Since the 1980s nitric acid (HNO3)-containing polar stratospheric cloud (PSC) particles have been recognized to play a key role in the wintertime ozone (O3) destruction of the Antarctic and Arctic stratospheres [Peter, 1997]. First, PSC cloud particles provide sites for the heterogeneous reactions that convert inactive chlorine to active chlorine [Solomon et al., 1986]. Second, the sedimentation of HNO3-containing particles irreversibly removes reactive nitrogen in the form of HNO3 from air parcels in a process known as denitrification [Toon et al., 1986]. In a denitrified atmosphere, chlorine remains in its active forms longer and causes more ozone destruction. Although denitrification has been observed extensively in both polar vortices using both in situ [Fahey et al., 1990; Popp et al., 2001] and remote [Santee et al., 1995; Dessler et al., 1999; Kondo et al., 2000; Kleinböhl et al., 2002] techniques, important details of the process are still not well understood. In particular, our ability to predict denitrification accurately in ozone loss models has long been impeded by unanswered questions about PSC composition, nucleation mechanisms, and size.
 PSC composition is categorized as type I (containing HNO3) or type II (containing primarily water ice) [Poole and McCormick, 1988]. Laboratory data have identified three compositions for type I PSCs: supercooled ternary solution (STS) containing HNO3, sulfuric acid (H2SO4), and water (H2O) [Molina et al., 1993; Zhang et al., 1993]; solid nitric acid trihydrate (HNO3·3H2O) (NAT) [Hanson and Mauersberger, 1988]; and solid nitric acid dihydrate (HNO3·2H2O) (NAD) [Worsnop et al., 1993]. However, defining the roles of these PSC types in and near the edges of the polar vortices has been limited by a lack of appropriate observations. Only recently have in situ observations confirmed the composition of liquid [Schreiner et al., 1999] and solid [Voigt et al., 2000] nitric acid phases in PSCs. In most observations of type I PSCs, particles have relatively small sizes (<2 μm diameter) and large total number densities (>1 cm−3). Modeling studies have identified many of these high number density populations as STS particles [Carslaw et al., 1994; Drdla et al., 1994; Del Negro et al., 1997].
 A principle factor in understanding denitrification is the nucleation mechanism for each PSC phase. STS particles grow by condensation of HNO3 and H2O on background stratospheric nuclei (1–10 cm−3) [Wilson et al., 1990; Deshler and Oltmans, 1998], as ambient temperatures decrease below 200 K [Carslaw et al., 1997]. This process is not impeded by a nucleation barrier, so all nuclei generally activate. Particle sizes are limited to less than about 2 μm diameter by available HNO3 and H2O. Therefore STS particles have low fall velocities and are not effective at denitrifying the stratosphere [Tabazadeh et al., 2000]. Conversely, the nucleation mechanisms for NAT and NAD are not well understood [Peter, 1997; Zondlo et al., 2000]. Laboratory studies have shown that both NAD and NAT formation have nucleation barriers [Worsnop et al., 1993]. A number of studies have proposed that populations of large (>2 μm) NAT or NAD particles would have strong potential to denitrify parcels on the timescale of days [Salawitch et al., 1989; Toon et al., 1990; Waibel et al., 1999]. Because of the limited amount of HNO3 in the stratosphere, a selective nucleation mechanism is typically invoked to explain the limitation of particle concentrations to much less than the background nuclei concentrations [World Meteorological Organization (WMO), 1999]. Several theories such as the slow freezing of background particles to form nitric acid hydrates [Koop et al., 1995; Biermann et al., 1996; Salcedo et al., 2001; Tabazadeh et al., 2001] or special nuclei [Tolbert and Toon, 2001] have been proposed to explain the selective nucleation process. It has also been suggested that lee-wave clouds play a role in the formation of denitrifying NAT particles [Carslaw et al., 1998; Dhaniyala et al., 2002]. In situ measurements of large particles are critical in order to constrain the different nucleation processes.
 In the past, low number density populations of large HNO3-containing particles in the Arctic or Antarctic may have eluded detection due to measurement and interpretation limitations. The few particle instruments that have sampled polar stratospheric air generally have limits of detection near 10−3 cm−3. The meteorological conditions in some winters, especially in the Arctic, may have been such that large particles simply did not form. Some populations of large particles with low number densities have been measured in the polar stratospheres, particularly during the Arctic winter of 1989 [Dye et al., 1990; Wofsy et al., 1990; Hofmann and Deshler, 1991; Goodman et al., 1997]. However, without simultaneous measurements of composition in most cases, these particles were assumed to be ice or NAT-covered ice. This led to the speculation that denitrification could occur as ice particles fell through NAT-saturated regions of the stratosphere and acquired a layer of NAT through condensation [Wofsy et al., 1990]. Based on the results presented here, particles observed in these earlier studies may indeed have included large NAT particles.
 The observations presented here derive from the 1999/2000 joint Sage III Ozone Loss and Validation Experiment (SOLVE)/Third European Stratospheric Experiment on Ozone (THESEO 2000). Extensive measurements of large (>2 μm) HNO3-containing particles were made during SOLVE/THESEO 2000 with three instruments on board the NASA ER-2 [Fahey et al., 2001]. The formation and sedimentation rates of these large particles suggest a viable mechanism for the observed severe and widespread denitrification of the 1999/2000 Arctic winter [Popp et al., 2001]. Extensive ozone loss was also observed during the SOLVE/THESEO 2000 campaign [Sinnhuber et al., 2000; Richard et al., 2001]. The combined measurements of large HNO3-containing particles, widespread irreversible denitrification, and ozone loss during the SOLVE/THESEO 2000 campaign offer a unique opportunity to answer remaining questions about PSC nucleation and composition, denitrification, and the role of PSCs in chemical ozone loss [Gao et al., 2001].
 A preliminary analysis of large particle observations in SOLVE/THESEO 2000 has been presented by Fahey et al.,  using two NOy (NOy = HNO3 + NO + NO2 + N2O5 + ClONO2 + …) data intervals taken during the flight of 20 January in the polar vortex. Assuming NAT composition, this study estimated particle sizes up to 20 μm diameter. Although the total number concentration is low (near 10−4 cm−3), the calculated instantaneous HNO3 flux values associated with the largest particles are sufficient to cause appreciable denitrification of the lower stratosphere if sustained for a matter of days. An atmospheric model with microphysical processes was used to simulate the growth, sedimentation, and advection of NAT particles with sizes up to 20 μm. Growth times for 2–20 μm diameter NAT particles from nucleation were estimated to be from less than a day up to 6 days. In a further study, particle nucleation process were constrained by comparing results obtained using different nucleation mechanisms with large particle observations [Carslaw et al., 2002]. Results indicate that synoptic-scale ice saturation conditions do not likely play a role in the highly selective nucleation process of large NAT or NAD particle formation.
 In this study the discussion of large HNO3-containing particles is expanded to include all of the 1999/2000 Arctic winter data set. The primary objectives of this paper are (1) to describe the sampling and detection of large HNO3-containing particles using an NOy detector with two heated inlets; (2) to provide diagnostics for the analysis of large particle data; and (3) to characterize all the observations of large particles in the Arctic 1999/2000 winter stratosphere. A combination of analytical methods is used to determine sizes and concentrations for the wide range of large particle populations that was observed during the SOLVE/THESEO 2000 mission. These methods are used to demonstrate that large particles with sizes up to 20 μm diameter were present on several days over a 48-day observation period, over large areas inside the vortex, and over a considerable range of altitudes. The observations will be discussed in the context of the vortex meteorological fields and areas of NAT saturation.
 The study begins in section 2 with a description of the NOy instrument and its aerosol sampling characteristics. Section 3 presents the large particle observations taken with the NOy instrument during the 1999/2000 Arctic winter. Section 4 presents a diagnostic for interpreting particle sizes and number densities from NOy time series.