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.
Coastal southeast Florida experiences a wide range of aerosol conditions, including African dust, biomass burning (BB) aerosols, as well as sea salt and other locally-emitted aerosols. These aerosols ...are important sources of cloud condensation nuclei (CCN), which play an essential role in governing cloud radiative properties. As marine environments dominate the surface of Earth, CCN characteristics in coastal southeast Florida have broad implications for other regions with the added feature that this site is perturbed by both natural and anthropogenic emissions. This study investigates the influence of different air mass types on CCN concentrations at 0.2% (CCN0.2%) and 1.0% (CCN1.0%) supersaturation (SS) based on ground site measurements during selected months in 2013, 2017, and 2018. Average CCN0.2% and CCN1.0% concentrations were 373 ± 200 cm−3 and 584 ± 323 cm−3, respectively, for four selected days with minimal presence of African dust and BB (i.e., background days). CCN concentrations were not elevated on the four days with highest influence of African dust (289 ± 104 cm−3 0.2% SS and 591 ± 302 cm−3 1.0% SS), consistent with high dust mass concentrations comprised of coarse particles that are few in number. In contrast, CCN concentrations were substantially enhanced on the five days with the greatest impact from BB (1408 ± 976 cm−3 0.2% SS and 3337 ± 1252 cm−3 1.0% SS). Ratios of CCN0.2%:CCN1.0% were used to compare the hygroscopicity of the aerosols associated with African dust, BB, and background days. Average ratios were similar for days impacted by African dust and BB (0.54 ± 0.17 and 0.55 ± 0.17, respectively). A 29% higher average ratio was observed on background days (0.71 ± 0.14), owing in part to a strong presence of sea salt and reduced presence of more hydrophobic species such as those of a carbonaceous or mineral-dust nature. Finally, periods of heavy rainfall were shown to effectively decrease both CCN0.2% and CCN1.0% concentrations. However, the rate varied at which such concentrations increased after the rain. This work contributes knowledge on the nucleating ability of African dust and BB in a marine environment after varying periods of atmospheric transport (days to weeks). The results can be used to understand the hygroscopicity of these air mass types, predict how they may influence cloud properties, and provide a valuable model constraint when predicting CCN concentrations in comparable situations.
•African dust plumes do not perturb CCN concentrations in southeast Florida.•CCN concentrations are greatly increased on days influenced by biomass burning.•Heavy rainfall reduces CCN concentrations.•The duration varies for CCN concentrations to restore after periods of heavy rain.
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.
This study focuses on the long-term aerosol and precipitation chemistry measurements from colocated monitoring sites in Southern Florida between 2013 and 2018. A positive matrix factorization (PMF) ...model identified six potential emission sources impacting the study area. The PMF model solution yielded the following source concentration profiles: (i) combustion; (ii) fresh sea salt; (iii) aged sea salt; (iv) secondary sulfate; (v) shipping emissions; and (vi) dust. Based on these results, concentration-weighted trajectory maps were developed to identify sources contributing to the PMF factors. Monthly mean precipitation pH values ranged from 4.98 to 5.58, being positively related to crustal species and negatively related to SO42−. Sea salt dominated wet deposition volume-weighted concentrations year-round without much variability in its mass fraction in contrast to stronger seasonal changes in PM2.5 composition where fresh sea salt was far less influential. The highest mean annual deposition fluxes were attributed to Cl−, NO3−, SO42−, and Na+ between April and October. Nitrate is strongly correlated with dust constituents (unlike sea salt) in precipitation samples, indicative of efficient partitioning to dust. Interrelationships between precipitation chemistry and aerosol species based on long-term surface data provide insight into aerosol–cloud–precipitation interactions.
A hollow fiber MD module was tested at various air and brine flow rates and temperatures. A model based on heat and mass transport was developed to predict permeate production rates. The dependence ...of permeate production rate on brine temperature, air flow rate and brine flow rate was successfully simulated. Numerical simulations support the selection of membrane characteristics and air and brine flow conditions for optimal performance in water desalination. Condensation was predicted to occur on the air side of the membrane due to saturation of the sweeping gas and is accounted for in the model. In the absence of condensation, temperature profiles in the module could not be predicted correctly. The ratio of length to diameter of the membrane module is of particular significance as it dictates the permeation rate for a specific pore size membrane. Small pores require higher aspect ratios than large pores to obtain the same permeate production rate. The membrane module used in this study has an effective pore size of 0.1μm, which renders membrane transport the dominant source of mass transfer resistance to through-pore water vapor transport. A module with a larger pore size and appropriate aspect ratio should produce permeate at a significantly higher rate.
•Performance of hollow fiber, sweeping gas MD over a broad design/operational space.•Model calibration using membrane tortuosity as sole fitting parameter.•Inclusion of vapor recondensation in overall energy balances.•Dependence of water production on membrane and operational characteristics.•Mass transfer limitation from resistance to Knudsen diffusion.
To determine the impact of dynamic and aerosol processes on marine low clouds, we examine the seasonal impact of updraft speed w and cloud condensation nuclei concentration at 0.43 % supersaturation ...() on the cloud droplet number concentration (N(C)) of low-level clouds over the western North Atlantic Ocean. Aerosol and cloud properties were measured with instruments on board the NASA LaRC Falcon HU-25 during the ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment) mission in summer (August) and winter (February–March) 2020. The data are grouped into different loadings, and the density functions of NC and w near the cloud bases are compared. For low updrafts (w < 1.3 m s−1), NC in winter is mainly limited by the updraft speed and in summer additionally by aerosols. At larger updrafts (w > 3 m s−1), NC is impacted by the aerosol population, while at clean marine conditions cloud nucleation is aerosol-limited, and for high it is influenced by aerosols and updraft. The aerosol size distribution in winter shows a bimodal distribution in clean marine environments, which transforms to a unimodal distribution in high due to chemical and physical aerosol processes, whereas unimodal distributions prevail in summer, with a significant difference in their aerosol concentration and composition. The increase of is accompanied with an increase of organic aerosol and sulfate compounds in both seasons. We demonstrate that NC can be explained by cloud condensation nuclei activation through upwards processed air masses with varying fractions of activated aerosols. The activation highly depends on w and thus supersaturation between the different seasons, while the aerosol size distribution additionally affects NC within a season. Our results quantify the seasonal influence of w and on NC and can be used to improve the representation of low marine clouds in models.
New particle formation (NPF) is the dominant contributor to total particle number concentration and plays an important role in the cloud condensation nuclei budget. Airborne data from Aerosol Cloud ...meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) are used to address seasonal NPF statistics and factors related to NPF in and around clouds. Higher ratios of particle concentrations greater than 3 versus 10 nm (N3/N10) were mainly observed above boundary layer cloud tops during winter as compared to summer. Cold dry air and low aerosol surface area concentration facilitate NPF over the ACTIVATE region; these conditions are especially prevalent during flights coinciding with cold air outbreaks.
Plain Language Summary
Airborne data collected during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) campaign's first year of research flights during the winter (February 14 to March 12, 2020) and summer (August 13 to September 30, 2020) provide insight on new particle formation over the Western North Atlantic Ocean. The formation of new particles in the atmosphere is the primary contributor to total particle number concentration and plays a key role in the cloud condensation nuclei budget. Airborne observations reveal more active new particle formation during the winter than summer, especially just above boundary layer cloud tops. Influential conditions coinciding with new particle formation include cold and dry air, along with low aerosol surface area concentration and high levels of precursor gases from continental outflow. These conditions are shown to be most prevalent during cold air outbreaks in the winter off the United States East Coast.
Key Points
New particle formation is more prevalent off the U.S. East Coast in winter rather than summer
Ratio of particle number above 3 versus 10 nm peaks above cloud top regardless of season
Cold and dry conditions during cold air outbreaks coincide with new particle formation
Fireworks degrade air quality, reduce visibility, alter atmospheric chemistry, and cause short-term adverse health effects. However, there have not been any comprehensive physicochemical and optical ...measurements of fireworks and their associated impacts in a Southeast Asia megacity, where fireworks are a regular part of the culture. Size-resolved particulate matter (PM) measurements were made before, during, and after New Year 2019 at the Manila Observatory in Quezon City, Philippines, as part of the Cloud, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex). A high-spectral-resolution lidar (HSRL) recorded a substantial increase in backscattered signal associated with high aerosol loading ∼440 m above the surface during the peak of firework activities around 00:00 (local time). This was accompanied by PM2.5 concentrations peaking at 383.9 µg m−3. During the firework event, water-soluble ions and elements, which affect particle formation, growth, and fate, were mostly in the submicrometer diameter range. Total (>0.056 µm) water-soluble bulk particle mass concentrations were enriched by 5.7 times during the fireworks relative to the background (i.e., average of before and after the firework). The water-soluble mass fraction of PM2.5 increased by 18.5 % above that of background values. This corresponded to increased volume fractions of inorganics which increased bulk particle hygroscopicity, kappa (κ), from 0.11 (background) to 0.18 (fireworks). Potassium and non-sea-salt (nss) SO42- contributed the most (70.9 %) to the water-soluble mass, with their mass size distributions shifting from a smaller to a larger submicrometer mode during the firework event. On the other hand, mass size distributions for NO3-, Cl−, and Mg2+ (21.1 % mass contribution) shifted from a supermicrometer mode to a submicrometer mode. Being both uninfluenced by secondary aerosol formation and constituents of firework materials, a subset of species were identified as the best firework tracer species (Cu, Ba, Sr, K+, Al, and Pb). Although these species (excluding K+) only contributed 2.1 % of the total mass concentration of water-soluble ions and elements, they exhibited the highest enrichments (6.1 to 65.2) during the fireworks. Surface microscopy analysis confirmed the presence of potassium/chloride-rich cubic particles along with capsule-shaped particles in firework samples. The results of this study highlight how firework emissions change the physicochemical and optical properties of water-soluble particles (e.g., mass size distribution, composition, hygroscopicity, and aerosol backscatter), which subsequently alters the background aerosol's respirability, influence on surroundings, ability to uptake gases, and viability as cloud condensation nuclei (CCN).
Leveraging aerosol data from multiple airborne and surface‐based field campaigns encompassing diverse environmental conditions, we calculate statistics of the oxalate‐sulfate mass ratio (median: ...0.0217; 95% confidence interval: 0.0154–0.0296; R = 0.76; N = 2,948). Ground‐based measurements of the oxalate‐sulfate ratio fall within our 95% confidence interval, suggesting the range is robust within the mixed layer for the submicrometer particle size range. We demonstrate that dust and biomass burning emissions can separately bias this ratio toward higher values by at least one order of magnitude. In the absence of these confounding factors, the 95% confidence interval of the ratio may be used to estimate the relative extent of aqueous processing by comparing inferred oxalate concentrations between air masses, with the assumption that sulfate primarily originates from aqueous processing.
Plain Language Summary
The extent of atmospheric chemical processing remains an uncertain aspect of air mass characterization. Addressing this uncertainty is important because chemical reactions in the atmosphere in the presence of water (aqueous processing) produce a large fraction of global aerosol mass. The oxalate‐to‐sulfate ratio has been proposed as an indicator of aqueous processing, where higher values point to increased processing of an air mass. In this study, we quantify a range in the oxalate‐to‐sulfate mass ratio (0.0154–0.0296) using data from multiple field campaigns encompassing a diverse set of environments. This range is robust near the surface for particles below 1 micrometer in diameter. Larger particles, especially dust, and biomass burning particles significantly affect the oxalate‐to‐sulfate ratio and thus may confound the interpretation of a high oxalate‐to‐sulfate ratio as a signal of aqueous processing. In the absence of dust and biomass burning particles, the oxalate‐to‐sulfate ratio range may be used to compare the relative extent of aqueous processing between different air masses.
Key Points
Oxalate‐sulfate mass ratios show similarity across multiple environments (95% confidence interval: 0.0154–0.0296; R = 0.76; N = 2,948)
Oxalate‐sulfate mass ratio is biased toward higher values in presence of coarse aerosol particles and/or biomass burning
Ground‐based, size‐resolved measurements reveal that the ratio can be robust within the mixed layer for the submicrometer mode
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