摘要
The UCD/CIT model was modified to include a process analysis(PA)scheme for gas and particulate matter(PM)to study the formation of secondary nitrate aerosol during a stagnant wintertime air pollution episode during the California Regional PM_(2.5)/PM_(10) Air Quality Study(CRPAQS)where detailed measurements of PM components are available at a few sites.Secondary nitrate is formed in the urban areas from near the ground to a few hundred meters above the surface during the day with a maximum modeled net increase rate of 4μg·m^(-3)·d^(-1) during the study episode.The secondary nitrate formation rate in rural areas is lower due to lower NO_(2).In the afternoon hours,near-surface temperature can be high enough to evaporate the particulate nitrate.In the nighttime hours,both the gas phase N_(2)O_(5) reactions with water vapor and the N_(2)O_(5) heterogeneous reactions with particle-bound water are important for secondary nitrate formation.The N_(2)O_(5) reactions are most import near the surface to a few hundred meters above surface with a maximum modeled net secondary nitrate increase rate of 1μg·m^(-3)·d^(-1) and are more significant in the rural areas where the O3 concentrations are high at night.In general,vertical transport during the day moves the nitrate formed near the surface to higher elevations.During the stagnant days,process analysis indicates that the nitrate concentration in the upper air builds up and leads to a net downward flux of nitrate through vertical diffusion and a rapid increase of surface nitrate concentration.
基金
This research was partially supported by the California Air Resources Board and the San Joaquin Valleywide Air Pollution Study Agency under contract 2000-2005 PM.
