Wintertime Haze

Haze events are often observed in Montreal during the wintertime. Typically haze is observed when there is a shallow mixed layer with a capping temperature inversion associated with a surface high pressure system. These conditions, combined with calm winds, normally occuring in the morning, allow local pollution from the urban environment (e.g. automobile emissions) to accumulate in the shallow mixed layer. This results in haze formation and deteriorating air quality.

Graph: Temperature vs height

The following is an investigation into the structure of urban wintertime haze episodes in Montreal using observations from a UHF wind profiler radar UHF Profiler, RASS, and laser ceilometer.

The right plot in Fig. 1 is an hourly averaged profile of virtual temperature obtained from RASS observations. The capping inversion is well defined and below the base of the inversion (700 m) there is a unstable region down to 470 m.

The plot in Fig. 1 is the same profile as above plotted in terms of virtual potential temperature. This plot shows the strong capping stable layer associated with the inversion and the unstable region below.

A comparison between the RASS, UHF wind profiler, and laser ceilometer observations during a wintertime haze event is shown below in Fig. 2. The RASS was run continuously during this period. Figure 2a is a time-height plot of the virtual potential temperature obtained from a running hourly average of the RASS data, indicating the structure of the boundary layer. The dashed line indicates the base of the capping stable layer, as shown in Fig. 1. One region of interest is between 0945 and 1130 where there is heating in the lower layers. This produces an unstable region above it, at about 500 m, that is capped by a strong stable layer (temperature inversion) at 700 m.

Figure 2b is a time-height plot of the range-normalized signal-to-noise (SNR) obtained from UHF radar data. Overplotted on Fig. 2b is the base of the capping stable layer obtained from the virtual potential temperature profiles in Fig. 2a. The top of the mixed layer can be inferred from the peak in SNR profiles. The figure shows the altitude of maximum SNR follows closely to the base of the stable layer.

Figure 2c is a time-height plot of optical extinction obtained from laser ceilometer observations. Overplotted again is the dashed line indicating the base of the capping stable layer from Fig. 2a. The haze is concentrated near the base of the stable layer but overshoots around 1000 EST, when the air below is most unstable. As the mixing layer develops, eddies transport aerosols from pollution sources, generally near the earth's surface, up to the base of the stable layer, where vertical motions are suppressed and the aerosols are trapped.

This example is one of many observed wintertime haze events in Montreal. The unique comparison of measurements showing the trapping of haze: (i) provides further evidence of the utility of RASS and UHF profiler data in pollution studies; (ii) demonstrates the potential of the laser ceilometer for providing inexpensive, continuous monitoring of air quality.