Aerosol responses to precipitation along North American air trajectories arriving at Bermuda

Abstract

Abstract. North American pollution outflow is ubiquitous over the western North Atlantic Ocean, especially in winter, making this location a suitable natural laboratory for investigating the impact of precipitation on aerosol particles along air mass trajectories. We take advantage of observational data collected at Bermuda to seasonally assess the sensitivity of aerosol mass concentrations and volume size distributions to accumulated precipitation along trajectories (APT). The mass concentration of particulate matter with aerodynamic diameter less than 2.5 µm normalized by the enhancement of carbon monoxide above background (PM2.5/ΔCO) at Bermuda was used to estimate the degree of aerosol loss during transport to Bermuda. Results for December–February (DJF) show that most trajectories come from North America and have the highest APTs, resulting in a significant reduction (by 53 %) in PM2.5/ΔCO under high-APT conditions (> 13.5 mm) relative to low-APT conditions (< 0.9 mm). Moreover, PM2.5/ΔCO was most sensitive to increases in APT up to 5 mm (−0.044 µg m−3 ppbv−1 mm−1) and less sensitive to increases in APT over 5 mm. While anthropogenic PM2.5 constituents (e.g., black carbon, sulfate, organic carbon) decrease with high APT, sea salt, in contrast, was comparable between high- and low-APT conditions owing to enhanced local wind and sea salt emissions in high-APT conditions. The greater sensitivity of the fine-mode volume concentrations (versus coarse mode) to wet scavenging is evident from AErosol RObotic NETwork (AERONET) volume size distribution data. A combination of GEOS-Chem model simulations of the 210Pb submicron aerosol tracer and its gaseous precursor 222Rn reveals that (i) surface aerosol particles at Bermuda are most impacted by wet scavenging in winter and spring (due to large-scale precipitation) with a maximum in March, whereas convective scavenging plays a substantial role in summer; and (ii) North American 222Rn tracer emissions contribute most to surface 210Pb concentrations at Bermuda in winter (∼ 75 %–80 %), indicating that air masses arriving at Bermuda experience large-scale precipitation scavenging while traveling from North America. A case study flight from the ACTIVATE field campaign on 22 February 2020 reveals a significant reduction in aerosol number and volume concentrations during air mass transport off the US East Coast associated with increased cloud fraction and precipitation. These results highlight the sensitivity of remote marine boundary layer aerosol characteristics to precipitation along trajectories, especially when the air mass source is continental outflow from polluted regions like the US East Coast.

The effect of clouds and precipitation on the aerosol concentrations and composition in a boreal forest environment

“…Both Tunved et al (2013) and Kesti et al (2020) concluded that the particles in the accumulation mode size range show the strongest sensitivity to the precipitation along the air mass trajectories. Dadashazar et al (2021) observed the strongest sensitivity of the PM 2.5 mass to accumulated precipitation of up to 5 mm, while accumulated precipitation exceeding this limit had only minor effects on the PM 2.5 mass. Similar behaviour was described by Tunved et al (2013) -the particle number size distribution was clearly affected by up to 10 mm of accumulated precipitation, and a horizontal asymptote was achieved beyond that.…”

“…They observed that a greater reduction in the accumulation mode particle concentration usually coincided with precipitation along the trajectory. A recent study investigated how precipitation along air masses affects aerosol mass and vol-ume observed in Bermuda (Dadashazar et al, 2021). They concluded that remote marine boundary layer aerosol characteristics are relatively sensitive to the precipitation along the air mass trajectories.…”

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

“…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 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.…”

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

“…Here the aerosol processes which increase the particle size of Aitken and accumulation modes are negligible, and the difference in the H group suggests another aerosol source during summer. The WNAO is directly located in the Northern Hemisphere west wind band in winter, but during summertime the anticyclonic circulation driven by the Bermuda-Azores High influences the study region with a southwest wind component (Sorooshian et al, 2020;Painemal et al, 2021;Dadashazar et al, 2021a). Therefore the aerosol sources can change between the seasons.…”