DOI: http://dx.doi.org/10.1065/espr2002.11.139 --- Intention, Goal, Scope, Background -- Photochemical pollution
is a very complex process involving meteorological, topographic,
emission and chemical parameters. The most important chemical
mechanisms involved in the atmospheric process have already
been identified and studied. However, many unknown
parameters still exist because of the large number of participating
chemical reactions.
-- Objective -- The present study investigates the processes involved
in the photochemical pollution effect of an urban station located
in the greater area of the Athens basin and gives a plausible
explanation for the different seasonal ozone development
between that station and another rural one. Furthermore, the
distribution of the mean monthly surface ozone observed at the
urban station during 1987–2001 is examined in order to create
a relevant forecasting tool.
-- Methods -- Averaged hourly data of O3 and NOx observations
monitored at the above mentioned stations, during 1987–2001,
have been used in order to derive the daytime (7:00–15:00) values.
Trajectories calculated by using a 2D-trajectory code and
meteorological data, during the period 1988–1996, have also
been used.
-- Results and Discussion -- At the urban station, the percentage
negative trend of NO and NOx data in winter and summer is
higher than that in spring and autumn, while the percentage
ozone trend is maximum in the summer. On the contrary, the
negative surface ozone trend at the rural station exhibits a minimum
in summer and a maximum in autumn and winter. The
mean seasonal wind-rose for the selected months shows that
the northward wind flow dominates during June, the month of
the lowest negative ozone trend in the rural station. Finally, the
development of the forecasting tool shows that the mean monthly
surface ozone data during the period (1987–2001) demonstrates
a semi-log distribution.
-- Conclusions -- Air transport effect on the air pollution of the rural
station (not blocked by mountains) is deduced as a possible
reason for the different seasonal ozone development observed
between the rural and the urban station. Finally, the discrepancies
between the theoretical probabilities deduced by the model
and the empirical ones appear to be very small, and the corresponding
correlation coefficient is 0.99.
-- Recommendation and Outlook -- However, to interpret the aforementioned
statistical results about the negative trends in ozone
and its precursors, additional parameters can be taken into account.
Changes in NOx concentrations, for instance, can result
not only from changes in emissions or meteorological conditions.
There might also be a contribution through changes in
the atmospheric composition. A study of the contribution of
changes in atmospheric composition to trends of observed NOx
concentrations requires that a series of steps be taken (removal
of meteorological influence in the time series, calculation of
trends in OH concentrations, etc.). |
