With the increasing intensity of road transport the monitoring of air pollution caused by aromatic hydrocarbons is becoming relevant. The decisive source of atmospheric emissions of aromatic hydrocarbons – and namely of benzene and its alkyl derivates – are above all exhaust gases of petrol motor vehicles. Another source are loss evaporative emissions produced during petrol handling, storing and distribution. Mobile sources emissions account for approx. 85 % of total aromatic hydrocarbons emissions, while the prevailing portion is represented by exhaust emissions. It is estimated that the remaining 15 % of emissions come from stationary sources. Many of these are related to industries producing benzene and those industries that use benzene to produce other chemicals. Data based on monitoring show that aromatic hydrocarbons represent 20–40 % of the total amount of non-methane hydrocarbons in the atmosphere of European cities [10, 11].

As concerns health risks, the most dangerous pollutant is benzene with carcinogenic effects on human health. A great number of cases were described of myeloblastic and erythroblastic leukemia attributed to benzene exposure. For benzene concentration in the atmosphere equal to 1 μg.m^-3 the lifetime risk of chronic leukemia is estimated to 4.4–7.6x10^-6 [12].

The obtained data illustrate that benzene level in petrol is about 1.5 % while diesel fuels contain relatively insignificant levels of benzene. Exhaust benzene is produced primarily by unburned benzene from fuels. Non-benzene aromatics in the fuels can cause about 70 to 80 % of the exhaust benzene formed. Some benzene also forms from engine combustion of non-aromatic fuel hydrocarbons.

Benzene is a highly stable aromatic hydrocarbon resistant to chemical attack. In comparison with other aromatic hydrocarbons benzene is minimally reactive and ranks among least reactive particles in the atmosphere. The only reaction of benzene in the ground-level atmosphere is the reaction with hydroxyl radical (OH). The calculated residence times – taking into account mainly reactions with OH in gaseous phase and removal of benzene from the atmosphere by dry and wet deposition – range from 2 days in summer cloudless weather to several months in cloudy weather in winter season. The observed stable products of atmospheric benzene oxidation are phenols (phenol and nitrophenol) and aldehydes, particularly glyoxal (CHO)₂.

The reaction of toluene and xylenes with OH radicals is more rapid (5x and 19x respectively) than with benzene and thus participate significantly in reactions resulting in ozone accumulation in periods of photochemical smog occurrence.

In 1998 regular monitoring of the above hydrocarbons, and namely benzene, toluene, o-, m- and p-xylene and ethylbenzene, started at four AIM CHMI stations: Most, Prague 4-Libuš, Pardubice-Rosice and Mikulov-Sedlec. In 1999 AIM CHMI station in Ostrava-Přívoz and early in 2000 AIM CHMI station in Prague 5-Smíchov were also included. BTX continual analysers based on the method of gas chromatography are used for the measurements at AIM stations. During 1999 the National Health Institute started to deliver data on aromatic hydrocarbons measurements to the ISKO database from 5 stations operated by Public Health Service. The measurements are based on 24-hour samples taken in special canisters in a six-day interval. Fig. 2-72 shows the annual courses of daily average concentrations of the above aromatic hydrocarbons at the stated stations. Fig. 2-70 illustrates the average weekly course of half-hour benzene concentrations in 2000 winter and summer at Prague 4-Libuš (urban background station) and Prague 5-Smíchov (traffic station). Fig. 2-71 shows the average diurnal courses of half-hour benzene concentrations at these stations on working days, Saturdays and Sundays in 2000 winter and summer periods. The comparison of the weekly and diurnal courses of benzene concentrations at the above localities illustrates the decisive influence of mobile sources emissions. It also demonstrates the difference in traffic regimes on working days and on weekends.

The annual average benzene concentrations at selected stations are shown in Table 2-76. In November 2000 EU Directive 2000/69/EC relating to limit values for benzene and carbon monoxide in ambient air was published. This Directive sets the limit value for annual average benzene concentrations (5 μg.m^-3) and this level should not be exceeded in 2010.

The CHMI will increase, with regard to the financial means allocated, the number of monitoring sites providing continuous measuring of aromatic hydrocarbons, primarily in agglomerations, pursuant with the requirements of EU Directives and the prepared Clean Air Act.

Tab. 2-76 Annual average benzene concentration at selected stations, 2000