SmallMonitoringStation.jpgThe design of the air quality monitoring network basically involves determining the number of stations and their location, and monitoring methods, with a view to the objectives, costs and available resources. The typical approach to the network design, appropriate over the city-wide or national scale, involves placing monitoring stations or sampling points at carefully selected representative locations, chosen on the basis of required data and known emission/dispersion patterns of the pollutants under study. This scientific approach will produce a cost effective air quality monitoring program. Sites must be carefully selected if measured data are to be useful. Moreover, modeling and other objective assessment techniques may need to be utilized to "fill in the gaps" in any such monitoring strategy.

An air monitoring network properly designed is a key component of any air quality control program. In practice, the operation and maintenance of air quality monitoring stations are expensive, so it is desirable to use as few stations as possible to meet monitoring goals. Another consideration in the basic approach to the network design is the scale of the air pollution problem:
  • The air pollution is of predominantly local origin. The network is then concentrated within the urban area to monitor nitrogen oxides (NO2), sulfur dioxide (SO2), particulate matter smaller than 10 um (PM10), particulate matter smaller than 2.5 um (PM2.5), carbon monoxide (CO) and volatile organic compounds (VOC).
  • There is a significant regional contribution to the problem and more emphasis will be on monitoring ozone (O3) and particulate matter..
  • Large-scale phenomena, such as winter or summer smog episodes or dust cloud (local impacts should be avoided).

The number of sites will depend upon the size and topography of the urban area, the complexity of the source mix and the monitoring objectives.[1][7] The U.S. Environmental Protection Agency (U.S. EPA) and the European Union (EU) Directives specify a minimum number of stations to be established dependent upon the population, and it also indicates what types of areas should be monitored.

Monitoring Objectives

The air quality monitoring program design will be dependent upon the monitoring specific objectives specified for the air quality management in the selected area of interest. What are the expected outputs of the monitoring activity? Which problems need to be addressed?

Defining the output will influence the design of the network and optimize the resources used for monitoring. It will also ensure that the network is specially designed to optimize the information on the problems at hand. There might be different objectives for the development of the environmental monitoring and surveillance system. Normally, the system will have to provide online data and information transfer with a direct /automatically/ on-line quality control of the collected data. Several monitors, sensors and data collection systems may be applied to make on-line data transfer and control possible.[7]

The main objectives for the development of an air quality measurement and surveillance program might be related to:
  • Population exposure and health impact assessment
  • Identifying threats to natural ecosystems
  • Determining compliance with national or international standards
  • Informing the public about air quality and establishing alert systems
  • Providing objective input to air quality management and to transport and land-use planning
  • Identifying and apportioning sources
  • Developing policies and setting priorities for management action
  • Developing and validating management tools such as models and geographical information systems
  • Quantifying trends to identify future problems or progress in achieving management or control targets.

Screening Studies and Operational Sequence

Before a final program design is presented it is also important to undertake a preliminary field investigation often referred to as a screening study. This may consist of some simple inexpensive measurements (e.g. using passive samplers) and simple dispersion models.[2] The data will give some information on the expected air pollution levels, highly impacted areas and the general background air pollution in the area.

The number of monitoring stations and the indicators to be measured at each station in the final permanent network may then be decided upon as based on the results of the screening study as well as on the knowledge of the sources and prevailing winds. Once the objective of air sampling is well-defined and some preliminary results of the screening study are available, a certain operational sequence has to be followed. The best possible definition of the air pollution
problem, together with the analysis of the personnel, budget and equipment available, represent the basis for the decision on the following questions:
  • What spatial density of sampling stations is required?
  • How many sampling stations are needed?
  • Where should the stations be located?
  • What kind of equipment should be used?
  • How many samples are needed and during what period?
  • What should the sampling (averaging) time and frequency be?
  • What additional background information is needed? (For example: meteorology, topography, population density, emission sources and rates)
    • Meteorology
    • Topography
    • Population density
    • Emission sources and rates
  • What is the best way to obtain the data (configuration of sensors and stations)?
  • How will the data be accessible, communicated, processed and used?

The answers to these questions will vary according to the particular need in each case. Most of the questions will have to be addressed in the site studies and in the selection of sites as discussed below.

Site selection

The urban air quality monitoring program will normally provide the information to support and facilitate the assessments of the air quality in a selected area and to meet the objectives as stated by the users. Some of the objectives have been presented above. This normally means that for designing a monitoring program in an urban area, several monitoring stations are needed for characterizing the air quality in the region. The areas are generally divided into urban, suburban and rural areas. Measurements should be undertaken in different microenvironments within these areas, where people are present. In a typical urban air pollution measurement program, the microenvironments selected are often classified as:
  • Urban including traffic, residential, commercial, and background
  • Suburban (traffic and industrial)
  • Rural sites (background areas)

When measuring air quality or analyzing the results from measurements it is important to bear in mind that the data you are looking at are a sum of impacts or contributions originating from different sources on different scales.
In any measurement point in the urban area the total ambient concentration is a sum of:
  • Natural background concentration
  • Regional background
  • City average background concentration
  • Local impact from traffic along streets and roads
  • Local impacts from small area sources like open air burning (waste and cooking)
  • Impact from large point sources such as Industrial emissions and power plants

To obtain the information about the importance of these different contributions it is therefore necessary to locate monitoring stations, so that they are representative for different impacts. In addition to the air pollution data, we will often need meteorological data to identify and quantify the sources contributing to the measurements. It is also important to carefully characterize the representativeness of the monitoring sites, and to specify what kind of stations we are reporting data from. More than one monitoring site is often needed in order to characterize the air quality in the urban area.[5][6]

Additional factors points to consider in this case when selecting specific site locations include:
  • Past and current monitoring results
  • Site accessibility
  • Power accessibility
  • Topographical effects
  • Local interferences
  • Security

When considering the location of individual samplers, it is essential that the data collected are representative for the location and type of area without undue influence from the immediate surroundings.

Air quality indicators

Air quality indicators have been selected for different environmental issues and challenges. Not all indicators are specific enough to address only one issue. The nature of the air pollution involves some indicators addressing several issues. Some of the issues that have to be addressed are:
  • Climate change
  • Ozone layer depletion
  • Acidification
  • Toxic contamination
  • Urban air quality
  • Traffic air pollution.

As it can be seen from the list, the indicators have to cover all scales of the air pollution problems (in space and time) to address different type of impacts and effects.[7]

The most commonly selected air quality indicators for urban and industrial air pollution are:
  • Nitrogen dioxide (NO2)
  • Sulfur dioxide (SO2)
  • Carbon monoxide (CO)
  • Particles with aerodynamic diameter less than 10 μm and 2.5 μm (PM10 and PM2.5)
  • Ozone (O3).
  • Volatile organic compounds (VOC)

The US EPA refers to the first five compounds listed above as the priority pollutants.[1] They are also given in the Air Quality Daughter Directives of the European Union with specific limit values for the protection of health and the environment.[7] The World Health Organization guideline values also include the above indicators.[3][4]

Other elements in the design

In the design of the air quality monitoring program we will also have to include the measurements of meteorology. Weather stations should be located in order to assess the general wind flow over the study area. Weather stations do not need to be placed at all air monitoring sites, but some co-locations will decrease the total cost of these measurements. Before the air quality data can be used to assess the situation in the area, it is important to assure that the data collected are real concentration values, which may be compared to similar information from other areas and countries. For each pollutant, which is measured as the input to the air quality assessment and evaluation, the following main questions may be asked:
  • Have the suitable quality assurance procedures been set up for all stages and activities?
  • Is technical advice available?
  • Is monitoring being carried out at suitable locations?
  • Have suitable arrangements for data handling and storage been made and implemented?

The documentation to support the credibility of data collection and the initial data quality assurance are the responsibility of the data provider. This includes the process of data collection, application of calibration factors, Quality Assurance procedures (QA/QC), data analysis, data “flagging”, averaging and reporting. A combination of data record notes, data quality flags and process documentation are all part of this first phase of processing. During the data collection phase, one role of the data provider is to assist in maintaining the process credibility and validity of the data. Good data quality is essential for adequate reporting of the
air quality.


  1. World Health Organization (WHO) Regional Office for Europe (1999), Monitoring ambient air quality for World health impact assessment, WHO Regional Publications, European Series, No. 85, ISBN 92-890-1351-6.
  2. M.R. Beychok (2005), Fundamentals of Stack Gas Dispersion, 4th Edition, ISBN 0-9644588-0-2.
  3. National Ambient Air Quality Standards (NAAQS), 1990, From the website of the U.S. EPA.
  4. "Methodology for assessment of exposure to environmental factors in application to epidemiological studies", World Health Organization (WHO) Regional Office for Europe, The Science of the Total Environment, Volume 168, Number 2, June 16, 1995, pp. 93-100(8).
  5. World Health Organization (WHO) Regional Office for Europe (2000), Air Quality Guidelines for Europe, 2nd Edition. WHO Regional Publications, European Series, No. 91, ISBN 92-890-1358-3.
  6. World Health Organization (WHO) Regional Office for Europe (2006), Air Quality Guidelines Global Update 2005, ISBN 92-890-2192-6.
  7. Criteria for EUROAIRNET - The EEA Air Quality Monitoring and Information Network, Technical Report No. 12, (1999).

Publishing Note:

This article was written by Massoud Estiri, a member of this wiki, and uploaded by him in a pdf format. It was then transformed into this wiki's format using the Wikitext markup language and the Wikitext Editor by Milton Beychok, the organizer of this wiki..