II.5 Aircraft measurements of ground-level ozone in Prague and its surroundings

Introduction

Aircraft measurements are irreplaceable for the monitoring of 3D spatial distribution of pollutants� concentrations and their long-range transport. Ambient air monitoring with the use of analyzers installed on board of the aircraft enables, under appropriately scheduled flight plans, to collect necessary information from vast area, both in vertical and horizontal dimensions, during a short time period. The aim of aircraft measurements for the summer period of the year 2005 was to map the ground-level ozone and NO_x concentrations on the leeward side of the Prague agglomeration and in the surroundings of significant sources of VOC.

Description of flight schedule

The measuring flights were scheduled for the days of expected increased ozone concentrations. From the meteorological aspect, it comes down to anticyclonic situations with prevailing warm and dry weather. The highest probability of the occurrence of such situations can be expected from early May to late August. With regard to the daily course of ozone concentrations the flights were realized during afternoon hours when ozone concentrations close to daily maximum can be expected under the mentioned meteorological conditions
The schedule of aircraft measurements of ambient air pollution distribution in Prague and its surroundings was focused on gaining information on ozone and NO₂ concentrations in horizontal dimension. The flights were realized at the flight level of about 150–250 m above ground level.
The planned flight paths covered one flyover on the windward side of the monitored emission sources of ozone precursors and several flyovers on the leeward side, the number of which was dependent on ad hoc situation.
The Cessna 205 airplane which was planned for the measurements, was seriously damaged when landing in May 2005, and therefore an alternative machine was used, and namely Cessna-TU 206G (Fig. II.5.1), operated by Jas Air, Ltd. České Budějovice, airport Hosín. Due to the change of the aircraft, the planned flights could be started as late as in the second half of August 2005. The measuring programme, usually realized by L 410-Turbolet, had to be considerably reduced, because Cessna 206 uses accumulators for power supply of analyzers (instead of board generators used in Turbolet). The first flight realized on 18 August was a testing one and it verified the function of air intake for analyzers, the installment and attachment of all instruments on the board, and the possibilities of using satellite navigation. The problems found during the first flight were eliminated before the following flight, i.e. before 1 September 2005.

Flight day 1. 9. 2005

Meteorological situation: Warm air from the south flowed in central Europe between the anticyclone of 1027 hPa with its centre above the Baltic Sea and the cyclone of 1004 hPa with its centre above Scotland.
Weather: almost clear, visibility 15–20 km, south-east wind up to 6 m.s^-1. Temperatures 27–30 �C.
NO₂ concentrations recorded by AIM in Prague and surroundings: 12–77 μg.m^-3 in Prague, Prague surroundings 4–7 μg.m^-3.
NO₂ concentrations recorded during the flight: about 8 μg.m^-3 on the windward side of Prague near Říčany and about 10 μg.m^-3 north-east of Dolní Břežany. Increased concentrations about 18 μg.m^-3 were measured during the first flyover on the leeward side of Prague, between Kladno and Slaný, and 17 μg.m^-3 during the flyover in greater distance from Prague, between Žatec and Louny (Fig. II.5.2).

Ozone concentrations as recorded by AIM in Prague and its surroundings: 62–112 μg.m^-3, with low values in the city centre.
Ozone concentrations measured during the flight: at the windward side of Prague near Říčany 120 μg.m^-3 and north-east of Břežany 125 μg.m^-3. Significantly increased levels of O₃ to 154 μg.m^-3 were recorded during the first flight on the leeward side of Prague between Kladno and Slaný and 169 μg.m^-3 during the other flyover, between Žatec and Louny (Fig. II.5.3). The centers of extreme values of ozone concentrations on windward and leeward sides are approximately in one line and correspond with wind direction. With regard to the results of previous aircraft measurements it can be stated that a characteristic �urban plume� was created; Prague can thus be specified as a source of ozone precursors. The graphs show that the difference between ozone concentrations on the windward and leeward sides is 49 μg.m^-3 (Fig. II.5.4). This value can be regarded as an estimate of the Prague agglomeration contribution to the regional level of ozone concentration in Prague and its surroundings.

Fig. II.5.1 Cessna-TU 206G aeroplane

Fig. II.5.2 Aircraft measurements of nitrogen dioxide, 1.9.2005, 12:49:00–14:20:00 CET

Fig. II.5.3 Aircraft measurements of ground-level ozone, 1.9.2005, 12:49:00–14:20:00 CET

Fig. II.5.4 O₃ and NO₂ concentration profiles in the direction of prevailing circulation, 1.9.2005, 12:59:00–14:20:00 CET