It is used as a preliminary assessment method in order to verify whether a zone is in exceedance or near-exceedance of the limit values, and determine the ongoing assessment regime that will be required.

Mobile laboratories or transportable measurement stations used for stationary measurements at fixed sites, usually combine the advantages of automated measurement methods (continuous, time-resolved measurements) with mobility or flexibility. For pollutants for which automated measurement methods are not available, mobile laboratories may also be equipped to perform non-automated measurements (PM10, heavy metals). The duration, the periods and the frequency of the campaigns or measuring periods will have to be established so as to be representative of the reference period of the limit value (1 hour, 24 hours, 1 year).

The location of maximum concentration levels in a zone will be chosen taking into account the source distribution, local meteorological conditions and topographical features of the area. The types of sources present in an area are very important when choosing a measuring site. Impact from elevated point sources is often difficult to measure at one point at ground level because both wind direction and wind speed, and their variation with height is important for the location of the maximum ground level impact. For monitoring the pollution from roads, the impact will decrease with the distance from the road, and the level of pollution will on average be proportional to the volume of traffic.

Time-series of hourly concentrations should reflect the pattern of traffic intensity. The highest concentrations for 24-hour periods should be expected to be located in areas where the road runs parallel to the most frequent wind-directions, or where the curvature of the road allows impact from several wind-directions.

• Establish the location of expected maximum concentration from either existing measurements, from information from similar zones, emissions inventories or modelling studies. The passive samplers (diffusive sampling technique) used as a tool to determine the spatial distribution of pollutants, may constitute an alternative technique to assess the areas of maximum concentration levels,
• From time series of existing measurements or from information from similar zones, determine the periods of maximum pollution levels,
• Perform the measurements as specified in the data quality requirements,
• Compare the obtained measurement results with the limit values of the Directive  and select the appropriate assessment regime.

When performing the measurements with a mobile laboratory, the maximum uncertainty of the measurements should be: ±15% for gaseous pollutants and ± 30% for particulate matter measurements (for single measurements averaged over the reference period and at the level of the limit value, taking into account errors of sampling, calibration and instrument performances). The measurements should be supported by an adequate QA/QC programme during the period of the campaigns (periodic in-situ calibration and calibration check, proper maintenance of instrumentation), and the quality of the measurements should be fully documented.