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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
Variation of the chemical composition of wet precipitation in
Mexico
Padilla, H.G., Belmont, R.D., and Baez, A.P.
Centro de Ciencias de la Atmósfera.
UNAM,
04510 Mexico, D.F.
Tel. 548-81-90, Fax 548-97-81
Rain water was collected in 2 mm rain fractions within entire
rain events, at Mexico City and Rancho Viejo, a wooded rural
area. The results indicated that as the rain event progressed,
the analysed ions except
H+
decreased. These decrements were more
pronounced on the f irst fourth fractions. From the f ifth up to
the last fraction the variation was less accentuated. It is
important to mention that the same variation pattern was
observed in the two years that wet precipitation was collected in
Mexico City, but in Rancho Viejo more variability was observed,
the ionic concentrations decreased, inqreased, or remained the
same within the rain event. The variability of the ionic
concentration through the entire rain event in explained by two
scavenging mechanisms: (1) The simultaneous action of water wapor
condensation within the cloud and the capture of air pollutants
as condensation nuclei (rainout), and (2) The capture of dust
particles, mainly alkaline, and some gases below cloud leve1
(washout). Due to the heavily polluted atmosphere of Mexico City,
much higher concentrations of the analysed ions were found in
samples collected in the city than in samples from Rancho Viejo,
indicating the effects that air pollutants have upon the
chemistry of wet precipitation.
When the Wilcoxon-Mann-Whitney
statistical test was applied, significant differences between
these two places were found. Finally, the rainout seems to be
the predominant scavenging mechanism in Rancho Viejo.
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