Intercomparison of field measurements of nitrous acid (HONO) during the SHARP Campaign
Because of the importance of HONO as a radical reservoir, consistent and accurate measurements of its concentration are needed. As part of the SHARP (Study of Houston Atmospheric Radical Precursors), time series of HONO were obtained by five different measurement techniques on the roof of the Moody Tower (MT) at the University of Houston. Techniques used were long path differential optical absorption spectroscopy (DOAS), long-path absorption photometry (LOPAP), mist chamber/ ion chromatography (MC/IC), quantum cascade-tunable infrared laser differential absorption spectroscopy (QC-TILDAS) and ion drift -chemical ionization mass spectrometry (ID-CIMS). Various combinations of techniques were in operation during the period from 15 April through 31 May 2009. This study comparing methods is unique in that it compares several techniques entirely at ambient conditions in a polluted atmosphere.
All instruments recorded a similar diurnal pattern of HONO concentrations with higher median and mean values during the night and lower values during the day throughout the entire measurement period. Highest values were observed in the final two weeks of the campaign. All pairs of instruments generally showed good agreement. The MC-IC, LOPAP (University of Miami), and pulsed QCL, and to a lesser extent the LP-DOAS, tracked each other most closely. Largest differences between pairs of measurements were evident during the day for concentrations less than ~100 ppt. Above ~ 200 ppt, concentrations from the Miami LOPAP, MC-IC and pulsed QCL converged to within about 20%, with slightly larger discrepancies when LP-DOAS was considered. Differences greater than 25 ppt between techniques were statistically significant (P-values < 0.05).