
                        Model Change Bulletin

                                MCB#8                      12/10/2012

                        AERMOD (dated 12345)

This Model Change Bulletin (MCB) documents changes made to the AERMOD
Dispersion Model. A brief description of the changes is provided
below.  Additional details are available in the Fortran source code.
Changes to the user instructions associated with this update are
documented in an updated AERMOD User's Guide Addendum.


This revised version of AERMOD (dated 12345) includes the following
modifications relative to the previous version (dated 12060):

--  Bug Fixes:

    1.  Adjustments to wind speeds based on the assumption that input
        wind speeds are vector (or resultant) mean winds have been 
        removed (see Eq. 112 on page 79 of the AERMOD Model Formulation 
        Document, EPA-454/R-03-004). This is considered a formulation bug
        fix since current EPA guidance for site-specific meteorological 
        monitoring (EPA-454/R-99-005) recommends that scalar mean wind 
        speeds be used in steady-state Gaussian dispersion models. 
        Furthermore, all wind speeds derived from NWS or FAA airport 
        data represent scalar mean wind speeds.  
        
        An option has also been included on the MODELOPT keyword 
        on the CO pathway (VECTORWS) that allows users to specify that 
        input wind speeds are vector means, in which case the previous 
        adjustments will be included. The new VECTORWS option is not 
        linked with the DFAULT option, but users should be able to 
        confirm that input speeds are vector means in order to justify 
        use of the option.  However, scalar mean wind speeds, if 
        available, should be used based on the current guidance.  
        This change affected several subroutines.  

    2.  Modified subroutine HRLOOP to correct a problem associated
        with missing hourly ozone data during MAXDCONT post-processing
        when no O3 values have been specified for substitution through
        the OZONEVAL or O3VALUES keywords on the CO pathway. The 
        MAXDCONT results in those cases were invalid and may have 
        shown up as negative concentrations.

    3.  Modified subroutines DAYRNG, METEXT, and SET_DATES to correct a 
        bug associated with use of the DAYRANGE keyword for multiple years
        of meteorological data, where the YR/MN/DY conversion to Julian
        may be incorrect.  Also included checks of the year specified on
        the SURFDATA keyword with the first year included in the surface
        meteorological data file. The ISYEAR variable is now adjusted to 
        match the first year of the data file, unless the DAYRANGE keyword 
        is being used. In that case, a fatal error is generated since the 
        ISYEAR variable is used to determine leap vs. non-leap years for 
        the YR/MN/DY conversion to Julian day for the DAYRANGE option.
        
    4.  Modified subroutine HRQREAD to check for large negative hourly 
        emissions (< -90), which may be used as missing indicators.
        Since AERMOD allows inputs of negative emissions for use in 
        emission credit calculations, negative values used as missing 
        indicators in the HOUREMIS file result in negative hourly 
        concentrations in the previous versions.  Warning messages are 
        generated and the emission rate is set to zero (0) for these cases.
        
    5.  Modified subroutine EVALFL to address a potential problem 
        with URBAN applications where the L_MorningTrans logical
        variable was not defined.

    6.  Modified MAIN program and subroutine PRESET to check for 
        duplicate STARTING keywords on the SO or RE pathways, since
        that would reset the array limits for setup arrays to zero
        during the PRESET phase, resulting in array subscript 
        out-of-bounds runtime errors.
        
        
--  Enhancements:

    1.  Incorporated two new BETA (non-Default) options to address
        concerns regarding model performance under low wind speed
        conditions.  The LOWWIND1 option increases the minimum value
        of sigma-v from 0.2 to 0.5 m/s and "turns off" the horizontal
        meander component. The LOWWIND2 option increases the minimum
        value of sigma-v from 0.2 to 0.3 m/s, and incorporates the 
        meander component, with some adjustments to the algorithm, 
        including an upper limit on the meander factor (FRAN) of 0.95.
        A new LOW_WIND keyword has been added to the CO pathway that
        allows users to adjust the minimum sigma-v value (within a
        range of 0.01 to 1.0 m/s), and the minimum wind speed value
        (within a range from 0.01 to 1.0 m/s), with a default value
        of 0.2828 m/s, consistent with the default applied in previous
        versions based on SQRT(2*SVmin*SVmin) with SVmin=0.2. The new
        LOW_WIND keyword also allows users to adjust the maximum value
        for the meander factor (FRAN) within a range of 0.50 to 1.0,
        inclusive, when the LOWWIND2 option is used.  Both of the new 
        LowWind BETA options also modify the the adjustment of vector 
        mean wind speeds (based on Eq. 112, p. 79, of the AERMOD Model 
        Formulation Document) to use the original values of sigma-
        v before they are adjusted based on SVmin. The new LOWWIND1
        and LOWWIND2 options are mutually exclusive and the model will
        issue a fatal warning message if both options are specified.

        These new LowWind BETA options can also be used in conjunction 
        with the new option in AERMET (v12345) to adjust the surface 
        friction velocity (U*) under low-wind/stable conditions (ADJ_U*), 
        based on Qian, W., and A. Venkatram, 2011: "Performance of Steady-
        State Dispersion Models Under Low Wind-Speed Conditions", Boundary-
        Layer Meteorology, 138, 475-491. The ADJ_U* option in AERMET is 
        also considered a non-Default option, and users must also specify 
        the BETA option on the MODELOPT keyword in AERMOD in order to 
        process meteorological inputs derived using the new ADJ_U* option. 
        See the AERMET Model Change Bulletin (MCB) #3 and AERMET User's 
        Guide Addendum for additional details regarding the ADJ_U* option 
        in AERMET.
        
        The new LowWind BETA options in AERMOD and the new ADJ_U*
        option in AERMET are considered to be non-Default options
        and are therefore subject to the alternative model provisions
        in Section 3.2 of Appendix W (40 CFR Part 51). Users should
        coordinate with the appropriate reviewing authority regarding
        the procedures and requirements for approval of these BETA
        options for regulatory modeling applications.

    2.  A new LINE source type has been included that allows users
        to specify line-type sources based on a start-point and 
        end-point of the line and the width of the line, as an 
        alternative to the current AREA source type for rectangular
        sources. The LINE source type utilizes the same routines as
        the AREA source type, and will give identical results for 
        equivalent source inputs.  The LINE source type also includes
        an optional initial sigma-z parameter to account for initial
        dilution of the emissions. As with the AREA source type, the
        LINE source type does not include the horizontal meander 
        component in AERMOD. This change affected several subroutines
        and also includes a new LPARM subroutine.
        

--  Miscellaneous:

    1.  Modified subroutine HRQEXT to include additional range checks
        on hourly stack exit velocities and exit temperatures input
        through the HOUREMIS option. A fatal error is generated if
        the hourly exit temperature is less than 200K (about -100F)
        and the exit velocity is greater than 200m/s, unless the hourly 
        emissions are zero (0) for that hour. This may indicate that 
        incorrect units for exit temperature have been used, or that 
        the order of exit temperature and exit velocity may have been 
        reversed in the HOUREMIS file. Also added a new warning message 
        for exit velocities larger than 250 m/s. Comparable changes 
        were incorporated also in subroutine PPARM for inputs on the 
        SRCPARAM keyword.

    2.  Modified subroutine MEOPEN to adjust the acceptable AERMET
        version date from 06341 to 11059. AERMOD will no longer run
        with met data based on version 06341 of AERMET.  AERMOD will 
        run using met data based on version 11059. However, a warning 
        message will be generated, and users are strongly encouraged 
        to update their meteorological data to version 12345 of AERMET
        due to the scope of changes included in that update.


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                        *   IMPORTANT   *
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This program will henceforth be identified by its name and a Julian date,
i.e.: AERMOD (dated 12345).  Future changes to this model will be identified
by an updated Julian date found at the beginning of the model source code
and followed by a reference to a SCRAM Model Change Bulletin (MCB#n)
which will explain the nature and content of the changes.  All Model Change
Bulletins for this program will always be available for back reference on
this website.  In the future, notification of changes (MCBs) to this program
will be made in the Recent Additions section of the SCRAM website.
