In:
Atmospheric Chemistry and Physics, Copernicus GmbH, Vol. 18, No. 8 ( 2018-04-18), p. 5265-5292
Kurzfassung:
Abstract. Aerosol optical properties (AOPs) and supporting parameters – particle
number size distributions, PM2.5 mass concentrations, and the
concentrations of trace gases (NOx and NOy) – were measured at
SORPES, a regional background station in Nanjing, China from June 2013 to
May 2015. The aerosol was highly scattering: the average scattering
coefficient was σsp=403 ± 314 Mm−1, the
absorption coefficient σap=26 ± 19 Mm−1, and
the single-scattering albedo SSA = 0.93 ± 0.03 for green light.
The SSA in Nanjing appears to be slightly higher than published values from
several other sites in China and elsewhere. The average Ångström
exponent of absorption (AAE) for the wavelength range 370–950 nm was 1.04
and the AAE range was 0.7–1.4. These AAE values can be explained with
different amounts of non-absorbing coating on pure black carbon (BC) cores and different
core sizes rather than contribution by brown carbon. The AOPs had typical
seasonal cycles with high σsp and σap in
winter and low ones in summer: the averages were σsp=544 ± 422 and σap=36 ± 24 Mm−1 in
winter and σsp=342 ± 281 and σap=20 ± 13 Mm−1 in summer. The intensive AOPs had no
clear seasonal cycles, the variations in them were rather related to the
evolution of pollution episodes. The diurnal cycles of the intensive AOPs
were clear and in agreement with the cycle of the particle number size
distribution. The diurnal cycle of SSA was similar to that of the air
photochemical age, suggesting that the darkest aerosol originated from fresh
traffic emissions. A Lagrangian retroplume analysis showed that the
potential source areas of high σsp and σap are
mainly in eastern China. Synoptic weather phenomena dominated the cycle of
AOPs on a temporal scale of 3–7 days. During pollution episodes, modeled
boundary layer height decreased, whereas PM2.5 concentrations and
σsp and σap typically increased gradually and remained
high during several days but decreased faster, sometimes by even more than
an order of magnitude within some hours. During the growth phase of the
pollution episodes the intensive AOPs evolved clearly. The mass scattering
efficiency MSE of PM2.5 grew during the extended pollution episodes
from ∼ 4 to ∼ 6 m2 g−1 and the mass fraction of BCe decreased from
∼ 10 to ∼ 3 % during the growth phase of the episodes.
Particle growth resulted in the backscatter fraction decreasing from more than
0.16 to less than 0.10, SSA growing from less than 0.9 to more than 0.95, and
radiative forcing efficiency (RFE) changing from less than −26 W m−2 to more than −24 W m−2, which means that
the magnitude of RFE decreased. The RFE probability distribution at SORPES
was clearly narrower than at a clean background site which is in agreement
with a published RFE climatology.
Materialart:
Online-Ressource
ISSN:
1680-7324
DOI:
10.5194/acp-18-5265-2018
DOI:
10.5194/acp-18-5265-2018-supplement
Sprache:
Englisch
Verlag:
Copernicus GmbH
Publikationsdatum:
2018
ZDB Id:
2092549-9
ZDB Id:
2069847-1
