Nitric acid uptake and decomposition on black carbon (soot) surfaces: Its implications for the upper troposphere and lower stratosphere

Abstract

The uptake and decomposition of HNO3 on black carbon (soot) surfaces were investigated in order to evaluate the proposal that HNO3 decomposition on aircraft-generated soot aerosols may alter the NOx/NOy partitioning in the upper troposphere and lower stratosphere. The experimental measurements were performed by using a fast flow-tube reactor coupled to a quadrupole mass spectrometer. Black carbon samples used as surrogate material for aircraft soot in this study included Degussa FW2 (an amorphous carbon black comprising medium oxides), graphite, hexane soot, and kerosene soot. The measurements of uptake were performed by varying P(HNO3) in the range of 5 × 10-7 to 5 × 10-4 Torr at 220 and 295 K. The results are summarized as follows. Significant HNO3 decomposition was observed on FW2 at 295 K with P(HNO3) ≥ 1 × 10-4 Torr, while it did not occur at 220 K. Similar HNO3 decomposition behavior on graphite was also observed under the condition of P(HNO3) ≥ 10-4 Torr and T = 295 K, although the extent of the decomposition was much smaller than that on FW2. The decomposition of HNO3 on soot produced NO, NO2, H2O, oxidized soot surface, and some unidentified volatile products. To explain the observed decomposition behavior at higher partial pressures of HNO3, a bimolecular HNO3 decomposition mechanism on soot surfaces was proposed. However, HNO3 immediately decomposed on an FW2 surface at 503 K even at lower partial pressure (∼10-6 Torr). On flame-deposited hexane and kerosene soot film, no HNO3 decomposition was observed up to P(HNO3) = 5 × 10-4 Torr. Moreover, the uptake and desorption of HNO3 were reversible at 295 K and irreversible at 220 K. Adsorbed HNO3 molecules on hexane soot film were saturated to a monolayer coverage at P(HNO3) ∼ 2 × 10-4 Torr according to Langmuir adsorption isotherm; further increase in P(HNO3) resulted in multilayer adsorption. Under the experimental conditions (P(HNO3) = 5 × 10-7 Torr and T = 220 K), the uptake of HNO3 was found to involve purely physical adsorption without showing any sign of irreversible decomposition over all black carbon samples. Subsequent heating of the sample following the uptake at 220 K desorbed most of the adsorbed HNO3 molecules. Physical adsorption of HNO3 was found to take place on the surface of concentrated H2SO4-coated soot at 230 K, but decomposition of HNO3 took place at 296 K. Finally, the present results suggest that the HNO3 decomposition on soot aerosols through a direct gas-solid interaction, which was proposed as a possible NOy-reactivation mechanism in the atmospheric modeling of upper troposphere and lower stratosphere, should be dismissed.