Impacts of electronically photo-excited NOsub2/sub on air pollution in the South Coast Air Basin of California

摘要

A new path for hydroxyl radical formation via photo-excitation of nitrogendioxide (NO₂) and the reaction of photo-excited NO₂ with water isevaluated using the UCI-CIT model for the South Coast Air Basin ofCalifornia (SoCAB). Two separate studies predict different reaction rates,which differ by nearly an order of magnitude, for the reaction ofphoto-excited NO₂ with water. Impacts of this new chemical mechanism onozone and particulate matter formation, while utilizing both reaction rates,are quantified by simulating two summer episodes. First, sensitivitysimulations are conducted to evaluate the uncertainty in the rate ofreaction of photo-excited NO₂ with water reported in the literature.Results indicate that the addition of photo-excited NO₂ chemistryincreases peak 8-h average ozone and particulate matter concentrations.The importance of this new chemistry is then evaluated in the context ofpollution control strategies. A series of simulations are conducted togenerate isopleths for ozone and particulate matter concentrations, varyingbaseline nitrogen oxides (NO_x) and volatile organic compounds (VOC)emissions. Isopleths are obtained using 1987 emissions, to represent pastconditions, and 2005, to represent current conditions in the SoCAB. Resultsshow that the sensitivity of modeled pollutant control strategies due tophotoexcitation decreases with the decrease in baseline emissions from 1987to 2005. Results show that including NO₂ photo-excitation, increasesthe sensitivity of ozone concentration with respect to changes in NO_xemissions for both years. In particular, decreasing NO_x emissions in2005 when NO₂ photo-excitation is included, while utilizing the higherreaction rate, leads to ozone relative reduction factors that are 15%lower than in a case without photo-excited NO₂. This implies thatphotoexcitation increases the effectiveness in reducing ozone throughNO_x emissions reductions alone, which has implications for theassessment of future emission control strategies. However, there is stilldisagreement with respect to the reaction rate constant for the formation ofOH. Therefore, further studies are required to reduce the uncertainty in thereaction rate constant before this new mechanism is fully implemented inregulatory applications.