Surface-induced oxidation of organics in the troposphere (SOOT)
| Project Period: | 04/01/2008 - 08/31/2009 |
| Total Budget: | $350,000 |
| Sub-Contractors: | TAMU
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Surface-induced heterogeneous reactions play a central role in partitioning of trace species in the troposphere, but the detailed kinetics and mechanism of the heterogeneous processes remain poorly understood. In the urban atmosphere, heterogeneous conversion of NOx on the surface of soot particles has been suggested to occur efficiently to form nitrous acid (HONO). This process leads to accumulation of elevated levels of HONO at night. Subsequent photolysis of HONO during the morning results in a sudden rise in the hydroxyl radical (OH) concentration and, hence, promotes efficient oxidation of volatile organic compounds (VOCs) and rapid ozone production. Model calculations have demonstrated that inclusion of the heterogeneous conversion of NO2 to HONO on the surfaces of soot aerosols accelerates the O3 production by about 1 hour in the morning and leads to a noticeable increase of about 7 ppb on average in the daytime O3 level over the Houston area [Lei et al., 2005].
Heterogeneous chemistry also influences the nitrogen oxide budget. Most notably, hydrolysis of dinitrogen pentoxide (N2O5) in aqueous aerosols can significantly impact the nighttime nitrogen chemistry. Nitrate radical (NO3), formed from the oxidation of NO2 by O3, is the dominant nighttime free radical. Similar to OH, NO3 reacts with VOCs through H-atom abstraction or addition reactions, leading to nighttime peroxy radical formation. Self reactions between peroxy radicals form the peroxides, which serve as an OH source the following day. NO3 also reacts with NO2 to form N2O5. Although N2O5 is not very reactive in the gas phase, it is taken up efficiently by aqueous aerosols or droplets to form HNO3. N2O5 also thermally decomposes to NO2 + NO3. Hence, N2O5 serves as either a sink or a temporary reservoir for NO3 and impacts the NOy budget and ozone formation on the subsequent day.
This project will consist of two phases: (1) experimental studies of heterogeneous reactions using a flow reactor and an aerosol chamber and (2) field measurements of several key nitrogen compounds and aerosols during 2008/2009 in Houston.
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