Total Peroxy Nitrates (Sigma PNs) in the atmosphere: the Thermal Dissociation-Laser Induced Fluorescence (TD-LIF) technique and comparisons to speciated PAN measurements

Peroxyacetyl nitrate (PAN) and its chemical analogues are increasingly being quantified in the ambient atmosphere by thermal dissociation (TD) followed by detection of either the peroxyacyl radical or the NO2 product. Here we present details of the technique developed at University of California, Berkeley which detects the sum of all peroxynitrates (Sigma PNs) via laser-induced fluorescence (LIF) of the NO2 product. We review the various deployments and compare the Berkeley Sigma PNs measurements with the sums of PAN and its homologue species detected individually by other instruments. The observed TD-LIF Sigma PNs usually agree to within 10% with the summed individual species, thus arguing against the presence of significant concentrations of unmeasured PAN-type compounds in the atmosphere, as suggested by some photochemical mechanisms. Examples of poorer agreement are attributed to a sampling inlet design that is shown to be inappropriate for high NOx conditions. Interferences to the TD-LIF measurements are described along with strategies to minimize their effects.