Abstract. We use a 3-D global chemical transport model (GEOS-Chem) to interpret aircraft observations of nitrate and sulfate partitioning in transpacific dust plumes during the INTEX-B campaign of April-May 2006. The model includes explicit transport of size-resolved mineral dust and its alkalinity, nitrate, and sulfate content. The observations show that particulate nitrate is primarily associated with dust, sulfate is primarily associated with ammonium, and Asian dust remains alkaline across the Pacific. This can be reproduced in the model by using a reactive uptake coefficient for HNO 3 on dust (γ (HNO 3 ) ∼10 −3 ) much lower than commonly assumed in models and possibly reflecting limitation of uptake by dust dissolution. The model overestimates gasphase HNO 3 by a factor of 2-3, typical of previous model studies; we show that this cannot be corrected by uptake on dust. We find that the fraction of aerosol nitrate on dust in the model increases from ∼30% in fresh Asian outflow to 80-90% over the Northeast Pacific, reflecting in part the volatilization of ammonium nitrate and the resulting transfer of nitrate to the dust. Consumption of dust alkalinity by uptake of acid gases in the model is slow relative to the lifetime of dust against deposition, so that dust does not acidify (at least not in the bulk). This limits the potential for dust iron released by acidification to become bio-available upon dust deposition. Observations in INTEX-B show no detectable Correspondence to: T. D. Fairlie (t.d.fairlie@nasa.gov) ozone depletion in Asian dust plumes, consistent with the model. Uptake of HNO 3 by dust, suppressing its recycling to NO x , reduces Asian pollution influence on US surface ozone in the model by 10-15% or up to 1 ppb.

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We use our forward domain filling trajectory model to explore the impact of tropical convection on stratospheric water vapor (H2O) and tropical tropopause layer cloud fraction (TTLCF). Our model results are compared to winter 2008/2009 TTLCF derived from Cloud‐Aerosol Lidar with Orthogonal Polarization and lower stratospheric H2O observations from the Microwave Limb Sounder. Convection alters the in situ water vapor by driving the air toward ice saturation relative humidity. If the air is subsaturated, then convection hydrates the air through the evaporation of ice, but if the air is supersaturated, then convective ice crystals grow and precipitate, dehydrating the air. On average, there are a large number of both hydrating and dehydrating convective events in the upper troposphere, but hydrating events exceed dehydrating events. Explicitly adding convection produces a less than 2% increase in global stratospheric water vapor during the period analyzed here. Tropical tropopause temperature is the primary control of stratospheric water vapor, and unless convection extends above the tropopause, it has little direct impact. Less than 1% of the model parcels encounter convection above the analyzed cold‐point tropopause. Convection, on the other hand, has a large impact on TTLCF. The model TTLCF doubles when convection is included, and this sensitivity has implications for the future climate‐related changes, given that tropical convective frequency and convective altitudes may change.

An examination of 2 yr of Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) lidar observations and CloudSat cloud radar observations shows that ice clouds at temperatures below about 2458C frequently fall below the CloudSat radar's detection threshold yet are readily detectable by the lidar. The CALIPSO ice water content (IWC) detection threshold is about 0.1 versus 5 mg m 23 for CloudSat. This comparison emphasizes the need for developing a lidar-only IWC retrieval method that is reliable for high-altitude ice clouds at these temperatures in this climatically important zone of the upper troposphere. Microphysical measurements from 10 aircraft field programs, spanning latitudes from the Arctic to the tropics and temperatures from 2868 to 08C, are used to develop relationships between the IWC and volume extinction coefficient s in visible wavelengths. Relationships used to derive a radiatively important ice cloud property, the ice effective diameter D e , from s are also developed. Particle size distributions (PSDs) and direct IWC measurements, together with evaluations of the ice particle shapes and comparisons with semidirect extinction measurements, are used in this analysis. Temperaturedependent D e (s) and IWC-s relationships developed empirically facilitate the retrieval of IWC from lidar-derived s and D e values and for comparison with other IWC observations. This suite of empirically derived relationships can be expressed analytically. These relationships can be used to derive IWC and D e from s and are developed for use in climate models to derive s from prognosed values of IWC and specified PSD properties.

Using the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis winds, temperatures, and anvil cloud ice, we use our domain-filling, forward trajectory model combined with a new cloud module to show that convective transport of saturated air and ice to altitudes below the tropopause has a significant impact on stratospheric water vapor and upper tropospheric clouds. We find that including cloud microphysical processes (rather than assuming that parcel water vapor never exceeds saturation) increases the lower stratospheric average H 2 O by 10-20%. Our model-computed cloud fraction shows reasonably good agreement with tropical upper troposphere (TUT) cloud frequency observed by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument in boreal winter with poorer agreement in summer. Our results suggest that over 40% of TUT cirrus is due to convection, and it is the saturated air from convection rather than injected cloud ice that primarily contributes to this increase. Convection can add up to 13% more water to the stratosphere. With just convective hydration (convection adds vapor up to saturation), the global lower stratospheric modeled water vapor is close to Microwave Limb Sounder observations. Adding convectively injected ice increases the modeled water vapor to~8% over observations. Improving the representation of MERRA tropopause temperatures fields reduces stratospheric water vapor by~4%. Trajectory models have demonstrated success at simulating many aspects of stratospheric H 2 O [e.g., Fueglistaler et al., 2005; Schoeberl and Dessler, 2011, hereafter SD11; Schoeberl et al., 2012, hereafter S12; Schoeberl et al., 2013, hereafter S13;Ueyama et al., 2014]. In the SD11 forward domain-filling trajectory model, winds determine the parcel motion and temperature determines the H 2 O content through instant adjustment of the parcel water vapor to not exceed predefined saturation limit. The assumption in this formulation is that any ice that forms quickly falls to lower atmospheric layers. We refer to this adjustment as instantaneous dehydration (ID).Air parcels moving slowly upward across the tropopause will have their water vapor concentration fixed as they pass through the cold point. However, convective systems can bypass the cold point and deposit ice directly into the lower stratosphere. Hydration through convection was parameterized in SD11 through a system described by Dessler et al. [2007]. In that system, parcels coincident with convection were set to the local saturation mixing ratio. Methane oxidation can also add water to the air parcel, but this process is unimportant in the lower tropical stratosphere where methane oxidation rates are slow (SD11).In SD11 and S12, we compared the modeled stratospheric water vapor to Microwave Limb Sounder (MLS) observations. These papers identified a number of processes that controlled stratospheric water vapor. These processes include the following: (1) the level of supersaturation permitted before ID, (2) the level of c...