor site, a suburban zone with the highest PM2.5 concentration. Additionally, we observed higher PM2.5 concentration in the dry-cold season compared to dry-hot season in almost all sites. This can be explained by the frequency of calm winds during the cold-season, concomitant with the major frequency of thermal inversion according to the high altitude and recurrent in winter or fall seasons.
We observed different 24h-PM2.5 concentrations among the four sites between the dry-cold and the dry-hot seasons. The SC site exceeded the 24-hour PM2.5 concentration limit of 45 µg/m3, established by the Mexican government (NOM-025-SSA1-2014). However, at least one day in the sampling period was out of the concentration limit for the rest of the sampling sites. Nevertheless, according to international 24-hour PM2.5 limit of 25 and 20 µg/m3, established by World Health Organization (WHO) and European Community (WHO 2013a, OJEU, 2008) respectively, only OX is under the international concentration limit, indicating that the rest of the population is breathing unhealthy air.
Differences among element concentrations between the two seasons were observed, likely due to the seasonal weather parameters mentioned above that determine the element distribution and its presence, such as temperature, WD and WS, as well as the topographical conditions of the region.
The highest metalloid (Sb) and transition metals (Co, Cu, Mn) concentrations were found at the SM site, which did not have the highest PM2.5 concentrations. SM is located in the southeast of MATV, bordered by Lerma-Tenango del Valle and Toluca México highways, neighboring to the Lerma-Toluca industrial park, with the main economic activity being shoes and clothing manufacturing.
The AP and SC sites share a similar trend in metalloid (Sb) and transition metals (Co, Cu, Mn) concentrations. Both sites are located north of the Toluca valley, with different economic activities; in addition to airplane transit on AP site, there is an industrial settlement, land dedicated to agriculture, and San Antonio roadway. However, SC is considered an urban settlement without natural barriers, the predominant wind coming from the east, and with a vector wind in the dry-cold season from the southeast that is influenced by the emissions generated in Toluca downtown. It is possible that additional emissions, mainly produced by industrial plants located in the south and south-east AP and SM sites, as well as by the resuspension of road dust and biomass combustion (Quiterio et al., 2005; Mansha et al., 2012) contribute to local pollution.
The station with the lowest PM2.5 concentration was OX site, located to the west of Toluca downtown, is characterized by high population density; it is considered a residential area with low levels of vehicular traffic, without industrial plants. The main difference from the rest of the sites was the air flow direction that comes from the southwest, opposite to the downtown and industrial areas. Near the area, at the north of OX site, there are mountain systems with elevations between 2800–3000 m.a.s.l. that run from west towards east to the limit of Toluca downtown that probably acting as a barrier to the pollutants that flow from Toluca downtown to OX. Additionally, the OX site is a place where calm winds are less frequent, suggesting an efficient removal of pollutants.
In the MATV we observed that in the dry-hot season the metal and metalloid concentrations had more important decrease compared to the dry cold-season. When calculating the enrichment of the element concentration of the dry-cold season in contrast to the dry-hot season, we detected that in some sites the increment was a hundred or thousand-fold. However, the PM2.5 mass did not change in the same magnitude, suggesting that in the dry-cold season the frequency of calm winds and probably the thermal inversion at high altitude induce a major permanence of metals and metalloids, associated with the increment in the economic activity of the area.
Comparisons between PM2.5 and element concentrations detected in our study, during the dry-hot season in the MATV, and those found in other countries showed that the concentrations are similar in range to those reported in other cities in the world (Table 5).
On the other hand, the content of metalloid and transition metals in PM2.5 in the dry-cold season at the SM, SC, and AP sites were above those reported in all other cities around the world (Table 5) the concentrations of metalloid and transition metals observed in the MATV were in the order of micrograms with respect to other cities with concentrations expressed in nanograms. The concentrations observed in the dry-cold season were higher than the limits recommended by regulatory agencies (e. g. EPA) to prevent human health.
Our study has important methodological and study design differences respect local and international studies, including the sampling time, which are relatively short. Moreover, the metal(loid)s extraction method varies among the studies which define the chemical form of the elements and the biological availability (Espinosa et al., 2002). The use of efficient and alternative tools, such the ICP-MS, for the analysis of PM2.5 can help to detect, quantify and design options to address pollution problems in large cities (Saldarriaga et al., 2009; Murillo et al., 2015), although the sensitivity of different detection systems of analytical methods can influence the data observed.
Cobalt (Co), which is a recurring pollutant of wastewater, is a metal that is released into the atmosphere as a particle, its main use is in the petrochemical and plastic industry as a catalyst, it can be released in scrap metal recycling, foundry and metal refining, additionally, Co can be released after burning fossil fuel. Co content in PM2.5 samples from MATV in AP, SC and OX sites were observed in the occupational setting concentration reported (Kim et al., 2006), but Co speciation its needed to be determine to explain the human health adverse effect.
Chromium (Cr) is released in the commercial and residential fossil fuel, natural gas, oil and coal combustion in addition to emissions in the metallurgical industries (e.g. ferrochromium or chromium), chromium platers, and paper industry (Kimbrough et al., 1999; Xu, et al., 2013). The Cr content in MATV PM2.5 is higher respect the annual standard stablished by WHO, and those reported to other countries (Table 5).
Copper (Cu) atmospheric emission sources include copper smelters, copper and iron ore processing, iron and steel production, combustion sources, municipal incinerators, copper sulfate production, brass and bronze production, carbon black production, cooling systems, brake wear particles, both by direct emissions and by suspended road dust (Georgopoulos et al., 2001; Keuken et al., 2013). According with the Cu concentration found in the PM2.5 from MATV, this metal was under the maximum annual concentration and copper concentration for 24-hour period stablished by EPA (1987a), 30 and 100 µg/m3, respectively (Table 5).
Manganese (Mn) is part of particle component, the crustal rock is the major natural source, other sources include forest fires, vegetation (e.g. leaching from plant tissues and dead plants), volcanic activity, and animal excrement. Anthropogenic source includes mining and mineral processing (e.g. nickel), emission from alloy (e.g. steel), the combustion of fossil fuel and in minor degree from combustion of fuel additives. Mn compounds have many applications such as the production of dry-cell batteries, matches, fireworks, porcelain and glass-bonding materials, as a catalyst in the chlorination of organic compounds, in animal feed to supply essential trace minerals, among others (Howe et al., 2004). PM2.5 collected in MATV has a higher contend in Mn respect other reported countries. However, concentrations are below annual standard limit stablished by WHO, but Mn content in PM2.5 from MATV is upper respect the minimal risk level for neurological effects by chronic inhalation (ATSDR, 2012) (Table 5).
Antimony (Sb) is incorporated in textiles, paper and plastics as coadjutant of fire retardants; the primary emissions sources are related with plastic manufacturing, petroleum industry, and structural metal products (Belzile et al., 2011; Tian et al., 2012). Sb contend in PM2.5 in SM, AP, and SC was > 1µg/m3, value referred as industrial area (Table 5), however, Sb has not been classified as carcinogenic in humans by U.S. EPA but ATSDR has placed antimony trioxide as possible human carcinogen. According with the high levels found of Sb in MATV further studies are need to describe the chemical speciation and the potential risk for human health.
Comparison of element concentrations in PM2.5 air samples reported in different countries
Metal(loid)s elements (ng/m3)
Method / sampling
Toluca Valley, Mexico ꬷ
ICP-MS / 24 h; every 6 days, > 8 weeks
Aztatzi et al.,
ICP-MS / 24 h
Remoundaki et al., 2013
24.9 − 33.2
ICP-MS / 12 h
Kfoury et al., 2016
ICP-MS / 24 h; every 3 days
Manousakas et al., 2014
USA, 187 countries
14.0 ± 0.22
ICP-MS / five years/monitor frequency 3–12 days
Bell et al., 2007
ICP-MS / 24 h; every 3 days
Murillo-Tovar et al., 2015
Mexico City, Mexico
ICP-MS / 24 h; every 6 days
Garza-Galindo et al., 2019
dos Santos Souza et al., 2021
ICP-MS /24 h in 10 consecutive days
Feng et al., 2009
ICP-AES / 48 h
Wang et al., 2013
ICP-MS / 24 h
Lee Jin-Hong et al., 2013
ICP-MS / 18–22 h
Dai et al., 2015
Concentration Criteria or limits
† <1 to2 ng/m3; ⸸ 1×104 to 1.7 × 106 ng/m3
* 100 and
‡ 12 ng/m3
§ 30 and 100 µg/m3
* 52 and
⁑ 0.3 µg/m3
⸙ <20 ng/m3. > 1 µg/m3 for industry.
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ꬷ Results of the dry-cold season of Metropolitan area from Toluca Valley, Mexico state, Mexico. † Unpulled sites ⸸ Air concentration range of Cobalt in occupational settings, Kim et al., 2006. ‡ EPA calculated inhalation unit risk estimate. § Copper maximum annual concentration and copper concentration for 24-hour period at a location within one-half mile of a major source, EPA 1987a. * Annual standard, WHO, 2016. ⁑ Minimal Risk Level, as an estimate of a chronic inhalation exposure that is likely to be without appreciable risk of adverse non-cancer effects during a lifetime; for Mn was based on impairment of neurobehavioral function in people, ATSDR (2012b). ⸙ antimony ambient air range, and in industry area observed data, ATSDR 1992.