| Abstrakt: |
Peroxy radicals were measured in a mixed deciduous forest atmosphere in Bloomington, Indiana, USA, during the Indiana Radical, Reactivity and Ozone Production Intercomparison (IRRONIC) during the summer of 2015. Total peroxy radicals ([XO2]≡[HO2]+Σ[RO2]) were measured by a newly developed technique involving nitric oxide (NO) - ethane (C2H6) chemical amplification followed by NO2 detection by cavity attenuated phase shift spectroscopy (hereinafter referred to as ECHAMP). The sum of hydroperoxy radicals (HO2) and a portion of organic peroxy radicals ([HO2*]=[HO2]+Σαi[RiO2], 0<α<1) was measured by the Indiana University Laser-Induced Fluorescence/Fluorescence Assay by Gas Expansion instrument (LIF-FAGE). Additional collocated measurements include concentrations of NO, NO2, O3, and a wide range of volatile organic compounds (VOCs); and meteorological parameters. XO2 concentrations measured by ECHAMP peaked between 13:00 to 16:00 local time, with campaign average concentrations of 41±15ppt (1σ) at 14:00. Daytime concentrations of isoprene averaged 3.6±1.9ppb (1σ) whereas average concentrations of NOx ([NO]+[NO2]) and toluene were 1.2ppb and 0.1ppb, respectively, indicating a low impact from anthropogenic emissions at this site. We compared ambient measurements from both instruments and conducted a calibration source comparison. For the calibration comparison, the ECHAMP instrument, which is primarily calibrated with an acetone photolysis method, sampled the output of the LIF-FAGE calibration source which is based on the water vapor photolysis method and, for these comparisons, generated a 50-50% mixture of HO2 and either butane or isoprene-derived RO2. A bivariate fit of the data yields the relation [XO2]ECHAMP=0.88 ([HO2]+RO2])IU_cal+6.6ppt, with an R2 of 0.99. This level of agreement is within the combined analytical uncertainties for the two instruments' calibration methods. A linear fit of all the 30-minute averaged [XO2] ambient data with the 1-minute averaged [HO2*] data (one point per 30 minutes) yields the relation [XO2]=0.85 [HO2*]+3.3ppt, with an R2 of 0.67. Day to day variability in the [XO2]/[HO2*] ratio was observed. The lowest [XO2]/[HO2*] ratios between 13:00 and 16:00 were 0.8 on 13 and 18 July, whereas the highest ratios of 1.1 to 1.3 were observed on 24 and 25 July - the same two days on which the highest concentrations of isoprene and ozone were observed. Although the exact composition of the peroxy radicals during IRRONIC is not known, 0-dimensional photochemical modeling of the IRRONIC dataset using the RACM2, RACM2-LIM1, MCM 3.2, and MCM 3.3.1 chemical mechanisms predicts afternoon [XO2]/[HO2*] ratios of between 1.1 to 1.5 depending on mechanism. Differences between the observed ambient [XO2]/[HO2*] ratio and that predicted with the 0-D modeling can be attributed to deficiencies in the model, errors in the two measurement techniques, or both. Although these comparison results are encouraging and demonstrate the viability of using the new ECHAMP technique for field measurements of peroxy radicals, further research investigating the overall accuracy of the measurements and possible interferences from both methods is warranted. [ABSTRACT FROM AUTHOR] |
