Introduction

  • Use the table of contents below to learn about the contents of our air quality systems
  • AQF Flow Chart
    How gases are measured in the atmosphere
    What trace species are
    How to read the pressure/temperature/wind profile
    The Aerosol Optical Depth


    AQF Flow Chart

    The flow chart above is the detailed mechanism behind our forecasting system. Data, whether it comes from emissions or meteorology, is imported to the model (SMOKE, WRF) and is uploaded to CMAQ so it could produce concentrations of pollutants. Processes within each model (i.e. chemistry, advection) are configured for maximum resolution.

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    How gases are measured in the atmosphere

    Gases in Earth's atmosphere are measured using the mixing ratio, number density, and/or partial pressure. For these plots, we measure our constituents using mixing ratios. Mixing ratio, or mole fraction, is the amount of a specific gas in relation to all gas species in air. Units such as ppm, ppb, ppt, and ppq represent parts per million, billion, trillion, quadrillion, respectively.

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    What the plot above shows is the overall mixing ratio of formaldehyde (HCHO) in ppbv. In this case, the scale ranges from 1 - 10 ppbv. All mixing ratios of our species are in units of ppbv, which means parts per billion by volume. Making measurments in ppbv means for every billion air moles in a volume of air, a certain number of moles on that scale represented on that plot are HCHO moles in that volume of air.

    Analyzing this plot further, you can determine the large purple area to the northeast of Houston has a mixing ratio of at least 10 parts of HCHO within 1 billion parts of air in that region, and in Conroe, roughly 4-6 parts of HCHO within 1 billion parts of air! Even though 1-10 ppbv does not look like a heavy concentration, that amount serves a significant contribution to the chemical mechanisms behind air pollution.

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    What trace species are

    Scientists have measured gases in our atmosphere since the 18th century. With technology advancing over the years, more species have been discovered at very low levels. These species occur at high reactivity rates and are harmful to the environment. Trace species, or tracers, leave their marks behind when present, making it possible to enhance our research when it comes down to their hazards. Molecular nitrogen (N2, ~78%), molecular oxygen (O2, ~20%), and argon (Ar, ~1%) make up the most abundant gases in our atmosphere. The remaining (tracers) exist in small amounts that make up <1% in today's conditions. Tracers we plot are those that cause the most production for air pollution, which the U.S. Environmental Protection Agency (EPA) establishes standards for. The maximum level for ozone in the 8-hour averaging time is 75 ppb and 35 micrograms/m3 for the 24-hour averaging time of particulate matter with a 2.5 micrometer diameter. More information about the National Ambient Air Quality Standards (NAAQS) and the EPA is located in http://www.epa.gov/air/criteria.html.

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    How to read the pressure/temperature/wind profile

    The profile we include the pressure, temperature, and wind features in is presented as one plot. Below is what one of these plots looks like:

    Geopotential height, for an air parcel, is the 4-digit number inside the white boxes. It represents the altitude where objects are at rest on a rotating planet. Since the Earth is always rotating, these atmospheric dynamics are observed through a geocentric frame of reference. Notice how 3,000 meters above the surface is equivalent to the air pressure at 700 halo pascals.

    The scale below the plot represents temperature in degrees Fahrenheit. Cooler temperatures exist above the surface since the temperature decreases with respect to height in the troposphere, the lowest level in the atmosphere (0 - ~11 km in north/south poles and 0 - ~16km in equator).

    The arrows pointing southwest resemble windspeed magnitude and direction. On the top-right corner, the box with the 10 above the arrow represents 10 m/s based on the length of that arrow.

    Below, you will see another example of the meteorological profile at the surface level.

    This plot contains the pressure (in halo pascals) instead of geopotential height in the white boxes. The mean sea level pressure (standard atmosphere) is 1013.25 hPa. Notice the different conditions between land and sea.

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    The Aerosol Optical Depth

    Aerosols exist as fine liquid/solid droplets that are in colloidal suspension within a gas. In the atmosphere, they have the ability to scatter and absorb energy through electromagnetic radiation. With efficient tranferable energy, a significant amount of these aerosols can grow in size and number density to reduce visibility. The aerosol optical depth (AOD) in this case is the measure of how polluted the air is. Matheatically, it is defined as the product of aerosol mass concentration and the absorption/scattering coefficients per unit mass of the aerosol. The AOD is a dimensionless value that ranges from 0.05 to 1.0. Below is an example plot of AOD.

    Conditions at 0.05, such as the Houston area, correlate with pristine environments (clean air), while conditions at 0.5-1.0 correlate with heavier sources, such as forest fire emissions, intense plumes, etc. (dirty air). Conditions at 0.05 can also represent regions in the remote environment with less emission sources (rural towns). Denser air masses, such as the one located along east Texas-west Louisiana (AOD ~0.2), can be transported overtime from its primary, urban source.

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