A physicochemical model of sorption processes in NO2 passive sampling with air humidity effects

Aqueous triethanolamine (TEA) solutions are widely used as sorption medium for passive sampling of ambient NO₂, with NO₂ trapped and accumulated as nitrite ion. The results of test measurements of ambient NO₂ concentrations using passive sampling method showed that the simple approach commonly used to describe passive sampling process might lead to substantial systematic errors. Presented in the article is a new physicochemical model of the process of passive sampling of gaseous NO₂, with aqueous TEA solution used as a trapping medium. The model is based on the available results of experimental studies of interaction of gaseous NO₂ with TEA/water solutions. The key principles underlying the model are: (1) when absorbed by a trapping solution, NO₂ forms nitrite ion only on the condition that TEA is hydrated; (2) coefficient of conversion of NO₂ to NO ₂ ^? is equal to one when reacting with hydrated TEA; and (3) the fraction of hydrated TEA molecules depends on air humidity at the moment of measurement. Validation of the model was made using the data of the field measurements carried out in the Middle Urals in 2007–2009. The new model was used to calculate average NO₂ concentrations. Concentrations calculated agreed well with the results obtained by reference methods. The difference between the datasets was statistically insignificant.