scholarly journals Size-resolved cloud condensation nuclei (CCN) activity and closure analysis at the HKUST Supersite in Hong Kong

2014 ◽  
Vol 14 (18) ◽  
pp. 10267-10282 ◽  
Author(s):  
J. W. Meng ◽  
M. C. Yeung ◽  
Y. J. Li ◽  
B. Y. L. Lee ◽  
C. K. Chan
Keyword(s):  
Particle Size ◽  
Hong Kong ◽  
Atmospheric Aerosols ◽  
Cloud Condensation Nuclei ◽  
Particle Diameter ◽  
Volume Ratio ◽  
Condensation Nuclei ◽  
Mixing State ◽  
Scanning Mobility Particle Sizer ◽  
The Mean

Abstract. The cloud condensation nuclei (CCN) properties of atmospheric aerosols were measured on 1–30 May 2011 at the HKUST (Hong Kong University of Science and Technology) Supersite, a coastal site in Hong Kong. Size-resolved CCN activation curves, the ratio of number concentration of CCN (NCCN) to aerosol concentration (NCN) as a function of particle size, were obtained at supersaturation (SS) = 0.15, 0.35, 0.50, and 0.70% using a DMT (Droplet Measurement Technologies) CCN counter (CCNc) and a TSI scanning mobility particle sizer (SMPS). The mean bulk size-integrated NCCN ranged from ~500 cm−3 at SS = 0.15% to ~2100 cm−3 at SS = 0.70%, and the mean bulk NCCN / NCN ratio ranged from 0.16 at SS = 0.15% to 0.65 at SS = 0.70%. The average critical mobility diameters (D50) at SS = 0.15, 0.35, 0.50, and 0.70% were 116, 67, 56, and 46 nm, respectively. The corresponding average hygroscopic parameters (κCCN) were 0.39, 0.36, 0.31, and 0.28. The decrease in κCCN can be attributed to the increase in organic to inorganic volume ratio as particle size decreases, as measured by an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The κCCN correlates reasonably well with κAMS_SR based on size-resolved AMS measurements: κAMS_SR = κorg × forg + κinorg × finorg, where forg and finorg are the organic and inorganic volume fractions, respectively, κorg = 0.1 and κinorg = 0.6, with a R2 of 0.51. In closure analysis, NCCN was estimated by integrating the measured size-resolved NCN for particles larger than D50 derived from κ assuming internal mixing state. Estimates using κAMS_SR show that the measured and predicted NCCN were generally within 10% of each other at all four SS. The deviation increased to 26% when κAMS was calculated from bulk PM1 AMS measurements of particles because PM1 was dominated by particles of 200 to 500 nm in diameter, which had a larger inorganic fraction than those of D50 (particle diameter < 200 nm). A constant κ = 0.33 (the average value of κAMS_SR over the course of campaign) was found to give an NCCN prediction within 12% of the actual measured values. We also compared NCCN estimates based on the measured average D50 and the average size-resolved CCN activation ratio to examine the relative importance of hygroscopicity and mixing state. NCCN appears to be relatively more sensitive to the mixing state and hygroscopicity at a high SS = 0.70% and a low SS = 0.15%, respectively.

scholarly journals Cloud condensation nuclei (CCN) and HR-ToF-AMS measurements at a coastal site in Hong Kong: size-resolved CCN activity and closure analysis

2014 ◽  
Vol 14 (7) ◽  
pp. 9067-9107 ◽  
Author(s):  
J. W. Meng ◽  
M. C. Yeung ◽  
Y. J. Li ◽  
B. Y. L. Lee ◽  
C. K. Chan
Keyword(s):  
Particle Size ◽  
Hong Kong ◽  
Cloud Condensation Nuclei ◽  
Particle Diameter ◽  
Volume Ratio ◽  
Condensation Nuclei ◽  
Mixing State ◽  
Scanning Mobility Particle Sizer ◽  
Coastal Site ◽  
The Mean

Abstract. The cloud condensation nuclei (CCN) properties of atmospheric aerosols were measured on 1–30 May 2011 at a coastal site in Hong Kong. Size-resolved CCN activation curves, the ratio of number concentration of CCN (NCCN) to aerosol concentration (NCN) as a function of particle size, were obtained at supersaturation (SS) = 0.15%, 0.35%, 0.50%, and 0.70% using a DMT CCN counter (CCNc) and a TSI scanning mobility particle sizer (SMPS). The mean bulk size-integrated NCCN ranged from ∼500 cm−3 at SS = 0.15% to ∼2100 cm−3 at SS = 0.70%, and the mean bulk NCCN / NCN ratio ranged from 0.16 at SS = 0.15% to 0.65 at SS = 0.70%. The average critical mobility diameters (D50) at SS = 0.15%, 0.35%, 0.50%, and 0.70% were 116 nm, 67 nm, 56 nm, and 46 nm, respectively. The corresponding average hygroscopic parameters (κCCN) were 0.39, 0.36, 0.31, and 0.28. The decrease in κCCN can be attributed to the increase in organic to inorganic volume ratio as particle size decreases, as measured by an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The κCCN correlates reasonably well with κAMS based on size-resolved AMS measurements: κAMS = κorg × forg + κinorg × finorg, where forg and finorg are the organic and inorganic volume fractions, respectively, κorg = 0.1 and κinorg = 0.6, with a R2 of 0.51. In closure analysis, NCCN was estimated by integrating the measured size-resolved NCN for particles larger than D50 derived from κ assuming internal mixing state. Estimates using κAMS from size-resolved AMS measurements show that the measured and predicted NCCN were generally within 10% of each other at all four SS. The deviation increased to 26% when κAMS was calculated from bulk PM1 AMS measurements of particles because PM1 was dominated by particles of 200 nm to 500 nm in diameter, which had a larger inorganic fraction than those of D50 (particle diameter < 200 nm). A constant κ = 0.33 (the average value of size-resolved κAMS over the course of campaign) was found to give an NCCN prediction within 12% of the actual measured values. We also compared NCCN estimates based on the measured average D50 and the average size-resolved CCN activation ratio to examine the relative importance of hygroscopicity and mixing state. NCCN was found to be relatively more sensitive to the mixing state and hygroscopicity at a high SS = 0.70% and a low SS = 0.15%, respectively.

scholarly journals Hygroscopic mixing state of urban aerosol derived from size-resolved cloud condensation nuclei measurements during the MEGAPOLI campaign in Paris

2013 ◽  
Vol 13 (1) ◽  
pp. 2035-2075 ◽  
Author(s):  
Z. Jurányi ◽  
T. Tritscher ◽  
M. Gysel ◽  
M. Laborde ◽  
L. Gomes ◽  
...  
Keyword(s):  
Water Uptake ◽  
Cloud Condensation Nuclei ◽  
Pure Water ◽  
Particle Diameter ◽  
Complex Mixture ◽  
Average Particle ◽  
Hygroscopic Growth ◽  
Condensation Nuclei ◽  
Mixing State ◽  
Ambient Aerosols

Abstract. Ambient aerosols are a complex mixture of particles with different physical and chemical properties and consequently distinct hygroscopic behaviour. The hygroscopicity of a particle determines its water uptake at subsaturated relative humidity (RH) and its ability to form a cloud droplet at supersaturated RH. These processes influence the Earth's climate and the atmospheric lifetime of the particles. Cloud condensation nuclei (CCN) number size distributions (i.e. CCN number concentrations as a function of dry particle diameter) were measured close to Paris during the MEGAPOLI campaign in January–February 2010 covering 10 different supersaturations (SS = 0.1–1.0%). The time-resolved hygroscopic mixing state with respect to CCN activation was also derived from these measurements. Simultaneously, a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) was used to measure the hygroscopic growth factor (ratio of wet to dry mobility diameter) distributions at RH = 90%. The aerosol was highly externally mixed and its mixing state showed significant temporal variability. The average particle hygroscopicity was relatively low at subsaturation, RH = 90% (mean hygroscopicity parameter κ = 0.12–0.27) and increased with increasing dry diameter in the range 35–265 nm. The mean κ value, derived from the CCN measurements at supersaturation, ranged from 0.08 to 0.24 at SS = 1.0–0.%. Two types of mixing state resolved hygroscopicity closure studies were performed comparing the water uptake ability measured below and above saturation. In the first type the CCN counter was coupled with the HTDMA and closure was achieved over the whole range of probed dry diameters, growth factors and supersaturations using the κ-parameterisation for the water activity and assuming surface tension of pure water in the Köhler theory. In the second closure type we compared hygroscopicity distributions derived from parallel monodisperse CCN measurements and HTDMA measurements. Very good agreement was found at all supersaturations which shows that monodisperse CCN measurements are a reliable alternative to determine the hygroscopic mixing state of ambient aerosols.

scholarly journals Hygroscopic mixing state of urban aerosol derived from size-resolved cloud condensation nuclei measurements during the MEGAPOLI campaign in Paris

2013 ◽  
Vol 13 (13) ◽  
pp. 6431-6446 ◽  
Author(s):  
Z. Jurányi ◽  
T. Tritscher ◽  
M. Gysel ◽  
M. Laborde ◽  
L. Gomes ◽  
...  
Keyword(s):  
Water Uptake ◽  
Cloud Condensation Nuclei ◽  
Pure Water ◽  
Particle Diameter ◽  
Complex Mixture ◽  
Average Particle ◽  
Hygroscopic Growth ◽  
Condensation Nuclei ◽  
Mixing State ◽  
Ambient Aerosols

Abstract. Ambient aerosols are a complex mixture of particles with different physical and chemical properties and consequently distinct hygroscopic behaviour. The hygroscopicity of a particle determines its water uptake at subsaturated relative humidity (RH) and its ability to form a cloud droplet at supersaturated RH. These processes influence Earth's climate and the atmospheric lifetime of the particles. Cloud condensation nuclei (CCN) number size distributions (i.e. CCN number concentrations as a function of dry particle diameter) were measured close to Paris during the MEGAPOLI campaign in January–February 2010, covering 10 different supersaturations (SS = 0.1–1.0%). The time-resolved hygroscopic mixing state with respect to CCN activation was also derived from these measurements. Simultaneously, a hygroscopicity tandem differential mobility analyser (HTDMA) was used to measure the hygroscopic growth factor (ratio of wet to dry mobility diameter) distributions at RH = 90%. The aerosol was highly externally mixed and its mixing state showed significant temporal variability. The average particle hygroscopicity was relatively low at subsaturation (RH = 90%; mean hygroscopicity parameter κ = 0.12–0.27) and increased with increasing dry diameter in the range 35–265 nm. The mean κ value, derived from the CCN measurements at supersaturation, ranged from 0.08 to 0.24 at SS = 1.0–0.1%. Two types of mixing-state resolved hygroscopicity closure studies were performed, comparing the water uptake ability measured below and above saturation. In the first type the CCN counter was connected in series with the HTDMA and and closure was achieved over the whole range of probed dry diameters, growth factors and supersaturations using the κ-parametrization for the water activity and assuming surface tension of pure water in the Köhler theory. In the second closure type we compared hygroscopicity distributions derived from parallel monodisperse CCN measurements and HTDMA measurements. Very good agreement was found at all supersaturations, which shows that monodisperse CCN measurements are a reliable alternative to determine the hygroscopic mixing state of ambient aerosols.

scholarly journals Distribution Characteristics of Aerosol Size and CCN during the Summer on Mt. Tian and Their Influencing Factors

Atmosphere ◽  
2020 ◽  
Vol 11 (9) ◽  
pp. 912
Author(s):  
Ankang Liu ◽  
Honglei Wang ◽  
Yuanyuan Li ◽  
Yan Yin ◽  
Bin Li ◽  
...  
Keyword(s):  
Particle Size ◽  
Size Distribution ◽  
Influencing Factors ◽  
Cloud Condensation Nuclei ◽  
Number Concentration ◽  
Distribution Characteristics ◽  
Condensation Nuclei ◽  
High Supersaturation ◽  
Aerosol Number Concentration ◽  
The Mean

The aerosol size distribution and cloud condensation nuclei (CCN) number concentration were measured using a wide-range particle spectrometer (WPS) and a cloud condensation nuclei counter (CCNC) on Mt. Tian from 31 July to 9 September, 2019. Combined with meteorological data, distribution characteristics of aerosol size and CCN and their influencing factors were analyzed. The results indicated that the mean aerosol number concentration was 5475.6 ± 5636.5 cm−3. The mean CCN concentrations were 183.7 ± 114.5 cm−3, 729.8 ± 376.1 cm−3, 1630.5 ± 980.5 cm−3, 2162.5 ± 1345.3 cm−3, and 2575.7 ± 1632.9 cm−3 at supersaturation levels of 0.1%, 0.2%, 0.4%, 0.6%, and 0.8%, respectively. The aerosol number size distribution is unimodal, and the dominant particle size is 30–60 nm. Affected by the height of the boundary layer and the valley wind, the diurnal variation in aerosol number concentration shows a unimodal distribution with a peak at 17:00, and the CCN number concentration showed a bimodal distribution with peaks at 18:00 and 21:00. The particle size distribution and supersaturation have a major impact on the activation of the aerosol into CCN. At 0.1% supersaturation (S), the 300–500 nm particles are most likely to activate to CCN. Particles of 100–300 nm are most easily activated at 0.2% (S), while particles of 60–80 nm are most likely activated at high supersaturation (≥0.4%). The concentrations of aerosol and CCN are higher in the northerly wind. Ambient relative humidity (RH) has little relationship with the aerosol activation under high supersaturation. According to N = CSk fitting the CCN spectrum, C = 3297 and k = 0.90 on Mt. Tian, characteristic of the clean continental type.

scholarly journals Summertime observations of ultrafine particles and cloud condensation nuclei from the boundary layer to the free troposphere in the Arctic

2016 ◽  
Author(s):  
Julia Burkart ◽  
Megan D. Willis ◽  
Heiko Bozem ◽  
Jennie L. Thomas ◽  
Kathy Law ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Ultrafine Particles ◽  
Cloud Condensation Nuclei ◽  
Particle Diameter ◽  
Open Ocean ◽  
The Arctic ◽  
Condensation Nuclei ◽  
Aerosol Composition ◽  
Low Level ◽  
20 Nm

Abstract. The Arctic is extremely sensitive to climate change. Shrinking sea ice extent increases the area covered by open ocean during Arctic summer, which impacts the surface albedo and aerosol and cloud properties among many things. In this context extensive aerosol measurements (aerosol composition, particle number and size, cloud condensation nuclei, and trace gases) were made during 11 flights of the NETCARE July, 2014 airborne campaign conducted from Resolute Bay, Nunavut (74N, 94W). Flights routinely included vertical profiles from about 60 to 3000 m a.g.l. as well as several low-level horizontal transects over open ocean, fast ice, melt ponds, and polynyas. Here we discuss the vertical distribution of ultrafine particles (UFP, particle diameter, dp: 5–20 nm), size distributions of larger particles (dp: 20 nm to 1 μm), and cloud condensation nuclei (CCN, supersaturation = 0.6 %) in relation to meteorological conditions and underlying surfaces. UFPs were observed predominantly within the boundary layer, where concentrations were often several hundreds to a few thousand particles per cubic centimeter. Occasionally, particle concentrations below 10 cm−3 were found. The highest UFP concentrations were observed above open ocean and at the top of low-level clouds, whereas numbers over ice-covered regions were substantially lower. Overall, UFP formation events were frequent in a clean boundary layer with a low condensation sink. In a few cases this ultrafine mode extended to sizes larger than 40 nm, suggesting that these UFP can grow into a size range where they can impact clouds and therefore climate.

scholarly journals Organic acids as cloud condensation nuclei: Laboratory studies of highly soluble and insoluble species

2003 ◽  
Vol 3 (1) ◽  
pp. 949-982 ◽  
Author(s):  
P. Pradeep Kumar ◽  
K. Broekhuizen ◽  
J. P. D. Abbatt
Keyword(s):  
Organic Acids ◽  
Room Temperature ◽  
Cloud Condensation Nuclei ◽  
Particle Diameter ◽  
Diffusion Chamber ◽  
Laboratory Studies ◽  
Condensation Nuclei ◽  
Gradient Diffusion ◽  
Kohler Theory ◽  
Good Agreement

Abstract. The ability of sub-micron-sized organic acid particles to act as cloud condensation nuclei (CCN) has been examined at room temperature using a newly constructed continuous-flow, thermal-gradient diffusion chamber (TGDC). The organic acids studied were: oxalic, malonic, glutaric, oleic and stearic. The CCN properties of the highly soluble acids – oxalic, malonic and glutaric – match very closely Kohler theory predictions which assume full dissolution of the dry particle and a surface tension of the growing droplet equal to that of water. In particular, for supersaturations between 0.3 and 0.6, agreement between the dry particle diameter which gives 50% activation and that calculated from Kohler theory is to within 3 nm on average. In the course of the experiments, considerable instability of glutaric acid particles was observed as a function of time and there is evidence that they fragment to some degree to smaller particles. Stearic acid and oleic acid, which are both highly insoluble in water, did not activate at supersaturations of 0.6% with dry diameters up to 140 nm. Finally, to validate the performance of the TGDC, we present results for the activation of ammonium sulfate particles that demonstrate good agreement with Kohler theory if solution non-ideality is considered. Our findings support earlier studies in the literature that showed highly soluble organics to be CCN active but insoluble species to be largely inactive.

scholarly journals Physical properties of the sub-micrometer aerosol over the Amazon rain forest during the wet-to-dry season transition - comparison of modeled and measured CCN concentrations

2004 ◽  
Vol 4 (8) ◽  
pp. 2119-2143 ◽  
Author(s):  
J. Rissler ◽  
E. Swietlicki ◽  
J. Zhou ◽  
G. Roberts ◽  
M. O. Andreae ◽  
...  
Keyword(s):  
Size Distribution ◽  
Biomass Burning ◽  
Time Resolution ◽  
Large Scale ◽  
Transition Period ◽  
Cloud Condensation Nuclei ◽  
Particle Diameter ◽  
Hygroscopic Growth ◽  
Condensation Nuclei ◽  
Number Size Distribution

Abstract. Sub-micrometer atmospheric aerosol particles were studied in the Amazon region, 125 km northeast of Manaus, Brazil (-1°55.2'S, 59°28.1'W). The measurements were performed during the wet-to-dry transition period, 4-28 July 2001 as part of the LBA (Large-Scale Biosphere Atmosphere Experiment in Amazonia) CLAIRE-2001 (Cooperative LBA Airborne Regional Experiment) experiment. The number size distribution was measured with two parallel differential mobility analyzers, the hygroscopic growth at 90% RH with a Hygroscopic Tandem Mobility Analyzer (H-TDMA) and the concentrations of cloud condensation nuclei (CCN) with a cloud condensation nuclei counter. A model was developed that uses the H-TDMA data to predict the number of soluble molecules or ions in the individual particles and the corresponding minimum particle diameter for activation into a cloud droplet at a certain supersaturation. Integrating the number size distribution above this diameter, CCN concentrations were predicted with a time resolution of 10 min and compared to the measured concentrations. During the study period, three different air masses were identified and compared: clean background, air influenced by aged biomass burning, and moderately polluted air from recent local biomass burning. For the clean period 2001, similar number size distributions and hygroscopic behavior were observed as during the wet season at the same site in 1998, with mostly internally mixed particles of low diameter growth factor (~1.3 taken from dry to 90% RH). During the periods influenced by biomass burning the hygroscopic growth changed slightly, but the largest difference was seen in the number size distribution. The CCN model was found to be successful in predicting the measured CCN concentrations, typically within 25%. A sensitivity study showed relatively small dependence on the assumption of which model salt that was used to predict CCN concentrations from H-TDMA data. One strength of using H-TDMA data to predict CCN concentrations is that the model can also take into account soluble organic compounds, insofar as they go into solution at 90% RH. Another advantage is the higher time resolution compared to using size-resolved chemical composition data.

scholarly journals Biomass burning emissions of trace gases and particles in marine air at Cape Grim, Tasmania

2015 ◽  
Vol 15 (23) ◽  
pp. 13393-13411 ◽  
Author(s):  
S. J. Lawson ◽  
M. D. Keywood ◽  
I. E. Galbally ◽  
J. L. Gras ◽  
J. M. Cainey ◽  
...  
Keyword(s):  
Particle Size ◽  
Biomass Burning ◽  
Cloud Condensation Nuclei ◽  
Trace Gases ◽  
Particle Growth ◽  
Dominant Mode ◽  
Condensation Nuclei ◽  
Substantial Impact ◽  
Marine Air ◽  
Cape Grim

Abstract. Biomass burning (BB) plumes were measured at the Cape Grim Baseline Air Pollution Station during the 2006 Precursors to Particles campaign, when emissions from a fire on nearby Robbins Island impacted the station. Measurements made included non-methane organic compounds (NMOCs) (PTR-MS), particle number size distribution, condensation nuclei (CN) > 3 nm, black carbon (BC) concentration, cloud condensation nuclei (CCN) number, ozone (O3), methane (CH4), carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), nitrous oxide (N2O), halocarbons and meteorology. During the first plume strike event (BB1), a 4 h enhancement of CO (max ~ 2100 ppb), BC (~ 1400 ng m-3) and particles > 3 nm (~ 13 000 cm-3) with dominant particle mode of 120 nm were observed overnight. A wind direction change lead to a dramatic reduction in BB tracers and a drop in the dominant particle mode to 50 nm. The dominant mode increased in size to 80 nm over 5 h in calm sunny conditions, accompanied by an increase in ozone. Due to an enhancement in BC but not CO during particle growth, the presence of BB emissions during this period could not be confirmed. The ability of particles > 80 nm (CN80) to act as CCN at 0.5 % supersaturation was investigated. The ΔCCN / ΔCN80 ratio was lowest during the fresh BB plume (56 ± 8 %), higher during the particle growth period (77 ± 4 %) and higher still (104 ± 3 %) in background marine air. Particle size distributions indicate that changes to particle chemical composition, rather than particle size, are driving these changes. Hourly average CCN during both BB events were between 2000 and 5000 CCN cm-3, which were enhanced above typical background levels by a factor of 6–34, highlighting the dramatic impact BB plumes can have on CCN number in clean marine regions. During the 29 h of the second plume strike event (BB2) CO, BC and a range of NMOCs including acetonitrile and hydrogen cyanide (HCN) were clearly enhanced and some enhancements in O3 were observed (ΔO3 / ΔCO 0.001–0.074). A short-lived increase in NMOCs by a factor of 10 corresponded with a large CO enhancement, an increase of the NMOC / CO emission ratio (ER) by a factor of 2–4 and a halving of the BC / CO ratio. Rainfall on Robbins Island was observed by radar during this period which likely resulted in a lower fire combustion efficiency, and higher emission of compounds associated with smouldering. This highlights the importance of relatively minor meteorological events on BB emission ratios. Emission factors (EFs) were derived for a range of trace gases, some never before reported for Australian fires, (including hydrogen, phenol and toluene) using the carbon mass balance method. This provides a unique set of EFs for Australian coastal heathland fires. Methyl halide EFs were higher than EFs reported from other studies in Australia and the Northern Hemisphere which is likely due to high halogen content in vegetation on Robbins Island. This work demonstrates the substantial impact that BB plumes can have on the composition of marine air, and the significant changes that can occur as the plume interacts with terrestrial, aged urban and marine emission sources.

scholarly journals Snow particle characteristics in the saltation layer

2014 ◽  
Vol 60 (221) ◽  
pp. 431-439 ◽  
Author(s):  
Christof Gromke ◽  
Stefan Horender ◽  
Benjamin Walter ◽  
Michael Lehning
Keyword(s):  
Particle Size ◽  
Particle Number ◽  
Particle Diameter ◽  
Wind Tunnel Experiments ◽  
Scale Parameters ◽  
Particle Characteristics ◽  
Maximum Particle ◽  
The Mean ◽  
Snow Particle ◽  
Measurement Area

AbstractShadowgraphy was employed to study snow saltation in boundary-layer wind tunnel experiments with fresh, naturally deposited snow. The shadowgraphy method allowed for a temporally and spatially high-resolution investigation of snow particle characteristics within a measurement area of up to 50 mm × 50 mm. Snow particle size and number characteristics, and their variation with height in the saltation layer, were analysed. The following observations and findings were made for the saltation layer: (1) the particle number decreases exponentially with height, (2) the mean particle diameter is fairly constant, with a very slight tendency to decrease with height, (3) the maximum particle diameter decreases linearly with height, and (4) the snow particle size distribution can be adequately described by gamma probability density functions. The shape and scale parameters of the gamma distribution were found to vary systematically, though only slightly, with height over ground and between experiments with different snowpack characteristics.

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