Carbonaceous aerosol (soot) measured in the lower stratosphere during POLARIS and its role in stratospheric photochemistry

Abstract

in the laboratory, Thlibi and Petit [1994] found that the reduction of HNO3 to NO occurred on carbon. Rogaski et al. [1997] found that the major products of the heterogeneous reaction of HNO3 with BCA were H20, NO2, and NO and reported an uptake coefficient of 3.8 x 10 •. They measured two NOx molecules produced for every three HNO3 molecules adsorbed. More recently, however, Choi and Leu [1998] on carbon between 10 -3 and 10 -5 depending on carbon sample surface history, with CO, CO2, and 02 as products. The presence of these products suggests that the ozone reaction on carbon is, at lest in part, noncatalytic.This paper discusses the measurement of BCA by the Ames Wire Impactors and assesses the role of BCA in stratospheric photochemistry. A new method of analyzing impactor samples is described that accounts for particle bounce and models the BCA as fractal aggregates to modify the aerodynamic collection efficiency and determine particle surface area. Results from the present analysis are compared with previous techniques and with BCA surface area values used in previous model simulations. The photochemical trajectory model used in the present simulations is described. An assessment of the importance of heterogeneous BCA reactions is made by comparing modeled and measured NOJNOy ratios and effects on ozone loss. Finally, the implications of mass-balancing these reactions on the lifetime of BCA particles in the stratosphere will be discussed. The Ames Wire ImpactorThe Ames Wire Impactor (AWI) has been used extensively in the past on such missions as AAOE, AASE, AASE II, ASHOE/MAESA, and POLARIS to sample stratospheric aerosol. It has proven to be an accurate and reliable means for obtaining atmospheric aerosol size distributions, and sulfate and BCA burdens. The sampler consists of palladium and/or gold wires of 75-and 500-p,m diameter that are strung across support rings. The wires are carbon coated and modules are assembled in a class 100 clean room. Up to six modules are mounted on the wingtip of the ER-2. Each module has the capability of holding several rings with several wires on each ring. The modules are exposed to the free stream by preselected triggers of time, altitude, or aircraft position. Aerosol particles suspended in the ambient air impact on the wires. A new automated sample actuation system was designed, fabricated, and successfully used to retrieve data during the POLARIS campaign. Samples were exposed typically for 3 min. Particles stick to the wire upon impact by virtue of van der Waals forces. After exposure, the wires are retracted into sealed modules containing concentrated ammonia vapor. The ammonia combines with the sulfuric acid aerosol to form ammonium sulfate which is very stable under laboratory conditions and allows for the sample to be stored indefinitely. Particles are manually identified, counted, and sized using a Hitachi S-4000 field emission scanning electron microscope. Under the conditions used for imaging, the lateral resolution of the microscope is better than 10 nm....

Simulating the global atmospheric black carbon cycle: a revisit to the contribution of aircraft emissions

“…Furthermore, Blake and Kato (1995) stressed that small BC particles tend to be underrepresented in the wire impactor samples. Black carbon particles immersed within larger spherical liquid aerosols are generally not detected by the BK95 and the Strawa et al (1999) method. These uncertainties have to be carefully considered when simulated BC concentrations are compared to the BK95 data.…”

“…2.2, this number-to-mass ratio is derived from the assumption that the surface-related BC particles contain n=10 spherically shaped primary particles on average. The morphologies of BC particles collected in the tropopause region (Blake and Kato, 1995;Pueschel et al, 2000) and the lower stratosphere (Strawa et al, 1999) reveal that much larger agglomerates can reside at these altitudes. Hence, the average n may be larger than assumed.…”

“…The analysis method used by BK95 to derive BC concentrations from wire impactor samples did not take into account the fractal shape of BC particles and did not account for particle bounce. Strawa et al (1999) showed with a modified method that these simplifications lead to an underestimation of the BC concentration by a factor of 6 on average. Furthermore, Blake and Kato (1995) stressed that small BC particles tend to be underrepresented in the wire impactor samples.…”

“…The number n of primary particles encountered in single atmospheric BC particles can be highly variable even in the upper troposphere and the stratosphere (e.g. Blake and Kato (1995) or Pueschel et al (2000) for UTLS BC; Strawa et al (1999) for stratospheric BC). Observational information on n at these altitudes is too sparse to enable robust assumptions on a representative value of n. The available measurements suggest that n has an order of magnitude of 10-100.…”

Chemical analysis of refractory stratospheric aerosol particles collected within the arctic vortex and inside polar stratospheric clouds

“…Soot particles were also found in the stratosphere by other researchers (e.g., Renard et al, 2008;Cziczo et al, 2002;Pueschel et al, 1992Pueschel et al, , 1997Sheridan et al, 1994;Strawa et al, 1999). As the most probable source aviation fuel, rocket launches and forest fires are discussed.…”