Realtors who belong to the Mainstreet Organization of Realtors and the Realtor Association of NorthWest Chicagoland approved a merger of the two groups, creating the fourth largest Realtors' association in the United States. Combined the merged agencies will have 16,000 members serving 185 communities in Cook County, DuPage County and Lake County, Ill. Operations for both associations will continue under the Mainstreet brand. The Realtor groups will officially become one entity on Oct. 1, bringing associated real estate agents that serve the west, south and northwest suburbs of Chicago under one umbrella.  The merger is expected to improve expenses while ensuring the association continues to provide quality products and services to members. “We will be able to enhance service to our members, while putting our organization in an extremely solid position moving forward,” Mainstreet President Christine Chase said.  “Pending home sales are up nationwide, and we want to be ready when the market turns.” The merger was first proposed a few months ago. Members overwhelming approved the merger after Realtors began voting on the propose deal last week. Write to Kerri Panchuk.