Acid precursor concentrations above the northeastern United States during summer 1987: Three case studies

Two aircraft measured the spatial distribution of SO2, H2O2, O3, and NOy above the northeast United States during summer. The aircraft flew three missions between Columbus, Ohio and Saranac Lake, New York. The sulfur dioxide concentration averaged 1–2 ppbv within the planetary boundary layer (pbl) and <1 ppbv in the free troposphere. Superimposed on this background concentration were surface source regions with elevated concentrations as high as 14 ppbv. The hydrogen peroxide concentration averaged 2–4 ppbv, with the highest values found above the top of the planetary boundary layer. The H2O2 pattern increased in both size and intensity above the top of the pbl during the afternoon. The ozone concentration varied between 30 and 110 ppbv. The concentration of O3 increased by as much as 30 ppbv during the day. Nitrogen oxide concentrations were in the range <1–8 ppbv. The pattern of SO2 and Noy concentrations remained substantially the same during the daytime hours. The O3 and NOy concentrations were linked to surface emissions and photochemical processes. The ratio between H2O2 and SO2 was >1, except in the lower pbl and near surface SO2 source regions.