David J. Hofmann

Thirty years of in situ stratospheric aerosol size distribution measurements from Laramie, Wyoming (41°N), using balloon‐borne instruments

[1]xa0Vertical profiles of size-resolved aerosol concentrations above Laramie, Wyoming (41°N), have been measured for the past thirty years, 1971-2001. During this period, two somewhat different optical particle counters have been used to measure particles with radii ≥0.15 μm, whereas the instrument to measure condensation nuclei (CN) has not changed significantly since the late 1970s. The two optical particle counters measure aerosols ≥0.15, 0.25 μm and aerosols ≥0.15–2.0 μm in twelve size classes. These measurements have concentration (N) uncertainties ∝ ±N−0.5, but with a minimum of ±10%. Sizing uncertainties are about ±10%. The impact of these uncertainties on size distribution fitting parameters and aerosol moments are approximately ±30% and ±40%. The long-term record from these measurements indicates that volcanoes have controlled stratospheric aerosol abundance for 20 of the past 30 years. The present period, beginning in 1997, represents the longest volcanically quiescent period in the record. These and other measurements clearly show that stratospheric aerosol are now in a background state, a state rarely occurring in recent times, and that this background state is not significantly different than observations in 1979. Aerosol volumes and surface areas, inferred from size distributions fit to the measurements, are compared with SAGE II satellite estimates of surface area and volume. For volume the measurements are in agreement within measurement error throughout the record. For surface area there is good agreement for a volcanic aerosol laden stratosphere, but for background aerosol conditions the SAGE II estimates are about 40% less than the in situ measurements. Present aerosol surface areas are ∼1.0 (0.6) μm2 cm−3 in the 15–20 (20–25) km layer based on in situ measurements. The Laramie size distribution record is now available to the community over the internet.

The increase in stratospheric water vapor from balloonborne, frostpoint hygrometer measurements at Washington, D.C., and Boulder, Colorado

Stratospheric water vapor concentrations measured at two midlatltude locations in the northern hemisphere show water vapor amounts have increased at a rate of 1-1.5% yr1 (0.05-0.07 ppmv yr1) for the past 35 years. At Washington, D.C., meas- urements were made from 1964-1976, and at Boulder, Colorado, observations began in 1980 and continue to the present. While these two data sets do not comprise a single time series, they individually show increases over their respective measurement periods. At Boulder the trends do not show strong seasonal differences; significant increases are found throughout the year in the altitude range 16-28 km. In winter these trends are significant down to about 13 km.

Balloonborne measurements of the Pinatubo aerosol size distribution and volatility at Laramie, Wyoming during the summer of 1991

Measurements using balloonborne optical particle counters at Laramie, Wyoming during the summer of 1991 are used to study the particle size distribution and volatility of the aerosol which formed in the stratosphere following the mid-June eruptions of Mt. Pinatubo. Enhanced aerosol layers were observed below 20 km as early as 16 July, about 1 month after the eruption. During late July, a transient though substantial particle layer was observed in the 23 km region. High concentrations of large particles in this high altitude layer resulted in aerosol mass mixing ratios as large as 0.5 ppm, considerably larger than observed following the eruption of El Chichon. Aerosol volatility tests indicated that well over 90% of the particles were composed of an H2SO4/H2O solution in all layers observed, indicating rapid conversion of SO2 to H2SO4 and subsequent droplet growth. High concentrations of droplets suggest homogeneous or ion nucleation as the most likely aerosol production mechanism.

Ten years of ozonesonde measurements at the south pole: Implications for recovery of springtime Antarctic ozone

Ten years of ozonesonde data at the south pole are used to investigate trends and search for indicators that can be used to detect Antarctic ozone recovery in the future. These data indicate that there have been no systematic winter temperature trends at altitudes of 7–25 km and thus no expected changes in stratospheric cloud particle surface area, which would affect heterogeneous chemistry. Springtime ozone depletion has been very severe since about 1992, with near-total loss of ozone in the 14- to 18-km region, but has lessened somewhat in 1994 and 1995. probably because of the decay of the sulfate aerosol from the Mount Pinatubo eruption which was present at 10–16 km. Sulfate aerosol particles from the Pinatubo eruption resulted in new ozone depletion in 1992 and 1993 in the 10- to 12-km region where it is too warm for polar stratospheric clouds (PSCs) to form. The volcanic aerosol also augmented depletion related to PSCs at 12–16 km. Although ozone depletion was not as severe in 1995 as in 1993, the depleted region remained intact longer than ever, with record low values throughout December in 1995. Since about 1992, a pseudo-equilibrium seems to have been reached in which springtime ozone depletion, as measured by the total column or the ozone in the 12- to 20-km main stratospheric cloud region, has remained relatively constant. Independent of volcanic aerosol, ozone depletion has extended into the upper altitudes at 22–24 km since about 1992. There is some indication that ozone depletion has also worsened at the bottom of the depletion region at 12–14 km. Extensions of the ozone hole in the vertical dimension are believed to be the result of increases in man-made halogens and not due to changes in particle surface area or dynamics. A quasi-biennial component in the ozone destruction rate in September, especially above 18 km, is believed to be related to variations in the transport of halogen-bearing molecules to the polar region. A number of indicators for recovery of the ozone hole have been identified. They include an end to springtime ozone depletion at 22–24 km, a 12- to 20-km mid-September column ozone loss rate of less than about 3 Dobson Units (DU) per day, and a 12- to 20-km ozone column value of more than about 70 DU on September 15. It is estimated that if the Montreal protocol and its amendments, banning and/or limiting substances that deplete the ozone layer, is adhered to, recovery of the Antarctic ozone hole may be conclusively detected from the aforementioned changes in the vertical profile of ozone as early as the year 2008. Future volcanic eruptions would affect ozone at 10–16 km, making detection more difficult, but indicators such as depletion in the 22- to 24-km region will be immune to these effects.

Lidar measurements of stratospheric aerosol over Mauna Loa Observatory

Dual-wavelength aerosol lidar backscatter measurements at Mauna Loa Observatory are used to monitor and characterize the 15–30 km stratospheric aerosol layer. The decay of aerosol loading following the El Chichon, Mexico (17°N) and Pinatubo, Philippine Islands (15°N) volcanic eruptions of 1982 and 1991, respectively, depends on the phase of the quasibiennial oscillation (QBO) in tropical stratospheric winds. Averaged over a 3-year period, these effects are removed and an exponential decay with a characteristic (e−1) decay time of about 1 year is observed for both eruptions. By the end of 1996, about 5 ½ years after the Pinatubo eruption, stratospheric aerosol levels at Mauna Loa had decayed to pre-eruption levels, approximately matching the lowest aerosol levels seen here in the past 17 years (about 6 × 10−5 sr−1 at 694 nm integrated between 15.8 and 33 km). However, this background stratospheric aerosol level at Mauna Loa may depend on the QBO, being slightly lower during the westerly phase. Analyses of aerosol backscatter, backscatter wavelength dependence, and trajectories provide evidence for a minor injection of aerosol from the Rabaul eruption in Papua, New Guinea (4°S) in September of 1994.

Critical issues for long-term climate monitoring

Even after extensive re-working of past data, in many instances we are incapable of resolving important aspects concerning climate change and variability. Virtually every monitoring system and data set requires better data quality, continuity, and homogeneity if we expect to conclusively answer questions of interest to both scientists and policy-makers. This is a result of the fact that long-term meteorological data, (both satellite and conventional) both now and in the past, are and have been collected primarily for weather prediction, and only in some cases, to describe the current climate. Long-term climate monitoring, capable of resolving decade-to-century scale changes in climate, requires different strategies of operation. Furthermore, the continued degradation of conventional surface-based observing systems in many countries (both developed and developing) is an ominous sign with respect to sustaining present capabilities into the future. Satellite-based observing platforms alone will not, and cannot, provide all the necessary measurements.Moreover, it is clear that for satellite measurements to be useful in long-term climate monitoring much wiser implementation and monitoring practices must be undertaken to avoid problems of data inhomogeneity that currently plague space-based measurements. Continued investment in data analyses to minimize time-varying biases and other data quality problems from historical data are essential if we are to adequately understand climate change, but they will never replace foresight with respect to ongoing and planned observing systems required for climate monitoring. Fortunately, serious planning for a Global Climate Observing System (GCOS) is now underway that provides an opportunity to rectify the current crisis.

Four decades of ozonesonde measurements over Antarctica

[1]xa0Ozonesonde observations from Syowa and the South Pole over more than 40 years are described and intercompared. Observations from the two sites reveal remarkable agreement, supporting and extending the understanding gained from either individually. Both sites exhibit extensive Antarctic ozone losses in a relatively narrow altitude range from about 12 to 24 km in October, and the data are consistent with temperature-dependent chemistry involving chlorine on polar stratospheric clouds as the cause of the ozone hole. The maximum October ozone losses at higher altitudes near 18 km (70 hPa) appear to be transported to lower levels near the tropopause on a timescale of a few months, which is likely to affect the timing of the effects of ozone depletion on possible tropospheric climate changes. Both sites also show greater ozone losses in the lowermost stratosphere after the volcanic eruption of Mt. Pinatubo, supporting the view that surface chemistry can be enhanced by volcanic perturbations and that the very deep ozone holes observed in the early 1990s reflected such enhancements. Sparse data from the Syowa station in the early 1980s also suggest that enhanced ozone losses due to the El Chichon eruption may have contributed to the beginning of a measurable ozone hole. Observations at both locations show that some ozone depletion now occurs during much if not all year at lower altitudes near 12–14 km. Correlations between temperature and ozone provide new insights into ozone losses, including its nonlinear character, maximum effectiveness, and utility as a tool to distinguish dynamical effects from chemical processes. These data also show that recent changes in ozone do not yet indicate ozone recovery linked to changing chlorine abundances but provide new tools to probe observations for the first such future signals.

Trends in the nonvolcanic component of stratospheric aerosol over the period 1971–2004

[1]xa0The six longest records of stratospheric aerosol (in situ measurements at Laramie, Wyoming, lidar records at: Garmisch-Partenkirchen, Germany; Hampton, Virginia; Mauna Loa, Hawaii; Sao Jose dos Campos, Brazil, and SAGE II measurements) were investigated for trend by (1) comparing measurements in the 3 volcanically quiescent periods since 1970 using standard analysis of variance techniques, and (2) analyzing residuals from a time/volcano dependent empirical model applied to entire data sets. A standard squared-error residual minimization technique was used to estimate optimum parameters for each measurement set, allowing for first order autocorrelation, which increases standard errors of trends but does not change magnitude. Analysis of variance over the 3 volcanically quiescent periods is controlled by the end points (pre-El Chichon and post-Pinatubo), and indicates either no change (Garmisch, Hampton, Sao Jose dos Campos, Laramie-0.15 μm) or a slight, statistically insignificant, decrease (Mauna Loa, Laramie-0.25 μm), −1 ± 0.5% yr−1. The empirical model was applied to the same records plus 1020 nm SAGE II data separated into 33 latitude/altitude bins. No trend in stratospheric aerosol was apparent for 31 of 33 SAGE II data sets, 3 of 4 lidar records, and in situ measurements at 0.15 μm. For Hampton and Laramie-0.25 μm, the results suggest a weak negative trend, −2 ± 0.5% yr−1, while 2 SAGE II data sets (30–35 km, 30° and 40°N) suggest a positive trend of similar magnitude. Overall we conclude that no long-term change in background stratospheric aerosol has occurred over the period 1970–2004.

Electron microscope studies of Mt. Pinatubo aerosol layers over Laramie, Wyoming during summer 1991

Stratospheric aerosol layers resulting from the June 1991 eruptions of Mt. Pinatubo were first observed over Laramie, Wyoming in July 1991. Atmospheric particles were collected from these layers during three balloon flights in July and August using cascade impactors. Analytical electron microscope analysis of the aerosol deposits indicated that a large majority (> 99%) of the fine particles in all three samples were collected as submicrometer aqueous H2SO4 droplets, which changed to (NH4)2SO4 particles over time. Other particles observed in the aerosol were larger, and consisted of supermicrometer sulfate particles and composite sulfate/crustal particles which ranged up to ∼10 μm in size. Peak aerosol concentrations for r > 0.15 μm diameter particles (determined by optical particle counters) in the layers were higher for the July flights than for the August sounding. This was reflected in the electron microscope results, which showed that the July impactor samples had particulate loadings on the fine particle stages which were 20–30% higher than those from the corresponding substrate from the August sample. A detailed analysis of the fine sulfate aerosol was performed to assess whether the sulfate particles contained small condensation nuclei. Nearly all analyzed sulfate particles showed no evidence of a solid or dissolved nucleus particle, which suggests that the volcanic H2SO4 aerosol formed through homogeneous nucleation processes. These data support heated-inlet optical particle counter data from the balloon flights which suggest that 95–98% of the volcanic particles were aqueous H2SO4.

Midlatitude lidar backscatter conversions based on balloonborne aerosol measurements

Aerosol size distributions derived from balloon-borne particle counter data from Laramie, WY, are used to calculate ratios of extinction, mass, and surface area to lidar backscatter at the widely used lidar wavelength of 532 nm. The results cover the range of the stratospheric aerosol layer from the tropopause to 30 km. These ratios may be used to infer particle extinction, mass, and surface area from midlatitude lidar backscatter data for the period late 1979 to 1993. This period includes the major volcanic eruptions of El Chichon and Pinatubo. The wavelength dependence of aerosol backscatter in the visible was calculated for the period 1991 to 1993 to allow conversions of the results to other lidar wavelengths. The wavelength dependence is similar to estimates from southern hemisphere midlatitude measurements indicating that these conversions may also be applied to southern hemisphere midlatitude lidar measurements.