Field observations during a series of 24 atmospheric sampling flights in winter, spring, summer, and fall of 1987 provided a preliminary climatology of selected trace chemicals above the central United States. Flights were along the 91.5°W meridian between 29° and 41°N latitude. The data set includes continuous measurements of trace gases (O3, SO2, H2O2, and NO/NOy), aerosol number and size distributions, meteorological variables, and position. Filter samples produced SO42−, NO3−, SO2, and trace metal data. Flask air samples yielded methane, hydrocarbon (C2‐C5), and CO concentrations. Mean concentrations of the measured species at 2450±150 m and 1450±150 m represent each season. These data are discussed as functions of season, location, and air mass origin. Solar energy (821–991 w m−2), temperature (18°–11.6°C) and water vapor mixing ratio (13.5–10.1 g kg−1) peaked during summer at low and high altitude. Carbon monoxide levels 88–160 parts per billion by volume (ppbv) peaked in spring and were characteristic of the planetary boundary layer during both spring and summer. Methane concentrations were maximized during spring (1770–1744 ppbv) and fall (1774–1733 ppbv) and minimized during winter (1747–1730 ppbv) and summer (1736–1705 ppbv) at low and high altitude. Spring had the highest (21.3–21.4 ppbv) and summer the lowest (7.1–5.3 ppbv) hydrocarbon concentrations at low and high altitude. Sulfur dioxide concentrations were highest in summer (1.0–2.3 ppbv) and winter (0.9–1.6 ppbv) at low altitude. SO2 concentrations at high altitude had no seasonal trend and averaged less than 0.9 ppbv. Sulfate concentrations were highest in summer (3.2–1.7 μg m−3) at low and high altitude. The average hydrogen peroxide concentration varied by a factor of 16 (0.3–4.8 ppbv) between winter and summer. Ozone concentrations were between 49 and 70 ppbv and were highest in spring and summer. The ratio of sulfate to sulfur dioxide increased slightly with altitude during winter, spring, and summer. This is probably due to SO2 oxidation in clouds. The ratio of H2O2 to SO2 is >1 during spring and summer and <1 during winter. This indicates that the conversion of sulfur dioxide to sulfate by reaction with hydrogen peroxide is not oxidant‐limited during spring and summer.

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Measurements of sulfur dioxide (S02) were made at the National Oceanic and Atmospheric Administration's Mauna Loa Observatory in Hawaii, during a 12‐month period beginning in December 1988. SO2 concentrations varied from background levels of less than 0.05 ppbv to a maximum of 50 ppbv, during episodes that lasted from 2 to 24 hours. Emissions from the Kilauea crater, approximately 35 km southeast of the observatory at an elevation of about 1000 m above sea level (asl), and the current eruption at Puu O′o 50 km east‐southeast, are the most likely sources for the higher concentrations. These episodes occurred 10–25 times each month, mostly during the day; peak concentrations were usually recorded at mid‐day. The SO2 concentrations can be grouped into three periods; low (June–September), high (October–January) and intermediate (February–May). A clear diurnal cycle of SO2 concentration exists throughout the year, although day‐night changes were greatest during October–January and were barely detectable during the June–September period. The highest SO2 concentrations were recorded when the predominant wind direction was northerly to northwesterly, even though the apparent sources are in the southeastern sector. Nighttime concentrations were usually at background levels; however, many exceptions were observed. A few cases of higher than background SO2 were observed when free tropospheric (FT) conditions were identified. The possibility that long‐range transport was the cause for elevated SO2 concentrations under FT conditions was examined using air mass back trajectories analyses. The highest nighttime SO2 concentrations, under FT conditions, were observed during periods with slow easterly trajectories, and the lowest concentrations were found during westerly flows. Twenty‐four nighttime free tropospheric events were recorded when the SO2 concentration exceeded 0.2 ppbv. During 18 of these episodes, unusually high CO2 concentrations were observed.

Vertical profiles of H2O2 mixing ratios were obtained for each season from a site in central Arkansas during 1988. Aircraft‐based measurements indicated that H2O2 mixing ratios followed an annual cycle, peaking during the summer at >6 parts per billion by volume (ppbv). The minimum occurred in winter when mixing ratios for H2O2 averaged about 0.2 ppbv. The H2O2 mixing ratio generally peaked at an altitude of about 800 mbar (2 km), although there may have been some seasonal dependence. The annual cycle followed variations in solar intensity, water mixing ratio, and temperature. Within a season, strong variations could be related to meteorological events. A daily cycle was inferred in which the H2O2 mixing ratio varied by a factor of 2 to 3; the peak observed values were at night. H2O2 mixing ratios at altitudes higher than 0.7 km were generally greater than local SO2 values above 0.7 km during all but the winter season.

Based on two case studies, the process of large-scale transport of a polluted air mass northward of Tokyo is described. Comparison of oxidant concentrations with meteorological data reveals that the polluted air mass produced along the coastal area of Tokyo is transported about 200km by a low-level jet whose core is located around the 950mb level. The stable layer formed between the 800mb and 900mb levels also plays an important role in this process.