Tropospheric water vapor profiles obtained with FTIR: comparison with balloon-borne frost point hygrometers and influence on trace gas retrievals

Retrievals of vertical profiles of key atmospheric gases provide a critical long-term record from ground-based Fourier transform infrared (FTIR) solar absorption measurements. However, the characterization of the retrieved vertical profile structure can be difficult to validate, especially for gases with large vertical gradients and spatial&ndash;temporal variability such as water vapor. In this work, we evaluate the accuracy of the most common water vapor isotope (H162O, hereafter WV) FTIR retrievals in the lower and upper troposphere&ndash;lower stratosphere. Coincident high-quality vertically resolved WV profile measurements obtained from 2010 to 2016 with balloon-borne NOAA frost point hygrometers (FPHs) are used as reference to evaluate the performance of the retrieved profiles at two sites: Boulder (BLD), Colorado, and at the mountaintop observatory of Mauna Loa (MLO), Hawaii. For a meaningful comparison, the spatial&ndash;temporal variability has been investigated. We present results of comparisons among FTIR retrievals with unsmoothed and smoothed FPH profiles to assess WV vertical gradients. Additionally, we evaluate the quantitative impact of different a priori profiles in the retrieval of WV. An orthogonal linear regression analysis shows the best correlation among tropospheric layers using ERA-Interim (ERA-I) a priori profiles and biases are lower for unsmoothed comparisons. In Boulder, we found a negative bias of 0.02&plusmn;1.9&thinsp;% (r=0.95) for the 1.5&ndash;3&thinsp;km layer. A larger negative bias of 11.1&plusmn;3.5&thinsp;% (r=0.97) was found in the lower free troposphere layer of 3&ndash;5&thinsp;km attributed to rapid vertical change of WV, which is not always captured by the retrievals. The bias improves in the 5&ndash;7.5&thinsp;km layer (1.0&plusmn;5.3&thinsp;%, r=0.94). The bias remains at about 13&thinsp;% for layers above 7.5&thinsp;km but below 13.5&thinsp;km. At MLO the spatial mismatch is significantly larger due to the launch of the sonde being farther from the FTIR location. Nevertheless, we estimate a negative bias of 5.9&plusmn;4.6&thinsp;% (r=0.93) for the 3.5&ndash;5.5&thinsp;km layer and 9.9&plusmn;3.7&thinsp;% (r=0.93) for the 5.5&ndash;7.5&thinsp;km layer, and we measure positive biases of 6.2&plusmn;3.6&thinsp;% (r=0.95) for the 7.5&ndash;10&thinsp;km layer and 12.6&thinsp;% and greater values above 10&thinsp;km. The agreement for the first layer is significantly better at BLD because the air masses are similar for both FTIR and FPH. Furthermore, for the first time we study the influence of different WV a priori profiles in the retrieval of selected gas profiles. Using NDACC standard retrievals we present results for hydrogen cyanide (HCN), carbon monoxide (CO), and ethane (C2H6) by taking NOAA FPH profiles as the ground truth and evaluating the impact of other WV profiles. We show that the effect is minor for C2H6 (bias &lt;0.5&thinsp;% for all WV sources) among all vertical layers. However, for HCN we found significant biases between 6&thinsp;% for layers close to the surface and 2&thinsp;% for the upper troposphere depending on the WV profile source. The best results (reduced bias and precision and r values closer to unity) are always found for pre-retrieved WV. Therefore, we recommend first retrieving WV to use in subsequent retrieval of gases.</p

HOVERCAT: a novel aerial system for evaluation of aerosol-cloud interactions

Aerosols have a profound impact on cloud microphysics through their ability to serve as ice nucleating particles (INPs). As a result, cloud radiative properties and precipitation processes can be modulated by such aerosol&ndash;cloud interactions. However, one of the largest uncertainties associated with atmospheric processes is the indirect effect of aerosols on clouds. The need for more advanced observations of INPs in the atmospheric vertical profile is apparent, yet most ice nucleation measurements are conducted on the ground or during infrequent and intensive airborne field campaigns. Here, we describe a novel measurement platform that is less expensive and smaller (&lt;&thinsp;5&thinsp;kg) when compared to traditional aircraft and tethered balloon platforms and that can be used for evaluating two modes of ice nucleation (i.e., immersion and deposition). HOVERCAT (Honing On VERtical Cloud and Aerosol properTies) flew during a pilot study in Colorado, USA, up to 2.6&thinsp;km above mean sea level (1.1&thinsp;km above ground level) and consists of an aerosol module that includes an optical particle counter for size distributions (0.38&ndash;17&thinsp; &micro;m in diameter) and a new sampler that collects up to 10 filter samples for offline ice nucleation and aerosol analyses on a launched balloon platform. During the May 2017 test flight, total particle concentrations were highest closest to the ground (up to 50&thinsp;cm&minus;3 at &lt;&thinsp;50&thinsp;m above ground level) and up to 2 in 10 2 particles were ice nucleation active in the immersion mode (at &minus;23&thinsp;∘C). The warmest temperature immersion and deposition mode INPs (observed up to &minus;6 and &minus;40.4&thinsp;∘C, respectively) were observed closest to the ground, but overall INP concentrations did not exhibit an inverse correlation with increasing altitude. HOVERCAT is a prototype that can be further modified for other airborne platforms, including tethered balloon and unmanned aircraft systems. The versatility of HOVERCAT affords future opportunities to profile the atmospheric column for more comprehensive evaluations of aerosol&ndash;cloud interactions. Based on our test flight experiences, we provide a set of recommendations for future deployments of similar measurement systems and platforms.</p

UAS Chromatograph for Atmospheric Trace Species (UCATS) – a versatile instrument for trace gas measurements on airborne platforms

UCATS (the UAS Chromatograph for Atmospheric Trace Species) was designed and built for observations of important atmospheric trace gases from unmanned aircraft systems (UAS) in the upper troposphere and lower stratosphere (UTLS). Initially it measured major chlorofluorocarbons (CFCs) and the stratospheric transport tracers nitrous oxide (N2O) and sulfur hexafluoride (SF6), using gas chromatography with electron capture detection. Compact commercial absorption spectrometers for ozone (O3) and water vapor (H2O) were added to enhance its capabilities on platforms with relatively small payloads. UCATS has since been reconfigured to measure methane (CH4), carbon monoxide (CO), and molecular hydrogen (H2) instead of CFCs and has undergone numerous upgrades to its subsystems. It has served as part of large payloads on stratospheric UAS missions to probe the tropical tropopause region and transport of air into the stratosphere; in piloted aircraft studies of greenhouse gases, transport, and chemistry in the troposphere; and in 2021 is scheduled to return to the study of stratospheric ozone and halogen compounds, one of its original goals. Each deployment brought different challenges, which were largely met or resolved. The design, capabilities, modifications, and some results from UCATS are shown and described here, including changes for future missions.Support was provided for HIPPO by NSF award no. AGS-0628452, for ATTREX by NASA Earth Venture program award no. NNA11AA55I, and for ATom by NASA award no. NNH17AE26I; additional support was provided by NASA Upper Atmosphere Research Program award no. NNH13AV69I. This work was also supported in part by the NOAA Cooperative Agreement with CIRES, NA17OAR4320101

Controlled weather balloon ascents and descents for atmospheric research and climate monitoring

In situ upper-air measurements are often made with instruments attached to weather balloons launched at the surface and lifted into the stratosphere. Present-day balloon-borne sensors allow near-continuous measurements from the Earth's surface to about 35km (3–5hPa), where the balloons burst and their instrument payloads descend with parachutes. It has been demonstrated that ascending weather balloons can perturb the air measured by very sensitive humidity and temperature sensors trailing behind them, particularly in the upper troposphere and lower stratosphere (UTLS). The use of controlled balloon descent for such measurements has therefore been investigated and is described here. We distinguish between the single balloon technique that uses a simple automatic valve system to release helium from the balloon at a preset ambient pressure, and the double balloon technique that uses a carrier balloon to lift the payload and a parachute balloon to control the descent of instruments after the carrier balloon is released at preset altitude. The automatic valve technique has been used for several decades for water vapor soundings with frost point hygrometers, whereas the double balloon technique has recently been re-established and deployed to measure radiation and temperature profiles through the atmosphere. Double balloon soundings also strongly reduce pendulum motion of the payload, stabilizing radiation instruments during ascent. We present the flight characteristics of these two ballooning techniques and compare the quality of temperature and humidity measurements made during ascent and descent.ISSN:1867-1381ISSN:1867-854