Organic enrichment in droplet residual particles relative to out of cloud over the northwestern Atlantic: analysis of airborne ACTIVATE data

Abstract

Abstract. 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.

Dropsonde observations during the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment

“…Dropsonde data help characterize meteorological conditions and derive large-scale divergence, vertical velocities, and surface heat fluxes useful for intercomparison and validation when compared to common reanalysis data sources, e.g., the fifth-generation European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA5), the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), and models using large eddy simulation 7 – 9 . A subset of ACTIVATE studies have classified wintertime flights into whether they qualify as a “cold air outbreak” 6 , 10 , 11 , for which dropsonde data are used along with published criteria requiring knowledge of the vertical thermodynamic structure of the atmosphere 12 . As demonstrated by another recent campaign investigating aerosol-cloud interactions (CAMP 2 Ex), dropsonde data such as vertically-resolved relative humidity can be used for model simulations of aerosol extinction and aerosol optical thickness 13 and for assistance in interpreting remote sensing data from HSRL-2 and RSP deployed on the King Air.…”

Tropospheric aerosols over the western North Atlantic Ocean during the winter and summer campaigns of ACTIVATE 2020: Life cycle, transport, and distribution

“…Observed sulfate concentrations exhibited a strong vertical gradient with much higher levels in the BL compared to winter along with enhancements at 3-4 km. The generally higher sulfate concentrations throughout the troposphere in summer reflect stronger oxidation of SO2 in both the gas phase and in-cloud (Dadashazar et al, 2022;Tai et al, 2010). The observed nitrate concentrations in the BL have a median value close to that in winter but show much smaller variability.…”

“…Submicron non-refractory aerosol composition was measured by the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS; Aerodyne) (DeCarlo et al, 2008;Hilario et al, 2021), operated in 1 Hz Fast-MS mode and averaged to 30 s time resolution. AMS data were collected downstream of an isokinetic double diffuser inlet (BMI, Inc.) and also sampled downstream of a counterflow virtual impactor (CVI) inlet (BMI, Inc.) when in cloud (Dadashazar et al, 2022;Shingler et al, 2012); only the former is used in this study. AMS measurements are reported at the standard temperature (273.15 K) and pressure (1013.25 hPa).…”

“…The ACTIVATE flights in the two-box region sampled this continental outflow extensively. Flights to the north often occurred during postfrontal conditions, reflecting one of the mission objectives to sample and study MBL clouds, especially during cold air outbreaks in winter that have been the subject of recent work (Tornow et al, 2022;Seethala et al, 2021;Corral et al, 2022;Li et al, 2022;Chen et al, 2022). In addition to lower temperature, the post-frontal air was characterized by subsiding motion with lower relative humidity and precipitation (dry air) as well as lower BL height.…”

Spatially coordinated airborne data and complementary products for aerosol, gas, cloud, and meteorological studies: the NASA ACTIVATE dataset

“…New HSRL-2 cloud retrieval products include cloud-top height, cloud-top extinction, and the cloud-top lidar ratio at horizontal resolutions of 75, 150, and 150 m, respectively (Johnathan W. Hair, personal communication, 2023). Relevant to ocean-air interactions, such as marine biogenic emissions (Corral et al, 2022a), ocean subsurface particulate backscatter coefficients at 532 nm are estimated at a depth of 10 m (Schulien et al, 2017) and made available for selected flights.…”