Resumen
An annular denuder/filter-pack sampling technique has been employed to get an insight of the formaldehyde partitioning between gas phase and particles (PM2.5) in México City using short time sampling periods. Formaldehyde was trapped using DNPH derivatives and analyzed by HPLC/UV. One annular denuder was used to trap formaldehyde from the gas phase followed by a Teflon filter to trap PM2.5, and a second backup denuder was used to trap degassed formaldehyde from the filter. Just after finishing the sampling, filters were soaked in a DNPH solution to fix the PM2.5 formaldehyde, and the denuders where extracted with another DNPH solution. The sum of the concentrations in both phases ranged from 7.90 to 44.60 µg/m3 during the sampling periods from 8 a.m. to noon in several ocassions from 2000 to 2003. In average, the formaldehyde PM2.5/gas-phase ratio was about 0.15, one hundred times higher than previously observed. A large fraction of the formaldehyde in particles is easily lost by a combination of three processes: two of these, blow off from the trapped particles and displacement of the phase equilibrium, both well known and confirmed in other studies. A third process called extrussion of particles is described. Thus, PM2.5 may act as a temporal sink or a slow releasing source of formaldehyde in México City?s photochemical smog. This findings may have strong implications for air quality modeling.