T. Novakov

The aethalometer — An instrument for the real-time measurement of optical absorption by aerosol particles☆

Abstract We describe an instrument that measures the concentration of optically absorbing aerosol particles in real time. This absorption is normally due to black carbon, which is a good tracer for combustion emission. The minimum resolving times range from seconds in urban environments to minutes in remote locations. We present results obtained during operation on an aircraft. Due to the time resolution capability, we can determine the spatial distributions of absorbing aerosol. From the Greek word “αιϑλoυν,” “to blacken with soot,” we have named this instrument the aethalometer .

Sulfates as Pollution Particulates: Catalytic Formation on Carbon (Soot) Particles

Experimental evidence (obtained by electron spectroscopy for chemical analysis) is presented which shows that finely divided carbon (soot) particles may play a major role in the catalytic oxidation of sulfur dioxide to sulfate in polluted atmospheres. The results obtained with sulfates produced in the laboratory by the oxidation of sulfur dioxide on graphite particles and combustion-produced soot particles are compared with the properties and behavior of ambient sulfates. The proposed sulfur dioxide oxidation mechanism is qualitatively consistent with field observations.

Large historical changes of fossil-fuel black carbon aerosols

Anthropogenic emissions of fine black carbon (BC) particles, the principal light-absorbing atmospheric aerosol, have varied during the past century in response to changes of fossil-fuel utilization, technology developments, and emission controls. We estimate historical trends of fossil-fuel BC emissions in six regions that represent about two-thirds of present day emissions and extrapolate these to global emissions from 1875 onward. Qualitative features in these trends show rapid increase in the latter part of the 1800s, the leveling off in the first half of the 1900s, and the re-acceleration in the past 50 years as China and India developed. We find that historical changes of fuel utilization have caused large temporal change in aerosol absorption, and thus substantial change of aerosol single scatter albedo in some regions, which suggests that BC may have contributed to global temperature changes in the past century. This implies that the BC history needs to be represented realistically in climate change assessments.

Global atmospheric black carbon inferred from AERONET

AERONET, a network of well calibrated sunphotometers, provides data on aerosol optical depth and absorption optical depth at >250 sites around the world. The spectral range of AERONET allows discrimination between constituents that absorb most strongly in the UV region, such as soil dust and organic carbon, and the more ubiquitously absorbing black carbon (BC). AERONET locations, primarily continental, are not representative of the global mean, but they can be used to calibrate global aerosol climatologies produced by tracer transport models. We find that the amount of BC in current climatologies must be increased by a factor of 2–4 to yield best agreement with AERONET, in the approximation in which BC is externally mixed with other aerosols. The inferred climate forcing by BC, regardless of whether it is internally or externally mixed, is ≈1 W/m2, most of which is probably anthropogenic. This positive forcing (warming) by BC must substantially counterbalance cooling by anthropogenic reflective aerosols. Thus, especially if reflective aerosols such as sulfates are reduced, it is important to reduce BC to minimize global warming.

The relationship between optical attenuation and black carbon concentration for ambient and source particles

Light absorption provides the basis for a fast and nondestructive method for determining concentrations of black carbon (BC). The laser transmission technique measures the attenuation (ATN) of visible light as it passes through filter samples. We have measured [BC] and ATN simultaneously for a large number of solvent-extracted source and ambient particle samples, using temperature-programmed evolved gas analysis with continuous light attenuation measurement. For all data with ATN ≤ 200, ATN is directly proportional to [BC], with the proportionality constant = 25.4 ± 1.7 cm2μg−1. Because the relationship between ATN and [BC] does not depend on the origin of the carbonaceous particles, measurement of ATN alone can provide a good estimate of the black carbon concentration.

SOOT IN THE ARCTIC

Abstract Substantial concentrations of graphitic carbon and its associated large optical absorption coefficient are observed in the Arctic. The graphitic content shows a dramatic increase from late fall to early spring, reaching levels that are comparable to those found in urban environments (i.e., the peak values in February are only about a factor of 10 less than the average levels found in New York City and a factor of three less than those found in Berkeley, California, and Denver, Colorado). If one ignores the possible contribution of natural burning processes which are expected to be small during this time of the year in the northern hemisphere, this graphitic component can be attributed directly to anthropogenic activity.

THE ROLE OF SOOT AND PRIMARY OXIDANTS IN ATMOSPHERIC CHEMISTRY

This paper discusses the role of soot in atmospheric sulfur chemistry. A methodology for estimating the primary and secondary particulate carbon concentrations is outlined. The relationship between ambient SO2, sulfate, and primary carbon is examined; and certain regularities in their behavior are defined. An explanation of these regularities is proposed, based on a mechanism involving oxygen chemisorbed on combustion-generated carbon particles and its subsequent reaction with aqueous sulfite.

Soot in the Atmosphere

Carbonaceous particles in the atmosphere consist of two major components — graphitic or black carbon and organic material. The organic component can either be directly emitted from sources (primary organics) or be produced by atmospheric reactions from gaseous precursors (secondary organics). We define soot as the total primary carbonaceous material, i.e., the sum of black carbon and primary organics. The complex set of questions concerning the origin and the chemical and physical characterization of carbonaceous particles has been central to the research of the Atmospheric Aerosol Research group at Lawrence Berkeley Laboratory since the group’s beginning in 1972. This paper will present an overview of our efforts to quantitate the amount of soot in a variety of urban locations. The results will demonstrate that soot is a major component of the carbon aerosol in all locations and can be the dominant component in many.

Determination of the chemical states of sulfur in ambient pollution aerosols by X-ray photoelectron spectroscopy☆

Abstract The chemical states of sulfur present in ambient paniculate matter have been identified by use of X-ray photoelectron spectroscopy (X.P.S.-E.S.C.A.) Seven separate chemical species of sulfur were found in atmospheric samples taken in California. These, in terms of net sulfur charge, are SO3.SO42−, SO2. SO32−, S0 and two kinds of sulfides. Sulfates were found to be the dominant species, although reduced forms of sulfur were at times comparable to the sulfate concentrations.

THE IMPORTANCE OF SOOT PARTICLES AND NITROUS ACID IN OXIDIZING SO2 IN ATMOSPHERIC AQUEOUS DROPLETS

Abstract Soot particles catalyze the oxidation of SO 2 H 2 O, HSO 3 − , and SO 2− 3 species at the same rate; however, the rate of production of sulfate is not constant at a constant partial pressure of SO 2 . The rate decreases as the pH decreases because the total concentration of S(IV) in aqueous droplets decreases as the pH decreases. The sulfate production rate has a complex dependence on the concentration of S(IV) and is not very sensitive to a change in SO 2 concentration at normal atmospheric conditions. The oxidation of SO 2 by HNO 2 in atmospheric water droplets first produces hydroxylamine disulfonate, which undergoes acid-catalyzed hydrolysis to form sulfate and hydroxylamine monosulfonate. The latter can react with HNO 2 and would then end up as nitrous oxide and sulfate Model calculations have been made comparing the relative importance of the sulfate production mechanism by soot particles and nitrous acid with other mechanisms involving liquid water. The results indicate that both soot and HNO 2 mechanisms can be very important when the lifetime of the atmospheric aqueous droplets is long.