Intercomparison of Nitrous Acid (HONO) Measurement Techniques during SHARP

HONO is regarded as a potentially important radical precursor in a number of diverse environments ranging from polar to semi-tropical. As part of the SHARP (Study of Houston Atmospheric Radical Precursors), time series of HONO were obtained by five different measurement techniques. Techniques used were long path differential optical absorption spectroscopy (DOAS), long-path absorption photometry (LoPAP), mist chamber (MC), quantum cascade laser and ionization detection-chemical ionization mass spectrometry. Various combinations of techniques were in operation during the whole period from 15 April through 31 May 2009 with a common measurement period extending from 16 to 28 May. All instruments recorded a similar diurnal pattern of HONO concentrations with higher mean values from the in-situ techniques than either the low- or mid-path DOAS. The largest differences among techniques were found during the afternoon with measurements from the in-situ techniques higher than either the low- or mid-path DOAS. Principal components analysis using measurements of trace species was used to identify possible sources of interference in the chemical measurements. Two major components were identified: one associated with primary, mainly traffic related pollutants and the other with photochemical species. The afternoon differences between DOAS and MC and the U Miami LoPAP were found to be most strongly associated with the photochemical component. The results for comparison between DOAS and MC are in accord with those found previously during August-September 2006. All instruments showed some association between measurement differences and the primary component. Further details and associations with air coming from different areas of the Houston airshed will also be presented.