Marine boundary layer aerosol in the eastern North Atlantic: seasonal variations and key controlling processes
2018
Abstract. The response of marine low cloud systems to changes in aerosol concentration
represents one of the largest uncertainties in climate simulations. Major
contributions to this uncertainty are derived from poor understanding of aerosol
under natural conditions and the perturbation by anthropogenic emissions. The
eastern North Atlantic (ENA) is a region of persistent but diverse marine
boundary layer (MBL) clouds, whose albedo and precipitation are highly
susceptible to perturbations in aerosol properties. In this study, we examine
MBL aerosol properties, trace gas mixing ratios, and meteorological
parameters measured at the Atmospheric Radiation Measurement Climate Research
Facility's ENA site on Graciosa Island, Azores, Portugal, during a 3-year
period from 2015 to 2017. Measurements impacted by local pollution on
Graciosa Island and during occasional intense biomass burning and dust events
are excluded from this study. Submicron aerosol size distribution typically
consists of three modes: Aitken (At, diameter D p ∼ 100 nm),
accumulation (Ac, D p within ∼100 to ∼300 nm), and
larger accumulation (LA, D p > ∼ 300 nm) modes, with average
number concentrations (denoted as N At , N Ac , and
N LA below) of 330, 114, and 14 cm −3 , respectively.
N At , N Ac , and N LA show contrasting seasonal
variations, suggesting different sources and removal processes.
N LA is dominated by sea spray aerosol (SSA) and is higher in winter and
lower in summer. This is due to the seasonal variations of SSA production,
in-cloud coalescence scavenging, and dilution by entrained free troposphere
(FT) air. In comparison, SSA typically contributes a relatively minor
fraction to N At (10 %) and N Ac (21 %) on an annual basis. In
addition to SSA, sources of Ac-mode particles include entrainment of FT
aerosols and condensation growth of Aitken-mode particles inside the MBL, while
in-cloud coalescence scavenging is the major sink of N Ac . The observed
seasonal variation of N Ac , being higher in summer and lower in winter,
generally agrees with the steady-state concentration estimated from major
sources and sinks. N At is mainly controlled by entrainment of FT
aerosol, coagulation loss, and growth of Aitken-mode particles into the
Ac-mode size range. Our calculation suggests that besides the direct
contribution from entrained FT Ac-mode particles, growth of entrained FT
Aitken-mode particles in the MBL also represent a substantial source of
cloud condensation nuclei (CCN), with the highest contribution potentially
reaching 60 % during summer. The growth of Aitken-mode particles to CCN size
is an expected result of the condensation of sulfuric acid, a product from
dimethyl sulfide oxidation, suggesting that ocean ecosystems may have a
substantial influence on MBL CCN populations in the ENA.
Keywords:
- Correction
- Source
- Cite
- Save
- Machine Reading By IdeaReader
113
References
25
Citations
NaN
KQI