Glenn E. Shaw

Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze

Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo-Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one- and four-dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo-Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (-20±4 W m^(−2)) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.

Arctic haze: current trends and knowledge gaps

Trend analyses were performed on several indicators of Arctic haze using data from sites located in the North American, Norwegian, Finnish and Russian Arctic for the spring months of March and April. Concentrations of nonseasalt (nss) SO4 = in the Canadian, Norwegian and Finnish Arctic were found to have decreased by 30–70% from the early 1990s to present. The magnitude of the decrease depended on location. The trend in nss SO4 = at Barrow, Alaska from 1997 to present, is unclear. Measurements at Barrow of light scattering by aerosols show a decrease of about 50% between the early 1980s and the mid-1990s for both March and April. Restricting the analysis to the more recent period of 1997 to present indicates an increase in scattering of about 50% during March. Aerosol NO3− measured at Alert, Canada has increased by about 50% between the early 1990s and 2003. Nss K+ and light absorption, indicators of forest fires, have a seasonal maximum during the winter and spring and minimum during the summer and fall at both Alert and Barrow. Based on these data, the impact of summertime forest fire emissions on low-altitude surface sites within the Arctic is relatively small compared to winter/spring emissions. Key uncertainties about the impact of long range transport of pollution to the Arctic remain including the certainty of the recent detected trends; sources, transport and trends of soot; and radiative effects due to complex interactions between aerosols, clouds and radiation in the Arctic. 1. The Arctic haze phenomenon It has been more than 50 yr since observations of a strange haze, of unknown origin, were reported by pilots flying in the Canadian and Alaskan Arctic (Greenaway, 1950; Mitchell, 1956). Based on measurements at McCall Glacier in Alaska, Shaw and Wendler (1972) noted that the turbidity maximized in spring. First measurements of the vertical structure of the haze were made in an Alaskan “bush” airplane with a hand-held sunphotometer (Shaw, 1975). At that time the origin of the haze was uncertain and was attributed to ice crystals seeded by open leads or blowing dust from riverbeds. It was only through “chemical fingerprinting” of the haze that its anthropogenic source was revealed (Rahn et al., 1977; Rahn and McCaffrey, 1979; Ottar et al., 1986; Rahn 1989). By the late 1970s the anthropogenic origin was clear but surprising since it was widely believed that aerosol was generally not transported more than a few hundreds of kilometres from its source regions. Experts from Europe and America convened at the first Arctic Air Chemistry Symposium at Lillestrom, Norway in 1978 and an informal measurement network was agreed upon. ∗Corresponding author. e-mail: patricia.k.quinn@noaa.gov DOI: 10.1111/j.1600-0889.2006.00238.x Spatial gradients soon showed the direction of flow and the surprisingly large extent of this anthropogenic cloud of pollution. A combination of intensive field programs and long-term measurements extending over the past 30 yr confirmed the early conclusions that the haze is anthropogenic in origin due to emissions from Europe and the former Soviet Union that are transported to and trapped in the Arctic air mass during the winter and early spring. The haze is composed of a varying mixture of sulphate and particulate organic matter (POM) and, to a lesser extent, ammonium, nitrate, dust, and black carbon (e.g. Li and Barrie, 1993; Quinn et al., 2002). It also is rich in distinct heavy metals, which has allowed for the identification of particular industrial sources (e.g. Shaw, 1983; Polissar et al., 1998, 2001). Particles within the haze are well aged with a mass median diameter of about 0.2 microns or less (e.g. Heintzenberg, 1980; Hoff et al., 1983; Pacyna et al., 1984; Shaw, 1984; Clarke, 1989; Leaitch et al., 1989; Trivett et al., 1989; Hillamo et al., 1993). This particle size range is very efficient at scattering visible solar radiation since the peak in the particle surface-area size distribution is near the maximum efficiency for Mie scattering (Waggoner and Weiss, 1980; Shaw, 1987). The haze also is weakly absorbing due to the presence of black carbon (e.g. Hansen and Rosen, 1984; Noone and Clarke, 1988; Kahl and Hansen, 1989; Hopper et al., 1994). Tellus 59B (2007), 1 99 100 P. K. QUINN ET AL. The result of the strong scattering and weaker absorption is a noticeable reduction in visibility to a few kilometres or less. The “weak” absorption may have large climatic influences when the dark coloured haze spreads out over the highly reflecting snow and ice pack of the Arctic. The highly reflecting surface enhances aerosol-radiative interactions due to multiple scattering between the surface and the haze (e.g. Shaw and Stamnes, 1980). Several seasonally dependent mechanisms contribute to the formation of Arctic haze. Strong surface-based temperature inversions form in the polar night causing the atmosphere to stabilize. This cold and stable atmosphere inhibits turbulent transfer between atmospheric layers as well as the formation of cloud systems and precipitation, the major removal pathway for particulates from the atmosphere (Barrie et al., 1981; Shaw, 1981a, 1995; Heintzenberg and Larssen, 1983). In addition, meridional transport from the midlatitudes to the Arctic intensifies during the winter and spring (Iversen and Joranger, 1985). The combination of these factors results in the transport of precursor gases and particulates to the Arctic and the trapping of the pollutant haze for up to 15 to 30 d (Shaw, 1981a, 1995). Aircraft and lidar measurements throughout the 1980s and 1990s revealed that the haze occurs primarily in the lowest 5 km of the atmosphere and peaks in the lowest 2 km (Leaitch et al., 1984; Hoff, 1988; Pacyna and Ottar, 1988; Barrie, 1996). Throughout the haze season, the pollution layers are highly inhomogeneous both vertically (tens of metres to 1 km thick) and spatially (20–200 km in horizontal extent) (Radke et al., 1984; Brock et al., 1989). Recent aircraft measurements of sulphate aerosol using a high time resolution technique revealed detailed information about the evolution of the vertical structure of the haze between February and May (Scheuer et al., 2003). During early February, significant enhancements in sulphate aerosol are confined near the surface (<2 km) as long-range transport from northern Eurasia occurs along low level, sinking isentropes (Klonecki et al., 2003). As the haze season progresses, enhanced sulphate occurs at higher altitudes (up to at least 8 km). Since vertical mixing is prohibited by the persistent low-level inversion (Kahl, 1990), the higher-altitude haze layers are thought to be due to transport into the Arctic along vertically higher isentropes tracing back to increasingly warmer source regions in northern Eurasia. During early April, sulphate layers below 3 km begin to dissipate due to the beginning of solar heating and resulting mixing near the surface. However, more stable isentropic transport continues at higher altitudes. By the end of May, both the lowerand higheraltitude sulphate enhancements are significantly decreased due to the continued break-up of the inversion and return of wet deposition. Recent studies also have provided evidence for an influence of natural climate variability on interannual changes in levels of Arctic haze. Modeling the dispersion of anthropogenic emissions from northern hemisphere continents, Eckhardt et al. (2003) found that the North Atlantic Oscillation (NAO) influences pollution transport into the Arctic during the winter–spring haze season. During positive phases of the NAO, surface concentrations of modelled tracers in the Arctic winter were found to be elevated by about 70% relative to negative phases. This difference was mainly due to a change in pathways of European pollution and, to a lesser extent, North American pollution to the Arctic both of which are enhanced during positive NAO phases. In addition, during positive NAO phases, significant positive correlations between the NAO and measured CO concentrations were found at three Arctic stations (Spitsbergen, Barrow and Alert) confirming enhanced poleward transport of pollution from Europe, Asia and North America. Similar but weaker correlations between the NAO and measured CO were found for spring. Low correlations were found during summer and fall. During transport from the source regions to the Arctic, the pollutant-containing air masses have a high probability of reaching saturation and nucleating and precipitating clouds. It is not understood how so much material gets through a strongly scavenging system (Bowling and Shaw, 1992). Arctic haze has been the subject of much study because of its potential to change the short and longwave radiation balance of the Arctic, affect visibility, and provide a source of contaminants to Arctic ecosystems. The near surface concentration of aerosols at most places in the Arctic are about an order of magnitude lower than those found at more polluted and industrialized locations. At the same time, however, the affected areas are much larger in size and the affected ecosystems in the high Arctic are thought to be quite sensitive to gaseous and aerosol contamination. It is not known what fraction of Arctic haze contaminants leave the Arctic and what fraction is deposited within the Arctic on land and sea surfaces. As the polar night ends, some of the pollution that has accumulated is released to the mid-latitudes (Penkett et al., 1993; Heintzenberg et al., 2003). It is known that haze contaminants (e.g. acidic sulphate and organics) end up in Arctic ecosystems (Meijer et al., 2003; Wania, 2003) but the timing and mechanism of the scavenging from the atmosphere is not well understood. In general, seasonality of the concentrations of anthropogenic species in surface snow and atmospheric aerosols corresponds for regions of the Arctic with available relevant data. Sharp et al. (2002) found that the seaso

Investigations of Atmospheric Extinction Using Direct Solar Radiation Measurements Made with a Multiple Wavelength Radiometer

Abstract A multiple wavelength solar radiometer designed for the purpose of measuring atmospheric optical depth at discrete wavelengths through the visible region is described. Experimental techniques including sample observations, are presented for obtaining atmospheric optical depth from radiometer measurements. These techniques apply for conditions where the optical depth is either temporally variant or invariant during the course of a day. The influence of the aerosol she distribution on optical depth is investigated. Theoretical calculations of the wavelength dependency of the aerosol optical depth contribution are presented for several representative aerosol size distributions. Methods are also presented for estimating the aerosol size distribution and aerosol man loading from multi-wavelength optical depth measurements.

The arctic haze phenomenon

The arctic atmosphere is the repository for surprisingly high concentrations of pollutants throughout the winter months. The polluted air mass in question includes virtually all the atmosphere above the Arctic Circle and also two great lobes that extend down over Eurasia and North America. In extent, this generally polluted airmass system is about as large as the African continent. The rather severe pollution throughout this airmass system in winter is, to a large extent, a result of the lowered rates of particle and gas removal in this cold, dark, and rather stable system. The arctic haze possibly has important climatic and ecological and global change implications that are coming under investigation in a number of planned studies.

Bio-controlled thermostasis involving the sulfur cycle

The Gaia hypothesis proposed by Lovelock and Margulis presumes the existence of an unspecified biological means of ameliorating climate that has operated since the emergence of life 3500 Myr ago: Recently it was suggested that the mechanism of thermostasis may involve biospheric cycling of atmospheric carbon dioxide.We suggest an alternative hypothesis of biothermostasis operating through the sulfur cycle, rather than the carbon cycle. The mechanism would operate by altering planetary albedo through the selective creation of biospheric organic sulfide gases which go on to metamorphize into submicron particles and introduce cooling. In contrast to the carbon-cycle mechanism, sulfur-based cooling would have the ability to ameliorate climate well into the future, in principle over stellar Main Sequence time intervals. The main feature of interest is that the S cycle represents a particularly favorable thermodynamic pathway, involving three to four orders of magnitude less mass of active material cycled through the biospheric-atmospheric system (in response to a given temperature-imposed stress) than would be the case for a greenhouse gas hypothesis.There is no evidence that the suggested biospheric controlled particle-albedo change mechanism is actually operating, but we speculate that the probability of its rising importance and perhaps eventual dominance will improve when the partial pressure of atmospheric CO2 drops low enough to impose stress on metabolic processes. The intriguing thing about the process is its extremely high efficiency.

New determination of Rayleigh scattering in the terrestrial atmosphere

New Rayleigh-scattering optical thickness values for the terrestrial atmosphere in the 260 < lambda < 1500-nm wavelength range have been calculated using updated data on atmospheric optical parameters. The calculations include molecular scattering from water vapor and take into account varying atmospheric composition with altitude. The new Rayleigh-scattering coefficients average 4.5% lower than those listed by Penndorf in 1955.

Transport of Asian Desert Aerosol to the Hawaiian Islands

Abstract A cloud of aerosol with optical thickness τ ≈ 0.18 (500 nm wavelength), passed over the Hawaiian Islands from late April to early May 1979. Vertical profiles, taken by evaluating the optical extinction coefficient by sun photometry, showed that 80–90% of the aerosol was confined to a 1 km thick layer centered at 3.5–4km altitude. Trajectory analysis at the 500 mb pressure level (∼5 km) indicated that the aerosol probably had its origin in sandstorms in the eastern deserts of Asia nine days before the event. The dust cloud was first observed passing over Japan where sand particles fell out at Nagasaki on 21 April. By the time dust from the sandstone reached Hawaii, it has spread out to ∼1500 km and contained an estimated 1011 g of sand material, mainly in the 0.5 < r < 5.0 μm size range. This episode clearly indicates that substantial quantities of primary aerosol are being transported on global distance scales. The episode is used to obtain a desert aerosol surface particle flux which agrees to a...

Error analysis of multi-wavelength sun photometry

The error terms involved in precision multi-wavelength sun photometry, as used to study atmospheric aerosols, are analyzed. The error terms treated include instrumental errors, calibration errors, and errors imposed by the atmosphere. It is shown that in order to derive accurate aerosol parameters, one must exercise great care in the photometer calibration. A procedure for accurate calibration is described, based on an intercalibration between extrapolations of the extraterrestrial solar spectral irradiance and irradiance of a standard lamp. Methods are described to assess, and reduce, uncertainties brought about by diffuse radiation in the photometers field of view, temporal variations in aerosol optical depth, and gaseous absorption features at the operating wavelength. It is shown that if care is taken sun photometry can be used to derive monochromatic aerosol optical depth to an accuracy of several thousandths.

Atmospheric Turbidity in the Polar Regions

Abstract Analysis is presented of 800 measurements of atmospheric monochromatic aerosol optical depth made poleward of ∼65° latitude. The atmosphere of the southern polar region appears to be uncontaminated but is charged with a background aerosol having a mean size of 0.1 μm radius, an almost constant mixing ratio throughout the troposphere, a sea level optical depth (λ = 500 nm) of ∼0.025 and an inferred columnar mass loading of 4-15 × 10−7 g cm−2. At around the time of spring equinox the northern polar region (all longitudes) is invaded with Arctic Haze, an aerosol showing a strong anthropogenic chemical fingerprint. The optical depth anomaly introduced by this man-caused haze is τ0 ≈ 0.110 and the associated columnar mass loading is ∼1.5 × 10−6 g cm−2. Turbidity measured seven decades ago at the solar observatory at Uppsala (60°N), suggests that Arctic optical depth has been rising at a rate of dτ/dt ≈ 0.01 ± 0.005 per decade.

Long-Range Tropospheric Transport of Pollution Aerosols into the Alaskan Arctic

Abstract Noncrustal vanadium and manganese are used as chemical tracers for pollution-derived aerosols (collected over a period of four years in the near-surface air at Barrow, Alaska), in order to investigate tropospheric long-range transport of anthropogenic pollution from midlatitudes to the Alaskan Arctic. The analysis is based upon subjectively identifying characteristic transport pathway types using daily circumpolar weather maps. The transport occurs when the midlatitudinal and Arctic atmospheric circulations manifest quasi-persistent circulation patterns. Rapid transport of aerosols, on the order of 7–10 days, is dominated by quasi-stationary anticyclones and takes place along their peripheries where pressure gradients are relatively strong. The seasonal variation in concentration of the Arctic pollution-derived aerosol is related to the seasonal variation in the occurrence and position of midlatitude blocking anticyclones, of the Arctic anticyclone and of the Asiatic anticyclone. The positions of...