Lee-wave clouds and denitrification of the polar stratosphere

We present a hypothesis that the known formation of nitric acid hydrates in lee-wave ice clouds produces a “standing crop” of nitric acid trihydrate (NAT) particles, and that growth and sedimentation of these particles efficiently denitrifies the polar stratosphere. Simulations using a simple two-dimensional model illustrate that the large NAT number concentrations produced in lee waves (>0.1 cm^(−3)) lead to low gas-phase HNO_3 concentration in the cloud layer, limiting subsequent particle growth. Provided the NAT existence temperature is not exceeded, these particles slowly sediment out of the cloud layer, at which point further growth is initiated. As a result of the long residence time and vertical wind shear, NAT particles produced in geographically selective regions over a short duration can cause extensive denitrification throughout the lower stratosphere. Our model illustrates that a single hypothetical lee-wave nucleation event [10 hours, 1000 km width (along a longitude) × 100 m altitude] is sufficient to produce significant denitrification (∼25%) vortex-wide

Particle nucleation in a forested environment

Peer reviewe

Observations of bromine monoxide transport in the Arctic sustained on aerosol particles

The return of sunlight in the polar spring leads to the production of reactive halogen species from the surface snowpack, significantly altering the chemical composition of the Arctic near-surface atmosphere and the fate of long-range transported pollutants, including mercury. Recent work has shown the initial production of reactive bromine at the Arctic surface snowpack; however, we have limited knowledge of the vertical extent of this chemistry, as well as the lifetime and possible transport of reactive bromine aloft. Here, we present bromine monoxide (BrO) and aerosol particle measurements obtained during the March 2012 BRomine Ozone Mercury EXperiment (BROMEX) near Utqiaġvik (Barrow), AK. The airborne differential optical absorption spectroscopy (DOAS) measurements provided an unprecedented level of spatial resolution, over 2 orders of magnitude greater than satellite observations and with vertical resolution unable to be achieved by satellite methods, for BrO in the Arctic. This novel method provided quantitative identification of a BrO plume, between 500 m and 1 km aloft, moving at the speed of the air mass. Concurrent aerosol particle measurements suggest that this lofted reactive bromine plume was transported and maintained at elevated levels through heterogeneous reactions on colocated supermicron aerosol particles, independent of surface snowpack bromine chemistry. This chemical transport mechanism explains the large spatial extents often observed for reactive bromine chemistry, which impacts atmospheric composition and pollutant fate across the Arctic region, beyond areas of initial snowpack halogen production. The possibility of BrO enhancements disconnected from the surface potentially contributes to sustaining BrO in the free troposphere and must also be considered in the interpretation of satellite BrO column observations, particularly in the context of the rapidly changing Arctic sea ice and snowpack

COSPAR Sample Safety Assessment Framework (SSAF)

The Committee on Space Research (COSPAR) Sample Safety Assessment Framework (SSAF) has been developed by a COSPAR appointed Working Group. The objective of the sample safety assessment would be to evaluate whether samples returned from Mars could be harmful for Earth's systems (e.g., environment, biosphere, geochemical cycles). During the Working Group's deliberations, it became clear that a comprehensive assessment to predict the effects of introducing life in new environments or ecologies is difficult and practically impossible, even for terrestrial life and certainly more so for unknown extraterrestrial life. To manage expectations, the scope of the SSAF was adjusted to evaluate only whether the presence of martian life can be excluded in samples returned from Mars. If the presence of martian life cannot be excluded, a Hold & Critical Review must be established to evaluate the risk management measures and decide on the next steps. The SSAF starts from a positive hypothesis (there is martian life in the samples), which is complementary to the null-hypothesis (there is no martian life in the samples) typically used for science. Testing the positive hypothesis includes four elements: (1) Bayesian statistics, (2) subsampling strategy, (3) test sequence, and (4) decision criteria. The test sequence capability covers self-replicating and non-self-replicating biology and biologically active molecules. Most of the investigations associated with the SSAF would need to be carried out within biological containment. The SSAF is described in sufficient detail to support planning activities for a Sample Receiving Facility (SRF) and for preparing science announcements, while at the same time acknowledging that further work is required before a detailed Sample Safety Assessment Protocol (SSAP) can be developed. The three major open issues to be addressed to optimize and implement the SSAF are (1) setting a value for the level of assurance to effectively exclude the presence of martian life in the samples, (2) carrying out an analogue test program, and (3) acquiring relevant contamination knowledge from all Mars Sample Return (MSR) flight and ground elements. Although the SSAF was developed specifically for assessing samples from Mars in the context of the currently planned NASA-ESA MSR Campaign, this framework and the basic safety approach are applicable to any other Mars sample return mission concept, with minor adjustments in the execution part related to the specific nature of the samples to be returned. The SSAF is also considered a sound basis for other COSPAR Planetary Protection Category V, restricted Earth return missions beyond Mars. It is anticipated that the SSAF will be subject to future review by the various MSR stakeholders

Characterizations of Cloud Droplet Shatter Artifacts in Two Airborne Aerosol Inlets

Aircraft-based aerosol sampling in clouds is complicated by the generation of shatter artifact particles from aerodynamic or impaction breakup of cloud droplets and ice particles in and around the aerosol inlet. Aerodynamic breakup occurs when the Weber number of a droplet, which primarily depends on the droplet size and the magnitude of the relative motion of the droplet and the local air mass, exceeds a critical value. Impaction breakup of a droplet occurs when the droplet’s impaction breakup parameter, K, which is a combination ofWeber and Ohnesorge numbers, exceeds a critical value. Considering these two mechanisms, the critical breakup diameters are estimated for two aerosol inlets of different designs—a conventional forward-facing solid diffuser inlet (SDI) and a cross-flow sampling sub-micron aerosol inlet (SMAI). From numerical simulations, it is determined that cloud droplets of all sizes will experience impaction breakup in SDI, while only droplets larger than ~16 μm will experience impaction breakup in SMAI. The relatively better in-cloud sampling performance of SMAI is because of its cone design that slows the flow just upstream of the sample tube. The slowing upstream flow, however, causes aerodynamic breakup of drops larger than ~100μm. The critical breakup diameters determined from analysis of field data largely validate numerical predictions. The cross-flow sampling design of SMAI is seen to ensure that shatter artifacts in the inlet are minimal even when there are a significant number of particles larger that the critical breakup size. The study results, thus, suggest that the SMAI design presents an effective approach to sample interstitial particles from aircraft

Aerosol Filtration with Mobility-Classified Particles: Role of Multiply Charged Particles in Skewing Penetration Measurements

<div><p>Filter media exhibit strong particle-size dependent capture characteristics that have a complicated dependence to filter properties and operating conditions. As simulation-based determination of the filtration characteristics of real media is very challenging, accurate experimental approaches are critically necessary. Conventionally, filter penetration measurements were made using test particles extracted from a differential mobility analyzer (DMA) and measuring the ratio of particle counts downstream of the filter to that upstream. Size-dependent filter penetration can be directly determined from these measurements if the DMA was operated at high resolution and the contribution of multiply charged particles was negligible. The accuracy of this well-established approach has, however, never been fully assessed. In this study, the influence of multiply charged particles to the filter efficiency measurements is determined using a system-modeling approach. The model results suggest that the contribution of multiply charged particles could significantly influence penetration results when the mode size of the test particles entering the DMA or the most penetrating particle size (MPPS) of the test filter are larger than ∼100 nm. Under such conditions, even if multiply charged particles constitute a small fraction of the test aerosol, they can significantly skew filter penetration results. For accurate calculation of filter penetration, two approaches are proposed: an improved filter test protocol and a new multiple-charge correction approach for calculation of size-dependent filter penetration from mobility-based measurements. The validity of the proposed approaches is established with experimental data.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

Climate Literacy—Imperative Competencies for Tomorrow’s Engineers

Engineers must take a leading role in addressing the challenges of mitigating climate change and adapting to the inevitable changes that our world is facing. To improve climate literacy, technical education must include problem formulation and solutions that consider complex interactions between engineered, Earth, and societal systems, including trade-offs among benefits, costs, and risks. Improving engineering students’ climate literacy must also inspire students’ motivation to work toward climate solutions. This paper highlights the content and pedagogical approach used in a class for engineering students that helped contribute to significant gains in engineering students’ climate literacy and critical thinking competencies. A total of 89 students fully participated in a pre/post climate literacy questionnaire over four years of study. As a whole, students demonstrated significant gains in climate-related content knowledge, affect, and behavior. Substantial differences were observed between students in different engineering disciplines and male vs. female students. Assessment of critical thinking showed that students did an excellent job formulating problem statements and solutions in a manner that incorporated a multidimensional systems perspective. These skills are critical for students to address climate change effectively in their eventual professions

Characterization of filter performance under low-pressure operation

<p>Particulate gas filters are a critical element in the purification systems used to ensure defect-free manufacturing in semiconductor industry. In atomic layer deposition (ALD) processes, these filters are typically operated under sub-atmospheric pressure conditions, but their filtration characteristics are, often, only known at atmospheric pressure. In this study, performance of a metal filter that is typically used in low-pressure ALD precursor delivery systems is studied experimentally and theoretically down to 4.5 kPa. The experimental procedure was designed to minimize the presence of multiply charged particles in the test aerosol for different operating pressures and flowrates. The experimental results suggest that most penetrating particle size only slightly varies with pressure, but the shape of the penetration curve and the maximum value of the penetration changes significantly with pressure. The experimental data are used to test predictions of filter performance at low pressures made using classical theory. The comparison results suggest that the combination of classical theory and manufacturer-specified parameters results in large errors in calculated penetration values at low pressures. Accurate predictions are seen to be possible for particle Stokes numbers less than 0.1, when an inhomogenous filtration model is used in combination with effective filter parameters that are obtained from experimental measurements of filter efficiency and pressure drop at atmospheric pressures.</p> <p>Copyright © 2016 American Association for Aerosol Research</p