A novel approach to measurement of atmospheric nitrate and dinitrogen pentoxide

The nitrogen oxides NO3 and N2O5 play a central role in the chemistry of the troposphere at night. NO3 is a potent atmospheric scavenger, oxidizing volatile organic compounds, especially biogenic alkenes and terpenes. Oxidation of NOx (NOx ≡ NO + NO2) to form N2O5 and subsequent heterogeneous hydrolysis of N2O5 to form nitric acid (HNO3) is a non-photochemical mechanism through which NOx is removed from the atmosphere. This dissertation describes a novel method for measurement of the nocturnal nitrogen oxides NO3 and N2O3 based on laser-induced fluorescence (LIF). The instrument is based on electronic excitation of NO3 using 662 nm light emitted by a continuous wave (cw) diode laser. N2O5 is detected as NO3 after thermal dissociation to NO2 and NO3. The calibration of the instrument is based on rapid conversion of laboratory-produced NO 3 into NO2 by reaction with NO, followed by quantification of NO2 by LIF using a tunable diode laser. The first successful field deployment of this instrument was executed in Contra Costa County, California during December 2003--January 2004. Measurements of N2O 5 along with complementary measurements of O3, NO2, and meteorology were used to quantify the atmospheric lifetime of N2 O5 and to asses the role of N2O5 in the nocturnal removal of NO, from the terrestrial boundary layer. These measurements indicated an atmospheric steady state lifetime of N2O5 of 30 minutes or less. This short lifetime indicates that almost all N 2O5 produced at night is effectively converted to HNO 3 on the surface of deliquescent aerosol. The amount of HNO3 produced during the night in this environment is comparable to ambient concentrations of NO2 (1--5 ppbv).