Long-range transported North American wildfire aerosols observed in marine boundary layer of eastern North Atlantic

Zheng, Guangjie; Sedlacek, Arthur J.; Aiken, A. C.; Feng, Yan; Watson, Thomas; Raveh‐Rubin, Shira; Uin, Janek; Lewis, Ernie R.; Wang, Jian

doi:10.1016/j.envint.2020.105680

Cited by 62 publications

(69 citation statements)

References 136 publications

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“…Compared to NCCN, the vertical profiles of Npre-CCN,bg are very similar to those of Npre-CCN, implying that, on average, continental emissions have relatively weaker impact on Npre-CCN. This is consistent with the picture that aged aerosols in long-range transported continental plumes are dominated by accumulation-mode particles (Zheng et al, 2020b) and a substantial fraction of Aitken-mode particles in the FT is produced by NPF in the outflow of convective and frontal clouds followed by the coagulation and condensational growth (Clarke et al, 1998;Andreae et al, 2018;Williamson et al, 2019;McCoy et al, 2020) .…”

Section: Differences Between Mbl and Ft Aerosolssupporting

confidence: 90%

“…In North America, biomass burning is more frequent and its emissions are stronger during summertime. A large fraction of the biomass burning emissions are lofted into the FT (Zheng et al, 2020b). In addition, due to the warmer land surface temperature compared to that of the ocean, the lower atmosphere is destabilized and continental outflow is often lifted above the MBL as it is transported over the Atlantic Ocean.…”

Section: Vertical Profiles Of Trace Gas Mixing Ratiosmentioning

confidence: 99%

“…The higher Npre-CCN and NCCN in the FT contribute to the elevated values in the MBL during the summer through entrainment. The higher MBL NCCN during summer is also partially due to the increased growth rate of nucleation and Aitken-mode particles as a result of stronger oceanic VOC emission (Zawadowicz et al, 2020;Zheng et al, 2020b). Stronger precipitation and thus coalescence scavenging of CCN can also contribute to the seasonal variation of NCCN in the MBL.…”

Section: Seasonal Variationmentioning

confidence: 99%

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Vertical profiles of trace gas and aerosol properties over the Eastern North Atlantic: Variations with season and synoptic condition

Zheng¹,

Jensen²,

Knopf³

et al. 2021

Preprint

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Abstract. Because of their extensive coverage, marine low clouds greatly impact the global climate. Presently, the response of marine low clouds to the changes in atmospheric aerosols remains a major source of uncertainty in climate simulations. One key contribution to this large uncertainty derives from the poor understanding of the properties and processes of marine aerosols under natural conditions, and the perturbation by anthropogenic emissions. The Eastern North Atlantic (ENA) is a region of persistent but diverse subtropical marine boundary layer (MBL) clouds, where cloud albedo and precipitation are highly susceptible to perturbations in aerosol properties. Here we examine the key processes that drive the cloud condensation nuclei (CCN) population in the MBL using comprehensive characterizations of aerosol and trace gas vertical profiles during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign. During ACE-ENA, a total of 39 research flights were conducted in the Azores, 20 during summer 2017, and 19 during winter 2018. During summer, long-range transported aerosol layers were periodically observed in the lower free troposphere (FT), leading to elevated FT CCN concentrations (NCCN). Both biomass burning and pollution from North America contribute to submicron aerosol mass in these layers, with pollution likely the dominant contributor. In contrast, long-range transported continental emissions have a much weaker influence on the aerosol properties in the ENA during the winter season. While the entrainment of FT air is a major source of particle number in the MBL for both seasons, on average, it does not serve as a direct source of CCN in the MBL because the average FT NCCN is the same or even lower than that in the MBL. The particle number flux due to FT entrainment is dominated by pre-CCN (particles that are too small to form cloud droplets under typical conditions, i.e., particles with sizes below the Hoppel minimum) due to the elevated Npre-CCN in the lower FT. Once these pre-CCN are entrained into the MBL, they can grow and reach CCN size range through condensational growth, representing an indirect and major source of MBL CCN at ENA. The impact of synoptic condition on the aerosol properties is examined. Under pre-front and post-front conditions, shallow convective activity often leads to a deep and decoupled boundary layer. Coalescence scavenging and evaporation of drizzle below clouds leads to much reduced NCCN and larger accumulation-mode particle sizes in the upper, cloud-containing decoupled layer, indicating that surface measurements overestimate the NCCN relevant to the formation of MBL clouds under decoupled conditions.

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Section: Differences Between Mbl and Ft Aerosolssupporting

confidence: 90%

Section: Vertical Profiles Of Trace Gas Mixing Ratiosmentioning

confidence: 99%

Section: Seasonal Variationmentioning

confidence: 99%

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Vertical profiles of trace gas and aerosol properties over the Eastern North Atlantic: Variations with season and synoptic condition

Zheng¹,

Jensen²,

Knopf³

et al. 2021

Preprint

Self Cite

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“…Second, this study represents a single research flight during the campaign and does not suggest that the same microphysical behavior should persist in marine stratocumulus clouds for every day of the year. While the entrainment‐mixing and drizzling behavior observed here is quite representative of our understanding of marine stratocumulus clouds (Wang et al., 2009; Wood, 2005; Yum et al., 2015), certain environmental perturbations such as cold fronts (Kazemirad & Miller, 2020) and increased aerosol loading (Zheng et al., 2020) can affect cloud microphysics significantly. Further studies involving all available research flights during multiple such IOPs to include seasonal variations should provide a better understanding of microphysical variability in marine stratocumulus clouds.…”

Section: Discussionsupporting

confidence: 60%

Vertical Variation of Turbulent Entrainment Mixing Processes in Marine Stratocumulus Clouds Using High‐Resolution Digital Holography

et al. 2021

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Marine stratocumulus clouds contribute significantly to the Earth's radiation budget due to their extensive coverage and high albedo. Yet, subgrid variability in cloud properties, such as aerosol concentration, droplet number, and precipitation rates, lead to considerable errors in global climate models. While these clouds usually have small vertical extent, turbulent entrainment‐mixing and precipitation can generate significant variations in droplet number, size, and relative dispersion with altitude. Here, we analyze turbulent entrainment‐mixing processes and the variability in cloud microphysical properties as a function of height within a warm marine stratocumulus cloud layer over the Eastern North Atlantic. We use high‐resolution airborne holographic measurements and compare them with local turbulence measurements. We find that entrainment‐mixing is primarily inhomogeneous near cloud top and homogeneous near cloud base. Further analysis of Damköhler number and transition scale number are able to explain the mixing mechanisms at different cloud heights using phase relaxation but not droplet evaporation as the microphysical time scale. A modified droplet evaporation time scale that considers local saturation deficit using a simple linear mixing model is developed, and it is able to reliably explain the observed mixing mechanisms. This study reinforces the importance of turbulent mixing and the use of appropriate microphysical time scales in determining cloud microphysical processes.

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“…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).…”

Section: Introductionmentioning

confidence: 99%

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

Silva

Ridley

Heald

2020

Earth and Space Science

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Atmospheric aerosol over the North Atlantic Ocean impacts regional clouds and climate. In this work, we use a set of sun photometer observations of aerosol optical depth (AOD) located on the Graciosa and Cape Verde islands, along with the GEOS-Chem chemical transport model to investigate the sources of these aerosol and their transport over the North Atlantic Ocean. At both locations, the largest simulated contributor to aerosol extinction is the local source of sea-salt aerosol. In addition to this large source, we find that signatures consistent with long-range transport of anthropogenic, biomass burning, and dust emissions are apparent throughout the year at both locations. Model simulations suggest that this signal of long-range transport in AOD is more apparent at higher elevation locations; the influence of anthropogenic and biomass burning aerosol extinction is particularly pronounced at the height of Pico Mountain, near the Graciosa Island site. Using a machine learning approach, we further show that simulated observations at these three sites (near Graciosa, Pico Mountain, and Cape Verde) can be used to predict the simulated background aerosol imported into cities on the European mainland, particularly during the local winter months, highlighting the utility of background AOD monitoring for understanding downwind air quality.

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Long-range transported North American wildfire aerosols observed in marine boundary layer of eastern North Atlantic

Cited by 62 publications

References 136 publications

Vertical profiles of trace gas and aerosol properties over the Eastern North Atlantic: Variations with season and synoptic condition

Vertical profiles of trace gas and aerosol properties over the Eastern North Atlantic: Variations with season and synoptic condition

Vertical Variation of Turbulent Entrainment Mixing Processes in Marine Stratocumulus Clouds Using High‐Resolution Digital Holography

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

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