The brown haze in this photo is particle pollution
We've all noticed a brown haze in metropolitan areas, usually during the cooler months of the year, in the late afternoon or the early morning when there’s little or no wind.
This is particle pollution. It occurs when tiny airborne particles are formed in the atmosphere and, without any wind to scatter them, they reach high concentrations and affect visibility.
These particles are generally less than 50 micrometers (µm) in diameter - one micrometer is equal to one thousandth of a millimetre. To put it into context, the average human hair is about 70µm in diameter.
You’d think these microscopic particles were too small to do much damage. Unfortunately, size is their strength. Fine particles are light and tend to stay floating in the air for days or weeks until they’re removed by rain. During this time they can travel remarkable distances from their source and affect air quality right across a region.
Larger, heavier particles stay in the air for a shorter time before being deposited locally, sometimes soiling or damaging materials, but certainly adding to the grimy look of some built-up areas.
Together, these particles are referred to as total suspended particulates (TSP).