average for purposes of modeling demonstrations of compliance with the NAAQS.
163.2.1. NWS data
18NWS data are available from the National Climatic Data Center (NCDC) in many
19formats, with the most common one in recent years being the Integrated Surface Hourly data
20(ISH). Most available formats can be processed by AERMET. As stated in Section 3.1, when
21using data from an NWS station alone or in conjunction with site-specific data, the data should
22be spatially and temporally representative of conditions at the modeled sources.
24A recently discovered issue with ASOS is that 5-second wind data that are used to
25calculate the 2-minute average winds are truncated rather than rounded to whole knots. For
26example, a wind of 2.9 knots is reported as 2 knots, not 3 knots. To account for this truncation of
27NWS winds (either standard observation or AERMINUTE output), an adjustment of ½ knot or
280.26 m/s is added to the winds in stage 3 AERMET processing. For more details refer to the
29AERMET User’s Guide Addendum (U.S. EPA, 2012b) and/or the appropriate EPA Regional
32 126.96.36.199. AERMINUTE
34 In AERMOD, concentrations are not calculated for variable wind (i.e., missing wind
35 direction) and calm conditions, resulting in zero concentrations for those hours. These light wind
36 conditions may be the controlling meteorological circumstances in some cases because of the
37 limited dilution that occurs under low wind speeds which can lead to higher concentrations. The
38 exclusion of a greater number of instances of near-calm conditions from the modeled
39 concentration distribution may therefore lead to underestimation of 24-hour average PM2.5 and
40 annual PM2.5 concentrations for design value calculation.
42 To address the issues of calm and variable winds associated with the use of NWS
43 meteorological data, the EPA has developed a preprocessor to AERMET, called AERMINUTE
44 (U.S. EPA, 2011d), that can read 2-minute ASOS winds and calculate an hourly average.
45 Beginning with year 2000 data, NCDC has made the 1-minute wind data, reported every minute
46 from the ASOS network freely available. The AERMINUTE program reads these 2-minute
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