Policy Air Quality Standards Pollutants Impacts Monitoring Monitoring Air Pollution Air monitoring methodologies can be divided into five main types, covering a wide range of costs and performance levels. The methods and their relative merits are shown in the table below and discussed in the following section.   It is also important to choose the most appropriate monitoring location for investigating a specific air pollution source or problem.   Method Advantages Disadvantages Passive sampling Low cost / simple. Useful for screening and base-line studies and in support of automatic monitoring for more detailed assessments. Unproven for some pollutants. Laboratory analysis required. In general, only provides weekly or longer averages, often with long data turn around times. Photochemical and optical sensor systems Can be used portably. Low sensitivity may only provide spot measurements. Active (semi-automatic) sampling Low cost / easy to operate - reliable. Historical datasets available. Provide daily averages. Some methods are labour intensive. Laboratory analysis required, often with long data turn around times. Automatic point monitoring Provide high resolution data. On-line data collection possible. Provide path or range-resolved data. Relatively expensive. Trained operator required. Regular service and maintenance costs. Remote optical/long-path monitoring Useful near sources. Multi-component measurements possible. Relatively expensive. Trained operator required. Data not readily comparable with point measurements.   Since monitoring instrumentation covers a wide range in capital and running costs, it is usually advisable to choose the simplest method available to meet the specified monitoring objectives. Many baseline monitoring, spatial screening and indicative surveys can be served perfectly well by inexpensive active or passive sampling methods. Only proven and generally accepted measurement methods should be considered.   Passive sampling methods (diffusion tubes) These represent a simple and cost-effective method of monitoring air quality in an area, to give a good general indication of average pollution concentrations. They are, therefore, particularly useful for assessment against annual mean objectives. A sample integrated over the exposure time is collected by either deposition or diffusion to a sampler. For Diffusion tubes, the low cost per tube permits sampling at a number of points in the area of interest; this is useful in highlighting 'hot spots' of high concentrations, such as alongside major roads or near power generation. They are less useful for identifying 'hot spots' around point sources or near to industrial locations where greater objectives requiring greater temporal resolution is required (ie investigating hourly or daily objectives). Diffusion tube surveys are simple to undertake and minimal operator training is required. Diffusion tubes are used to monitor the following pollutants in Gibraltar: • Nitrogen dioxide • BTX (benzene, toluene, xylene) The tubes must be analysed by laboratories that can offer suitable quality assurance and quality control measures to ensure the results meet the data quality objectives defined for the method. The diffusion tube data may be corrected for bias in the analytical results.   Active (semi-automatic) sampling sampler methods These methods collect pollutant samples either by physical or chemical means for subsequent analysis in a laboratory. Typically, a known volume of air is pumped through a collector such as a filter or chemical solution for a known period of time, for subsequent laboratory analysis. These include bubbler samplers for sulphur dioxide and gravimetric filter samplers for PM10, PM2.5 and lead. Sequential instruments with automatic sample changers are available.   Automatic real-time point analyser methods These produce high-resolution measurements (typically hourly or shorter period averages) for oxides of nitrogen, sulphur dioxide, carbon monoxide and ozone. Gas chromatography (GC) analysers also provide high-resolution data on benzene, and 1,3-butadiene. The sample is analysed on-line and in real-time. In order to ensure that the data produced are accurate and reliable, a high standard of maintenance, calibration, operational and QA/QC procedures is required.   Remote optical/long-path analysers These are instruments that use a long-path spectroscopic technique to make real-time measurements of the concentration of a range of pollutants integrated along a path between a light source and a detector. Instruments using the Differential Optical Absorption Spectroscopy (DOAS) system can be used to monitor data for nitrogen dioxide, sulphur dioxide and benzene. In order to ensure that the data produced are accurate and reliable, a high standard of maintenance, calibration, operational and quality assurance and quality control procedures is required. Photochemical and optical sensor systems A range of relatively low-cost automatic analysers has been developed specifically for portable and personal exposure monitoring applications. These are battery or mains operated electrochemical or solid-state sensor based systems which can continuously monitor a range of pollutants (carbon monoxide, nitrogen dioxide, sulphur dioxide) with a time average of 15-minutes or less. These sensors are of low sensitivity and mostly suitable for identifying hotspots at roadsides and near point sources etc. Portable sensors using the light scattering principle are available for PM10 monitoring. Monitoring Locations Monitoring sites can be classified according to the type of environment in which they are located, in order to permit more meaningful evaluation of data. The site description will generally reflect the influence of a particular pollutant source or of overall land use. Typical monitoring location types are described in the table below.   Type Description Source Influences Objectives ROADSIDE A site sampling between 1m of the kerbside of a busy road and the back of the pavement. Typically this will be within 5m of the road, but could be up to 15m. Local traffic. Assessing worst case population exposure. Evaluating impacts of vehicle emission controls. Determining impacts of traffic planning/calming schemes. KERBSIDE A site sampling within 1m of the kerb of a busy road. Local traffic. Identifying vehicle pollution blackspots. Assessing worst case scenarios. Evaluating impacts of vehicle emission control technologies. Determining impacts of traffic planning/calming schemes. URBAN BACKGROUND 50m from any busy road and typically in a residential area Vehicle, commercial, space heating. Trend analysis, Urban planning, Traffic and land-use planning. SUBURBAN A location type situated in a residential area on the outskirts of a town or city. Traffic, commercial, space heating, regionaltransport, urban plume downwind of a city. Traffic and land-use planning. Investigating urban plumes. For further information about this web site please email: info@gibraltarairquality.gi © Copyright Home | About Air Quality | Data | Links | About This Site This website is prepared and hosted by AEA Energy & Environment, on behalf of the Government of Gibraltar and the Gibraltar Environmental Agency