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. ***************** * IMPORTANT * ***************** 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.