We evaluate the sensitivity of the size calibrations of
two commercially available, high-resolution optical particle sizers to
changes in aerosol composition and complex refractive index (RI). The
...Droplet Measurement Technologies Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) and the TSI, Inc. Laser Aerosol Spectrometer (LAS) are
two commonly used instruments for measuring the portion of the aerosol size
distribution with diameters larger than nominally 60–90 nm. Both instruments
illuminate particles with a laser and relate the single-particle light
scattering intensity and count rate measured over a wide range of angles to
the size-dependent particle concentration. While the optical block geometry
and flow system are similar for each instrument, a significant difference
between the two models is the laser wavelength (1054 nm for the UHSAS and
633 nm for the LAS) and intensity (about 100 times higher for the UHSAS), which
may affect the way each instrument sizes non-spherical or absorbing
aerosols. Here, we challenge the UHSAS and LAS with laboratory-generated,
mobility-size-classified aerosols of known chemical composition to quantify
changes in the optical size response relative to that of ammonium sulfate
(RI of 1.52+0i at 532 nm) and NIST-traceable polystyrene latex spheres
(PSLs with RI of 1.59+0i at 589 nm). Aerosol inorganic salt species are
chosen to cover the real refractive index range of 1.32 to 1.78, while
chosen light-absorbing carbonaceous aerosols include fullerene soot,
nigrosine dye, humic acid, and fulvic acid standards. The instrument
response is generally in good agreement with the electrical mobility
diameter. However, large undersizing deviations are observed for the
low-refractive-index fluoride salts and the strongly absorbing nigrosine dye and fullerene soot particles. Polydisperse size distributions for both fresh
and aged wildfire smoke aerosols from the recent Fire Influence on Regional
to Global Environments Experiment and Air Quality (FIREX-AQ) and the Cloud,
Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex)
airborne campaigns show good agreement between both optical sizers and
contemporaneous electrical mobility sizing and particle time-of-flight mass
spectrometric measurements. We assess the instrument uncertainties by
interpolating the laboratory response curves using previously reported RIs
and size distributions for multiple aerosol type classifications. These
results suggest that, while the optical sizers may underperform for strongly
absorbing laboratory compounds and fresh tailpipe emissions measurements,
sampling aerosols within the atmospherically relevant range of refractive
indices are likely to be sized to better than ±10 %–20 % uncertainty over the submicron aerosol size range when using instruments calibrated with
ammonium sulfate.
The Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) field campaign provides accurate data for aerosol characterization and trace gas profiles, and establishes ...knowledge of the relationships between aerosols and water. The dropsonde dataset provides an in situ characterization of the vertical thermodynamic structure of the atmosphere during 165 research flights by NASA Langley's King Air research aircraft between February 2020 and June 2022 and four test flights between December 2019 and November 2021. The research flights covered the western North Atlantic region, off the coast of the Eastern United States and around Bermuda and covered all seasons. The dropsonde profiles provide observations of temperature, pressure, relative humidity, and horizontal and vertical winds between the surface and about 9 km. 801 dropsondes were released, of which 796 were processed and 788 provide complete profiles of all parameters between the flight level and the surface with normal parachute performance. Here, we describe the dataset, the processing of the measurements, general statistics, and applications of this rich dataset.
Due to their fast evolution and large natural variability in macro- and microphysical properties, the accurate representation of boundary layer clouds in current climate models remains a challenge. ...One of the regions with large intermodel spread in the Coupled Model Intercomparison Project Phase 6 ensemble is the western North Atlantic Ocean. Here, statistically representative in situ measurements can help to develop and constrain the parameterization of clouds in global models. To this end, we performed comprehensive measurements of boundary layer clouds, aerosol, trace gases, and radiation in the western North Atlantic Ocean during the NASA Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) mission. In total, 174 research flights with 574 flight hours for cloud and precipitation measurements were performed with the HU-25 Falcon during three winter (February–March 2020, January–April 2021, and November 2021–March 2022) and three summer seasons (August–September 2020, May–June 2021, and May–June 2022). Here we present a statistical evaluation of 16 140 individual cloud events probed by the fast cloud droplet probe and the two-dimensional stereo cloud probe during 155 research flights in a representative and repetitive flight strategy allowing for robust statistical data analyses. We show that the vertical profiles of distributions of the liquid water content and the cloud droplet effective diameter (ED) increase with altitude in the marine boundary layer. Due to higher updraft speeds, higher cloud droplet number concentrations (Nliquid) were measured in winter compared to summer despite lower cloud condensation nucleus abundance. Flight cloud cover derived from statistical analysis of in situ data is reduced in summer and shows large variability. This seasonal contrast in cloud coverage is consistent with a dominance of a synoptic pattern in winter that favors conditions for the formation of stratiform clouds at the western edge of cyclones (post-cyclonic). In contrast, a dominant summer anticyclone is concomitant with the occurrence of shallow cumulus clouds and lower cloud coverage. The evaluation of boundary layer clouds and precipitation in the Nliquid ED phase space sheds light on liquid, mixed-phase, and ice cloud properties and helps to categorize the cloud data. Ice and liquid precipitation, often masked in cloud statistics by a high abundance of liquid clouds, is often observed throughout the cloud. The ACTIVATE in situ cloud measurements provide a wealth of cloud information useful for assessing airborne and satellite remote-sensing products, for global climate and weather model evaluations, and for dedicated process studies that address precipitation and aerosol–cloud interactions.
The NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) ship
and aircraft field campaign deployed to the western subarctic Atlantic
between the years 2015 and 2018. One of the primary ...goals of NAAMES is to
improve the understanding of aerosol–cloud interaction (ACI) over the
Atlantic Ocean under different seasonal regimes. ACIs currently represent the
largest source of uncertainty in global climate models. During three NAAMES
field campaigns (NAAMES-1 in November 2015, NAAMES-2 in May 2016, and
NAAMES-3 in September 2017), multiple 10 h science flights were conducted
using the NASA C-130 aircraft to measure marine boundary layer aerosol and
cloud properties. The standard flight pattern includes vertical spirals
where the C-130 transitioned from high altitude to low altitude (and vice
versa), collecting in situ measurements of aerosols, trace gases, clouds, and
meteorological parameters as a function of altitude. We examine the data
collected from 37 spirals during the three NAAMES field campaigns, and we
present a comprehensive characterization of the vertical profiles of aerosol
properties under different synoptic conditions and aerosol regimes. The
vertical distribution of submicron aerosol particles exhibited strong
seasonal variation, as well as elevated intra-seasonal variability depending
on emission sources and aerosol processes in the atmospheric column.
Pristine marine conditions and new particle formation were prevalent in the
wintertime (NAAMES-1) due to low biogenic emissions from the surface ocean
and reduced continental influence. Higher concentrations of submicron
aerosol particles were observed in the spring (NAAMES-2) due to strong
phytoplankton activity and the arrival of long-range-transported continental
plumes in the free troposphere with subsequent entrainment into the marine
boundary layer. Biomass burning from boreal wildfires was the main source of
aerosol particles in the region during the late summer (NAAMES-3) in both
the marine boundary layer and free troposphere.
Wildfire smoke influences on air quality and atmospheric chemistry have been underscored by the increasing fire prevalence in recent years, and yet, the connection between fire, smoke emissions, and ...the subsequent transformation of this smoke in the atmosphere remains poorly constrained. Toward improving these linkages, we present a new method for coupling high time‐resolution satellite observations of fire radiative power with in situ observations of smoke aerosols and trace gases. We apply this technique to 13 fire plumes comprehensively characterized during the recent FIREX‐AQ mission and show that changes in fire radiative power directly translate into changes in conserved smoke tracers (CO2, CO, and black carbon aerosol) observed in the downwind smoke plume. The correlation is particularly strong for CO2 (mean r > 0.9). This method is important for untangling the competing effects of changing fire behavior versus the influence of dilution and atmospheric processing on the downwind evolution of measured smoke properties.
Key Points
Geostationary satellite observations of fire radiative power are highly correlated with in situ airborne measurements of primary‐emission smoke tracers
High‐resolution satellite observations are needed to disentangle how fire activity and plume dilution impact the downwind evolution of smoke
Diurnal fire activity for wildfires observed during FIREX‐AQ is best parameterized using a bimodal Gaussian distribution to inform models
Cloud droplet chemical composition is a key observable property that can aid understanding of how aerosols and clouds interact. As part of the Clouds, Aerosols and Monsoon Processes – Philippines ...Experiment (CAMP2Ex), three case studies were analyzed involving collocated airborne sampling of relevant clear and cloudy air masses associated with maritime warm convection. Two of the cases represented a polluted marine background, with signatures of transported East Asian regional pollution, aged over water for several days, while the third case comprised a major smoke transport event from Kalimantan fires.
Sea salt was a dominant component of cloud droplet composition, in spite of fine particulate enhancement from regional anthropogenic sources. Furthermore, the proportion of sea salt was enhanced relative to sulfate in rainwater and may indicate both a propensity for sea salt to aid warm rain production and an increased collection
efficiency of large sea salt particles by rain in subsaturated environments. Amongst cases, as precipitation became more significant, so too did the variability in the sea salt to (non-sea salt) sulfate ratio. Across cases, nitrate and ammonium were fractionally greater in cloud water than fine-mode aerosol particles; however, a strong
covariability in cloud water nitrate and sea salt was suggestive of prior uptake of nitrate on large salt particles.
A mass-based closure analysis of non-sea salt sulfate compared the cloud water air-equivalent mass concentration to the concentration of aerosol particles serving as cloud condensation nuclei for droplet activation. While sulfate found in cloud was generally constrained by the sub-cloud aerosol concentration, there was significant
intra-cloud variability that was attributed to entrainment – causing evaporation of sulfate-containing droplets –and losses due to precipitation. In addition, precipitation tended to promote mesoscale variability in the sub-cloud aerosol through a combination of removal, convective downdrafts, and dynamically driven convergence. Physical
mechanisms exerted such strong control over the cloud water compositional budget that it was not possible to isolate any signature of chemical production/loss using in-cloud observations. The cloud-free environment surrounding the non-precipitating smoke case indicated sulfate enhancement compared to convective mixing quantified by a stable gas tracer; however, this was not observed in the cloud water (either through use of ratios
or the mass closure), perhaps implying that the warm convective cloud timescale was too short for chemical production to be a leading-order budgetary term and because precursors had already been predominantly exhausted.
Closure of other species was truncated by incomplete characterization of coarse aerosol (e.g., it was found that only 10 %–50% of sea salt mass found in cloud was captured during clear-air sampling) and unmeasured gasphase abundances affecting closure of semi-volatile aerosol species (e.g., ammonium, nitrate and organic) and soluble volatile organic compound contributions to total organic carbon in cloud water.
Marine biogenic particle contributions to atmospheric aerosol concentrations are not well understood though they are important for determining cloud optical and cloud-nucleating properties. Here we ...examine the relationship between marine aerosol measurements (with satellites and model fields of ocean biology) and meteorological variables during the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). NAAMES consisted of four field campaigns between November 2015 and April 2018 that aligned with the four major phases of the annual phytoplankton bloom cycle. The FLEXible PARTicle (FLEXPART) Lagrangian particle dispersion model is used to spatiotemporally connect these variables to ship-based aerosol and dimethyl sulfide (DMS) observations. We find that correlations between some aerosol measurements with satellite-measured and modeled variables increase with increasing trajectory length, indicating that biological and meteorological processes over the air mass history are influential for measured particle properties and that using only spatially coincident data would miss correlative connections that are lagged in time. In particular, the marine non-refractory organic aerosol mass correlates with modeled marine net primary production when weighted by 5 d air mass trajectory residence time (r=0.62). This result indicates that non-refractory organic aerosol mass is influenced by biogenic volatile organic compound (VOC) emissions that are typically produced through bacterial degradation of dissolved organic matter, zooplankton grazing on marine phytoplankton, and as a by-product of photosynthesis by phytoplankton stocks during advection into the region. This is further supported by the correlation of non-refractory organic mass with 2 d residence-time-weighted chlorophyll a (r=0.39), a proxy for phytoplankton abundance, and 5 d residence-time-weighted downward shortwave forcing (r=0.58), a requirement for photosynthesis. In contrast, DMS (formed through biological processes in the seawater) and primary marine aerosol (PMA) concentrations showed better correlations with explanatory biological and meteorological variables weighted with shorter air mass residence times, which reflects their localized origin as primary emissions. Aerosol submicron number and mass negatively correlate with sea surface wind speed. The negative correlation is attributed to enhanced PMA concentrations under higher wind speed conditions. We hypothesized that the elevated total particle surface area associated with high PMA concentrations leads to enhanced rates of condensation of VOC oxidation products onto PMA. Given the high deposition velocity of PMA relative to submicron aerosol, PMA can limit the accumulation of secondary aerosol mass. This study provides observational evidence for connections between marine aerosols and underlying ocean biology through complex secondary formation processes, emphasizing the need to consider air mass history in future analyses.
Atmospheric marine particle concentrations impact cloud
properties, which strongly impact the amount of solar radiation reflected
back into space or absorbed by the ocean surface. While satellites ...can
provide a snapshot of current conditions at the overpass time, models are
necessary to simulate temporal variations in both particle and cloud
properties. However, poor model accuracy limits the reliability with which
these tools can be used to predict future climate. Here, we leverage the
comprehensive ocean ecosystem and atmospheric aerosol–cloud dataset
obtained during the third deployment of the North Atlantic Aerosols and
Marine Ecosystems Study (NAAMES3). Airborne and ship-based measurements were
collected in and around a cold-air outbreak during a 3 d (where d stands for day) intensive
operations period from 17–19 September 2017. Cold-air outbreaks are of keen
interest for model validation because they are challenging to accurately
simulate, which is due, in part, to the numerous feedbacks and sub-grid-scale processes that influence aerosol and cloud evolution. The NAAMES
observations are particularly valuable because the flight plans were
tailored to lie along Lagrangian trajectories, making it possible to
spatiotemporally connect upwind and downwind measurements with the
state-of-the-art FLEXible PARTicle (FLEXPART) Lagrangian particle dispersion
model and then calculate a rate of change in particle properties. Initial
aerosol conditions spanning an east–west, closed-cell-to-clear-air
transition region of the cold-air outbreak indicate similar particle
concentrations and properties. However, despite the similarities in the
aerosol fields, the cloud properties downwind of each region evolved quite
differently. One trajectory carried particles through a cold-air outbreak,
resulting in a decrease in accumulation mode particle concentration
(−42 %) and cloud droplet concentrations, while the other remained outside
of the cold-air outbreak and experienced an increase in accumulation mode
particle concentrations (+62 %). The variable meteorological conditions between these two adjacent trajectories result from differences in the local sea surface temperature in the Labrador Current and surrounding waters, altering the stability of the marine atmospheric boundary layer. Further
comparisons of historical satellite observations indicate that the observed
pattern occurs annually in the region, making it an ideal location for
future airborne Lagrangian studies tracking the evolution of aerosols and
clouds over time under cold-air outbreak conditions.
Aerosol optical depth was retrieved from two airborne remote sensing instruments, the Research Scanning Polarimeter (RSP) and Second Generation High Spectral Resolution Lidar (HSRL-2), during the ...National Aeronautics and Space Administration (NASA) Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE). The field campaign offers a unique opportunity to evaluate an extensive 3-year dataset under a wide range of meteorological conditions from two instruments on the same platform. However, a long-standing issue in atmospheric field studies is that there is a lack of reference datasets for properly validating field measurements and estimating their uncertainties. Here we address this issue by using the triple collocation method, in which a third collocated satellite dataset from the Moderate Resolution Imaging Spectroradiometer (MODIS) is introduced for comparison. HSRL-2 is found to provide a more accurate retrieval than RSP over the study region. The error standard deviation of HSRL-2 with respect to the ground truth is 0.027. Moreover, this approach enables us to develop a simple, yet efficient, quality control criterion for RSP data. The physical reasons for the differences in two retrievals are determined to be cloud contamination, aerosols near the surface, multiple aerosol layers, absorbing aerosols, non-spherical aerosols, and simplified retrieval assumptions. These results demonstrate the pathway for optimal aerosol retrievals by combining information from both lidars and polarimeters for future airborne and satellite missions.
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