GEOPHYSICAL RESEARCH LETTERS, VOL. 20, NO. 22, PAGES 2563-2566, NOVEMBER 19, 1993 STR.ATOSPHERIC NOy MEASUREMENTS ONTHENASA DC-8 DURING AASE Ii A. J.Weinheimer, J.G. Walega, B. A. Ridley National Center for Atmospheric Research G. W. Sachse, B. E. Anderson NASA Langley Research Center J. E. Collins, Jr. Science and Technology Corp. Abstract. We survey measurements of stratospheric NOy on the NASA DC-8 during AASE II in early 1992. Emphasis ison correlations with otherlong-lived species:03, N20, and CH4.No cases of extremely high NOy/O 3 ratios were seen in 1992. This suggests that denitrification at higher altitudes was less pervasive than in 1989. Values of NOy/O3 in 1992 arecomparable to, though perhaps lower than, those measured in 1989.The correlation of NOy with N20 changes over the course of the mission. Relative to the correlation for the whole season, lowvalues of NOy occur in February, most notably on the 22 ncl. The reason for the low NOy is unknown. It isgenerally toowarm at DC-8 altitudes for the presence of PSC particles, but denitrification at higher altitudes, followed by descent, might account for the low values seen on 22 February. However, this may be unlikelysince the NASA ER-2, flying duringthe sameperiod,saw no evidence of denitrification at higher altitudes [Loewenstein eta!., 1993]. Introduction Reactive nitrogen (NOy) species playmany roles in the chemistry of the stratosphere. The NOx (NO and NO2) catalytic cycleis important for the photochemical destruction of 03. Also, NOy species interact with the other major cycles that destroy ozone, those involving C1Ox and HOx. In addition, heterogeneous processes are influenced by, as well as influence, NOyspecies. Themost abundant NOy species, HN03,condenses with H20 tOform polarstratospheric cloud (PSC) particles,providing surfacesfor the reactions that liberate chlorine. The active NOx is converted to inactive HN03 by reactions on both sulfate aerosols and PSC particles, thereby not only (1) reducing the influence of the NOx catalytic cycle,but also (2) enhancing the HOx cycle through increased photo!ysis of HNO3 and(3) enhancing the ClOx cycles by diminishing the ability of NO2 to remove ClO. The sum, NOy, of all the reactive nitrogen species (NO, NO2, HNO3, C1ONO2, 2N205, HO2NO2, etc.) is nearly conserved in the lower stratosphere (in the absence of denitrification). As a result, it is usefullyemployed in correlations withother tracers. Pairs of long-lived species are very strongly correlated, and deviations from the correlations may be used to infer the occurrence of unexpected (and relatively fast) chemical processes [e.g., Fahey et al., 1990b; Proffitt et al., 1992]. Furthermore, fitsderived from scatter plots of pairsof long-lived species reflectthe species lifetimes and vertical fluxes [Plumb and Ko, 1992]. An advantage to considering ratios and correlations is that, in a Copyright 1993 bythe American Geophysical Union. Paper number 93GL02627 0094-8534/93/93 GL-02627503.00 region of very strong altitude gradients suchas the lower stratosphere, ratios andcorrelations arefar lesssensitive to changes in altitude, and this facilitates thecomparison of a set of measurements either with models or with other sets of measurements. In this paper wedescribe thegeneral features of the NOy measurements obtainedon the DC-8 during AASE II, and relate them to previous measurements, especially those from the first AASEmission (1989). Thecorrelations of NOywith 03, N20, and CH4 are shown; other 03 correlations are presented in a companion paper [Collinset al., 1993]. The data in this paper arefrom 15 AASE !I flights overnorthern mid-,subpolar, and polarlatitudes, including a 5-flightseries in each of January, February, and March of 1992, with gaps of 2-3 weeks in between. These flights cover latitudes of 40- 90øN and longitudes of 150øW-10øE [Anderson and Toon, 1993]. Emphasis is on stratospheric measurements obtained at cruise pressure altitudes of 10 to !2 kin. In Situ Techniques N Oy is measured via catalytic conversion of its component species to NO whichin turnis detected by means of its chemiluminescentreaction with reagent 03 that is added to the airstream [Walegaet al., 1991 ]. 03 is measured via the same chemiluminescent reaction, but produced by the addition of reagent NO to the airstream [Ridley et al., 1992]. NOy and 03 aresampled through a common inlet,which faces aft to discriminate against the intake of large particles. For 2-s 03 values,the precisionis better than 1%, and the overall uncertainty is about 5%. For 2-s NOy values of 1000 pptvand greater, theprecision (lB) is better than 2%, andthe overall uncertainty is betterthan 10%. This latteruncertainty excludes errors associatedwith aerosols. However, based on measured temperatures and measured NOy, nitric acid trihydrate PSC particles should not be present for the flights reported here. N20 and CH4 are measured with a tunable diode laser described in Collins et al. [1993]. Precisions (213) are 2 and 7 ppbv, respectively, for the 5-s data, and accuracies are 2%. The N20 and CH4 data have been shiftedby 25 s relative to the NOy and 03 toadjust fordelays inthe flowof sampled air through the entire TDL system. Data are then averaged ovei' 1-minute intervals for the present analysis. NOyversus 03 N Oy and 03 are strongly correlated in the lower stratosphere [Proffittet al., 1989; Kondoet al., 1990;Murphy et al., 1993]. Somedegreeof correlation is expected, based on the fact that the same high-ultraviolet-radiation environment that is responsible for theproduction of NOy, namely the tropical middle-to-upperstratosphere, is also 2563