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.
Monitoring and modeling aerosol particle life cycle in Southeast Asia (SEA) is challenged by high cloud cover, complex meteorology, and the wide range of aerosol species, sources, and transformations ...found throughout the region. Satellite observations are limited, and there are few in situ observations of aerosol extinction profiles, aerosol properties, and environmental conditions. Therefore, accurate aerosol model outputs are crucial for the region. This work evaluates the Navy Aerosol Analysis and Prediction System Reanalysis (NAAPS-RA) aerosol optical thickness (AOT) and light extinction products using airborne aerosol and meteorological measurements from the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP.sup.2 Ex) conducted in 2019 during the SEA southwest monsoon biomass burning season. Modeled AOTs and extinction coefficients are compared to those retrieved with a high spectral resolution lidar (HSRL-2). Agreement between simulated and retrieved AOT (R.sup.2 = 0.78, relative bias =-5 %, normalized root mean square error (NRMSE) = 48 %) and aerosol extinction coefficients (R.sup.2 = 0.80, 0.81, and 0.42; relative bias = 3 %, -6 %, and -7 %; NRMSE = 47 %, 53 %, and 118 % for altitudes between 40-500, 500-1500, and 1500 m, respectively) is quite good considering the challenging environment and few opportunities for assimilations of AOT from satellites during the campaign. Modeled relative humidities (RHs) are negatively biased at all altitudes (absolute bias =-5 %, -8 %, and -3 % for altitudes 500 500-1500 and 1500 m, respectively), motivating interest in the role of RH errors in AOT and extinction simulations. Interestingly, NAAPS-RA AOT and extinction agreement with the HSRL-2 does not change significantly (i.e., NRMSE values do not all decrease) when RHs from dropsondes are substituted into the model, yet biases all move in a positive direction. Further exploration suggests changes in modeled extinction are more sensitive to the actual magnitude of both the extinction coefficients and the dropsonde RHs being substituted into the model as opposed to the absolute differences between simulated and measured RHs. Finally, four case studies examine how model errors in RH and the hygroscopic growth parameter, γ, affect simulations of extinction in the mixed layer (ML). We find NAAPS-RA overestimates the hygroscopicity of (i) smoke particles from biomass burning in the Maritime Continent (MC) and (ii) anthropogenic emissions transported from East Asia. This work mainly provides insight into the relationship between errors in modeled RH and simulations of AOT and extinction in a humid and tropical environment influenced by a myriad of meteorological conditions and particle types. These results can be interpreted and addressed by the modeling community as part of the effort to better understand, quantify, and forecast atmospheric conditions in SEA.
Monitoring and modeling aerosol particle life cycle in Southeast Asia (SEA)
is challenged by high cloud cover, complex meteorology, and the wide range
of aerosol species, sources, and transformations ...found throughout the
region. Satellite observations are limited, and there are few in situ
observations of aerosol extinction profiles, aerosol properties, and
environmental conditions. Therefore, accurate aerosol model outputs are
crucial for the region. This work evaluates the Navy Aerosol Analysis and
Prediction System Reanalysis (NAAPS-RA) aerosol optical thickness (AOT) and
light extinction products using airborne aerosol and meteorological
measurements from the Cloud, Aerosol, and Monsoon Processes Philippines
Experiment (CAMP2Ex) conducted in 2019 during the SEA southwest monsoon
biomass burning season. Modeled AOTs and extinction coefficients are
compared to those retrieved with a high spectral resolution lidar (HSRL-2).
Agreement between simulated and retrieved AOT (R2= 0.78, relative
bias =-5 %, normalized root mean square error (NRMSE) = 48 %) and
aerosol extinction coefficients (R2= 0.80, 0.81, and 0.42; relative
bias = 3 %, −6 %, and −7 %; NRMSE = 47 %, 53 %, and 118 % for altitudes
between 40–500, 500–1500, and >1500 m, respectively)
is quite good considering the challenging environment and few opportunities
for assimilations of AOT from satellites during the campaign. Modeled
relative humidities (RHs) are negatively biased at all altitudes (absolute
bias =-5 %, −8 %, and −3 % for altitudes <500 500–1500
and >1500 m, respectively), motivating interest in the role of
RH errors in AOT and extinction simulations. Interestingly, NAAPS-RA AOT and
extinction agreement with the HSRL-2 does not change significantly (i.e.,
NRMSE values do not all decrease) when RHs from dropsondes are substituted
into the model, yet biases all move in a positive direction. Further
exploration suggests changes in modeled extinction are more sensitive to the
actual magnitude of both the extinction coefficients and the dropsonde RHs
being substituted into the model as opposed to the absolute differences
between simulated and measured RHs. Finally, four case studies examine how
model errors in RH and the hygroscopic growth parameter, γ, affect
simulations of extinction in the mixed layer (ML). We find NAAPS-RA
overestimates the hygroscopicity of (i) smoke particles from biomass burning
in the Maritime Continent (MC) and (ii) anthropogenic emissions transported
from East Asia. This work mainly provides insight into the relationship
between errors in modeled RH and simulations of AOT and extinction in a
humid and tropical environment influenced by a myriad of meteorological
conditions and particle types. These results can be interpreted and
addressed by the modeling community as part of the effort to better
understand, quantify, and forecast atmospheric conditions in SEA.
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 (CAMP.sup.2 Ex) 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.
This work examines particulate chloride (Cl–) and bromide (Br–) depletion in marine aerosol particles influenced by wildfires at a coastal California site in the summers of 2013 and 2016. Chloride ...exhibited a dominant coarse mode due to sea salt influence, with substantially diminished concentrations during fire periods as compared to nonfire periods. Bromide exhibited a peak in the submicrometer range during fire and nonfire periods, with an additional supermicrometer peak in the latter periods. Chloride and Br– depletions were enhanced during fire periods as compared to nonfire periods. The highest observed %Cl– depletion occurred in the submicrometer range, with maximum values of 98.9% (0.32–0.56 μm) and 85.6% (0.56–1 μm) during fire and nonfire periods, respectively. The highest %Br– depletion occurred in the supermicrometer range during fire and nonfire periods with peak depletion between 1.8–3.2 μm (78.8% and 58.6%, respectively). When accounting for the neutralization of sulfate by ammonium, organic acid particles showed the greatest influence on Cl– depletion in the submicrometer range. These results have implications for aerosol hygroscopicity and radiative forcing in areas with wildfire influence owing to depletion effects on composition.
This study reports on ambient measurements of organosulfur (OS) and methanesulfonate (MSA) over the western United States and coastal areas. Particulate OS levels are highest in summertime and ...generally increase as a function of sulfate (a precursor) and sodium (a marine tracer) with peak levels at coastal sites. The ratio of OS to total sulfur is also highest at coastal sites, with increasing values as a function of normalized difference vegetation index and the ratio of organic carbon to elemental carbon. Correlative analysis points to significant relationships between OS and biogenic emissions from marine and continental sources, factors that coincide with secondary production, and vanadium due to a suspected catalytic role. A major OS species, methanesulfonate (MSA), was examined with intensive field measurements, and the resulting data support the case for vanadium's catalytic influence. Mass size distributions reveal a dominant MSA peak between aerodynamic diameters of 0.32–0.56 µm at a desert and coastal site with nearly all MSA mass (≥84%) in submicrometer sizes; MSA:non‐sea‐salt sulfate ratios vary widely as a function of particle size and proximity to the ocean. Airborne data indicate that relative to the marine boundary layer, particulate MSA levels are enhanced in urban and agricultural areas and also the free troposphere when impacted by biomass burning. Some combination of fires and marine‐derived emissions leads to higher MSA levels than either source alone. Finally, MSA differences in cloud water and out‐of‐cloud aerosol are discussed.
Key Points
Organosulfur (OS) and methanesulfonate (MSA) studied across western U.S. in aerosol and cloud water
Vanadium positively related with OS and MSA due to presumed catalytic effect
MSA levels are enhanced in urban areas and when biomass burning and marine emissions interact
This study uses airborne data from multiple field campaigns off the California coast to determine the extent to which a size distribution parameter and a cloud water chemical measurement can capture ...the effect of giant cloud condensation nuclei (GCCN), specifically sea salt, on marine stratocumulus cloud properties. The two GCCN proxy variables, near‐surface particle number concentration for diameters >5 µm and cloud water chloride concentration, are significantly correlated (95% confidence) with each other, and both exhibit expected relationships with other parameters (e.g., surface wind) that typically coincide with sea salt emissions. Factors influencing the relationship between these two GCCN proxy measurements include precipitation rate (R) and the standard deviation of the subcloud vertical velocity owing likely to scavenging effects and improved mixing/transport of sea salt to cloud base, respectively. When comparing 12 pairs of high and low chloride cloud cases (at fixed liquid water path and cloud drop number concentration), the average drop spectra for high chloride cases exhibit enhanced drop number at diameters exceeding 20 µm, especially above 30 µm. In addition, high chloride cases coincide with enhanced mean columnar R and negative values of precipitation susceptibility. The difference in drop effective radius between high and low chloride conditions decreases with height in cloud, suggesting that some GCCN‐produced raindrops precipitate before reaching cloud tops. The sign of cloud responses (i.e., R) to perturbations in giant sea salt particle concentration, as evaluated from Modern Era Retrospective Analysis for Research and Applications version 2 reanalysis data, is consistent with the aircraft data.
Key Points
Cloud water sea salt and near‐surface giant particle concentrations are related and represent GCCN in the marine atmosphere
Clouds with higher chloride concentrations coincide with enhanced rain rate (at fixed LWP) and negative precipitation susceptibility values
Differences in vertical drop size data between low and high chloride cases suggest GCCN‐produced raindrops precipitate before reaching tops
Cloud drop number concentrations (Nd) over the western North Atlantic Ocean (WNAO) are generally highest during the winter (DJF) and lowest in summer (JJA), in contrast to aerosol proxy variables ...(aerosol optical depth, aerosol index, surface aerosol mass concentrations, surface cloud condensation nuclei (CCN) concentrations) that generally peak in spring (MAM) and JJA with minima in DJF. Using aircraft, satellite remote sensing, ground-based in situ measurement data, and reanalysis data, we characterize factors explaining the divergent seasonal cycles and furthermore probe into factors influencing Nd on seasonal timescales. The results can be summarized well by features most pronounced in DJF, including features associated with cold-air outbreak (CAO) conditions such as enhanced values of CAO index, planetary boundary layer height (PBLH), low-level liquid cloud fraction, and cloud-top height, in addition to winds aligned with continental outflow. Data sorted into high- and low-Nd days in each season, especially in DJF, revealed that all of these conditions were enhanced on the high-Nd days, including reduced sea level pressure and stronger wind speeds. Although aerosols may be more abundant in MAM and JJA, the conditions needed to activate those particles into cloud droplets are weaker than in colder months, which is demonstrated by calculations of the strongest (weakest) aerosol indirect effects in DJF (JJA) based on comparing Nd to perturbations in four different aerosol proxy variables (total and sulfate aerosol optical depth, aerosol index, surface mass concentration of sulfate). We used three machine learning models and up to 14 input variables to infer about most influential factors related to Nd for DJF and JJA, with the best performance obtained with gradient-boosted regression tree (GBRT) analysis. The model results indicated that cloud fraction was the most important input variable, followed by some combination (depending on season) of CAO index and surface mass concentrations of sulfate and organic carbon. Future work is recommended to further understand aspects uncovered here such as impacts of free tropospheric aerosol entrainment on clouds, degree of boundary layer coupling, wet scavenging, and giant CCN effects on aerosol–Nd relationships, updraft velocity, and vertical structure of cloud properties such as adiabaticity that impact the satellite estimation of Nd.
North American pollution outflow is ubiquitous over the western North
Atlantic Ocean, especially in winter, making this location a suitable
natural laboratory for investigating the impact of ...precipitation on aerosol
particles along air mass trajectories. We take advantage of observational
data collected at Bermuda to seasonally assess the sensitivity of aerosol
mass concentrations and volume size distributions to accumulated
precipitation along trajectories (APT). The mass concentration of
particulate matter with aerodynamic diameter less than 2.5 µm
normalized by the enhancement of carbon monoxide above background
(PM2.5/ΔCO) at Bermuda was used to estimate the degree of
aerosol loss during transport to Bermuda. Results for December–February
(DJF) show that most trajectories come from North America and have the highest
APTs, resulting in a significant reduction (by 53 %) in PM2.5/ΔCO under high-APT conditions (> 13.5 mm) relative to low-APT
conditions (< 0.9 mm). Moreover, PM2.5/ΔCO was most
sensitive to increases in APT up to 5 mm (−0.044 µg m−3 ppbv−1 mm−1) and less sensitive to increases in APT over 5 mm.
While anthropogenic PM2.5 constituents (e.g., black carbon, sulfate,
organic carbon) decrease with high APT, sea salt, in contrast, was comparable
between high- and low-APT conditions owing to enhanced local wind and sea
salt emissions in high-APT conditions. The greater sensitivity of the fine-mode volume concentrations (versus coarse mode) to wet scavenging is evident
from AErosol RObotic NETwork
(AERONET) volume size distribution data. A combination of GEOS-Chem model
simulations of the 210Pb submicron aerosol tracer and its gaseous precursor
222Rn reveals that (i) surface aerosol particles at Bermuda are most
impacted by wet scavenging in winter and spring (due to large-scale
precipitation) with a maximum in March, whereas convective scavenging plays
a substantial role in summer; and (ii) North American 222Rn tracer
emissions contribute most to surface 210Pb concentrations at Bermuda in
winter (∼ 75 %–80 %), indicating that air masses arriving at
Bermuda experience large-scale precipitation scavenging while traveling from
North America. A case study flight from the ACTIVATE field campaign on 22 February 2020 reveals a significant reduction in aerosol number and volume
concentrations during air mass transport off the US East Coast associated
with increased cloud fraction and precipitation. These results highlight the
sensitivity of remote marine boundary layer aerosol characteristics to
precipitation along trajectories, especially when the air mass source is
continental outflow from polluted regions like the US East Coast.
Cloud processing is known to generate aerosol species such as sulfate and secondary organic aerosol, yet there is a scarcity of airborne data to
examine this issue. The NASA Aerosol Cloud meTeorology ...Interactions oVer the western ATlantic Experiment (ACTIVATE) was designed to build an
unprecedented dataset relevant to aerosol–cloud interactions with two coordinated aircraft over the northwestern Atlantic, with aerosol mass spectrometer data used from four deployments between 2020–2021 to contrast aerosol composition below, in (using a counterflow virtual impactor)
and above boundary layer clouds. Consistent features in all time periods of the deployments (January–March, May–June, August–September) include
the mass fraction of organics and relative amount of oxygenated organics (m/z 44) relative to total organics (f44) increasing in droplet
residuals relative to below and above cloud. Detailed analysis comparing data below and in cloud suggests a possible role for in-cloud aqueous
processing in explaining such results; an intriguing aspect though requiring more attention is that only approximately a quarter of the cloud
cases (29 of 110) showed higher organic mass fractions either below or above cloud. Of those 29 cases, the majority (25) showed higher organic mass
fraction below cloud base where the cloud processing signature is presumably more evident as compared to above cloud. These results are consistent
with the few past studies analyzing droplet residuals pointing to higher organic enrichment than in adjacent cloud-free areas. The data findings are
important as other datasets (e.g., reanalysis) suggest that sulfate is both more abundant than organics (in contrast to this work) and more closely
related to drop number concentrations in the winter when aerosol–cloud interactions are strongest. Here we show that organics are more abundant than
sulfate in the droplet residuals and that aerosol interaction with clouds potentially decreases particle hygroscopicity due to the increase in
organic:sulfate ratio for droplet residuals relative to surrounding cloud-free air. These results are important in light of the growing importance
of organics over the northwestern Atlantic in recent decades relative to sulfate owing to the success of regulatory activity over the eastern United States to cut sulfur dioxide emissions.
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