Nitric acid concentrations near the tropical tropopause: Implications for the properties of tropical nitric acid trihydrate clouds

Jensen, E. J.; Drdla, K.

doi:10.1029/2002gl015190

Cited by 20 publications

(22 citation statements)

References 20 publications

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“…They concluded that a subset of the clouds that were depolarizing and had color ratios significantly less than unity were possibly Nitric Acid Trihydrate (NAT) particles. This conclusion is implausible because the nitric acid concentration in the TTL is far too low to produce NAT clouds detectable by CALIPSO (Jensen and Drdla, 2002;Poole et al, 2009). What the CALIOP measurements do suggest is that some fraction of TTL clouds are composed of small (a few microns or less), nonspherical particles.…”

Section: Ice Crystal Concentrationsmentioning

confidence: 75%

Ice nucleation and cloud microphysical properties in tropical tropopause layer cirrus

et al. 2010

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Abstract.In past modeling studies, it has generally been assumed that the predominant mechanism for nucleation of ice in the uppermost troposphere is homogeneous freezing of aqueous aerosols. However, recent in situ and remotesensing measurements of the properties of cirrus clouds at very low temperatures in the tropical tropopause layer (TTL) are broadly inconsistent with theoretial predictions based on the homogeneous freezing assumption. The nearly ubiquitous occurence of gravity waves in the TTL makes the predictions from homogeneous nucleation theory particularly difficult to reconcile with measurements. These measured properties include ice number concentrations, which are much lower than theory predicts; ice crystal size distributions, which are much broader than theory predicts; and cloud extinctions, which are much lower than theory predicts. Although other explanations are possible, one way to limit ice concentrations is to have on the order of 50 L −1 effective ice nuclei (IN) that could nucleate ice at relatively low supersaturations. We suggest that ammonium sulfate particles, which would be dry much of the time in the cold TTL, are a potential IN candidate for TTL cirrus. However, this mechanism remains to be fully quantified for the size distribution of ammonium sulfate (possibly internally mixed with organics) actually present in the upper troposphere. Possible implications of the observed cloud microphysical properties for ice sedimentation, dehydration, and cloud persistence are also discussed.

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Section: Ice Crystal Concentrationsmentioning

confidence: 75%

Ice nucleation and cloud microphysical properties in tropical tropopause layer cirrus

et al. 2010

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“…Thus, for an airmass to form an amount of NAT corresponding to 0.13-2.3 ppbv, it would require an unexplained abnormally high HNO 3 abundance during the nucleation and growth of NAT particles. Jensen and Drdla (2002) have reported localized regions with higher concentration of HNO 3 at the tropical tropopause. It cannot be ruled out that the HNO 3 concentration may be enhanced sufficiently to produce NAT above convective areas, due to flash induced NO x formation in convective systems, but it is not necessary to assume that the particles were composed of NAT in order to explain their occurrence.…”

Section: Number Density and Compositionmentioning

confidence: 99%

Solid particles in the tropical lowest stratosphere

et al. 2007

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Abstract. We report in situ and remote observations proving occasional occurrence of solid particles in the tropical lowest stratosphere, 200 km from deep convective events. The particles were found during field campaigns in Southeast Brazil (49.03 W 22.36 S). They occur in the altitude range from 17.5 to 20.8 km, at temperatures up to at least 10 K above the expected frost point temperature. While stability of ice particles at these altitudes is unexpected from a theoretical point of view, it is argued that these observations are indications of tropospheric air masses penetrating into the stratosphere during convective overshoots. It is argued that the intrusion of tropospheric air must have carried a large amount of water with it, which effectively hydrated the lowest stratosphere, and consequently suppressed sublimation. This conclusion is further supported by a separate water vapor mixing ratio profile obtained at the same observation site.

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“…However, from the CVI-TDL (Counterflow Virtual-Impactor -Tunable Diode Laser, detection limit ≈0.2 ppb) measurements onboard Geophysica during the APE-THESEO campaign there is no evidence for HNO 3 in the particles (see Luo et al, 2003). The measurements made on ER-2 flights in the tropics show a HNO 3 mixing ratio typically 0.1-0.5 ppbv (Jensen and Drdla, 2002), which is too low to explain either the HALOE extinctions (Jensen and Drdla, 2002) or the UTTCs. Furthermore, a detailed investigation of the UTTC lidar data reveals that evidence for a distinct difference between ice clouds and HNO 3 -hydrate clouds, as known from lidar measurements in the polar regions (Type-1 versus Type-2 polar stratospheric clouds), is missing.…”

Section: The Uttc Paradoxmentioning

confidence: 99%

Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms

Luo¹,

Peter²,

Wernli³

et al. 2003

Atmos. Chem. Phys.

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Abstract. Mechanisms by which subvisible cirrus clouds (SVCs) might contribute to dehydration close to the tropical tropopause are not well understood. Recently Ultrathin Tropical Tropopause Clouds (UTTCs) with optical depths around 10 −4 have been detected in the western Indian ocean. These clouds cover thousands of square kilometers as 200-300 m thick distinct and homogeneous layer just below the tropical tropopause. In their condensed phase UTTCs contain only 1-5% of the total water, and essentially no nitric acid. A new cloud stabilization mechanism is required to explain this small fraction of the condensed water content in the clouds and their small vertical thickness. This work suggests a mechanism, which forces the particles into a thin layer, based on upwelling of the air of some mm/s to balance the ice particles, supersaturation with respect to ice above and subsaturation below the UTTC. In situ measurements suggest that Correspondence to: B. P. Luo (Beiping.Luo@ethz.ch) these requirements are fulfilled. The basic physical properties of this mechanism are explored by means of a single particle model. Comprehensive 1-D cloud simulations demonstrate this stabilization mechanism to be robust against rapid temperature fluctuations of ±0.5 K. However, rapid warming ( T > 2 K) leads to evaporation of the UTTC, while rapid cooling ( T < 2 K) leads to destabilization of the particles with the potential for significant dehydration below the cloud.

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Nitric acid concentrations near the tropical tropopause: Implications for the properties of tropical nitric acid trihydrate clouds

Cited by 20 publications

References 20 publications

Ice nucleation and cloud microphysical properties in tropical tropopause layer cirrus

Ice nucleation and cloud microphysical properties in tropical tropopause layer cirrus

Solid particles in the tropical lowest stratosphere

Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms

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