Laboratory Investigation of Heterogeneous Reaction of NO2 and HONO Formation on Soot Surfaces

Although formation of HONO on soot-containing aerosols is believed to represents a major source of radicals in polluted atmosphere, the detailed kinetics and mechanism of the heterogeneous processes leading to HONO formation remain poorly understood. Currently, the reaction probability of the NO2 reaction on soot is uncertain from available experimental studies, in the range from 1.1 x 10-2 to 3.3 x 10-4, decreasing with the aging of soot particles. As part of the 2009 SHARP/SOOT Campaign, we have conducted experiments of the heterogeneous chemistry of HONO using a flow reactor. A fast-flow, wall-coated flow-reactor was used to measure heterogeneous reactions of NO2 on soot surfaces. An inner glass tube, coated with soot, was inserted into the flow reactor. The measurements included uptake coefficients on fresh and aged soot surfaces with variable amounts of organics, sulfate, and nitrate. Gaseous NO2 was introduced through a movable injector and diluted with helium. The soot surface was exposed to organic acids, sulfuric acid, and nitric acid to produce internally mixed particles similar to those found in the atmosphere. Water vapor was added through a side port to adjust the relative humidity. The reactants and products in the uptake experiments were monitored using the ion drift – chemical ionization mass spectrometry (ID-CIMS). The kinetics and mechanism of the heterogeneous reaction were determined to obtain by observing the loss of reactant and formation of the product in the gas-phase. Implications of our laboratory experiments on HONO formation in polluted will be discussed.