I was looking for this on the code guru site and could not find it. But after some research have solved the issue. Here is the discussion.
This will set the AirTemp column to NULL in the table.
Use the PutValue(sval) method to set the ado parameter in the recordset to NULL. The sval is a variant class that encapsulates the VARIANT data type. According to the doc, a potential value for the VARIANT is VT_NULL. VT_NULL ... "A propagating null value was specified. (This should not be confused with the null pointer.) The null value is used for tri-state logic, as with SQL.".