Day-night ozone patterns found in the city of Cartagena

This paper presents the hourly evolution for ozone during the period 1995 to 1997. Data were provided by the Pollution Surveillance Network of the City of Cartagena from five stations spread in the metropolitan area Data were recorded as thirty-minute averages. Different patterns were found when comparing day vs. night. During day time O-3 behavior is characteristic as being clearly related to temperature. The descent in early morning is caused by a rise of nitrogen oxides, which at first consume O-3 and later, with solar radiation, produce O-3 by a feedback photochemical reaction. At night there are three different patterns for O-3: a constant concentration in La Aljorra, rising hourly averages in ETASA and decreasing hourly averages in the rest of the sampling points. The constant concentration encountered in La Aljorra is caused by the impossibility of participation of ozone in elimination reactions, Gunsten et al (1998)(1). The increase over time noted in ETASA has been found in Madrid, Zurita and Castro (1983)(2). Since a nearby O-3 source is not likely at dark, any increase in O-3 concentration, after the late evening low values might be attributed to advection. This transport could be either downward, by turbulent mixing with upper layers, or horizontal, from neighboring areas where the O-3 destruction is less marked or slower than in the urban air. In the third pattern observed, O-3 reduction runs parallel to NO2 as a consequence of their nocturnal chemistry. Nocturnal oxidation of NO2 is conducted by O-3 leading to formation of nitrogen trioxide.