Simultaneous Measurement of NO2, NO3 and N2O5 by Cavity Ring-down Spectroscopy During NEAQS-ITCT 2004

We have constructed a cavity ring-down spectrometer (CaRDS) for the simultaneous measurement of nitrogen dioxide, NO₂, nitrate radical, NO₃, and dinitrogen pentoxide, N₂O₅, in the atmosphere. NO₃ is known to be the primary nighttime oxidant; during the day, NO₃ is efficiently destroyed by photolysis. N₂O₅ removal is a major pathway for loss of NO_x, a key ozone production ingredient, from the troposphere. The instrument employs three cavity ring-down cells. In the first cell, NO₂ is monitored at 532 nm using a frequency-doubled Nd-YAG laser operated in pulsed mode at 100 Hz. In the second channel, the ambient NO₃ radical concentration is measured at 662 nm, using the output of a dye laser. The third channel is heated to 75° C, which quantitatively converts N₂O₅ to NO₃, and the amount of N₂O₅ present is then determined from the increase in the absorption signal at 662 nm. Detection limits are <1 pptv for NO₃ and N₂O₅ and 35 pptv for NO₂ (1 s averages). With this instrument, it becomes possible to continuously characterize all the key nitrogen species involved in NO₃/N₂O₅ chemistry with high precision directly in the atmosphere. The spectrometer was fielded during the NEAQS-ITCT 2004 campaign on board of the research vessel Ron Brown. In this presentation, we compare our CaRDS NO₂ to that measured using the more traditional method and present some initial findings from this campaign, with particular emphasis on the chemical evolution of NO_x species in NO_x-rich urban plumes in the marine boundary layer.