Terahertz wave—evaluate emergency measures during red alert period

March 14, 2017, Science China Press
A terahertz-2DCOS analysis of PM2.5 during red alert period (a) and normal condition (b). Credit: ©Science China Press

In the Beijing-Tianjing-Hebei (BTH) region of China, haze has become especially serious in recent years, which has attracted much attention. To protect public health, the red alert has been implemented since 7 a.m. Tuesday (8th Dec. 2015), and the warning is due to be lifted at noon on Thursday (10th Dec. 2015). Since this is the first-ever red alert for smog, PM2.5 monitoring, including both concentration and pollutants' source characterization, is of great significance for environmental departments and the public to validate the effect of red alert.

The concentration and pollution constituents of PM2.5 were identified by the combination of THz absorption and two-dimensional correlation spectroscopy (2DCOS). As expected, the samples with high PM2.5 are related to larger absorption at selected frequencies, and THz absorption of low PM2.5 is similarly small under low pollution. Therefore, the information gained by EM during the red alert period was effective in relieving the haze pollution by gradually decreasing the PM2.5 concentration.

During the first-ever red alert period, car use was limited—cars are allowed on the roads on alternating days depending on the odd or even numbers of their license plates. Construction garbage and muck transport vehicles, concrete tank trucks and gravel transport vehicles are prohibited on the road. Outdoor operations of construction sites are banned and some industrial plants are required to limit or stop production. Kindergartens, primary and high schools are advised to suspend classes.

Based on the comparison of THz peaks of PM2.5 collected in the periods with and without red alert, features can be attributed to two conditions. By comparing features in synchronous and asynchronous plots of these spectra, we found that sulfate types were different under the two conditions, and that metallic oxides belonging to PM2.5 sources in the normal high-pollution condition were absent during the red alert period.

In summary, it would be useful to develop a real-time monitoring system for air quality in Beijing, given that haze conditions vary with time and location. In the future, metamaterials should be considered to measure relatively low densities of PM2.5 in real time.

Explore further: Beijing issues red alert for severe air pollution

More information: HongLei Zhan et al, Terahertz assessment of the atmospheric pollution during the first-ever red alert period in Beijing, Science China Physics, Mechanics & Astronomy (2017). DOI: 10.1007/s11433-016-0469-4

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