"INTRODUCTION The results of the second year program of a projected three-year study entitled ""Reactions of Sulfur in Stack Flames"" conducted by GCA Technology Division are presented in this report. This work has been conducted under Contract No. PH-S6-67-125 supported jointly by the Public health Service@ National Air Pollution Control Administration: American Petroleum Institute: Bituminous Coal research@ Inc.; and the Edison Electric Institute. We have restated in the following paragraphs the basic objectives and major tasks as outlined in title First Annual Report (Ref. 1) ""The overall objective of the three-year program is to provide an improved rationale for predicting the concentration levels of sulfur compounds in the atmosphere. The emphasis in this study is directed to the physical and/or chemical reactions of sulfur compounds in fossil fuel combustion gases during or after release to atmosphere which must be considered in the development of useful meteorological diffusion models. The scope of this program has been constrained mainly to the investigation of those chemical and physical processes leading to the conversion of sulfur dioxide to sulfur trioxide and sulfuric acid@ since these compounds are the major constituents in most polluted atmospheres. In order to meet the program objectives@ both field and laboratory experiments have been incorporated in the research plan. The field approach defines the composition changes in real fossil fuel effluents from stack source to extend downwind travel distances by measurement of stack and plume SO2 concentrations and associated metrological parameters. Laboratory experiments with fossil fuel stack effluents and synthetic gas mixtures have been designed to permit detailed studies of those physical and chemical reactions believed to occur in stack plumes. The analysis of field measurements@ which depict the proper order of plume concentration levels despite statistical limitations@ and supporting laboratory data@ which permit improved resolution of system variables@ is intended to furnish a better understanding of the behavior of suffer oxides in the atmosphere."" During the second year of this study@ it was decided to supplement and emphasis the flight program effort so that sufficient data could be obtained to improve statistical significance for predicting the concentration levels of sulfur compounds in stack plumes. The formal decision by the Steering Committee to extend the flight program was made in a meeting at GCA Technology Division on January 7@ 1969. The stack and plume sampling techniques described in Reference 1 were used with modification during the second year program. Actual evaluation of effluents from the pilot plant combustion system in the stirred reactor were not performed during the second year because of the increased emphasis on flight measurements. The progress made in the various tasks has been described in the following sections of this report. Despite difficulties from the point of view of finding acceptable metrological conditions for flight measurements@ the plume sampling flights were accomplished. Metrological calculations were prepared for sampling guidelines in the design of field experiments. During the flight program it was observed that the background SO2 concentrations arising from multiple sources in the Greater Boston area exert considerable bearing upon the analysis of experimental results."