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Regional Air Quality Index - Frequently Asked
Questions
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1. |
What is the Regional Air
Quality Index (RAQI)? |
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The RAQI is a measure of the
aggregate level of major air pollutants. The RAQI for the Pearl River Delta
(PRD) regional air quality monitoring network is derived from the
concentrations of 4 major air pollutants, namely respirable suspended
particulates (RSP or PM10), sulphur dioxide (SO2),
nitrogen dioxide (NO2) and ozone (O3).
The higher the index value, the higher the overall level of regional air
pollution.
The RAQI is divided into 5 grades. The categorization of the RAQI grades, their
corresponding index values and air quality conditions are as shown below:
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Grade
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Value
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Air Quality Conditions in the Monitored Area
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I
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0-1
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Concentrations of all pollutants are well
within the National Ambient Air Quality
Standards (NAAQS) [#]
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II
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1-2
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Concentrations of all pollutants are
generally within the NAAQS
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III
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2-3
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Concentrations of individual pollutants may
approach or exceed the NAAQS
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IV
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3-4
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The NAAQS are generally exceeded
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V
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> 4
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The NAAQS are significantly exceeded
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[#] Refer to the Class 2 NAAQS (GB 3095 – 1996
– revised version), which are applicable to residential, mixed
commercial/residential, cultural, industrial and village areas.
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| 2. |
How to calculate the RAQI? |
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The formula for calculating the RAQI is as
follows:
where Ic stands for the RAQI, an indicator of the aggregate
pollution level of four pollutants, namely, SO2, NO2,
PM10 and O3. With respect to SO2,
NO2 and PM10, Ci means the daily average
concentration while Ri represents the daily average concentration
limit of the corresponding pollutants as specified in the Class 2 NAAQS (GB
3095 – 1996 – revised version). With respect to O3,
Ci means the highest hourly average of a day while Ri
represents the 1-hour average concentration limit in the Class 2 NAAQS.
The Class 2 NAAQS (GB 3095 – 1996 – revised version) apply to residential,
mixed commercial/residential, cultural, industrial and village areas. The
concentration limits for various pollutants in the standards are listed in the
table below.
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Standard for various pollutants
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Pollutant
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Sampling duration
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NAAQS (GB 3095–1996–revised
version )
Class 2 standards
(mg/m3)
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SO2
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Daily average
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150
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NO2
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Daily average
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120
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PM10
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Daily average
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150
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O3
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Hourly average
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200
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| 3. |
Why is the RAQI calculated
based on the NAAQS but not the Hong Kong Air Quality Objectives (HKAQO)? How do
the NAAQS compare with the HKAQO? |
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The NAAQS are categorized into 3 classes. The Class 2
NAAQS is applicable to inhabited areas. Geographically, cities in Guangdong
constitute the major part of the PRD region. It is therefore more appropriate
to adopt the NAAQS for calculating the RAQI. The NAAQS are largely comparable
with the HKAQO. |
| 4. |
What are the differences
between the RAQI and the Air Pollution Index (API)? Why are there two indices? |
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The RAQI and API are different air quality
indicators and they are not comparable.
The RAQI is a measure of the aggregate level of 4 major regional air pollutants
(RSP, SO2, NO2 and O3),
and is useful for indicating large scale regional air pollution levels.
Regional air pollution in the PRD is complex in nature, characterised by the
formation of the 4 major air pollutants and their transportation and
inter-reaction (for example, SO2 can be transformed to
sulphate particulates through oxidization in the air, while NO2
can react with volatile organic compounds to form O3 through
the photochemical process) over an extensive area. We therefore adopt the RAQI,
which is an amalgamation of the concentrations of all these 4 pollutants, for
representing the regional air pollution in the PRD area.
On the other hand, the API report issued by an individual city reflects the
concentration of the most significant pollutant (known as the “critical
pollutant”) which poses the greatest health risk to the local residents. It
serves to inform the residents of the local air pollution information and alert
them when the level of critical air pollutant in the area where they live has
exceeded the air quality standard so that they could take precautionary
measures.
The API focuses on the critical pollutant in the local area. Among all the
pollutants being measured, the critical pollutant has a concentration closest
to the air quality standard limit or exceeding the limit most, and therefore
poses the greatest health impact to the local residents. When the API of a
particular place exceeds 100, it means that the concentration of the critical
pollutant has exceeded the air quality standard and may pose an immediate
health risk to those susceptible to air pollution.
The API is useful for describing the state of local air quality and alerting
the local residents in the events of high local air pollution.
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| 5. |
Which index should I refer
to? |
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In general, the public should refer to the API report
which reflects the concentration of the critical pollutant posing the greatest
health risk to them. The API can help to alert the local residents when the
level of critical air pollutant in the place where they live has exceeded the
air quality standard, so that the public can take the necessary precautionary
measures.
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| 6. |
Then, how is the RAQI of the
PRD region useful to me? |
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The RAQI report tells you about the overall
levels and spatial distribution of regional air pollution in the PRD area. The
RAQI report is more useful for the appraisal and management of regional air
pollution problems.
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| 7. |
In case the RAQI reaches
Grade IV or above, which means that the air quality standards have been
generally exceeded in the measured areas, should the public take precautionary
measures? |
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The RAQI report mainly serves to indicate the
overall levels and spatial distribution of regional air pollution over a large
area. The public are advised to refer to the API reports issued by individual
cities for information on the local air pollution situation and whether there
are exceedances of the air quality standards. They should take necessary
precautionary measures in case the API value of the city/district where they
live is above 100, which means that the concentration of the critical pollutant
has exceeded the air quality standard.
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