Factors that Contribute to the Formation of Ozone and Particulate Matter
In addition to the presence of VOCs and NOx, there are other factors which contribute to the formation of ground-level ozone. These are the types of factors that MDEQ meteorologists take into consideration when forecasting Clean Air Action Days for our area.
Solar insulation, or high levels of ultraviolet radiation (sunlight), is required to initiate the ozone-forming photochemical reactions. Sunlight stimulates the VOCs and NOx chemicals to recombine to create ground-level ozone.
In addition, the amount of cloud cover contributes to ozone formation. When there are few or no clouds, or only high transparent clouds, solar insulation is more able to penetrate to ground level, enabling the photochemistry that generates ground-level ozone to occur. When cloud cover increases, the likelihood of elevated ozone levels decreases.
Temperatures over 80° also influence the formation of ozone. Higher temperatures enhance the ozone formation chemistry and increase the evaporative emissions of volatile organic compounds. The higher the temperature, the more likely elevated levels of ozone will occur.
Wind direction, especially winds out of the south-southwest, can contribute to ground-level ozone. South or southwesterly winds typically bring warmer weather to West Michigan, or transport existing ozone from upwind locales, such as Chicago.
Low wind speeds (less than 10 MPH) are necessary for the accumulation of precursors of ozone formation (VOCs and NOx) and the subsequent formation of ozone. Higher wind speeds tend to dilute or disperse emissions. However, they can still transport ozone from other locations.
Dry weather allows ozone to remain in the air; therefore low levels of precipitation can contribute to ozone formation. Scattered showers do not produce enough precipitation to completely eliminate ozone, but widespread rain will cleanse the atmosphere of ozone.
The positions of fronts are also considered in ozone formation because of their potential to affect cloud cover, precipitation, and changes in the air mass.
High particulate matter episodes are less understood than high ozone episodes because of the large variety of compounds that can chemically react and transform into this pollutant (secondary particulate). Three pollutants that often play a roll in the formation of secondary particulate matter are nitrates, sulfates, and organic carbon. Particulate matter can also be emitted directly from a source (primary particulate). Although meteorological conditions and atmospheric chemistry for high particulate matter days vary seasonally, certain characteristics have been noted. MDEQ meteorologists take these factors into account when forecasting Clean Air Action Days for our area.
Like ozone, low wind speeds are also necessary for the accumulation of particulate matter. The longer the period of low wind speeds in an area, the greater the likelihood that particulate matter will accumulate. Higher wind speeds tend to dilute or disperse emissions, but they can still transport them to other locations.
High pressure air masses that last several days may create stagnant conditions that allow for the buildup of particulate matter levels. Accumulation will continue until a change in weather patterns brings less polluted air into the region.
Wind direction, especially winds out of the south-southwest, can elevate particulate matter levels. South or southwesterly winds typically transport existing pollution from upwind locales, such as Chicago.
High relative humidity enhances the formation of nitrate and sulfate aerosol particles. The chemistry for forming aerosols is limited in a restricted moisture environment.
Cooler temperatures promote nitrate formation in the winter, while warmer temperatures promote sulfate formation in the summer. This is why concentrations of particulate matter tend to peak in both the winter and summer months in the upper Midwest.
Mixing heights define the depth of the atmosphere where surface-borne air pollutants are trapped. The deeper the depth of potential mixing, the more atmospheric mixing and dilution can occur. If the mixing depth is shallow, mixing is restricted and particulate matter levels will become more concentrated.
Atmospheric stability controls the vertical turbulence of the atmosphere. An unstable atmosphere (i.e. low pressure systems) will tend to have strong vertical movement air which increases the mixing depth and dilution potential. A stable atmosphere (i.e. high pressure) tends to limit vertical turbulence and mixing potential. Therefore, mixing depths in a stable atmosphere tend to be very shallow, which can lead to rapid particulate accumulation.
Winds aloft can originate from different geographic regions than surface winds. Since the vertical profile of wind direction tends to veer clockwise with height, surface winds can be southwest while upper air winds can be northwest. Thus surface air can be polluted while upper air can be clean. If the depth of atmospheric mixing reaches into upper layers of the atmosphere, cleaner air can mix down and dilute the more polluted air near the surface.