A. D. A. Hansen

Identification of the optically absorbing component in urban aerosols

Raman spectroscopy is used to determine the absorptivity of urban particulates for combustion sources.(AIP)

Real-time measurement of the absorption coefficient of aerosol particles.

The authors describe the development of an aethalometer. This portable device measures aerosol absortion in real time. (AIP)

Black carbon (soot) aerosol in the lower stratosphere and upper troposphere

As determined by impactor samplers flown on ER-2 and DC-8 aircraft, black carbon aerosol (BCA) mass loadings in the stratosphere average 0.6 nanograms per standard cubic meter, or 0.01% of the total aerosol. Upper tropospheric BCA increases to 0.3%. Low stratospheric concentration is commensurate with present commercial air traffic fuel consumption, given the following assumptions: the BCA emissions are 0.1 grams per kilogram of fuel consumed, 10% of route mileage is above the tropopause, and average BCA stratospheric residence time is about one year. Taking BCA into account, the stratospheric single scatter albedo is ≈0.99. Using parameters for planned supersonic commercial aircraft, whose emissions will be predominantly in the stratosphere, we show that such traffic will double stratospheric BCA concentration. This would reduce the aerosol single scattering albedo by one percent, and double the BCA surface area that is available for heterogeneous chemistry.

Correlations among combustion effluent species at Barrow, Alaska: Aerosol black carbon, carbon dioxide, and methane

As part of the second Arctic Gas and Aerosol Sampling Program (AGASP II) continuous measurements of atmospheric aerosol black carbon (BC) were made at the NOAA/GMCC observatory at Barrow, Alaska (71°19′N, 156°36′W) during the period March 21–April 22, 1986. Black carbon is produced only by incomplete combustion of carbonaceous materials and so is a particularly useful atmospheric indicator of anthropogenic activities. The BC data have been analyzed together with the concurrent measurements of carbon dioxide (CO2), methane (CH4), and condensation nuclei (CN) that are routinely made at the observatory. All four species showed elevated and highly variable concentrations due to local human activities, principally in the township of Barrow, 7 km to the southwest, and at the DEW Line radar installation 1 km to the northwest. We distinguish between those periods of the record that are affected by local activities and those that are not, on the basis of the short-term (periods of up to 1 hour) variability of the continuous CO2 and CN records, with large short-term variabilities indicating local sources. We identified seven periods of time (events) with durations ranging from 13 to 37 hours when the BC, CO2, and CH4 concentrations changed smoothly over time, were highly correlated with each other, and were not influenced by local activities. These events had BC/CO2 ratios in the range (50–103)×10−6. These ratios are dimensionless since we convert the CO2 concentrations to units of ng m−3 of carbon. Such values of BC/CO2 are characteristic of the combustion effluent from large installations burning heavy fuel oil or coal, automobiles, and domestic-scale natural gas usage. We conclude that these events are indicative of air masses that have been polluted with combustion emissions in a distant location and then transported to the Arctic. In the absence of species-selective loss mechanisms, these air masses will maintain their combustion effluent signatures during the transport. The BC/CO2 ratios found for the local combustion activities are consistent with those expected from known combustion processes.

Optical absorption by aerosol black carbon and dust in a desert region of Central Asia

Abstract In September 1989 a joint U.S.S.R.-U.S. study of the causes and effects of desert dust on the environment was conducted in the Tadzhik S.S.R. in Soviet Central Asia. The objectives of the study included measurements of optical absorption by suspended material, both windblown dust and aerosol “black” carbon. This latter material is a combustion effluent, prevalent in emissions from poorly controlled burning, with a long atmospheric lifetime and a large cross-section for the absorption of visible radiation. The measurements obtained from the analysis of filter samples indicate that only during periods of active dust production was there a significant contribution of dust to total absorption. At other times, the presence of black carbon from local and regional sources accounted for approximately 90° of the total aerosol optical absorption. The conclusions are that fuel combustion may produce a greater optical impact on the atmosphere in less-developed areas of the world than that arising from the effects of desert dust production.

Chemical composition of fog water and interstitial aerosol in Berkeley, California

Abstract Fog water and interstitial particles were sampled at Berkeley, California, during foggy periods in the summer of 1986. The sampling site received marine fog advected from the San Francisco Bay. Samples of ambient particulate material (total aerosol) were collected using a filter sampler with a heated inlet. Concentrations of Na + , K + , NH 4 , Cl, NO 3 and SO 4 2 were measured in the particles and fog water, for which pH was also measured. Black carbon aerosol was determined on the filter samples using optical attenuation. The marine fog showed the influence of mixing and chemical reaction with anthropogenic pollutants Twenty-six percent of the black carbon was present in fog droplets, indicating that incorporation of combustion particles into fog droplets had occurred. Anthropogenic particulate species were not scavenged by fog water as efficiently as sea-salt components. Non-sea-salt SO 4 2 was enhanced in the total aerosol during fog compared to dry conditions, and this observation has been interpreted as evidence for heterogeneous production of SO 4 2 in fog. The fog had elevated SO 4 2 and NO 4 levels compared to sites closer to the ocean. Sufficient NH 3 (g) was available to neutralize the increased acidity from anthropogenic H 2 SO 4 and HNO 3 so that the median pH was 4, as compared to pH 2.2 observed in Los Angeles fog water under more polluted conditions. Loss of Cl from the fog or its precursor aerosol was enhanced compared to other measurements of San Francisco Bay Area fog at more remote locations.

Aerosol black carbon measurements in the Arctic haze during AGASP-II

During the second Arctic Gas and Aerosol Sampling Program conducted in April 1986, we performed measurements of the optically absorbing carbonaceous component of the ambient aerosol from the NOAA WP-3D aircraft operating between sea level and 10 km altitude. We collected the aerosol of filters that were exposed for several hours; we also operated the aethalometer to measure the concentration of aerosol black carbon in real time. The filter analyses represent averages over the altitude range and time span during which the filter was collecting. The real-time results were sorted by altitude to calculate vertical profiles of black carbon concentration. Values typically ranged from 300 to 500 ng m−3 at lower altitudes, decreasing gradually to 25 to 100 ng m−3 at 8–10 km. Strong stratification at lower altitudes was frequently observed. The magnitude of these concentrations suggests that the sources are distant regions of considerable fuel consumption. The presence of this material in the tropospheric column and its probable deposition to the high-albedo surface may result in perturbations of the solar radiation balance. The concentrations measured at the highest altitudes may mean that particulate carbon and accompanying emissions for which it is a tracer are mixing into the stratosphere.

Sulfur dioxide oxidation in laboratory clouds

Abstract SO2 oxidation in the presence of NH3 was studied in a mixing-type continuous-flow cloud chamber. NaCl and soot particles (∼5–15 μ m−3) were used as cloud condensation nuclei. Cloud liquid water content was varied between 0.2 and 3 g m−3. SO2 and NH3 concentrations were 0.6 and 1.1 ppm, respectively. The contact time between the SO2 and the cloud drops was varied from 8 s to 3 min. Up to 80% of the input SO2 can be oxidized within short contact times in the presence of NH3 and when the water is in the condensed cloud-drop phase. Negligible sulfate formation was observed in the absence of the liquid phase regardless of the presence or absence of NH3. No significant dependence of the oxidation on the cloud condensation nuclei type nor the contact time was found. This in-cloud SO2 oxidation is much faster than predicted by S(IV) oxidation by molecular oxygen measured in bulk solutions.