[1] The validation of version 2.2 (v2.2) H 2 O measurements from the Earth Observing System (EOS) Microwave Limb Sounder (Aura MLS) on the Aura satellite are presented.Results from comparisons made with Aqua Atmospheric Infrared Sounder (AIRS), Vaisala radiosondes, frost point hygrometer, and WB57 aircraft hygrometers are presented. Comparisons with the Aura MLS v1.5 H 2 O, Goddard global modeling and assimilation office Earth Observing System analyses (GEOS-5) are also discussed. For H 2 O mixing ratios less than 500 ppmv, the MLS v2.2 has an accuracy better than 25% between 316 and 147 hPa. The precision is 65% at 316 hPa that reduces to 25% at 147 hPa. This performance is better than expected from MLS measurement systematic error analyses. MLS overestimates H 2 O for mixing ratios greater than 500 ppmv which is consistent with a scaling error in either the calibrated or calculated MLS radiances. The validation of the accuracy of MLS v2.2 H 2 O from 121 to 83 hPa which is expected to be better than 15% cannot be confirmed at this time because of large disagreements among the hygrometers used in the AVE campaigns. The precision of the v2.2 H 2 O from 121 to 83 hPa is 10-20%. The vertical resolution is 1.5-3.5 km depending on height. The horizontal resolution is 210 Â 7 km 2 along and perpendicular to the Aura orbit track, respectively. Relative humidity is calculated from H 2 O and temperature. The precision, accuracy, and spatial resolution are worse than for H 2 O.Citation: Read, W. G., et al. (2007), Aura Microwave Limb Sounder upper tropospheric and lower stratospheric H 2 O and relative humidity with respect to ice validation,

A Single-Particle Soot Photometer (SP2) detects black refractory or elemental carbon (EC) in particles by passing them through an intense laser beam. The laser light heats EC in particles causing them to vaporize in the beam. Detection of wavelength-resolved thermal radiation emissions provides quantitative information on the EC mass of individual particles in the size range of 0.2-1 μm diameter. Non-absorbing particles are sized based on the amount of light they scatter from the laser beam. The time series of the scattering signal of a non-absorbing particle is a Gaussian, because the SP2 laser is in the TEM00 mode. Information on the scattering properties of externally and internally mixed EC particles as detected by the SP2 is lost in general, because each particle changes size, shape, and composition as it passes through the laser beam. Thus, scattered light from a sampled EC particle does not yield a full Gaussian waveform. A method for determining the scattering properties of EC particles using a two-element avalanche photodiode (APD) is described here. In this method, the Gaussian scattering function is constructed from the leading edge of the scattering signal (before the particle is perturbed by the laser), the Gaussian width, and the location of the leading edge in the beam derived from the two-element APD signal. The method allows an SP2 to determine the scattering properties of individual EC particles as well as the EC mass. Detection of polystyrene latex spheres, wellcharacterized EC particles with and without organic coatings, and Mie scattering calculations are used to validate the method.

Particle size and volume measurements obtained with the forward scattering spectrometer probe (FSSP), model 300 during January and February 1989 in the Airborne Arctic Stratospheric Experiment are presented and used to study processes important in the formation and growth of polar stratospheric cloud (PSC) particles. Comparisons of the observations with expected sulfuric acid droplet deliquescence suggest that in the Arctic a major fraction of the sulfuric acid droplets remain liquid until temperatures at least as low as 193 K. Arguments are presented to suggest that homogeneous freezing of the sulfuric acid droplets might occur near 190 K and might play a role in the formation of PSCs. The first suggestion of nitric acid trihydrate (NAT) particles appears near saturation ratios of HNO3 with respect to NAT of 1 (about 195 K) as an enhancement, of the large particles on the tail of the sulfuric acid droplet size distribution. The major increases in number and volume indicative of the main body of the NAT cloud are not seen in these Arctic investigations until 191 to 192 K, which corresponds to an apparent saturation ratio of HNO3 with respect to NAT of about 10, unlike the Antarctic where clouds were encountered at saturation ratios near 1. A decrease in the number of particles was observed in regions in which the airmass was denitrified, i.e. NOy, the sum of all reactive nitrogen species, was reduced. This was especially true for the larger particles on the upper tail of the sulfate size distribution. The loss of these largest particles supports the idea that denitrification may be the result of the preferential nucleation and growth of NAT on only the largest sulfate particles, which then sediment out of the airmass.

Measurements of nitric oxide, NO, and the sum of reactive nitrogen species, NOy, were made as part of the Airborne Antarctic Ozone Experiment (AAOE) on flights of the NASA ER‐2 aircraft over the Antarctic continent. Reactive nitrogen species include NO, NO2, NO3, N2O5, HNO3, and ClONO2. The technique utilized the conversion of NOy components to NO on a gold catalyst and the subsequent detection of NO by the chemiluminescent reaction of NO with O3. NO was measured on two of the flights by removing the catalyst from the sample line. The flights ranged from 53°S to 72°S latitude in the lower stratosphere, with the majority of flights following the 425 K (±10 K) or 450 K (±10 K) potential temperature surfaces. The boundary of a chemically perturbed region (CPR) above the continent occurred on average near 66°S as indicated by a sharp increase in the level of ClO. Outside or equatorward of the CPR, NOy mixing ratios ranged between 6 and 12 parts per billion by volume (ppbv), with values increasing with latitude. At the edge of the CPR, large latitude gradients of NOy and NO were found with values decreasing poleward. Total NOy levels dropped to 4 ppbv or less within 5° poleward of the boundary. NO values were 0.1–0.2 ppbv outside and below the detection limit of 0.03 ppbv inside the CPR. The levels of NO and NOy observed preclude a chemical loss of ozone due to reaction with NO. The NOy values outside the CPR are in accord with the results of two‐dimensional photochemical models that incorporate only homogeneous chemistry when allowance is made for enhanced diabatic descent of air parcels. NO is somewhat lower than the model predictions. Inside the CPR, low NOy values indicate denitrification, defined as the removal of NOy from an air parcel. Low H2O levels, which indicate dehydration, are observed to coincide with denitrification, suggesting that the respective processes are coupled. The partitioning of the remaining NOy inside the CPR likely favors HNO3 and ClONO2. Outside the CPR, the concurrent measurements of ClO, NO, NOy, and O3 along with photochemical steady state relations indicate that NO2 and ClONO2 are minor NOy components. Near the boundary, the variation of NO with ClO is shown to be consistent with heterogeneous reactions of HCl and ClONO2 producing reactive chlorine.