ANNEX II

EVENTS INFLUENCING STATION MEASUREMENTS

Flags are used for the indication of concentrations measured at times when unusual events occur near a station or when unusual events are detected during sampling or analysis.

The use of flags was introduced pursuant to the recommendation of the EMEP programme¹ and it follows the respective methodology (EMEP 1996). The EMEP has established 10 groups of flags, within which the flags are divided into two categories: valid and invalid data (i.e. a measured concentration can be marked with a flag). When data flagged as valid are subsequently used and during the evaluation of ambient air quality, it should be taken into account that the representativeness of measurement results from the respective station is influenced and limited.

The AQIS database introduced flags in 2010. They are used to flag concentrations measured at all manual measuring stations of the CHMI. At all automated CHMI stations, only two internal flag have been used so far: ‘Local influence’ and ‘Value replaced by half of the detection limit’. The use of additional flags is being considered. Flags improve the quality of analyses of episodes of unusual concentrations measured at a particular station. This yearbook for the second time includes flags in the evaluation of ambient air quality in the Czech Republic.

Significantly biased valid concentration measurements may be indicated by three flags according to the EMEP code list (EMEP 2014): Flag 652 indicates construction/building activity in the vicinity of a station, flag 199 indicates local influence, and flag 559 indicates non-specific contamination. Values are still regarded as valid even when these flags are used.

The extent to which measurements taken at a station are influenced is individual and depends on the nature of the particular local influence. An influence upon annual characteristics is considered significant when flags occur in more than 2 % measurements, i.e. if the number of flagged concentrations in the total number of measured concentrations at a station is equal to or greater than 2 %.

The following table presents a list of influenced measurements with comments by the data provider or laboratory.

 

Tab. 1 Summary of influenced measurements at CHMI stations in 2014
 

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¹The EMEP (European Monitoring and Evaluation Programme, www.emep.int) was founded with the aim to provide member states of the Convention on Long-range Trans-boundary Air Pollution (CLRTAP), signed in 1979, qualified scientific information on problems of air pollution caused by various pollutants and on the evaluation of the effects of their presence in the atmosphere on ecosystems and human health.