Investigation of day- and night-time NOx/VOCs coupling using thermal dissociation cavity ring-down spectroscopy
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Abstract
Reactive nitrogen species, so called NOy, are a class of compounds involved in
different atmospheric cycles that impact the global distribution of atmospheric trace gases. The NOz group of compounds , whose precursors are NOx (NO + NO2), includes short lived species like NO3, HONO, RO2NO2, ClNO2 and longer lived species like N2O5, HNO3, R(O)O2NO2 and RONO2. The fate of NOy (NOx + NOz) in the lowest layer of the atmosphere, the planetary boundary layer, has a strong influence on ozone levels, on HOx (OH + HO2 +RO2 ) species and on aerosol particles composition. The destruction of NOx through processes other than NO2 photolysis (which produces NO) like conversion to NOz and subsequent removal by dry or wet deposition, results in a decrease of the global ozone production. This thesis investigates atmospheric levels, lifetimes and production rates of different NOy species in the field using thermal dissociation cavity ring-down spectroscopy (abbreviated as TD-CRDS). Results using existing instruments and an instrument developed in the frame of this work are presented. The first chapter reports the current knowledge on NOy chemistry and describe the research objectives of this work. Chapter 2 describes the design and performances of a newly developed 5 channel TD-CRDS based instrument to measure NO2, NO3, N2O5, the sum peroxy nitrates (ƩRO2NO2, abbreviated as ƩPNs) and the sum of alkyl nitrates (ƩRONO2,abbreviated as ƩANs). We show that this method allows for precise measurement of those reactive nitrogen species at high time resolution and with a good accuracy. A number of design improvements compared to previously developed similar instruments which reduce the measurement uncertainties and facilitate field deployment are detailed. Chapter 3 reports field observations during two campaigns that took place in 2011 and 2015 on top of the “Kleiner Feldberg” mountain near Frankfurt am Main (Germany). NO3 mixing ratios and estimated lifetimes are analysed in regard to meteorological and chemical influences. It is demonstrated that the measurement site is occasionally sampling from the nocturnal residual layer which result in very high steady state NO3 lifetimes (up to 1 hour) which is unusual for ground based measurements. The estimated lifetimes assuming steady state are compared to the NO3 reactivity calculated using VOCs measurements (by GC-FID and GC-MS) and NO measurements (by CLD) and significant discrepancies are found. An unaccounted source of NO3 through the oxidation of NO2 by Stabilized Criegee Intermediates (SCIs) as well as the influence of the local topography are considered as the potential causes of the observed differences. In this chapter are also reported the measured mixing ratios and calculated production rates of organic nitrates during both campaign. The differences in lifetimes between OH and NO3 generated organic nitrates are discussed. The nitrogen partitioning ratio between organic nitrates (ƩPNs + ƩANs) and NOx is reported and found to
depend on NOx and temperature and to be as high as 80 % in aged, warm air masses. Finally the effective yield of alkyl nitrates for the PARADE campaign is estimated using O3 and ƩANs mixing ratios and is found to match well the calculated yield based on VOCs measurements and individual alkyl nitrates yields reported in the literature. The general conclusions of this work are presented in Chapter 4.