Source apportionment study on particulate air pollution in two high-altitude Bolivian cities: La Paz and El Alto

Abstract

Abstract. La Paz and El Alto are two fast-growing high-altitude Bolivian cities forming the second largest metropolitan area in the country, located between 3200 and 4050 m a.s.l. Together they host a growing population of around 1.8 million people. The air quality in this conurbation is strongly influenced by urbanization. However, there are no comprehensive studies that have assessed the sources of air pollution and their impacts on health. Despite being neighboring cities, the drastic change in altitude and topography between La Paz and El Alto together with different socio-economic activities lead to different sources, dynamics and transport of particulate matter (PM). In this investigation, PM10 samples were collected at two urban background stations located in La Paz and El Alto between April 2016 and June 2017. The samples were later analyzed for a wide range of chemical species including numerous source tracers (OC, EC, water-soluble ions, sugar anhydrides, sugar alcohols, trace metals, and molecular organic species). The US-EPA Positive Matrix Factorization (PMF v.5.0) receptor model was then applied for source apportionment of PM10. This is the first source apportionment study in South America that incorporates a large set of organic markers (such as levoglucosan, polycyclic aromatic hydrocarbons – PAH, hopanes and alkanes) together with inorganic species. The multisite PMF resolved 11 main sources of PM. The largest annual contribution to PM10 came from two major sources: the ensemble of the four vehicular emissions sources (exhaust and non-exhaust), together responsible for 35 % and 25 % of the measured PM in La Paz and El Alto, respectively, and dust contributing 20 % and 32 % to the total. Secondary aerosols contributed 22 % (24 %) in La Paz (El Alto). Agriculture-related smoke from biomass burning originated in the Bolivian lowlands and neighboring countries contributed to 8 % (7 %) of the total PM10 mass annually. This contribution increased to 17 % (13 %) between August–October. Primary biogenic emissions were responsible for 13 % (7 %) of the measured PM10 mass. Finally, it was possible to identify a profile related to open waste burning occurring between the months of May and August. Despite the fact that this source contributed only to 2 % (5 %) of the total PM10 mass, it constitutes the second largest source of PAHs, compounds potentially hazardous to health. Our analysis resulted in the identification of two specific traffic-related sources. In addition, we also identified a lubricant source (not frequently identified) and a non-exhaust emissions source. This study shows that PM10 concentrations in La Paz and El Alto region are mostly impacted by a limited number of local sources. In conclusion, dust, traffic emissions, open waste burning and biomass burning are the main sources to target in order to improve air quality in both cities.

Tropical tropospheric aerosol sources and chemical composition observed at high-altitude in the Bolivian Andes

“…However, a marine contribution seems also possible especially for a fraction of Mg 2+ and Na + , hinted by their clustering together (Figure S11), and in agreement with a marine source affecting this site as hypothesized by Adams et al (1977). Finally, and in agreement with our findings, SO4 -2 , Ca 2+ , Mg 2+ , and Na + were identified as a group of species influencing the region from the long-range (Mardoñez et al, 2022). Levoglucosan, K + , NO3 -, EC, OC, formate, oxalate, F -, Cl -, and Brconstitute another group.…”

“…The contribution of mineral dust is very important, especially for the dry season, accounting for 33-56% of PM10. This fraction is higher than in La Paz -El Alto, where mineral dust contributes to 20-35% of the mass (Mardoñez et al, 2022). Secondary inorganic aerosol (SIA= NH4 + + NO3 -+ SO4 -2 ) ranged from 15 to 26%, with all sulphur found in soluble form.…”

“…Burning emissions, along with other organic pollutants, are transported to the Andes (Magalhães et al, 2019;Estellano et al, 2008) through what would be the free troposphere from an Amazonian point of view (1-4 km a.s.l, Andreae et al, 1988;Kundu et al, 2010b), and a regional residual layer from the Altiplano point of view (> 5 km a.s.l., Figure S12). Additionally, smoke goes upslope in the valleys (Bourgeois et al, 2015), as observed on higher influence of smoke in aerosol mass in La Paz valley (17% in mass) than at the Altiplano (13%, Mardoñez et al, 2022). Once the smoke arrives to the Altiplano, it is picked up by convection and it is transported with the PBL development to the station (along with urban pollutants, natural emissions, etc.).…”

“…Note however, that mannosan and galactosan are usually below the quantification limit in the wet months and therefore, ratios between December and April cannot be calculated even if levoglucosan is present. For the wet season the burning source is hypothesized to be nearby smallscale fires (such as those observed in the close urban area, Mardoñez et al, 2022).…”

Impact of COVID-19 lockdown on particulate matter oxidative potential at urban backgroundversustraffic sites

“…9 A bigger percentage of particulate pollution is driven by anthropogenic emissions, especially traffic and residential burning, in urban areas 10 and even in background sites. 11,12 During the lockdown periods in China and amidst strict restrictions on anthropogenic activities, there were still occurrences of severe PM pollution events, attributed to stagnant meteorological conditions. [13][14][15][16][17] Reduction in NO x pollution levels was found in the United States during the lockdown period, 18,19 Europe, 20,21 and Canada; 22 however, this was not reected in PM levels, as exemplied in France.…”