Continuous wet denuder measurements of atmospheric nitric and nitrous acids during the 1999 Atlanta Supersite

Two different measurement methods for atmospheric nitric and nitrous acid during the Atlanta Supersite study are described and compared. Both approaches combined wet denuder collection coupled to ion chromatographic analysis. One of these utilized a rotating wet annular denuder maintained indoor with a very dilute Na2CO3 solution as an absorber, operated by the Energieonderzoek Centrum Nederland (ECN), ion chromatography (IC) being conducted with a carbonate eluent system. Data from this instrument was available for a 15 min sample every hour. The other wet denuder was of the parallel plate design and was deployed on the roof of the measurement shelter. This device used dilute H2O2 solution as an absorber and was coupled to an IC operated with a hydroxide eluent. Operated by Texas Tech University (TTU), this instrument provided data with 10 min time resolution. When both instruments were seemingly operating properly, data from TTU and ECN instruments were well correlated, although the peak HNO3 values during high NO2/NOy periods were lower for the TTU instrument. Daily peaks in HNO3, typically ranging in magnitude between 3 and 6 ppbv (7.8 ppbv registered by the ECN instrument on the highest NOy day) were observed. HONO results from both TTU and ECN instruments exhibited strong diurnal variations with nighttime peaks up to ~5 ppbv. Data from the middle of the study period for the two instruments were correlated with a r2 value of 0.78. The relationship was not statistically distinguishable from a 1:1 correspondence. A similar correlation of r2=0.76 was observed for the HNO3 data; in this case the peak concentrations occurring in day tim