The coarse mode of the atmospheric aerosol, containing mostly particles larger than 1 pm in diameter, can conveniently be measured by means of an optical-fonvardscattering spectrometer probe mounted on an airplane. Although the instrument is able to count single particles, at least 10,000 particles have to pass the sensitive volume in order to reduce errors due to statistical fluctuations of the counts, especially in the bins of the larger particles. The fitting of a lognormal curve to the measured particle counts is possible by means of a least-squares technique, described in this paper. The quality of the fit can be examined by determination of the best fit for the particle number distribution and the particle volume distribution, and an intercomparison of the two. Data for atmospheric samples show geometric mean diameters for the number size distribution between 1 and 2.5 pm and 5tandard deviations between 1.7 and 2.5. Aerosols dominated by one Fource have a small standard deviation. Standard deviations larger than 2.5 are an indication of an aerosol coming from several sources and in many cases a good fit can be obtained by using two lognormal distributions.

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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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