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.

A novel inlet has been designed for selective sampling of gas and aerosol phases of volatile species from high-speed aircraft. A multistage flow system brings the flow nearly isokinetically towards the sampling port. Two small airfoil-shaped "blades" are placed close to the sample port to provide the flow conditions required for aerosol and gas sampling. Aerosols are sampled when these blades are positioned to operate the inlet as a counterflow virtual impactor (CVI). The design enables sampling of particles as small as 0.1 w m from a high-speed aircraft under stratospheric conditions, a substantial improvement over that possible with previous CVI designs. For gas sampling, one of the blades is moved by a stepper motor to occlude the inlet opening and gas is sampled perpendicular to the bulk flow. Boundary layer suction is used to prevent the sampled gas from coming in contact with the impactor walls. This is one of the first designs of an inlet that enables gas sampling free of wall contact. The inlet was flown on the NASA ER-2 aircraft during the SOLVE 2000 campaign to study aerosol/gas partitioning of nitric acid in the lower stratosphere. Data from the flight tests show that the inlet flow characteristics are broadly in agreement with computational fluid dynamics (CFD) simulations.

In this study, several mechanistic models for particle removal from substrates were introduced and their predictive ability was tested. Empirically optimized parameters for a selected set of particle-substrate combinations were obtained and the ability of these models to predict resuspension for a large number of particle-substrate combinations and flow conditions using the published experimental data was explored. Our analysis showed that accurate predictions for a broad spectrum of particle-substrate combinations require accounting for substrate roughness and particle non-sphericity. From analysis of our compiled experimental data set, it was determined that the shear velocities required for particle resuspension follow a log-normal distribution, with geometric standard deviations in the range of 1.4 to 1.7. From comparison of the calculated and measured critical shear velocities, we found that a critical roughness parameter value of Δ_c = 0.774 is a reasonable estimate for most real substrates. We show that the combination of parameters determined from our study and models that account for roughness and particle non-sphericity, it is possible to predict particle resuspension from most substrates with reasonable accuracy.

The resuspension rate of particles from surfaces is significantly dependent on surface roughness. Existing models only consider the case of surface asperities much smaller than particle diameter, but often particle sizes of interest are comparable or smaller than substrate roughness. In this study, a model for particle removal from large-scale roughness is described. Substrate roughness is described using a 2D sinusoidal profile. The critical shear velocity is determined considering rolling detachment of particles with JKR adhesion model and hydrodynamic moments on particles residing on the rough substrates. Our model results show that the effect of surface roughness on critical shear velocity is strongly dependent on the value of roughness amplitude and wavelength relative to particle size. The model predicts four primary removal regions based on relative particle-surface characteristics: easier-to-remove than smooth substrate, more difficult to remove than smooth substrate, not possible to remove, and similar behavior as smooth surface. The model performance is validated by comparison of predictions with published experimental results for highly rough substrates.

Design of a new submicron aerosol inlet (SMAI) for airborne sampling of aerosol particles is introduced and its performance characteristics under a range of sampling conditions are presented. Analysis of inlet performance in clear-air and cloud systems shows that submicron aerosols are sampled representatively by the inlet, and in comparison with other types of inlets the SMAI has a relatively minor or nonexistent problem of droplet shatter contamination. The SMAI has a flow-through cone, with a perpendicular subsampling tube inside it. The cone acts as a virtual blunt body and decelerates the velocity directed toward a subsampling tube within the cone, resulting in reduced droplet impaction velocities and negligible artifact particle generation. The use of a perpendicular subsampling tube helps eliminate large shattered droplets from entering the sample volume, though it also results in lowering the aerosol sampling cut size. The SMAI sampling characteristics are determined from computational flu... AbstractDesign of a new submicron aerosol inlet (SMAI) for airborne sampling of aerosol particles is introduced and its performance characteristics under a range of sampling conditions are presented. Analysis of inlet performance in clear-air and cloud systems shows that submicron aerosols are sampled representatively by the inlet, and in comparison with other types of inlets the SMAI has a relatively minor or nonexistent problem of droplet shatter contamination. The SMAI has a flow-through cone, with a perpendicular subsampling tube inside it. The cone acts as a virtual blunt body and decelerates the velocity directed toward a subsampling tube within the cone, resulting in reduced droplet impaction velocities and negligible artifact particle generation. The use of a perpendicular subsampling tube helps eliminate large shattered droplets from entering the sample volume, though it also results in lowering the aerosol sampling cut size. The SMAI sampling characteristics are determined from computational flu...

There is limited understanding of the role of aerosols in the formation and modification of clouds, partly due to inadequate data on such systems. Aircraft-based aerosol measurements in the presence of cloud particles have proven to be challenging because of the problem of cloud droplet/ice particle shatter and the generation of secondary artifact particles that contaminate aerosol samples. Recently, the design of a new aircraft inlet, called the Blunt-body Aerosol Sampler (BASE), which enables sampling of interstitial aerosol particles, was introduced. Numerical modeling results and laboratory test data suggested that the BASE inlet should sample interstitial particles with minimal shatter particle contamination. Here, the sampling performance of the inlet is established from aircraft-based measurements. Initial aircraft test results obtained during the PLOWS (Profiling of Winter Storms) campaign indicated two problems with the original BASE design: separated flows around the BASE at high altitudes and a significant shatter problem when sampling in drizzle. The test data were used to improve the accuracy of flow and particle trajectory modeling around the inlet, and the results from the improved flow model were used to guide design modifications of the BASE to overcome the problems identified in its initial deployment. The performance of the modified BASE was tested during the ICE–T (Ice in Clouds Experiment – Tropics) campaign, and the inlet was seen to provide near shatter-free measurements in a wide range of cloud conditions. The initial aircraft test results, design modifications, and the performance characteristics of the BASE relative to another interstitial inlet, the submicron aerosol inlet (SMAI), are presented.