Analysis of the Effects of ASOS Data on Air Dispersion Modeling

In the early and middle 1990s, the National Weather Service (NWS) methods for collection of hourly observations changed from a manual observer to the Automated Surface Observing System (ASOS). This change has resulted in changes to individual parameters within the surface observation. New sampling methods, updated equipment, and data processing algorithms are among the factors which may contribute to differences between manual observations and ASOS observations for a given hourly parameter. Many of these parameters are important to computation of stability class when used in air dispersion modeling. Wind data is one of the parameters affected by the switch from manual observations to ASOS. Specifically, an increase in the frequency of calm wind reports has been observed. This paper will analyze the occurrence of calm winds in both pre- and post-ASOS data periods for 1990-1995. A quantitative anlysis will be made to show the frequency of calms for each station. For selected stations, an analysis will also be made using the Industrial Source Complex Short Term (ISCST3) model to determine the affects of ASOS calms on ground level concentrations at 1-hour, 24-hour, and annual averages for different locations across the United States.