Particle Pollution Hourly Trend

Particle pollution can be directly emitted into the atmospher or formed by chemical reactions and varies in time and space. Our analyses have demonstrated both hourly variations in fine particle levels as well as differences between regions. In the chart below, the hourly particle level for each monitor has been averaged across an entire year.

What we discovered is that there are two peaks during the day. There is a peak of a few hours first thing in the morning and a lesser peak over more hours later in the day at most of our monitors.

These peaks are associated with 'rush hour' traffic combined with lower mixing levels. The mixing level is the height upto which there is good vertical mixing. During the late morning and afternoon hours the sun heats the surface and that warm air rises. There is good vertical mixing, the mixing level is high. When the air is well mixed, pollution levels drop because the pollution is spread out over a larger area. During the evening and overnight hours the surface cools and the air near the surface tends to remain at the surface so there is little vertical mixing and the mixing level is low. When the air is NOT mixed, pollution levels tend to increase because the pollution is squeezed into a smaller area.

Another interesting trend is demonstrated in the chart as well. We see the differences between the regions. The three urban sites show a greater diurnal variation and higher values than the rural elevated monitor in the Mid-Coast region. We also note that for the latter monitor the afternoon peak is higher than the morning peak, although the value is still much lower than the urban monitoring values.

Air Quality Trends

Particle Pollution Hourly Trend