Introduction to Photochemical Assessment Monitoring


With Taiwan's economic progress and rapid population growth, large quantities of pollutants have been produced by industrial advancement and released into the atmosphere. Pollution concentration has become more severe due to rapid increase in the number of factories and automobiles. In the past decade, of the categories on the Pollutant Standards Index (PSI), SO2 has been reduced the most, while concentrations of PM10, CO, and NO2 have also been diminished greatly. However, the concentration of O3 has been increasing instead in recent years (Figure 1). O3 is a very reactive oxidizing agent and studies indicate it is harmful to the human body, building materials, and animals. It is especially damaging to the respiratory system. If a person is exposed to high concentration of ozone for a long time, symptoms such as headache, fatigue, coughing, and asthma may happen. It is also harmful to crops and also acts as greenhouse gas that contributes to climate change.

O3 is not emitted directly and is formed through nitroxides(NOx)and volatile organic molecules(VOCS)undergoing a series of photochemical reactions. Different volatile organic matters have different degrees of contribution to O3 formation. In order to understand the problem O3 causes, one needs to understand the mechanism behind O3 formation. One way to do that is to gather the information regards to the relationship between concentration of O3 precursors and O3 formations.

【Figure 1-Average O3 Concentration in Taiwan from 2003 to 2012 (unit: ppb)】

Establishment of Photochemical Assessment Monitoring Stations

After the Clean Air Act Amendments were passed by the United States in 1990, local governments are required to establish Photochemical Assessment Monitoring Stations (PAMS) to monitor O3, O3 precursors, and oxygen-containing volatile organic matters to understand the reason behind concentrated O3 formation. After the implementation of the Clean Air Act Amendments started, the US Environmental Protection Agency also established other laws in 18 months to reinforce monitoring of O3 precursors, and state governments were asked to establish PAMS to monitor O3 precursors in the atmosphere and to understand nitroxides and volatile organic compounds. When the laws were reviewed and passed, PAMS were established.

The primary goal of PAMS is to provide accurate, representative, and long-term data for O3 precursors in order to establish the relationship between O3, O3 precursors, and weather conditions, to find out the factors behind O3 formation, and to devise a strategy for O3 control.

Depending on O3 formation and transport characteristics, PAMS are basically separated into 4 types: upwind and background characteristics (type 1), maximum ozone precursor concentration (type 2), maximum O3 concentration (type 3), and far downwind (type 4). As shown in Figure 2, it can be introduced in 4 points.

  • 1Upwind and Background Characteristics (Type 1):The goal of this type of station is to evaluate the upwind ozone transportation information, including monitoring upwind ozone and its related precursors. This station type should be established in suitable upwind locations with high ozone precursor concentration.
  • 2Maximum Ozone Precursor Concentration (Type2):This station is primarily used to monitor the maximum O3 precursor output and formation in metropolitan areas, so it should be established in the downwind direction of most of the major prevailing winds in the morning from Type 1 stations and close to locations where O3 precursors are primarily generated in metropolitan areas. If the area covered is too large, an additional station should be set up near the second major morning prevailing wind direction.
  • 3Maximum O3 Concentration (Type 3): This station is mainly use to monitor the downwind area of locations where the highest concentration of O3 precursors are formed, amount of O3 formed after photochemical reactions, and VOCS remaining afterwards. It is usually set 20-40 km from city boundaries.
  • 4Far Downwind (Type 4):This station type is used to understand long-distance transport of O3 and O3 precursors. It is usually established in the direction of afternoon prevailing winds and close to the edge of downwind area.

Photochemical assessment stations may have multiple purposes in Taiwan due to high population density and close distance between residential area, business area and industrial area(Figure3).

PAMS Setup Principle
【Figure2-PAMS Setup Principle】
Multipurpose Photochemical Assessment Stations
【Figure3-Multipurpose Photochemical Assessment Stations】

Monitor Goal

The following points are used to explain the purpose and goals of monitoring O3 precursors.:

Specific information is closely related to emission types Each emission type, such as traffic, industrial or vegetation emission has its own unique composition. Therefore monitoring different VOCS is helpful in determining the contribution of different sources to O3 formation and provides essential reference information for O3 control.
Provides Direct Photochemical Evidence O3 is a secondary product of photochemical reactions. The relationship between reactants and products can be analyzed by monitoring the information regarding VOCS, NOx, and O3.
Confirmation Model Simulation O3 formation and control are usually simulated using models in order to understand O3 problems in large areas. Information for VOCS not only provides initial values for model calculation but provides actual values that can be compared with simulation values. The model’s parameters can be adjusted and the predictions can be made more accurate. Lack of actual information can make simulations very uncertain and unsuitable for decision making.
Provides a Base for Policy Change After receiving simulation results and experimental confirmation for photochemical reactions, control strategies can be formulated in different areas. With the assistance of actual test information, risk of inaccurate information is lowered, and this is the ultimate goal for data measurement. After a strategy is put into effect, actual observation data can be used to assess its effects. Therefore, information of VOCS and O3 is very valuable for evaluation of control strategies and necessary for O3 control.
Monitoring Poisonous Materials Aside from controlling O3, PAMS are also able to detect some toxic substances in the air, such as benzene, toluene and so forth. Instantaneous tracking information for VOCS provides concentration data for many trace substances in the atmosphere and can be used to analyze the composition of pollutants and be a reference for pollution sources.