Model Change Bulletin MCB#1 12/07/2006 AERMOD (dated 06341) This Model Change Bulletin documents changes made to the AERMOD Dispersion Model. A brief description of the changes is provided below. Additional details are also available in the Fortran source code. Changes to the user instructions associated with this update are documented in a separate AERMOD User's Guide Addendum. This revised version of AERMOD (dated 06341) includes the following modifications relative to the previous version (dated 04300): Bug Fixes: 1. A bug causing inconsistencies in estimated concentrations for area sources was fixed by initializing the effective plume height (HE) to equal the release height (HS) prior to first call to ADISY in subroutines ACALC and OCALC. 2. The logical variable lbd was undefined in calls to WAKE_XSIG. The calling argument lbd was replaced with .TRUE. to ignore BID for the "outside" cavity source in subroutine CAV_SRC. 3. Subroutine PRMDELH was modified to save L_INWAKE to a local variable to avoid potential problems on some computer systems. 4. Modified subroutines WAKE_DFSN and WAKE_DFSN2 to initialize dummy variables zkdum and ykdum to 0.0. Otherwise these variables may be undefined in subroutine WAKE_SIG under some circumstances. 5. Moved call to RSDUMP for MULTYR option from subroutine HIPER to MAIN to resolve minor discrepancy in ANNUAL averages for MULTYR option. 6. Added arrays to store WDSIN and WDCOS by source for use in determining the wind direction for the dominant source in the PVMRM option. 7. Deleted code in subroutine PCALC that adjusts DHCRIT based on distance to well-mixed layer for stable conditions for consistency with Model Formulation Document. 8. Added error checking for missing GASDEPOS inputs. 9. Corrected variable type from INTEGER to REAL for variable NEWRAD (effective radius) in subroutine GENCIR for AREACIRC sources. Use as an integer variable may have resulted in some truncation of the effective radius. Enhancements: 1. A new 'BETA' option has been added to the CO MODELOPT card to identify and allow for new features to be added to the model that are still in a draft BETA-test status. The BETA option is a non-DFAULT option, and will be overridden if the DFAULT option is specified. The following two draft enhancements are included in this update to AERMOD under the BETA option: a) options for capped stacks (source type = POINTCAP) and for horizontal releases (source type = POINTHOR); and b) the PSDCREDIT option for PVMRM to account for NO/NO2 plume chemistry in the computation of PSD increment consumption with credits. Inclusion of these draft BETA-test options does not imply any endorsement of their use for regulatory or non-regulatory applications of the model. In addition, the designation of BETA-test to these draft enhancements does not imply that these options have completed rigorous internal (Alpha) testing prior to being included in a public release of the model. 2. Additional options included under EMISFACT keyword to vary emissions by month, hour-of-day, and day-of-week (MHRDOW and MHRDOW7). 3. Expanded urban option to allow multiple urban areas to be defined in a single run through multiple URBANOPT cards. A new URBAN-ID parameter has been added to link sources with specific urban areas. Existing input files with a single urban area do NOT need to be modified to run with this new version of the model. 4. The following changes have been made to ensure consistency with the current PM NAAQS: a) Added special processing for PM-2.5 to calculate design values in accordance with the PM NAAQS. The design value for 24-hour averages is based on the high-eighth-high (H8H) averaged over N years, as an unbiased surrogate for the 98th percentile. The long-term design value for PM-2.5 is based on the highest annual average concentration averaged over N years using the ANNUAL keyword on the AVERTIME card. b) The "post-1997" PM-10 processing based on H4H averaged over N years has been removed since that standard was vacated. The PM-10 design value for 24-hour averages is based on the high-sixth-high (H6H) over five years (for NWS data), or more generally by the high-N+1-high value over N years. This can be accomplished using the existing CO MULTYEAR option with multiple 1-year input met data files, or using single five-year data files without the MULTYEAR option. 5. Added option to specify initial in-stack NO2 ratio for PVMRM and OLM options with CO NO2STACK card. Default ratio without the NO2STACK card for OLM is 0.10. No default ratio has been determined for PVMRM, so user must either use the CO NO2STACK card to initialize ratio for all sources, or specify ratio for each source using the SO NO2RATIO card. The SO NO2RATIO card can be used to override the value specified on the CO NO2STACK card for specific sources. 6. The maximum number of vertices for an AREAPOLY source is now allocated dynamically at runtime. The previous version of AERMOD allowed a maximum of 20 vertices. The maximum number of vertices allowed for AREACIRC sources is not allocated dynamically. However, if both AREAPOLY and AREACIRC sources are included in the same model run, then the maximum number of vertices identified for the AREAPOLY sources will also define the maximum number allowed for the AREACIRC sources. In addition, the maximum number of vertices allowed for AREACIRC sources will always be at least 50. Miscellaneous: 1. Added range check on value of vertical potential temperature gradient above ZI (variable VPTGZI), to avoid problems with data provided from sources other than AERMET. A minimum value of 0.005 K/m is applied for consistency with AERMET, and a warning message is also generated for values larger than 0.10 K/m. 2. Refined the process of dynamically allocating array storage to skip allocation of arrays that are not needed based on the model options selected, in order to reduce unnecessary memory usage. 3. Removed calls to subroutines VDP and SCAVRAT from subroutine PLUME_VOL. 4. Removed references to obsolete BOUNDARY keyword for receptors in RECSIZ and PREREINC subroutines to allocate receptor arrays, and in subroutines PRTANN, SPRTHT, and PRTPM25 of OUTPUT.FOR. 5. Changed AMAX1/AMIN1 intrinsics to MAX/MIN. ***************** * IMPORTANT * ***************** This program will henceforth be identified by its name and a Julian date, i.e.: AERMOD (dated 06341). 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 SCRAM ALERTS section of this website.