Heterogeneous reaction of NO2 with soot at different relative humidity.

The influences of relative humidity (RH) on the heterogeneous reaction of NO2 with soot were investigated by a coated wall flow tube reactor at ambient pressure. The initial uptake coefficient (γ initial) of NO2 showed a significant decrease with increasing RH from 7 to 70%. The γ initial on "fuel-rich" and "fuel-lean" soot at RH = 7% was (2.59 ± 0.20) × 10-5 and (5.92 ± 0.34) × 10-6, respectively, and it decreased to (5.49 ± 0.83) × 10-6 and (7.16 ± 0.73) × 10-7 at RH = 70%, respectively. Nevertheless, the HONO yields were almost independent of RH, with average values of (72 ± 3)% for the fuel-rich soot and (60 ± 2)% for the fuel-lean soot. The Langmuir-Hinshelwood mechanism was used to demonstrate the negative role of RH in the heterogeneous uptake of NO2 on soot. The species containing nitrogen formed on soot can undergo hydrolysis to produce carboxylic species or alcohols at high RH, accompanied by the release of little gas-phase HONO and NO.