Abstract
Atmospheric pollution is an important problem in the metropolitan area of
Mexico City. Concentrations of ozone, sulfur dioxide, carbon monoxide,
nitrogen dioxide and PM10 particles (particles smaller than 10 micrometers)
have been measured in approximately thirty monitoring stations in different
places of the city since 1986, although the data before 1990 are not very
confident. Each hour a concentration measure is measured. In this work we
have analyzed with nonlinear dynamics techniques the concentration time
series of atmospheric pollutants since 1990 to 2005. We have used detrended
fluctuation analysis (DFA) and the Higuchi's algorithm in order to
characterize the correlations in these time series. We have found that there
are long-large correlations in the time series of ozone, sulfur dioxide,
carbon monoxide, nitrogen dioxide and PM10 particles. However, there are
significant differences between the correlational dynamics of each
pollutant; we have found greater correlations in the time series of PM10 and
ozone. We have concluded that the permanence time of PM10 and ozone is
greater than the permanence time of sulfur dioxide, carbon monoxide and
nitrogen dioxide. We have also studied the annual variation of the
correlation exponents and we have obtained that there is a qualitative
correlation between the values of the correlation exponents and the
concentration values of the pollutants. In order of better characterize the
nonlinear properties of the time series of these complex systems; we
analyzed the time series by using the Chhabra and Jensen multifractal
formalism. All the time series (annual and total) are multifractal, however,
the multifractality degree (the width of the multifractal spectrum) is
different for each pollutant. The degree of multifractality has been
associated with the complexity of the time series, in these sense we have
encountered that time series of sulfur dioxide are more complex than time
series of other pollutants. Although the width of the multifractal spectrum
is not a measure of the concentration levels, when we analyzed the plots of
the spectrum width of the annual time series versus the year, the greater
values of the width coincide with the greater values of concentration of the
atmospheric pollutants.
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