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The characteristics of daytime and nighttime mountain breezes have been analysed andcompared at three different sites: a) in the foothills of the Guadarrama Mountain range (ElEscorial, Spain); b) on a plateau close to The Pyrenees (Lannemezan, France);and c) in the Salt Lake Valley (SLV, Utah, US). A systematic algorithm, based onsynoptic and local meteorological conditions, has been used to detect automaticallynumerous events at each site. On the one hand, the wind characteristics of thesemountain breezes depend on the scale of the breeze detected at each site. Theirarrivals are observed approximately when the sensible heat flux changes sign, butthey are delayed in the sites that are farther away from the mountains. On the otherhand, the effects of these breezes on CO2 mixing ratios have been investigated.The typical increases and decreases of CO2 mixing ratios observed around theafternoon and morning transition do not always occur at the same time of the breezearrival to the tower site, which unlinks these drastic changes in CO2 from the directhorizontal advection produced by the breezes. However, the CO2 mixing ratio issensitive to changes in wind direction in highly heterogeneous sites, like the SLV site.Besides, the changes in surface turbulence produced by the breezes have an importanteffect on CO2. Indeed, a clear relationship is found for CO2 mixing ratio and theturbulent kinetic energy in the lowest atmospheric layers during the nighttime events. [ABSTRACT FROM AUTHOR]