Understanding the weather conditions leading to major winter particle pollution. Case study over Quebec region during February 2005

During the nine-day period from January 31 to February 8, 2005, all south western Quebec regions accounting for 6.6 millions of people or about 88% of the Quebec population experienced a severe particle pollution episode. Many other regions in the eastern-half of the U.S as well as the southern part of Ontario were also affected. Smog episode during winter time is not a rare event but this episode was unique because of high level of PM2.5 (Particle Matter with diameter less than 2.5 mm) recorded, the duration of the event and the number of people affected. Over Montreal areas the PM2.5 level were over the threshold value of 35 µg/m3 (poor air quality) during 197 hours in a row. During the episode the PM2.5 concentrations reached between 50 to 100 µg/m3 everyday over all Saint-Laurent Valley. Weather conditions played a significant part in the development of this episode. Particle levels were already high in the Midwest few days prior to the episode. During the seven-day period, the weather pattern was set up in a such way that disturbances were moving across the southern U.S. and central Canada. Consequently, there were minimal dynamics to promote vertical mixing and dispersion of PM2.5 in the atmosphere. At the surface, high pressure system over much of the eastern U.S. resulted in light winds, stagnant air mass conditions, and the build-up of pollutants. The studies of the back trajectories during the event confirmed those facts. Radiation and subsidence Inversion played a large role in this event. These conditions, combined with day to day accumulation of local emissions such as transportations and residential wood burning contributed to this episode. Visibility was significantly reduce in haze during all the period.

CAPMoN (Canadian Air and Precipitation Monitoring Network) a non-urban air quality monitoring networks not affected by local sources of air pollution collect air Particle and trace gas concentrations such as HNO3 and SO2 and rain samples like NO3-, SO4=, NH4+ also hourly HNO3, NH3 and SO2 concentrations from Provincial and Montreal City network have been collected and analyses during the episode as well as organic and elementary carbonic. Results show that nitrate and sulphate level were particularly high during the event.

AURAMS (A Unified Regional Air-quality Modelling System) is a complete numerical chemical model developed by the Meteorological Service of Canada for air quality research and management. The meteorological fields needed by the AURAMS model were obtained from the Global Environmental Multiscale model (GEM). AURAMS model was applied to simulate the entire PM2.5 episode. AURAMS hourly outputs show good agreement with observed PM2.5 data over Quebec region domain.

This case analysis will contribute to better predict in advance such events and to establish a more effective emission control strategy to prevent this type of severe PM2.5 episode.