In the U.S., atmospheric pressure is almost always reported in inches of mercury (Hg), corrected to standard conditions at sea level. The rare exceptions are encoutered at research sites, where the actual pressure (station pressure) may be reported in millibars. Calculations of aerosol optical thickness (AOT) require the station pressure expressed in millibars (mb). If you do not have access to a barometer that has been calibrated at sea level and NOT corrected for your site elevation, you can use this JavaScript applet to give you a good approximation of station pressure at your site based on reported atmospheric pressure and your site elevation in METERS. The correction is based just on your elevation, and does not include temperature effects, which are relatively small.
If you enter the "weather report" pressure in inches of mercury (in the range 29-31), the applet will convert this value to mb for you. The result is always given in mb.