Identifying a regional aerosol baseline in the eastern North Atlantic using collocated measurements and a mathematical algorithm to mask high-submicron-number-concentration aerosol events

Abstract

Abstract. High-time-resolution measurements of in situ aerosol and cloud properties provide the ability to study regional atmospheric processes that occur on timescales of minutes to hours. However, one limitation to this approach is that continuous measurements often include periods when the data collected are not representative of the regional aerosol. Even at remote locations, submicron aerosols are pervasive in the ambient atmosphere with many sources. Therefore, periods dominated by local aerosol should be identified before conducting subsequent analyses to understand aerosol regional processes and aerosol–cloud interactions. Here, we present a novel method to validate the identification of regional baseline aerosol data by applying a mathematical algorithm to the data collected at the U.S. Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) user facility in the eastern North Atlantic (ENA). The ENA central facility (C1) includes an aerosol observing system (AOS) for the measurement of aerosol physical, optical, and chemical properties at time resolutions from seconds to minutes. A second temporary supplementary facility (S1), located ∼0.75 km from C1, was deployed for ∼1 year during the Aerosol and Cloud Experiments (ACE-ENA) campaign in 2017. First, we investigate the local aerosol at both locations. We associate periods of high submicron number concentration (Ntot) in the fine-mode condensation particle counter (CPC) and size distributions from the Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) as a function of wind direction using a meteorology sensor with local sources. Elevated concentrations of Aitken-mode (< 100 nm diameter) particles were observed in correspondence with the wind directions associated with airport operations. At ENA, the Graciosa Airport and its associated activities were found to be the main sources of high-concentration aerosol events at ENA, causing peaks in 1 min Ntot that exceeded 8000 and 10 000 cm−3 at C1, in summer and winter, respectively, and 5000 cm−3 at S1 in summer. Periods with high Ntot not associated with these wind directions were also observed. As a result, the diverse local sources at ENA yielded a poor relationship between Ntot measurements collected at C1 and S1 (R2=0.03 with a slope =0.05±0.001). As a first approach to mask these events, the time periods when the wind direction was associated with the airport operations (west to northwest and southeast to south at C1 and east to south at S1) were applied. The meteorological masks removed 38.9 % of the data at C1 and 43.4 % at S1, and they did not significantly improve the relationship between the two sites (R2=0.18 with a slope =0.06±0.001). Due to the complexity of high-Ntot events observed at ENA, we develop and validate a mathematical ENA Aerosol Mask (ENA-AM) to identify high-Ntot events using 1 min resolution data from the AOS CPC at C1 and S1. After its parameterization and application, ENA-AM generated a high correlation between Ntot in the summer at C1 and S1 (R2=0.87 with a slope =0.84±0.001). We identified the regional baseline at ENA to be 428±228 cm−3 in the summer and 346±223 cm−3 in the winter. Lastly, we compared masked measurements from the AOS with the ARM Aerial Facility (AAF) during flights over C1 in the summer to understand submicron aerosol vertical mixing over C1. The high correlation (R2=0.71 with a slope of 1.04±0.01) observed between C1 and the AAF Ntot collected within an area of 10 km surrounding ENA and at altitudes < 500 m indicated that the submicron aerosol at ENA was well mixed within the first 500 m of the marine boundary layer during the month of July during ACE-ENA. Our novel method for determining a regional aerosol baseline at ENA can be applied to other time periods and at other locations with validation by a secondary site or additional collocated measurements.

Micro-spectroscopic and freezing characterization of ice-nucleating particles collected in the marine boundary layer in the eastern North Atlantic

“…For an IMF temperature of 240 K, we determined the activated INP fraction as given in Table 2. Applying mean total ambient particle concentrations for given sampling periods (Gallo et al, 2020) yields IMF INP number concentrations of about 15 to 40 INP L −1 , in accordance with the range of previous measurements of INP number concentrations (Knopf et al, 2021;DeMott et al, 2016DeMott et al, , 2010Mason et al, 2015). We further analyzed the IMF freezing data using the surface-based deterministic approach INAS and the surfaceand time-dependent approach of CNT expressed in our AB-IFM following closely our previous methodologies (China et al, 2017;Wang et al, 2012a, b).…”

“…Particles were collected by impaction using a Multi Orifice Uniform Deposition Impactor (MOUDI, 110-R). Since ambient particle numbers were low (between 330 and 540 cm −3 , Gallo et al, 2020) and sampling intervals for this ice nucleation study were limited, particle collection was conducted over several days intermittently. This ensured suffi-cient aerosol loading on substrates for single-particle chemical analyses and ice nucleation experiments based on our previous studies (China et al, 2017;Knopf et al, 2014;Wang et al, 2012a, b).…”

“…S2 in the Supplement provides more backward-trajectory calculations for the 2-4 d sampling periods.) The trajectories were calculated using global wind data from the ECMWF reanalyses ERA5 (Hersbach et al, 2020). The data are horizontally interpolated to a 0.5 • × 0.5 • grid, with 3-hourly time intervals and 137 vertical hybrid levels.…”

Long-range transported continental aerosol in the Eastern North Atlantic: three multiday event regimes influence cloud condensation nuclei

“…In addition to the AOS routine measurements, during two Intensive Operating Periods (IOPs) (June-July 5 2017 and January-February 2018) of the ARM Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign, the ARM Aerial Facility (AAF) Gulfstream-159 (G-1) research aircraft flew over the ENA site and provided in-situ characterizations of the marine boundary layer and lower free troposphere structure, as well as the vertical distribution and horizontal variability of low clouds and aerosols . High correlation (slope = 1.04 +/-0.01, r 2 = 0.7) between AOS submicron number concentrations of particles at the ENA fixed site and AAF measurements were found during 10 the summer indicating the broader regional representativeness of the AOS surface measurements when the boundary layer is well mixed (Gallo et al, 2020).…”

Exploring the Constraints on Simulated Aerosol Sources and Transport Across the North Atlantic With Island‐Based Sun Photometers

“…While previous studies have investigated the transport of aerosol species westward across the Equatorial Atlantic (e.g., Barkley et al, 2019) or the influence of specific events on transported species (Ancellet et al, 2016), we focus here on the North Atlantic with particular interest in understanding the long‐term sources of transported aerosol to continental Europe. Analyses of in situ observations indicate the importance of long‐range transport in the North Atlantic (e.g., Gallo et al, 2020; Zheng et al, 2020); however, there has been limited exploration and evaluation of model representations of aerosols over this region. Constraining this trans‐Atlantic transport is central to characterizing aerosol source contributions to the degradation of air quality over Europe as well as investigating how these aerosol species influence climate and biogeochemistry over the ocean (e.g., Carslaw et al, 2013; Foltz & McPhaden, 2008; Penner et al, 1994).…”