New calculation of the Việt Nam Air Quality Index

11/03/2020

     Nowadays, the public demand for update and announcement of air quality control is highly increasing. In attempt to enhance the management and use of environmental monitoring data and publicise air quality data in a more diverse manner, the Vietnam Environment Administration (VEA) on November 12^th, 2019 issued Decision 1459/QD-TCMT regulating the techniques for the calculation and publication of the Vietnam Air Quality Index (VN_AQI) that replaced Decision No. 878/QD-TCMT dated July 1^st, 2011.

     The Air Quality Index (AQI) included indicators that showed a clean or polluted environment and gave warnings and recommendations to people when air pollution increases. The index was calculated upon five major substances that could be highly polluted - which are ground ozone (O₃), PM2.5 and PM10 fine dust, carbon monoxide (CO), sulfur dioxide (SO₂) and nitro dioxide (NO₂).

     The first AQI manual was released by VEA in 2011, which had then helped people get better access to air quality control. However, after eight years, the AQI manual needed changes based on a new approach that were being broadly applied across the world.

 

The automatic air quality monitoring station at 566 Nguyen Van Cu, Ha Noi

 

     During the compilation of the new AQI manual, the group of authors studied AQI calculation methods that were available at the time, selected the most widely-used one and adjusted it to match Vietnam’s regulations. The selected method was called Break Point table, which was first used in the United States. Many countries had used the method such as China, Mexico, Brazil, Japan, South Korea, Thailand, India and Singapore. The authors then made some changes to make it meet the standards set by the Vietnamese Ministry of Natural Resources and Environment (Standard No. 05:2013/BTNMT).

     According to the Guide of the VN_AQI, there are different score levels, displayed by different colours to show the quality of the atmosphere and the impact on the health.

     Table 1. The AQI score ranges and evaluation

AQI score range	The quality of air	Colour	RBD colour code	Impact on the health
0 - 50	Good	Green	0; 228; 0	Good quality of air, no impact on the health.
51 - 100	Average	Yellow	255; 255; 0	Acceptable quality of air. But there may be some impact on old people, children and people with diseases.
101 - 150	Poor	Orange	255; 126;0	People who are sensitive to changes in the air may get health issues. Those with normal physical conditions get less impact.
151 - 200	Bad	Red	255; 0; 0	People with normal physical conditions start feeling unwell. Those are sensitive to changes get worse issues.
201 - 300	Very bad	Purple	143; 63; 151	Warning: All people may get serious health issues.
301 - 500	Harmful	Brown	126; 0; 35	Critical warning: All population get serious health issues.

     AQI calculation

     The AQI is divided into hour-based and day-based AQI:

     - Day-based AQI: is used to report the air quality updates for the whole day.

     - Hour-based AQI: is used to report hourly air quality updates.

     Calculation of hour-based AQI (AQI^h)

     The data used in AQI^h calculation is the one-hour average monitoring value. As there is no hourly calculation method for PM10 and PM25, so the monitor unit has to use the Nowcast method which is developed by the US Environmental Protection Agency. Nowcast is computed from the most recent 12 hours of PM monitoring data.

     To compute hourly AQI, first, calculate all AQI^h values of all indicators (AQI_x).

     The AQI^h values of indicators SO₂, CO,  NO₂, O₃ are computed by formula 1, the AQI^h values of indicators PM₁₀, PM_2.5 are computed by formula 2.:

  (Formula 1)

  (Formula 2)

     Of which:

     Table 2. BP_i values for each indicator

I	Ii	Regulated BPi  for each indicator (Unit: µg/m3)
		O3(1h)	O3(8h)	CO	SO2	NO2	PM10	PM2.5
1	0	0	0	0	0	0	0	0
2	50	160	100	10.000	125	100	50	25
3	100	200	120	30.000	350	200	150	50
4	150	300	170	45.000	550	700	250	80
5	200	400	210	60.000	800	1.200	350	150
6	300	800	400	90.000	1.600	2.350	420	250
7	400	1.000	-	120.000	2.100	3.100	500	350
8	500	≥1.200	-	≥150.000	≥2.630	≥3.850	≥600	≥500

     Note: - Use BPi value of O₃ to calculate hourly AQI (AQI^h) value of indicator O₃ (1h)

          - Use Bpi values of O₃ (1h) and O₃ (8h) to calculate daily AQI (AQI^d) values of O₃

     Combined hourly AQI: is the maximum AQI_x value of all indicators

Calculation of daily AQI (AQI^d)

     The formula used in computing daily AQI is similar to that of hourly AQI based on the Break Point table (table 2). But the inputs are:

     - Indicators for PM_2.5 and PM₁₀: 24-hour average values.

     - Indicator O₃: highest one-hour average value of the day and the highgest eight-hour average value of the day.

     - Indicators SO₂, NO₂ and CO: highest one-hour average values of the day.

     Combined daily AQI: the maximum AQI_x values of all indicators.

     The new rules of computing AQI in Viet Nam is highly appreciated by experts. After being released by the Vietnam Environment Administration, agencies and organisations apply the VN_AQI calculation to publish the results on air quality for people.

 

Lê Hoàng Anh, Vương Như Luân

Centre for Environmental Monitoring

(Source: Vietnam Environment Administration Magazine, English Edition III - 2019)