Seasonal updraft speeds change cloud droplet number concentrations in low level clouds over the Western North Atlantic

. Low level clouds over the Western North Atlantic show a seasonal cycle in cloud properties which anticorrelates to aerosol concentrations. To determinate the impact of dynamic and aerosol processes within marine low clouds we examine the seasonal impact of updraft speed w and cloud condensation nuclei concentration at 0.43% supersaturation (N CCN 0 . 43% ) 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 5 meTeorology Interactions oVer the western ATlantic Experiment) mission in summer (August) and winter (February-March) 2020. The data are grouped in different N CCN 0 . 43% loadings and the density functions of N C and w near the cloud bases are compared. For low updrafts ( w < 1 . 3 m s − 1 ), N C in winter are mainly limited by the updraft speed and in summer additionally by aerosols. At larger updrafts ( w > 3 m s − 1 ), N C are impacted by the aerosol population, while at clean marine conditions cloud nucleation is aerosol limited and for high pollution it is influenced by aerosols and updraft. The aerosol size distribution 10 in winter shows a bimodal distribution in clean marine environments, which transforms to a unimodal distribution in high pollution levels due to altering processes, whereas unimodal distributions prevail in summer with a significant difference in their aerosol concentration and composition. The increase in pollution level is accompanied with an increase of organic aerosol and sulfate compounds in both seasons. We demonstrate that N C 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 15 thus supersaturation between the different seasons, while the aerosol size distribution additionally affects N C within a season. Our results quantify the seasonal influence of w and N CCN 0 . 43% on N C and can be used to improve the representation of low marine clouds in models. The observational data presented in this study includes key parameters which are used in state-of-the-art aerosol-climate models to describe aerosol-induced cloud modifications. Consistent observations of the aerosol number concentration, size 405 distribution and composition, w as well as N C are provided for a wide range of conditions in the winter and summer seasons. Hence the data could serve as a valuable basis for evaluating and further improving the representation of aerosol-cloud interactions in future climate simulations.


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Company Incorporated (SPEC Inc.) is a forward-scattering probe, which counts single particles in the diameter size range 80 of 1.5 − 50 µm. In this analysis we use only particles with diameters larger than 3µm. The FCDP uses a laser beam at 785 nm wavelength to collect light scattered by particles passing through the laser beam according to Mie theory in a 4°− 12°c ollection angle. A 70:30 beam splitter is used to split the collected light to a signal and qualifier detector. The signal detector has a 800 µm pinhole for coincidence reduction (Lance, 2012) and a rectangular slit aperture with 800 µm length and 200 µm width. Both detectors convert the incoming light intensity into corresponding voltages and amplify them over two stages. The 85 beam diameter on the detectors depends on the distance of the measured particle from the focal plane of the collecting lens system. The ratio of the qualifier voltage to signal voltage is the so-called depth of field (DoF) criteria which can be used to limit the sample area of the probe, because the slit aperture width restricts the intensity on the qualifier detector depending on the magnification of the beam diameter. In this analysis we use a DoF criteria >0.6, which is equivalent to a calibrated sample area of 0.248 mm 2 (Lance et al., 2010;Faber et al., 2018). With a sampling rate of 25 ns the FCDP additionally 90 stores the transit time, inter-arrival time and waveform of each particle. These parameters are used for data corrections, see Baumgardner et al. (1985); SPEC inc (2012). Coincidence correction is applied by deriving a theoretical particle transit time, determined by particle air speed (PAS) and particle diameter, under consideration of a top hat intensity along the laser beam cross section. Measured particles with transit times larger than 125% of the theoretical transit time are deemed coincident and operated in two modes during ACTIVATE. The first is a continuous flow mode where ambient air enters a column shaped humidified chamber with a constant supersaturation of 0.43%. Aerosols are activated depending on their size and chemical properties. The droplets are measured afterwards by an optical particle counter. The second is a scanning flow mode where the flow rate in the chamber is changed while a constant temperature gradient is maintained (Moore and Nenes, 2009). Here an aerosol sample is exposed to a continuously changing supersaturation in the chamber and the concentration of activated 165 aerosols N CCN is measured depending on supersaturation. One scan is typically done in a 10 − 60 seconds time interval and in this analysis we use the mean of N CCN in a supersaturation range of 0.40−0.46% to appromximate N CCN 0.43% . The uncertainty in percent supersaturation is ±0.04 and in N CCN 0.43% ±10%.
Since the instrument supersaturation is fixed in continuous flow mode and artificially generated in scanning flow mode we have to estimate the supersaturation in cloud base. The maximum supersaturation S max is calculated according to Pinsky et al. 170 (2012) with where C is determined by cloud base temperature and pressure, and w and N C are the updraft speed and cloud droplet number concentration, respectively, measured in cloud base.

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Submicron non-refractory aerosol chemical composition was measured by a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS; Aerodyne Research Inc. DeCarlo et al., 2006;Hilario et al., 2021). Mass concentrations of sulfate, nitrate, chloride, ammonium, and organic matter were recorded at 1Hz and averaged to 30-s for all subsequent analyses.
Measurements were made isokinetically using a forward-facing dual-diffuser aircraft inlet (model 1200, Brechtel Manufacturing Inc.) and were pressure-controlled at 500 torr. Mass concentrations were processed using default relative ionization 180 efficiencies for each chemical component, with a collection efficiency of unity, and are reported at standard temperature and pressure (STP; 273.15 K and 1013.25 mb).
The particle-into-liquid sampler (PILS) obtained water-soluble aerosol composition data. Sampled aerosol particles were grown into droplets that were collected via inertial impaction and transported to vials on a rotating carousel. The liquid content of the vials was analyzed post-flight via ion chromatography for water-soluble ions (Sorooshian et al., 2006). This study reports Spectrometer (LAS; model 3340, TSI, Inc.; Moore et al., 2021) measured 100 − 3162 nm diameter particles at 1Hz time response. SMPS sizing is calibrated and frequently verified using NIST-traceable polystyrene latex spheres. LAS sizing is calibrated using lab-generated monodisperse ammonium sulfate (refractive index = 1.52). Each instrument sampled dried air 195 from the same common inlet as the HR-ToF-AMS and data are reported at STP.

Methodology
In this study we select the data a priori into pairs of series of below cloud base (BCB) and above cloud base (ACB) legs resulting in two pairs per ensemble (ensemble is a collection of legs below, in, and above clouds) flown during ACTIVATE, shown in Figure 2. This flight design intends for measurements to reflect the same environment. Closely spaced aerosol and 200 cloud measurements are ensured by taking the latest full N CCN scan or 60 seconds of continuous N CCN 0.43% measurements of the BCB leg and the last measurement of a cloud portion in the nearest ACB leg is restricted to never exceed a horizontal distance of 40 km to the aerosol measurement. Cloud periods are defined as seconds with a threshold of liquid water content > 0.02 gm −3 and N C > 20 cm −3 . We additionally excluded pairs with precipitation occurrences in the BCB leg by using the 2D-S size distribution and images, since the N CCN 0.43% measurements are influenced by the large particles shattering on 205 the aerosol inlet and precipitation indicates that the cloud is at a different point of its life cycle where agglomeration and coalescence altered N C and aerosol removal occurred below cloud. Each flight leg pair consists of a N CCN 0.43% distribution taken from the pair's BCB leg either in continuous flow or scanning flow mode, a mean aerosol loading derived from the N CCN 0.43% distribution, N C and positive vertical velocity measurements (updraft speeds w) in cloud portions of the pair's ACB leg.
For ensuring similar environmental conditions the pairs are classified with respect to their mean N CCN 0.43% into a low polluted (LP), medium polluted (MP) and high polluted (HP) group. For comparison both seasons share the boundaries separating the groups and the bin boundaries are chosen by identifying modes in the distribution of all winter pair mean N CCN 0.43% values.
The LP groups contains N CCN 0.43% from the minimum measured to 372 cm −3 , the MP group extends from > 372 − 769 cm −3 and the HP group is defined for > 769 cm −3 to the maximum measured in the respective season. The Probability Matching 215 Method (PMM) is used on each group's set of N C and w within a 2.5 to 97.5 percentile interval to quantify the impact of w for the different pollution levels.
We use the effective updraft speed w eff for approximating the updraft through the measured w density function in cloud base With the help of the w to N C relation from the PMM the corresponding N C to w eff can be derived and therefore a S max 220 estimate, which is representative for the supersaturation in cloud base of the respective group. We use the variability and magnitude of w with the related S max estimates, the aerosol size distribution and chemical composition to quantify their contribution to the activation of CCN in the winter and summer season 2020 for different pollution levels.

Probability Matching Method
The PMM was proposed by Calheiros and Zawadzki (1987) for a statistical comparison of radar reflectivity to rain rate. The 225 derived relationship is verified and performs significantly better than power law regression (Rosenfeld et al., 1994). Additional improvements by taking physical parameters into account for different rain type classification were done by Rosenfeld et al. (1995). The PMM is mathematically justified with an error estimation by Haddad and Rosenfeld (1997), and Braga et al. (2017a) showed that the PMM can be applied to get a reasonable relationship of w to N C . The PMM is based on the assumption that two related parameters taken in non-simultaneous measurements, sharing the same environment in terms of climatolog-230 ical and physical means, are increasing monotonically with each other. The relationship can be computed by matching the percentiles of the parameter's density functions, with more details on the mathematical background described in Haddad and Rosenfeld (1997). Braga et al. (2021) showed good agreement between measurements of N C at cloud bases of convective clouds and estimations from an adiabatic parcel model.   Mean values and standard deviation in parenthesis for w and NC from ACB cloud portions, and NCCN 0.43% from the BCB legs. Dmax is the maximal distance of cloud measurements to the aerosol measurements and hACB is the height above cloud base with standard deviation in parenthesis. All data ensemble pairs used in this study from the ACTIVATE winter February-March 2020 deployment are given in Table 1.

Results and Discussion
We use selected pairs with a minimum in-cloud time above or equal 10 seconds for sufficient statistics. N C is predicted to reach its maximum at a height above cloud base depending on w and subsequent S max estimates in an adiabatic parcel model (Braga et al., 2021). That the activation of CCN into cloud droplets had sufficient time is ensured by taking only pairs into account with 250 cloud measurements at a height above cloud base h ACB greater than 35 m in this analysis. The h ACB is gauged by calculating the middle of the difference between leg-mean values at BCB and ACB altitudes. In total we use 39 pairs from 10 RF, where all needed data are available for the PMM application, with a combined duration of 1786 seconds in cloud. The aerosol loading mean N CCN 0.43% values range from 96 cm −3 in clean conditions up to 1788 cm −3 in high polluted environments. The mean of N C is between 208 − 1367 cm −3 . The measured w distributional mean ranges from 0.25 up to 2.07 ms −1 . During RF02 255 15 February 2020 flight a distinct shift of N CCN 0.43% was measured between 17:42 to 17:57 UTC which can be attributed to a plume crossing event and affected pairs were excluded from the analysis, because the link between aerosol environment and measured N C through cloud formation is questionable. The horizontal distance between aerosol measurements below cloud and cloud measurements in cloud base is mainly below 30 km and never exceeds 40 km. Results derived from the PMM are more robust with a choice of narrow a priori boundaries for classifying similar environmental conditions.

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The same procedure was applied to the flights of the ACTIVATE August 2020 deployment resulting in the pairs listed in Table 2. We use a total of 16 pairs from 5 RF with a combined duration of 360 seconds in cloud. The full data set of the ACTIVATE August-September 2020 deployment including CCN measurements is only available for the August period limiting available pairs. The reduced fraction of time in cloud is in line with the observed lower cloud fraction and horizontal dimension of clouds during summer. In addition to excluding pairs affected by precipitation the pairs in RF28 were not used 265 in the analysis because of a smoke layer possibly altering the cloud formation process. The aerosol loading mean N CCN 0.43% values range from 122 cm −3 in clean conditions up to 1995 cm −3 in high polluted environments. The pairs in summer exhibit   peaking at around 20 nm and an accumulation mode (100 − 1000 nm) at 100 nm. In contrast the HP group has a unimodal 290 distribution with a flat peak at 40 − 100 nm at similar dNdlogD p concentrations to the LP and MP group and exhibits a plateau below 20 nm which hints to an overlapping ultra fine particle mode. The integrated number concentration for particles greater than 85 nm N gt85 in the BCB leg, depicted in Figure 5c, shows that the steady increase from 472 cm −3 (LP) over   Since the groups are categorized by their mean N CCN 0.43% and the group's N gt85 are constantly higher than their N CCN 0.43% , the activation radii of the size distribution at 0.43% supersaturation is probably between 85 − 93 nm for the MP and HP group and around 106 nm for the LP group. The winter groups differ for particles smaller than 40 nm, which contributes a high fraction to the available aerosol population for the LP and MP group. We consider particles smaller than 40 nm as irrelevant for the cloud formation process itself, but as a critical reservoir for the accumulation mode through altering processes, which 305 can be seen in the HP group's distribution. The MP group with its high fraction of particles below 20 nm could hint to the process of new particle formation (Zheng et al., 2021). However, the aerosol size distributions display that for a critical activation radii down to 40 nm the HP group has the highest amount of particles being possible CCN, followed by the MP group and finally the LP group.
During summertime the aerosol size distribution of the LP and HP group are comparable by adhering to a unimodal distri-310 bution, but differ significantly in the dNdlogD p concentrations between 10 nm and 400 nm (see Figure 5b). This difference is reflected in N gt85 in Figure 5d with mean values of 241 cm −3 (LP) compared to 1418 cm −3 (HP). The summer group's mean N gt85 is smaller than their mean N CCN 0.43% , suggesting a critical activation radius of the size distribution below 85 nm at 0.43% supersaturation for both groups. Here the altering processes of Aitken and accumulation modes are negligible and the difference in the HP group suggest another source of pollution during summer. The WNAO is directly located in the Northern 315 hemisphere west wind band in winter, but during summertime the anticyclonic circulation driven by the Bermuda-Azores High influences the study region with a south west wind component (Sorooshian et al., 2020;Painemal et al., 2021;Dadashazar et al., 2021a). Therefore the sources of pollution can change between the seasons.
The wintertime aerosol mass concentrations in the BCB legs are given in Table 3. Sea salt is the dominant species with respect to mass throughout the season and has a high variability day to day and within a research flight. The highest concentrations were 320 measured during RF16 on 6 March 2020, which can be attributed to the HP group and thus yield high N CCN 0.43% . However, there is no observable trend of sea salt mass concentration between the groups. On the other hand OA shows a significant increase from the LP to the MP/HP group. It can be deduced that the MP and HP group are influenced by pollution sources like the North East Coast American outflow, while the LP group represents natural marine conditions. The SO 2− 4 , NO − 3 and NH + 4 mass concentrations have a slight increase from the clean marine condition (LP) to polluted conditions (MP/HP) and i.e. RF01 325 on 14 February 2020 is an outlier and has the highest values, which decreases farther offshore during the flight.
In Table 4 is the BCB aerosol mass concentration below cloud depicted for the August 2020 summertime period. The sea salt mass concentration is highly variable like wintertime with low statistics in the LP group. Negative values for NH + 4 mean that the mass concentration is lower than the calibrated background concentration and thus real. A significant increase from the LP to the HP group is measured for all species expect sea salt and suggest more pollution in the summer season. The difference in 330 mass concentration is not equally distributed with the smallest rate of a doubling for SO 2− 4 , followed by a factor of 4 for NO − 3 and a factor of over 6(20) for OA(NH + 4 ). The chemical composition of the aerosol population alters N C (Hoose and Möhler, 2012), i.e. the organic carbon species have variable influences depending on solubility, molecular weight and surface tension (Ervens et al., 2005). high polluted (HP/red) group with their boundaries of mean NCCN 0.43% in parenthesis. The dark shaded areas represent the measurement uncertainty of 20% in addition with the relative error calculated according to Haddad and Rosenfeld (1997) with the assumption that the standard deviation of NC in each group represents the ratio of the noisy variation in the NC measurements to the true variation in NC. c) The Smax estimate of each group is given in 'x' markers for the same w spectrum with the error as shallow lined shaded area. The vertical lines are the w eff with associated Smax. The same PMM and Smax analysis in b) and d) for the summertime LP and HP group.
3.3 Seasonal impact of w and N CCN 0.43% on N C 335 Figure 6a shows the application of the PMM to all groups of the winter season. The w to N C relations shows the fraction of activated aerosol from the aerosol size distribution for a given updraft of supersaturation, respectively. The LP group, which has a mean N C of 315(±165) cm −3 , shows the highest impact of w to N C for w < 1.4 ms −1 and reaches saturation for higher w values. The MP group exhibits a similar trend with a mean of 518(±304) cm −3 , but the impact of w is decreasing slower compared to the LP group for higher w. The HP group shows the strongest impact for w < 1.6 ms −1 and as a mean N C of 340 930(±630) cm −3 . In addition, it has a second mode with a strong increase in N C for w > 3 ms −1 .
The two domains of w in the HP group could represent the activation of smaller aerosol particles from the aerosol population.
Since the critical diameter of aerosol activation depends on the supersaturation and is shifted towards smaller diameters for higher supersaturation, the positive correlation of w and supersaturation results in smaller aerosols getting activated for higher w (Köhler, 1936;Dusek et al., 2006;Schulze et al., 2020). N C are slightly smaller than the respective group's N CCN 0.43% 345 leading to a mean supersaturation below 0.43% in winter. The LP group exhibits some characteristics of an aerosol-limited regime with N C highly depending on the available aerosol population, while the HP group shows the characteristics of an updraft-limited regime with N C being directly proportional to w (Reutter et al., 2009). The MP group is between both regimes and tend to the characteristics of an updraft-limited regime, since N C does not reach saturation for high w.
The S max estimate for each group's w eff in Figure 6c is decreasing with increasing pollution level and are 0.33%(LP), 350 0.26%(MP) and 0.18%(HP), respectively. Since the variability of updraft speed is higher with larger w, the local supersaturation can deviate from the derived S max estimates. The reduction of S max for increasing pollution levels demonstrate the water vapor competition of more activated CCN and thus function as a buffer for preventing higher supersaturation. The LP group's mean N C is above its mean N CCN 0.43% although S max is near, and below 0.43%, which could be explained by a contribution of the soluble Aitken mode particles in the bimodal aerosol size distribution . However, the winter groups exhibit 355 mean N C near N CCN 0.43% with a trend of a reduced fraction of activated aerosol with increasing pollution level.
In Figure 6b the PMM is applied to the summer season in the same way. The impact of w on N C has a similar trend in summer and winter for the LP group up to the maximal measured w of 1.3 ms −1 during summer and has a mean N C of 196(±55) cm −3 . The HP group has a nearly constant impact for the full range of w up to 2.1 ms −1 and a mean N C of 642(±389) cm −3 . The w to N C relation coincides with the wintertime equivalent for w below 1.7 ms −1 . The S max estimate 360 for each group's w eff in Figure 6d is analogously reduced from the LP to the HP group in summer as in winter, while between the seasons a halfing of the S max takes place. N gt85 of the summer LP group is significantly lower than its winter counterpart, thus less aerosol for cloud formation is available in clean conditions during summer compared to winter. On the other hand N gt85 of the HP summer group is substantially higher than during winter. Another key feature is the lower mean N gt85 in comparison to the mean N CCN 0.43% ,

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showing a higher fraction of activated CCN in summer for a given supersaturation of 0.43%, which hints to a lower mean critical supersaturation needed for activation of the summer aerosol composition. Table 3 and Table 4 show an increased mass concentration of OA and SO 2− 4 between the respective groups. The high hygroscopicity of SO 2− 4 is most likely accountable for the observed lower mean N gt85 than mean N CCN 0.43% , because the raised OA mass concentrations from the LP to HP group is not reflected. Lower supersaturation in summer due to the smaller updrafts results in less activated CCN. The bisection of w eff 370 in Figure 6b propagates through derived S max estimates to N C .

Conclusions
In this study we examine the seasonal impact of w and N CCN 0.43% on N C over the WNAO from an in-situ perspective during the ACTIVATE campaign. The impact is determined by a statistical approach with the PMM where pairs of flight legs below and in cloud base are used to categorize in-situ measurements into similar environmental conditions and N CCN 0.43% . We also 375 give detailed information on the aerosol size distribution and composition below cloud base. Key findings are summarized and related to 2020 winter (February-March) and summer (August) conditions as follows: -N C in low clouds over the WNAO show a positive correlation with w and N CCN 0.43% . Updrafts smaller than 1.3 ms −1 have the highest impact on N C in both seasons. Polluted environments exhibit a stronger w impact over the full w distribution in a season, while in clean marine environments the available N CCN limit N C for higher w.

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-The WNAO exhibits an anti-correlated seasonal cycle of N C and N CCN 0.43% at cloud base with 25% less N C and 71% more N CCN 0.43% in their overall observed mean values in summer compared to winter. The seasonal cycle is consistent with the anti-correlated AOD and N C cycle measured by remote sensing and satellite instruments (Dadashazar et al., 2021b).
-The mean values of w at cloud bases are 33% lower in summer compared to winter. Simultaneously the variability of 385 updraft speeds is reduced by 31% in summer. Both indicate a higher dynamical influence during winter. A correlation of N C and w is observed in the seasonal cycle and suggest that the difference between the seasons is driven by dynamics.
-The winter N CCN 0.43% directly below cloud shows a broad distribution due to different aerosol sources and pollution levels, while only clear sky or high polluted conditions were measured in summer. For high polluted environments, summer exhibits a 46% increased mean N CCN 0.43% .

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-The aerosol size distribution during winter exhibits a bimodal distribution in clean marine and medium polluted condition, which transforms into a unimodal distribution for higher pollution levels. The Aitken mode acts as reservoir for the accumulation mode, since N gt85 increases while the aerosol number concentrations do not differ significantly. In contrast to the winter period, the summer period is characterized by unimodal distributions and a clear difference between the aerosol concentrations of the pollution levels.

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-The aerosol composition shows a constant proportion of sea salt in each season, with an increased aerosol mass concentration measured in winter, which could be related to the increased surface wind speeds resulting in more efficient wind-driven sea salt emissions (Painemal et al., 2021). With the increase in pollution levels, a concomitant increase in OA, SO 2− 4 , NO − 3 and NH + 4 mass concentrations is measured in summer. In winter, the increase is comparatively moderate.

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w and related S max determine the range of activated CCN and S max is reduced at increasing pollution levels. As shown, w dominantly affects the activation of CCN and determines the fraction of activated aerosol and thus explains generally higher N C values during winter compared to summer.
The observational data presented in this study includes key parameters which are used in state-of-the-art aerosol-climate models to describe aerosol-induced cloud modifications. Consistent observations of the aerosol number concentration, size 405 distribution and composition, w as well as N C are provided for a wide range of conditions in the winter and summer seasons. Hence the data could serve as a valuable basis for evaluating and further improving the representation of aerosol-cloud interactions in future climate simulations.