Occurrence and temperature dependence of atmospheric gas-phase organophosphate esters in high-mountain areas (Pyrenees).

The air concentrations of organophosphate esters (OPEs) were studied in a network of six remote high-mountain areas of the Pyrenees located along an altitudinal profile between 1619 m and 2453 m above sea level on a restricted planar surface to assess their vertical distribution based on long-range atmospheric transport and temperature gradients. Polyurethane foam passive samplers were used in five periods spanning over three years (September 2017-October 2020). The sum of concentrations of five OPEs were between 5.3 and 100 pg m, averaging 16-53 pg m across campaigns at the different locations. These concentrations were much lower than those observed in areas under anthropogenic influence but also than those found in low altitude remote continental sites. A significant progressive change in predominant compounds was observed along the altitudinal gradient, with prevalence of tris(1-chloro-2-propyl) phosphate (TCIPP) or tris(2-chloroethyl) phosphate (TCEP) below or above 2300 m above sea level, respectively. This trend was consistent with the higher volatility of TCEP, which was retained at greater extent at lower environmental temperatures (higher altitude). A significant temperature dependence of the gas phase concentrations was observed for TCEP, TCIPP and triphenyl phosphate (TPHP), which could be explained by retention in the cold periods, predominantly adsorbed in snow, and their release to the atmosphere during snowmelt. This mechanism was consistent with the good agreement found between the vaporization enthalpies measured under laboratory conditions and the experimental values obtained from the slopes of the significant linear regressions when representing the vertical gradients.