Vertical distribution of hydrocarbons in the low troposphere below and above the mixing height: tethered balloon measurements in Milan, Italy.

A novel approach for measuring vertical profiles of HCs and particle number concentrations was described and applied in the low troposphere over Milan (Italy) during typical spring and summer days. Particle profiles yielded nearly homogeneous concentrations below the mixing height, with level-to-ground concentration ratios of 92-97%, while HCs showed a more pronounced decrease (74-95%). Vertical mixing and photochemical loss of HCs were demonstrated to cause these gradients. Much lower concentrations were observed for the profiles above the mixing height, where the HC mixtures showed also a different composition, which was partially explained by the horizontal advection of air with HC sources different to those prevailing at the site. The application of pseudo-first order kinetics for reactions between HCs and the hydroxyl radical allowed for the estimation of the vertical mixing time scale in the order of 100 ± 20 min.