Version 5.20 provides the following additional capabilities and features over previous versions:

• The vertical transport algorithm has been improved. As part of this change, the top boundary condition is now an internal "zero-gradient" calculation; this alleviates the need for a top boundary condition input file. The original approach was first-order accurate and led to large vertical velocities and numerical diffusion, which were especially evident over complex terrain. The new approach is second-order accurate and mitigates this problem.
• PM chemistry has been linked to the SAPRC99 gas-phase mechanism (CAMx "Mechanism 5"). Both CF and CMU versions of PM chemistry, as well as mercury chemistry, can now be driven by SAPRC99. New chemistry parameters files are available for Mechanism 5 with PM.
• Uptake of nitric acid on mineral dust particles has been added as a new pathway for particle nitrate formation. Nitrate uptake by calcium on fine crustal PM is calculated external to ISORROPIA. CAMx outputs total particulate nitrate, i.e., the sum of particle nitrate determined by ISORROPIA and calcium nitrate.