Barry A. Bodhaine

Aerosol absorption measurements at Barrow, Mauna Loa and the south pole

Aerosol absorption (σap) has been measured continuously using aethalometers at Barrow, Alaska (1986 to present); Mauna Loa, Hawaii (1990 to present); and south pole, Antarctica (1987–1990). These three stations are part of a network of baseline monitoring stations operated by the Climate Monitoring and Diagnostics Laboratory (CMDL) of the National Oceanic and Atmospheric Administration (NOAA). Condensation nucleus (CN) concentration and multiwavelength aerosol scattering (σsp) have also been measured continuously for many years at these stations. Aethalometer measurements are usually reported in terms of atmospheric black carbon aerosol (BC) concentration using the calibration suggested by the manufacturer. Here we deduce the in situ σap(550 nm) from aethalometer measurements by assuming that the aerosol absorption on the aethalometer filter is enhanced by a factor of 1.9 over that in the atmosphere. This is consistent with using 19 m2 g−1 for the specific absorption of BC on the aethalometer filter and 10 m2 g−1 for the in situ specific absorption of BC in the atmosphere (the ratio of the two specific absorptions is 1.9). Although these values of specific absorption may vary significantly for different environments, the ratio might be expected to be relatively constant. The single-scattering albedo, defined by ω = σsp/(σsp + σap), has been estimated from the simultaneous measurements of σap and σsp. Furthermore, assuming a 1/λ dependence for σap in the 450 to 700-nm wavelength region, multiwavelength σsp measurements allow the estimation of the wavelength dependence of ω. Each station shows a considerable annual cycle in σap, σsp, and ω. The maximum in the Barrow annual cycle is caused primarily by the springtime Arctic haze phenomenon; the maximum in the Mauna Loa annual cycle is caused by the springtime Asian dust transport; and the maximum in the south pole annual cycle is caused by late winter transport from southern midlatitudes. It was found that annual mean values are σap = 4.1 × 10−7 m−1 (≈41 ng m−3 BC) and ω = 0.96 for Barrow; σap = 5.8 × 10−8 m−1 (≈5.8 ng m−3 BC) and ω = 0.97 for Mauna Loa; and σap = 6.5 × 10 −9 m−1 (≈0.65 ng m−3 BC) and ω = 0.97 for south pole. It was also found that the wavelength dependence of ω may be important at Barrow and south pole, but not important at Mauna Loa.

The aerosol at Barrow, Alaska: long-term trends and source locations

Abstract Aerosol data consisting of condensation nuclei (CN) counts, black carbon (BC) mass, aerosol light scattering (SC), and aerosol optical depth (AOD) measured at Barrow, Alaska from 1977 to 1994 have been analyzed by three-way positive matrix factorization (PMF3) by pooling all of the different data into one large three-way array. The PMF3 analysis identified four factors that indicate four different combinations of aerosol sources active throughout the year in Alaska. Two of the factors (F1, F2) represent Arctic haze. The first Arctic haze have factor F1 is dominant in January–February while the second factor F2 is dominant in March–April. They appear to be material that is generally ascribed to long-range transported anthropogenic particles. A lower ratio of condensation nuclei to scattering coefficient loadings is obtained for F2 indicating larger particles. Factor F3 is related to condensation nuclei. It has an annual cycle with two maxima, March and July–August indicating some involvement of marine biogenic sources. The fourth factor F4 represents the contribution to the stratospheric aerosol from the eruptions of El Chichon and Mt. Pinatubo. No significant long-term trend for F1 was detected while F2 shows a negative trend over the period from 1982 to 1994 but not over the whole measurement period. A positive trend of F3 over the whole period has been observed. This trend may be related to increased biogenic sulfur production caused by reductions in the sea-ice cover in the Arctic and/or an air temperature increase in the vicinity of Barrow. Potential source contribution function (PSCF) analysis showed that in winter and spring during 1989 to 1993 regions in Eurasia and North America are the sources of particles measured at barrow. In contrast to this, large areas in the North Pacific Ocean and the Arctic Ocean was contributed to observed high concentrations of CN in the summer season. Three-way positive matrix factorization was an effective method to extract time-series information contained in the measured quantities. PSCF was useful for the identification possible source areas and the potential pathways for the Barrow aerosol. The effects of long-distance transport, photochemical aerosol production, emissions from biogenic activities in the ocean, volcanic eruptions on the aerosol measurements made at Barrow were extracted using this combined methodology.

Altitude effects on UV spectral irradiance deduced from measurements at Lauder, New Zealand, and at Mauna Loa Observatory, Hawaii

Measurements from Lauder, New Zealand, and from the high-altitude Mauna Loa Observatory, Hawaii, are used to determine the altitude effects on spectral UV irradiance and to relate these altitude differences to other factors that influence UV radiation. The measured ratios UVMauna Loa/UVLauder are complex functions of both wavelength and solar zenith angle (SZA). Spectrally, the ratios tend to increase toward shorter wavelengths through most of the UV-A region. For small SZA (SZA ∼80°, local minima in the ratios are seen at shorter wavelengths in the UV-B region. For biologically weighted irradiances, the peak ratios occur near SZA = 70°, where UV-A, erythemally weighted UV, UV-B, and DNA-weighted UV irradiances at Mauna Loa Observatory exceeded those at Lauder by ∼17%, 26%, 27%, and 29% respectively. The ratios of irradiances at the two altitudes, as functions of SZA and wavelength, were related to differences expected from radiative transfer calculations. For small SZA, modeled and measured ratios agreed within the limits of experimental uncertainty without taking differences in altitude distributions of ozone and temperature into account. However, for larger SZA and shorter wavelengths these profile shapes had a significant effect. In the model calculations, satisfactory agreement with the measurements was achieved only when the contribution from radiation scattered from air or cloud tops below the observation height at Mauna Loa Observatory was included. To model this accurately, a three-dimensional radiative transfer code should be used in conjunction with a topographical model of the surrounding terrain.

Three-wavelength nephelometer suitable for aircraft measurement of background aerosol scattering coefficient

A new nephelometer suitable for aircraft measurements of aerosol scattering extinction coefficient (σsp) has been constructed and operated under field conditions. This instrument is vacuum tight for operation in a pressurized aircraft cabin and is capable of measuring background tropospheric σsp at an averaging time of 1 min. For example, in a typical atmospheric profile the instrument can measure values of about 10−4 m−1 with a time resolution of 2 s in a polluted region, and about 10−7 m−1 with a time resolution of 1 min in a clean region. This sensitivity is made possible by: (1) subtracting in real time the air Rayleigh scattering from the total scattering signal by continuously measuring pressure and temperature in the sampling volume of the instrument; (2) correcting for the dark count and sensitivity of the photomultipliers using a rotating shutter; and (3) using a beam splitter arrangement to allow simultaneous detection by three photomultipliers. A laboratory measurement of instrument noise suggests a 550-nm noise level of about 5 × 10−8 m−1 at an averaging time of 1 min.

UV measurements at Mauna Loa : July 1995 to July 1996

A UV spectroradiometer was installed at Mauna Loa Observatory (MLO), Hawaii, in July 1995. This instrument, based on a commercially available double monochromator, uses a diffuser mounted as a horizontal receptor inside a quartz dome and views the whole sky. The instrument scans over the 290–450 nm spectral range with a band pass of about 1 nm for each 5° of solar zenith angle (SZA). The UV irradiances measured at MLO are much more intense than at low-altitude midlatitude locations. For observations at SZA 45° the erythemally weighted UV irradiances can exceed 21 μW cm−2, which is approximately 15–20% greater than that seen at Lauder, New Zealand, for similar ozone amounts. The difference is primarily due to the higher altitude at MLO (3.4 km). For overhead Sun conditions at MLO the largest value of erythemal UV was 51.3±3.1 μW cm−2, which to our knowledge is the highest recorded any-where at the Earths surface. UV irradiance is strongly correlated (inversely) with Dobson spectrophotometer total ozone measurements at MLO, with higher correlations at shorter wavelengths. The radiative amplification factor (RAF) for erythema at MLO is about 1.33±0.2 at SZA 45°.

Aerosol measurements during the Mauna Loa Photochemistry Experiment 2

Aerosol measurements have been made continuously at Mauna Loa Observatory (MLO) from 1974 to the present. Condensation nucleus (CN) concentration has been measured using automatic CN counters, and aerosol scattering extinction (σsp) has been measured using a four-wavelength nephelometer. The Mauna Loa Observatory Photochemistry Experiment (MLOPEX) was conducted in 1991–1992 to study intensively many important variables in the field of atmospheric chemistry. Because of a strong diurnal cycle in nearly everything measured at MLO, caused by an upslope-downslope wind system, it is important to develop data-editing criteria that can safely identify background conditions as opposed to other conditions when the site may be contaminated by local sources. Ordinarily, background conditions occur during nighttime downslope wind conditions, and contaminated conditions occur during daytime upslope wind conditions. However, occasionally unusual weather conditions or contamination caused by a local source such as the Mauna Loa caldera can confuse the issue. It is recommended that background aerosol data be chosen during 0000–0800 Hawaiian standard time (HST) to generally avoid upslope wind conditions, and that wind direction and speed, CN, and SO2 data be used if available to further eliminate local pollution episodes. In addition, all data should be examined by a human editor, if possible, in order to recognize certain episodes that may not fit automated criteria.

Solar Irradiance Anomalies Caused by Clear-Sky Transmission Variations above Mauna Loa: 1958-99

Abstract The clear-sky transmission of the atmosphere contributes to determining the amount of solar irradiance that reaches various levels in the atmosphere, which in turn is fundamental to defining the climate of the earth. As of the end of 1999, sustained clear-sky solar transmission over the mid-Pacific, as viewed from Mauna Loa, Hawaii, reached its highest level of clarity since before the eruption of Mount Pinatubo in 1991 and appears to be continuing to increase toward baseline levels established during 1958–62 and not sustained since. This record is used to answer the question as to impact of transmission variations, which can be attributed to either upward scattering or absorption above the station, on the net solar irradiance at 3.4 km, the altitude of the isolated mountain-top observing site. Net solar irradiance at a given level describes the total solar irradiance absorbed below that level. Monthly mean net solar anomalies caused by transmission variations, relative to the 1958–62 baseline, r...

Calibrating Broadband UV Instruments: Ozone and Solar Zenith Angle Dependence

Abstract A UV spectroradiometer was installed at Mauna Loa Observatory (MLO), Hawaii, in July 1995. This instrument has been employed to characterize several broadband UV instruments of a type commonly used to estimate erythemal irradiance at many sites around the globe. One year of clear-sky data from MLO has been analyzed for solar zenith angles (SZAs) of 5°–85°, in steps of 5°, and for total ozone values in the range 220–310 DU measured with a Dobson spectrophotometer. Because the spectral responses of various broadband instruments can be quite different, and particularly because the erythemal response defined for human skin is significantly different than that of many broadband instruments, the calibration of a broadband instrument reporting in erythemal units is strongly dependent on total ozone and SZA. When a broadband instrument is placed in the field it is necessary to know the calibration as a function of ozone and SZA to determine accurate erythemal irradiance. However, the manufacturers of bro...

Measured and calculated optical property profiles in the mixed layer and free troposphere

Nearly simultaneous measurements of the physical and optical properties of mixed layer and free tropospheric aerosols near Boulder, Colorado, were made on several occasions using aircraft, balloon, and ground-based sensors. This effort (Front Range Lidar, Aircraft, and Balloon experiment (FRLAB)) was conducted with the purpose of obtaining a diverse, self-consistent data set that could be used for testing optical model calculations based on measured physical characteristics such as apparent size distribution, composition, and shape. It was found that even with the uncertainties involved, the model predictions are in good agreement with the measurements in the visible and near infrared wavelength regions. At CO2 lidar wavelengths there is considerably more uncertainty in both the calculated and measured values; however, within the estimated errors there appears to be satisfactory agreement except for the highest free tropospheric layer studied. The results also indicate that during FRLAB the aerosol in the boundary layer and free troposphere behaved as spherical particles for optical modeling purposes. The utility of the observations for determining the extinction-to-backscatter ratio relevant to aerosols in the boundary layer and free troposphere is described with typical measured values being in the 20 to 30 sr range.

Airborne measurements of aerosol optical properties over south-central new Mexico

Abstract We used a three wavelength nephelometer (449, 536 and 690 nm) and an aethalometer on board the NOAA King Air research aircraft to assess the contributions of aerosol optical scattering and absorption to shortwave extinction. The measurements were made over south-central New Mexico in February and July 1989. The winter measurements revealed a shallow, polluted planetary boundary layer with cleaner air above. The summer measurements showed a uniformly mixed planetary boundary layer extending from ground level to the operational ceiling of 4.5 km above ground. In both cases the total optical thickness values for the column were similar (0.03) and the fractional contribution of aerosol absorption to the extinction was between 5 and 10%. These results suggest that the aerosol extinction in summer and winter is similar, even though the planetary boundary layer thickness is quite different during the two seasons. They also demonstrate that a suitably instrumented light aircraft can profile the optical properties of the troposphere with high sensitivity and good spatial resolution.