Experts explain that Hanoi has a higher air pollution index at night, mainly due to the heat and calm winds that make it difficult for the air to diffuse to rise.
The Vietnam Environment Administration (Ministry of Natural Resources and Environment) recently released a report on air quality developments in Hanoi during the end of September and early October.
Accordingly, from September 27 to October 2, the air quality in Hanoi continued to have bad developments, the average value of PM2.5 exceeded the allowed level. By October 3, air quality was better when Hanoi welcomed a heavy rain in the early morning.
Notably, the report stated that the time to record PM2.5 values at high level is mainly at night and early morning, typically in the frame of 0-6h in many days.
The General Department of Environment explains the cause of this phenomenon because the night is a period of calm wind and the opposite of heat, causing pollutants to be dispersed and maintained at a high level.
Talking to PV, Dr. Hoang Duong Tung, President of Clean Air Network, said that the assessment results of the Vietnam Environment Administration were quite objective and realistic. Mr. Tung said that although there is the least amount of traffic at night, the movement of people is limited, but this is a good time to have a heat inversion .
"This phenomenon usually occurs at night because at this time the ground is cold, the lower atmosphere is compressed under the hard to evaporate," Tung explained.
The General Department of Environment stated that air pollution occurred mainly in the period of 0-6h.(Photo: Viet Hung).
Accordingly, normally the higher the air gets thinner and the temperature decreases. But when heat inversion occurs, the atmosphere at the upper floor will have a temperature equal to or sometimes higher than that of the ground. This makes the air in the lower level compressed, unable to evaporate.
In addition, at night is also a time when there is less wind, the air contains pollutants that are difficult to diffuse, causing pollution tends to be higher than other times of the day.
Experts recommend that people limit movement outside, exercise during the period of 5-6 am because at this time the air pollution can not diffuse to high levels, harmful to human health.
Hanoi air quality in the morning of October 7 was still poor.(Graphic: Minh Hong)
In addition, the air quality overview report of the General Department of Environment in September also provides the data to assess and confirm air pollution in Hanoi in the past 10 years of the downward trend.
"Following the monitoring of developments from 2010 to the present, PM10 and PM2.5 dust concentrations tend to decrease. Comparison of PM2.5 dust concentration in the past months from 2013-2019 shows that PM2.5 dust in the months of 2019 tends to decrease over the years " , the General Department of Environment stated.
Talking about this trend, Mr. Tung said that it is necessary to reconsider the source of data and consider explaining this trend. Because, in the period of 2013-2019, the volume of motorbikes, traffic density, construction, and urbanization speed of Hanoi increase each year, so the trend of air pollution is unlikely to decrease.
"Based on the assessment of AQI within a few years of an organization alone, it is difficult to identify the trend of reduced air pollution. It is necessary to have an analysis and linking information together. prize on this trend ", Mr. Tung made his point.
According to him, if the trend of pollution has decreased, it may be due to the climate changes in rainfall over years.