Abstract. Summertime Arctic aerosol size distributions are strongly controlled by
natural regional emissions. Within this context, we use a chemical transport
model with size-resolved aerosol microphysics (GEOS-Chem-TOMAS) to interpret
measurements of aerosol size distributions from the Canadian Arctic
Archipelago during the summer of 2016, as part of the “NETwork on Climate
and Aerosols: Addressing key uncertainties in Remote Canadian Environments”
(NETCARE) project. Our simulations suggest that condensation of secondary organic
aerosol (SOA) from precursor vapors emitted in the Arctic and near Arctic
marine (ice-free seawater) regions plays a key role in particle growth events
that shape the aerosol size distributions observed at Alert (82.5∘ N,
62.3∘ W), Eureka (80.1∘ N, 86.4∘ W), and
along a NETCARE ship track within the Archipelago. We refer to this SOA as
Arctic marine SOA (AMSOA) to reflect the Arctic marine-based and likely
biogenic sources for the precursors of the condensing organic vapors. AMSOA from a simulated flux (500 µgm-2day-1, north of
50∘ N) of precursor vapors (with an assumed yield of unity) reduces the
summertime particle size distribution model–observation mean fractional
error 2- to 4-fold, relative to a simulation without this AMSOA. Particle
growth due to the condensable organic vapor flux contributes strongly
(30 %–50 %) to the simulated summertime-mean number of particles with
diameters larger than 20 nm in the study region. This growth couples with
ternary particle nucleation (sulfuric acid, ammonia, and water vapor) and
biogenic sulfate condensation to account for more than 90 % of this
simulated particle number, which represents a strong biogenic influence. The simulated fit to
summertime size-distribution observations is further improved at Eureka and
for the ship track by scaling up the nucleation rate by a factor of 100 to
account for other particle precursors such as gas-phase iodine and/or amines
and/or fragmenting primary particles that could be missing from our
simulations. Additionally, the fits to the observed size distributions and total
aerosol number concentrations for particles larger than 4 nm improve with
the assumption that the AMSOA contains semi-volatile species: the
model–observation mean fractional error is reduced 2- to 3-fold for the Alert and
ship track size distributions. AMSOA accounts for about half of the
simulated particle surface area and volume distributions in the summertime
Canadian Arctic Archipelago, with climate-relevant simulated summertime
pan-Arctic-mean top-of-the-atmosphere aerosol direct (−0.04 W m−2) and
cloud-albedo indirect (−0.4 W m−2) radiative effects, which due
to uncertainties are viewed as an order of magnitude estimate. Future work
should focus on further understanding summertime Arctic sources of AMSOA.
Particle Size Distribution Dynamics Can Help Constrain the Phase State of Secondary Organic Aerosol
