1.1 Basic ideas behind the placement of monitoring stations

Air Pollution Continuous Monitoring Technology in Japan
Plan of air pollution continuous monitoring

1. Plan of general air pollution monitoring stations and roadside air pollution monitoring stations

The placement of stations, the notes on establishing stations and other necessary items for continuous monitoring with regard to general air pollution monitoring stations and roadside air pollution monitoring stations are introduced in this section.

1.1. Basic ideas behind the placement of monitoring stations

When establishing a network for continuous monitoring stations , it is desirable to understand spaciously the situation on air pollution in a target area, using the minimum number of monitoring stations. To accurately monitor the changes according to time, automatic monitoring instruments are very useful. However, it is necessary to effectively locate monitoring stations in order to accurately monitor with respect to the space,.

1.1.1. General air pollution monitoring station
When an air pollution monitoring station will be established, the regional representation as methods of placing the station in one location of a region can be adopted so that the extents and changes of air pollution for the region can be represented by measured data from the location. The size of above-mentioned region means 25 km², as this guideline is used by Environment Agency in Japan. However, the size of a region can vary depending on geographical characteristics (the configuration of forests, plains, lakes and marshes and rivers); the characteristics and distribution of a stationary source; population density; the weather situation. The size of a region depends also on the extent to the difference in the levels of concentration between each location in the same region. (In this connection, currently Nitrogen dioxide(NO2) is considered to be the most important in Japan. The concentration levels of NO2 of two locations are considered to be the same when the difference between annual averages of NO2 in two locations is within 5 ppb.) Those locations where extents and changes are considered to be similar are lumped. Then the location for a monitoring station within the region so that the concentration level is as representative as possible is established. A number of local governments use a regional average as the representative concentration level.
In practice, there are other considerations for placing a monitoring station.

(1) To place a monitoring station with consideration for the distribution of stationary sources on pollutants and for wind directions.

(2) The number of stations in densely populated areas should be large compared
to other areas.

(3) To place stations at the boundary areas by considering the distribution of stationary sources in neighboring areas controlled by other local governments, and weather patterns of neighboring areas.

(4) To place stations with consideration of land-use plans for the future.

(5) Not to place stations inclined toward a particular region.

In addition, if possible, background level of air pollution is tried to understand by establishing a monitoring station in a region where pollution is kept at a minimum. It also provides an opportunity to understand the extent to which the concentration of air quality has advanced in an area originally believed to be free of pollution. Also, it facilitates an understanding of pollution problems in a large area and over a long time span.
On determining the placement of monitoring stations, the following two methods are popular in Japan:

(a) To estimate the distribution of concentration in a target area through diffusion simulation. Based on results of the estimation, an area in which similar concentration levels are observed is determined. Then the locations of monitoring stations are determined. In this method, actual data monitored in various places in the target area are necessary to improve the precision of calculation.
(b) To investigate the similarity between existing data of monitoring points to determine an area in which similar concentration levels are observed. This method is useful for integrating existing monitoring stations into one station. However, it is not sufficient for determining the location of a new monitoring station.

Neither (a) nor (b) can be put into practice without existing data. When existing data within the target area are not available, the following methods can be used:

1) To divide the target area with a grid (such as a area of 5km ~5km) by considering its geographical features and the distribution of stationary sources.
2) To install the sampling device used for the Triethanolamine (TEA) method for measuring NO2 and the PbO2 methods for measuring Sulfur Dioxide(SO2) described in Chapter 6 at an appropriate location where the concentration level of the grid is estimated to be around the average.
3) To hang these devices under the eaves of a building or from a tree branch for a predetermined period in order to obtain concentration levels by collecting the devices and analyzing the data.
4) To obtain weather data, a mobile monitoring vehicle or the mobile station introduced in the examples of Osaka Prefectural and Osaka City Government in Chapter 3 are useful .
5) In this way, environmental and weather data are obtained. If grids next to each other show a large difference in concentration level, wind velocity or wind direction, the grids of that vicinity are subdivided into finer grids to repeat the survey.
6) Since these types of surveys can be easily influenced by weather, the survey should be conducted for different weather patterns throughout the year. For those countries where seasons are divided into wet and dry seasons, the division into periods should be made in accordance with the characteristic weather patterns of the country.

In this way, an area in which similar concentration levels of air pollution are observed can be determined.
Before above-mentioned methods started to be carry out the appropriate placement of monitoring stations around 1986 in Japan, in some cases a number of monitoring stations were established in a small area because of serious sulfur oxide(SOx) pollution in the area. As a result of data accumulation and a total analysis, placement methods were based on the scientific ideas described above other than methods through some experiences.

1.1.2. Roadside air pollution monitoring station
With regard to the placement of roadside air pollution monitoring stations, monitoring stations with consideration for the following purposes shall be placed:

(1) To effectively understand air pollution by analyzing traffic density; the proportion of large vehicles such as trucks in the total number of vehicles (large-vehicle mixture rate); the running speed of vehicles; road structure (number of lanes, whether the road is on the plane or situated in the overhead); geographical features and surrounding buildings; local and regional weather conditions; and population density near the road.
(2) The monitoring station is located near a road or an intersection with dense traffic. These places are near or right at places where people live and work from day to day. Considering the dispersion of monitored exhaust gas from automobiles due to distance, a monitoring station should be placed near the road(for example, distance of about 10m from edge of road).

The phrase "to effectively understand" in (1) means to place the monitoring station so as to have a representative concentration of pollutants for the roadside area. The representation for the area means the following: Suppose there are a few roads whose traffic densities, large-vehicle mixture rates, vehicle running speeds, road structures, geographical features and surrounding buildings, local or regional weather conditions, and roadside population densities are similar to each other. In such a case, the status of air pollution for one road can represent that for other roads. A traffic survey should be included when determining the location of a roadside air pollution monitoring station. Apart from adding a traffic survey, the method is almost the same as in the case of general air pollution monitoring station.