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We assemble data on the Pinatubo aerosol from space, air, and ground measurements, develop a composite picture, and assess the consistency and uncertainties of measurement and retrieval techniques. Satellite infrared spectroscopy, particle morphology, and evaporation temperature measurements agree with theoretical calculations in showing a dominant composition of H2SO4‐H2O mixture, with H2SO4 weight fraction of 65–80% for most stratospheric temperatures and humidities. Important exceptions are (1) volcanic ash, present at all heights initially and just above the tropopause until at least March 1992, and (2) much smaller H2SO4 fractions at the low temperatures of high‐latitude winters and the tropical tropopause. Laboratory spectroscopy and calculations yield wavelength‐ and temperature‐dependent refractive indices for the H2SO4‐H2O droplets. These permit derivation of particle size information from measured optical depth spectra, for comparison to impactor and optical‐counter measurements. All three techniques paint a generally consistent picture of the evolution of Reff, the effective radius. In the first month after the eruption, although particle numbers increased greatly, Reff outside the tropical core was similar to preeruption values of ∼0.1 to 0.2 μm, because numbers of both small (r < 0.2 μm) and large (r > 0.6 μm) particles increased. In the next 3–6 months, extracore Reff increased to ∼0.5 μm, reflecting particle growth through condensation and coagulation. Most data show that Reff continued to increase for ∼1 year after the eruption. Reff values up to 0.6–0.8 μm or more are consistent with 0.38–1 μm optical depth spectra in middle to late 1992 and even later. However, in this period, values from in situ measurements are somewhat less. The difference might reflect in situ undersampling of the very few largest particles, insensitivity of optical depth spectra to the smallest particles, or the inability of flat spectra to place an upper limit on particle size. Optical depth spectra extending to wavelengths λ > 1 μm are required to better constrain Reff, especially for Reff > 0.4 μm. Extinction spectra computed from in situ size distributions are consistent with optical depth measurements; both show initial spectra with λmax ≤ 0.42 μm, thereafter increasing to 0.78 ≤ λmax ≤ 1 μm. Not until 1993 do spectra begin to show a clear return to the preemption signature of λmax ≤ 0.42 μm. The twin signatures of large Reff (>0.3 μm) and relatively flat extinction spectra (0.4–1 μm) are among the longest‐lived indicators of Pinatubo volcanic influence. They persist for years after the peaks in number, mass, surface area, and optical depth at all wavelengths ≤1 μm. This coupled evolution in particle size distribution and optical depth spectra helps explain the relationship between global maps of 0.5‐ and 1.0‐μm optical depth derived from the Advanced Very High Resolution Radiometer (AVHRR) and Stratospheric Aerosol and Gas Experiment (SAGE) satellite sensors. However, there are important differences between the AVHRR ...

The Ames airborne tracking sunphotometer was operated at the National Oceanic and Atmospheric Administration (NOAA) Mauna Loa Observatory (MLO) in 1991 and 1992 along with the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) automated tracking sunphotometer and lidar. June 1991 measurements provided calibrations, optical‐depth spectra, and intercomparisons under relatively clean conditions; later measurements provided spectra and comparisons for the Pinatubo cloud plus calibration checks. June 1991 results are similar to previous MLO springtime measurements, with midvisible particle optical depth τp(λ = 0.526μm) at the near‐background level of 0.012 ± 0.006 and no significant wavelength dependence in the measured range (λ = 0.38 to 1.06μm). The arrival of the Pinatubo cloud in July 1991 increased midvisible particle optical depth by more than an order of magnitude and changed the spectral shape of τp(λ) to an approximate power law with an exponent of about −1.4. By early September 1991, the spectrum was broadly peaked near 0.5 μm, and by July 1992, it was peaked near 0.8 μm. Our optical‐depth spectra include corrections for diffuse light which increase postvolcanic midvisible τp values by 1 to 3% (i.e., 0.0015 to 0.0023). NOAA‐ and Ames Research Center (ARC)‐measured spectra are in good agreement. Columnar size distributions inverted from the spectra show that the initial (July 1991) post‐Pinatubo cloud was relatively rich in small particles (r<0.25μm), which were progressively depleted in the August‐September 1991 and July 1992 periods. Conversely, both of the later periods had more of the optically efficient medium‐sized particles (0.25

As determined in situ by impactor samplers flown on an ER‐2 at 16.5‐ to 20.7‐km pressure altitude and on a DC‐8 at 9.5‐ to 12.6‐km pressure altitudes, the 1991 Pinatubo volcanic eruption increased the particle surface area of stratospheric aerosols up to 50‐fold and the particle volume up to 2 orders of magnitude. Particle composition was typical of a sulfuric acid‐water mixture at ER‐2 altitudes. Ash particles coated with sulfuric acid comprised a significant fraction of aerosol at DC‐8 altitudes. Mie‐computed light extinction increased up to 20‐fold at midvisible and >100‐fold at near‐IR wavelengths. The optical thickness measured through the aerosol layer by an autotracking Sun photometer aboard a DC‐8 aircraft at 10.7‐ to 11.3‐km pressure altitudes shows a spectral shape that is similar to the Mie‐calculated spectral extinction at ER‐2 altitudes. Surface area distributions calculated by inversion of spectral optical depth measurements show characteristics that are similar to the mean surface area distribution resulting from 35 in situ measurements.

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Abstract. Simultaneous measurements of the oxides of hydrogen and nitrogen made during the NASA Subsonic Assessment, Ozone and Nitrogen Oxide Experiment (SONEX) afforded an opportunity to study the coupling between these two important families throughout the free troposphere and lowermost stratosphere. Moreover, the suite of measurements made during the campaign was unprecedented in its completeness, thus providing a uniquely detailed picture of the radical photochemistry that drives oxidation and ozone production in this part of the atmosphere. On average, observed hydrogen oxides (HOx = OH + HO2) agree well with both instantaneous and diel steady-state models; however, there is a persistent deviation of the observations that correlates with the abundance of nitrogen oxides (NOx = NO + NO2) in the sampled air mass. Specifically, the observed HO• tends to exceed the model predictions in the presence of high NO• concentrations, by as much as a factor of 5 (>500 pptv NOx), and is sometimes as little as half that expected by steady state at lower NOj levels. While many possibilities for these discrepancies are discussed, it is argued that an instrumental artifact is not probable and that the discrepancy may bespeak a shortcoming of our understanding of HO• chemistry. The consistently elevated HO• in the presence of elevated NOx leads directly to greater ozone production than expected, thereby extending the NOx-limited regime of the upper troposphere. These results could thus have bearing on the predicted impacts of increasing NO• emissions into this region of the atmosphere from, for example, the growth of global air traffic.

Abstract. We present here the chemical composition of outflow from the Asian continent to the atmosphere over the western Pacific basin during the Pacific Exploratory Mission -West (PEMWest B) in February-March 1994. Comprehensive measurements of important tropospheric trace gases and aerosol particulate matter were performed from the NASA DC-8 airborne laboratory. Backward 5 day isentropic trajectories were used to partition the outflow from two major source regions: continental north (>20 øN) and continental south (<20 øN). Air parcels that had not passed over continental areas for the previous 5 days were classified as originating from an aged marine source. The trajectories and the chemistry together indicated that there was extensive rapid outflow of air parcels at altitudes below 5 kin, while aged marine air was rarely encountered and only at