Abstract:

NO3 and N2 O5 are products of NO 2 oxidation that accumulate at night when NO3 photolysis is unimportant. Subsequent chemistry results in conversion to soluble species (e.g. HNO3 and hydroxyalkylnitrates) that are rapidly removed from the atmosphere by deposition which is a major pathway for removal of reactive nitrogen from the atmosphere. NO3 is also believed to be major nighttime oxidant of unsaturated and aldehydic atmospheric organic compounds in the surface layer. Reactive chemistry of the organic nitrates formed in these reactions may be competitive with deposition and result in return of NO2 to the available pool.

Observations that confirm these ideas are few because, at least until the last 5 years, techniques capable of simultaneous, co-located measurements of NO3 or N2 O5 , reactive volatile organic compounds (VOCs), aerosol surface area and aerosol composition were unavailable. Measurements of NO3 and N2 O5 are challenging because of their concentrations are often less than 10 ppt in the lowest 100 m of the atmosphere and because of their extremely high reactivity on surfaces which makes sampling without losses difficult. To address these issues, Wood et al. built an instrument based on continuous wave laser-induced fluorescence detection of NO3 and thermal dissociation of N2 O 5 followed by NO3 detection. This instrument was capable of studying NO3 and N2 O5 at concentrations of 100 pptv or more. In Chapter 2, I describe improvements to this instrument using a pulsed diode laser combined with a gated detection to achieve a sensitivity of 17 ppt/min.

In Chapters 3 and 4, I describe application of this new instrument to observations at two sites in California. In Chapter 3, the measurements were used to investigate the contribution of NO3 and N2 O 5 to HNO3 formation and thus to high NH4 NO 3 aerosol load in San Joaquin Valley (SJV). The measurements show that NO3 reactions with VOCs are the dominant NOx removal processes in the SJV and they are likely 75% of the HNO3 source. In Chapter 4, observations of the sum of NO3 and N2 O 5 and 1-d model calculations are combined to study the effects of NO 3 chemistry on the budget and partitioning of reactive nitrogen on the western slopes of the Sierra Nevada. The model analyses are capable of reproducing the observations and give some ideas for new specific constraints on chemistry controlling and total peroxy nitrates (?PNs), total alkyl nitrates (?ANs) and HNO3 chemistry.