average for purposes of modeling demonstrations of compliance with the NAAQS.


16 3.2.1. NWS data


18 NWS data are available from the National Climatic Data Center (NCDC) in many

19 formats, 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

21 using data from an NWS station alone or in conjunction with site-specific data, the data should

22 be spatially and temporally representative of conditions at the modeled sources.


24 A recently discovered issue with ASOS is that 5-second wind data that are used to

25 calculate the 2-minute average winds are truncated rather than rounded to whole knots. For

26 example, a wind of 2.9 knots is reported as 2 knots, not 3 knots. To account for this truncation of

27 NWS winds (either standard observation or AERMINUTE output), an adjustment of ½ knot or

28 0.26 m/s is added to the winds in stage 3 AERMET processing. For more details refer to the

29 AERMET User’s Guide Addendum (U.S. EPA, 2012b) and/or the appropriate EPA Regional

30 Modeling Contact.




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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