Knowledge on the microphysical properties of atmospheric aerosols is essential to better
evaluate their radiative forcing. This paper presents an estimate of
the real part of the refractive indices (
Aerosols represent the largest uncertainty in estimating radiative forcing, through strongly affecting the energy balance of the Earth by scattering and/or absorbing solar radiation (IPCC, 2007; Jacobson, 2001; Ramanathan and Carmichael, 2008), and cloud formation (Jacobson, 2006; Rosenfeld et al., 2008). They also strongly affect visibility, causing severe haze problems in polluted regions (Wu et al., 2005; Zhang et al., 2010). Submicron particles commonly make up the majority of total aerosol mass in polluted urban atmospheres (Tao et al., 2014; Shi et al., 2014), and contribute to the majority of light scattering and absorption (Seinfeld and Pandis, 2006; Bond and Bergstrom, 2006).
Optical properties of atmospheric aerosols are sensitive to their physical (e.g., size, density, and morphology) and chemical properties (Moffet and Prather, 2009; Moffet et al., 2008; Raut et al., 2009). Aerosols are generally internally mixed, composed of various compounds, and therefore uncertainties are inevitable when modeling their effects based on the assumption that they are composed of several individual species that are externally mixed (Sullivan and Prather, 2005). Therefore, knowledge of the direct link between aerosol optical properties and mixing state is required to accurately predict their radiative forcing (Bauer et al., 2013). However, the chemical composition, size, optical property, shape, and density of aerosols are generally measured by independent analytical techniques, which may inevitably introduce uncertainties when establishing their relationships.
Efforts have been made to embed light-scattering measurements into aerosol
mass spectrometry in order to simultaneously retrieve as much information as
possible for a single particle (Murphy et al., 2004; Moffet and Prather,
2005; Cross et al., 2007). For example, Moffet et al. (2008) have
successfully retrieved the real part of the refractive indices (
The relationship between the mixing state and optical properties of ambient
aerosols in China is still not well understood. Previous studies in China
typically performed model calculations, mostly based on the assumption of
particle mixing state (Ma et al., 2012; Tao et al., 2014), and no previous direct
measurements are available. Herein, we applied a real-time single-particle
aerosol mass spectrometer (SPAMS) with embedded light-scattering measurements
to explore the microphysical properties (i.e.,
Single-particle measurements were carried out at an urban site in Guangzhou
(Bi et al., 2011) from 13 October to 29 November 2012, using a SPAMS
developed by Hexin Analytical Instrument Co., Ltd (Guangzhou, China).
Temporal profiles of local meteorological parameters, including solar
radiation, temperature, relative humidity, wind direction, wind speed, and
air quality parameters (i.e., NO
A total of approximately 3 500 000 single-particle mass spectra were
characterized and statistically analyzed in the present study. The analysis
mainly covered particles with
A large data set, including size, chemical composition, and the LSS of each particle at wavelength of 532 nm, was collected by
the SPAMS. Scattering by spherical submicron particles can be well described
by Mie theory (Bohren and Huffman, 1981). In Mie theory, the refractive index
is given by
The methodology is briefly summarized herein. Firstly, theoretical response
(i.e., PSCS)
is firstly compared with the LSS measured by SPAMS
from PSL particles with sizes ranging from 150 to 2000 nm. Then a
calibration curve is constructed to transform the LSS (at the 90th
percentiles, i.e., upper limit) measured by SPAMS to the PSCS, enabling a
quantitative comparison between the measured and theoretical PSCS. Before
performing the scattering calculation, the detected particles were grouped into
17 particle types on the basis of chemical compositions. Finally, a series of
The calibration curve is provided in Fig. S2 to show the relationship between
the experimental LSS and the theoretical PSCS (
Retrieved
SSE distribution of fitting between measured and theoretical PSCS
as a function of
The 17 particle types, resulted from ART-2a clustering, are in four
categories of similar chemical characteristics, namely: organics (OC),
elemental carbon (EC), internally mixed elemental carbon and organics (ECOC),
and Metal-rich. The majority of the single-particle types and their mass spectra
throughout the study were similar to those described in our previous publication
(Zhang et al., 2015b). Their mass spectra are provided in Fig. S3 and also
described in the Supplement. It is pointed that assuming
negligible absorption for the internally mixed EC particle types (including
EC and ECOC group) might introduce uncertainties for the estimation of
To retrieve
The OC group was characterized by three organic-rich particle types,
including organics dominantly internally mixed with sulfate and limited
nitrate (OC-S), organics internally mixed with both sulfate and nitrate (OC-SN),
and high-mass OC (HMOC). Figure 1 (left panel) exhibits the SSE between measured and
theoretical PSCS varying with the two variables
(top) Mass spectra, and (bottom) measured and best-fit theoretical PSCS for HMOC.
Mass spectra, and measured and best-fit theoretical PSCS for LC-EC, SC-EC, and NaK-EC, respectively.
The EC group contained three particle types, consisting of EC with more
carbon cluster ions (C
The ECOC group contained both OC and EC ion signatures in the mass spectra,
including potassium-rich particles internally mixed with sulfate/nitrate (K-S
for dominantly with sulfate, K-N for dominantly with nitrate, and K-SN for
both sulfate and nitrate), and ECOC internally mixed with sulfate/nitrate (ECOC-S
and ECOC-SN). The retrieved
The Metal-rich group, including Na-rich, Na-K, Fe-rich, Pb-rich,
Cu-rich, and internally mixed Fe–Cu–Pb types, also exhibits scattering curves
that are indicative of the existence of nearly spherical morphology (Figs. 4
and S5). The retrieved
(top) Mass spectra and (bottom) measured and best-fit theoretical PSCS for Na-rich type.
Number fraction, number of all the detected particles, and the
estimated average
The results show that the scattering curves for the majority of particle types
can be well modeled (with
For models that predict the radiative impact of aerosols, it is important to
be able to constrain the
The size, mass spectra information, and light-scattering signals were
simultaneously obtained for chemically segregated ambient aerosols in the
atmosphere of Guangzhou, China. Based on comparison between experimental
light-scattering measurements obtained by a SPAMS and Mie theoretical
calculation results,
The authors acknowledge financial support from the “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (XDB05020205), the National Nature Science Foundation of China (No. 41405131 and 41403091) and the Foundation for Leading Talents from Guangdong Province Government. Edited by: A. Hofzumahaus