Arnold Field is located just north of downtown Sonoma. Take Highway 12 through Kenwood and Agua Caliente towards Sonoma.  You will need to turn left onto West Spain Street which is just before Napa Street (the main street).  Stay on West Spain past the Historic Park and the State Park.  You can access the field by turning left onto either 1st Street West or 1st Street East (on either side of the Sonoma Plaza).