The visible consequences of sulfuric acid aerosol emissions-opaque stack emissions called "blue plumes"-are merely the tip of an iceberg. In sufficient concentration, SO3 can also increase corrosion and fouling of equipment and components downstream of the furnace while decreasing their efficiency and penalizing overall plant heat rate. This article-the first in a three-part series-details the negative impacts of SO3 on operations and maintenance of back-end plant equipment. Part II will explain and quantify the potential benefits of limiting the concentration of SO3 in flue gas to 3 ppm at the entrance to the air heater. Part III will describe the characteristics of an optimal SO3 removal technology and present the technical details and operating experience of one patented process that has worked successfully at a half-dozen plants for up to three years.