Why is Hong Kong suddenly blanketed in haze and how much longer will it last?

A dusty airstream linked to the northeast monsoon
left eyes watering and throats sore in Hong Kong on Sunday.
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According to the Observatory, the city experienced its driest April morning since an automatic weather station was set up in 1984, with relative humidity falling to as low as 21 per cent.
The Post examines the sudden arrival of severe air pollution and what precautions residents should take to minimise their exposure.
Conditions at Shing Mun River in Sha Tin. Photo: Sam Tsang
1. What triggered the haze?
Former Observatory assistant director Leung Wing-mo explained that both the low relative humidity and poor air quality were influenced by the weakened northeast monsoon.
'The low relative humidity is likely due to continental airflow and sinking air, similar to the hot and dry conditions before a typhoon, while the poor air quality is due to high levels of both sandstorm-related particulate matter and an unusually high ozone level,' Leung said.
He said the surging ozone level was likely caused by stratospheric ozone intrusion.
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'This occurs when ozone from the layer of the atmosphere about 10km [6.2 miles] to 30km high descends to the ground due to sinking air,' he said. 'Ozone combined with some particulate matter like PM10 and PM2.5 makes the air quality even worse.'
The Environmental Protection Department noted the airborne dust primarily consisted of PM10, or larger respirable suspended particulates. The level of PM2.5, the fine suspended particulates that pose a more direct threat to human health, was lower.

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