Reactive Nitrogen in the Tropical Troposphere

The aim of this project was to calibrate a thermal-decomposition chemiluminescence (TD-Chem) instrument, capable of measuring the composition of the reactive nitrogen pool at the Cape Verde Atmospheric Observatory (CVAO) in the remote tropical troposphere. This data will be used in global atmospheric models in an attempt to better understand the sources of NOx in the remote troposphere and how these could affect background ozone (O3) levels.
Existing nitrogen oxides (NOx = NO + NO2) data from the CVAO was analysed for the measurement period of October 2006 to December 2011. The aim of this analysis was to identify the cause of the NO2 diurnal, which exhibits a maximum mixing ratio at solar noon, in contrast to the minimum expected due to NO2 photolysis in a clean environment. This anomaly was referred to as ΔNO2. It was found that between 4.12.2007 and 28.2.2009, ΔNO2 was significantly higher and caused an average increase in ΔNO2 of 5.09 ± 0.94 pptv to the entire dataset. This period corresponded with the orientation of the inlet, resulting in the heating of the sample and potentially significant levels of thermal dissociation of peroxyacetyl nitrate (PAN) to produce the NO2 observed. Future speciated measurements of the reactive nitrogen pool will help address the ΔNO2 anomaly fully.
Calibrations of the inlet in the TD-Chem instrument were carried out using PAN, n-propyl nitrate (NPN) and nitric acid (HNO3) standards to represent peroxyacyl nitrate (PNs), alkyl nitrate (ANs) and HNO3 reservoirs. Quantification of the standards was achieved using a molybdenum oven and a gold oven in conjunction with a small flow of carbon monoxide, both heated to 300 ̊C. Both methods are known to cause ~ 100 % conversion of NOy compounds to NO, to allow detection via chemiluminescence (NOy = NOx + PNs, ANs, HNO3, aerosol nitrate, halogen nitrates etc.). Both ovens agreed on the concentration of the standards to > 99 %. Temperature ramp experiments quantified the temperature range at which each standard thermally dissociated to form NO2 and a companion radical in each of the quartz ovens used in the TD-Chem instrument. All experiments show thermal dissociation kinetics consistent with current understanding and kinetic theory. Deviations that do occur are either known and can be quantified, have been experimentally deduced, or a work schedule is in place in order to quantify them in the near future. Completion of the instrument calibration and subsequent installation of the TD-Chem instrument at the CVAO is projected to be in the summer of 2013.