Passive air and water sampling in high-altitude lakes: occurrence and distribution of legacy and emerging organic pollutants

Abstract

[eng] Remote high mountains allow the study of the environmental fate of legacy and emerging anthropogenic chemicals that form a worldwide diffuse background layer of pollution. Semi- volatile compounds like polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), organophosphate esters (OPEs), and other organochlorine compounds like hexachlorobenzene (HCB) have become ubiquitous by undergoing long-range atmospheric transport, penetrating other environmental compartments (e.g., water, soils, sediments, and biota), and often remaining unperturbed due to their high resistance to degradation. Moreover, pollutant levels in high mountains tend to be amplified because of cold-trapping, a series of physical and thermodynamical processes that occur at low temperatures and high altitudes. Toxic effects in living organisms have been linked to the exposure to these chemicals, thus constituting a threat to the health of some of the most fragile ecosystems of the world. However, high mountains pose a set of unique challenges that complicate the assessment of pollutant levels (e.g., inaccessibility and harsh environmental and meteorological conditions). In recent decades, passive samplers have provided an alternative way of monitoring pollutants with significant benefits for studying remote sites: low cost, ease of use, no energy supply required, deployment in environmental matrices (e.g., air and water) for extended periods, and time-integrated assessment of average pollutant levels. Here, passive samplers were used for the determination of semi-volatile and hydrophobic organic compounds in air and water from six alpine lacustrine areas forming an altitudinal gradient in the Pyrenees (1619 to 2453 m above sea level). Polyurethane foam (PUF) was used for the atmospheric gas-phase, and low-density polyethylene (LDPE) and silicone rubber (SR) were used for the dissolved phase of water. The performance and uncertainty of these methods were adequate, even in a remote environment with trace-level pollutant concentrations. Long-term trends in pollutant levels in air and water from the Pyrenees were studied. Persistent compounds like PCBs and HCB were found at levels very similar to those reported over two decades ago in the same area. PAHs, less persistent, were detected at half of their previously measured concentrations, agreeing with estimated reductions of their emissions. Atmospheric concentrations of half of the studied pollutants significantly increased with temperature, showing the influence of re-volatilization from surfaces (e.g., soils, water, snow).

Secondary emissions were prominent for OPEs and the heavier PCBs and PAHs, as their lower volatilities cause them to partition more prominently towards other environmental compartments. HCB and the lighter PAHs and PCBs did not present significant changes with temperature, as expected from their higher volatility. Diffusive exchange fluxes between air and water at the lakes showed deposition of actively emitted PAHs, slight deposition or near- equilibrium state for most PCBs, OPEs, and other chlorinated compounds, and slight volatilization of HCB and the diagenetically generated methylated PAH retene. Overall, this high-mountain region may be acting as a sink for several pollutants that accumulate in its surfaces, but some compounds may reach equilibrium between air and other compartments and may experience seasonal secondary emissions. Additionally, passive air samplers were used for the study of pollutant levels in the city of Barcelona. Most pollutants in air presented concentrations up to more than two orders of magnitude higher than in the Pyrenees. Since these measurements were performed before and during the implementation of restrictive lockdown measures at the beginning of the COVID-19 global pandemic, sudden changes in the composition of atmospheric organic pollution caused by an unprecedented halt in anthropogenic activities were also assessed. The concentrations of PAHs, PCBs, HCB, and other organochlorine compounds decreased by 60 to 91%, but OPE concentrations remained stable because their release is diffusive and does not depend directly on the activities that stopped during restrictions.

[cat] L’alta muntanya permet estudiar el destí ambiental de compostos químics que formen part d’una capa difusa de contaminació global. Compostos semi-volàtils tòxics com els bifenils policlorats (PCBs), els hidrocarburs aromàtics policíclics (PAHs), els èsters organofosforats (OPEs) i l'hexaclorobenzè (HCB) són sotmesos a un transport atmosfèric de llarga distància, penetren altres compartiments ambientals (aigua, sòls, sediments), resisteixen la degradació i s’acumulen a l’alta muntanya degut a processos físics que tenen lloc a baixes temperatures i altituds elevades. L’avaluació dels seus nivells en alta muntanya és difícil degut a la seva inaccessibilitat i dures condicions meteorològiques. Els mostrejadors passius proporcionen una alternativa per al seu estudi durant períodes prolongats amb baix cost, facilitat d'ús i sense subministrament d'energia. En aquest treball s’han utilitzat mostrejadors passius per a la determinació de compostos orgànics en aire i aigua de sis zones lacustres als Pirineus (entre 1619 i 2453 m): escuma de poliuretà (PUF) per a l’atmosfera i polietilè de baixa densitat (LDPE) i cautxú de silicona (SR) per l’aigua dels llacs. Els compostos més persistents (PCBs, HCB) s’han trobat a nivells similars als mesurats fa més de dues dècades, mentre que els menys persistents (PAHs) han decrescut com les seves emissions. Les concentracions atmosfèriques de la meitat dels contaminants mostren augments significatius amb la temperatura degut a la influència de la revolatilització des de sòls i neu (especialment OPEs, PCBs i PAHs menys volàtils). Els fluxos d'intercanvi entre aire i aigua als llacs mostren la deposició de PAHs, una lleugera deposició o equilibri de PCBs i OPEs i la volatilització d'HCB.

També s'han estudiat els nivells d’aquests contaminants a la ciutat de Barcelona, on les concentracions són fins a dos ordres de magnitud superiors a les dels Pirineus. Atès que aquestes mesures es van realitzar durant la implementació de mesures de confinament a l'inici de la pandèmia de COVID-19, s’han avaluat canvis en la composició de la contaminació atmosfèrica provocats per l’aturada d’activitats antropogèniques. Les concentracions de PAHs, PCBs i HCB van disminuir entre un 60 i un 91%, però les dels OPEs es van mantenir estables a causa del seu diferent mecanisme d’alliberament.