Contribución de fuentes y origen del material particulado atmosférico en Bogotá, Colombia

Abstract

Air pollution, primarily due to the presence of inhalable particulate matter (PM10), is currently one of the most relevant environmental and health issues worldwide. This pollutant has been associated with adverse effects on ecosystems, climate change, urban infrastructure and, particularly, human health. Exposure to PM10 is considered one of the leading risk factors for deaths and diseases, being associated mainly with respiratory and lung diseases, cerebrovascular accidents and lung cancer. Several anthropogenic activities responsible for the emission of PM10 are carried out in urban contexts. Some of these activities are vehicular transportation, construction and demolition works, power generation, industrial processes, burning of waste and biomass, among others. Large cities also concentrate a high number of inhabitants constantly exposed to poor air quality, due to the emission of particles from different sources with the potential to be inhaled. This has led to studies on the characteristics and origins of PM10, particularly in megacities, with the purpose of preventing and reducing atmospheric emissions, thus improving urban air quality. Bogota, capital of Colombia, is a megacity located in the Andes mountains. Its urban dynamics and weather conditions have contributed to air pollution, which has been recognized by environmental and sanitary authorities at the local level. PM10 has been identified as one of the air pollutants of greatest concern in Bogota, due to its high concentrations and impact on public health. Despite this, there are few detailed studies about the chemical characterization and emission sources of PM10, which is what led to this doctoral thesis. Within this context, and with the aim of carrying out a chemical characterization and a source apportionment study of the PM10 fraction in urban contexts in Bogota, two sampling campaigns were performed. The first focused on the outdoor PM10 at an urban background site, and the second targeted the PM10 of road dust. For the analysis of outdoor PM10, daily samples were collected (24±1 h) during a continuous year (June 2015 to May 2016), obtaining a total of 311 samples. Inorganic compounds were analyzed, such as organic (OC) and elemental carbon (EC), water soluble compounds (SO4 2–, Cl–, NO3 – and NH4 +), major elements (Al, Fe, Mg, Ca, Na, K and P) and traces (V, Cd, Pb, Sr, Ba, Cr, As, Ni, Zn, among others). The data obtained revealed that the carbonaceous fraction (~ 51%) and mineral dust (23%) were the main PM10 components. The results were interpreted in terms of their variability during rainy and dry seasons. Peak values of most of the elements were registered during the dry season due to photochemical activity and forest fires. Six factors (associated with origins of PM10) were identified through a source apportionment analysis, including fugitive dust (28%), road dust (23%), vehicular combustion (23%), secondary PM (21%), metal processing (4%) and industrial emissions (< 1%). Traffic (exhaust emissions + road dust) was the main source of PM10, accounting for ~ 50% of the mass. Considering that the carbonaceous fraction is the major component of PM10, a specific analysis about the characteristics and temporal variability of these aerosols was carried out. The results showed that the averages of OC and EC were, during the year of sampling, 8.92 ± 4.52 μg/m3 and 3.25 ± 1.59 μg/m3, respectively. January, February and March were the months with the highest OC average. This three-month period registered high temperatures and forest fires. EC concentrations had a lower level of variability during the year of study. The OC/EC ratio ranged from 1.66 (June) to 4.88 (March), with an annual average of 3.16 ± 2.01. The secondary organic carbon (SOC) accounted for 45% of the total OC and registered a positive correlation with temperature, suggesting that SOC formation was related to photochemical processes. The back-trajectory analysis revealed that days with air masses from E+NE recorded the highest OC concentrations. This can be explained by the emission of OC and SOC precursor gases from oil industry, forest fires and burning of biomass as an agricultural practice, all of which are typical activities of the northwest of Venezuela and the Arauca, Casanare and Meta departments. For the analysis of PM10 in road dust, identified as one of the major sources of PM10 in Bogota, a total of twenty samples were collected from representative industrial, residential and commercial areas. A portion of the samples was used to evaluate the particle size distribution by laser diffraction and its morphology by scanning electron microscopy. The samples were then resuspended to obtain the PM10 fraction, which was subject to an inorganic chemical characterization (OC, EC, water-soluble compounds, major elements and traces) and a source apportionment analysis. The data obtained revealed that the volume (%) of PM10 was higher in samples from industrial zones where heavy traffic, industrial emissions and deteriorated pavements prevailed. Elements with crustal affinity were the most abundant species, accounting for 49-62% of the PM10 mass, followed by OC (13-29%) and water-soluble ions (1.4-3.8%). Concentration maps and coefficients of divergence were obtained to analyze the spatial variability of the samples. Spherical and semispherical particles of Fe, Cu and Pb, as well as angular and subangular particles of Ba, Zn, Cu, Fe, Mn, Sn and Pb, were identified with size smaller than 10 μm. The source apportionment was analyzed considering the variability of chemical profiles, enrichment factors and ratios of Fe/Al, K/Al, Ca/Al, Ti/Al, Cu/Sb, Zn/Sb, OC/TC and OC/EC. Finally, by means of a principal component analysis, six factors were identified: local soils and pavement erosion (63%), construction and demolition activities (13%), industrial emissions (6%), brake wear (5%), vehicle exhaust emissions (4%) and other sources (9%). The results of this study provide novel data about the chemical composition of outdoor PM10, its sources and seasonal variability during a year. This could help the local government to manage air quality, by defining preventive and regulatory strategies for the main emission sources during the most critical periods. The findings also provide information to achieve a better understanding of road dust, which is one of the leading sources of PM10 emissions in Bogota. These analyzes provide elements to improve the city’s environmental policies, conduct future studies on human health and optimize air quality modeling processes.