Aerosol optical properties were measured with a seven-wavelength
aethalometer and a three-wavelength nephelometer at the suburban site SORPES
in Nanjing, China, in September 2013–January 2015. The aethalometer
compensation parameter

Aerosols affect both local, regional and global climate directly by
scattering and absorbing solar radiation and indirectly by modifying cloud
properties (e.g., IPCC, 2013). For the assessment of the direct radiative
forcing it is crucial that both light scattering and absorption are measured
accurately. Light-scattering measurements with the nephelometer are well
established, but absorption is more difficult. An ideal method would make the
measurement when the aerosol is in the suspended state. Such methods exist
but the simplest, relatively cheap and therefore the most commonly used light
absorption measurements are still based on collecting aerosols on a filter
tape and detecting the reduction of light transmittance through the filter.
Globally, probably the most widely used such filter-based method is the
aethalometer, produced by Magee Scientific. It converts the reduction of
light transmittance directly into black carbon (BC) mass concentrations by
assuming that BC is the only aerosol component affecting the reduction and by
assuming that the conversion is linear. The data are further used for
calculating the light absorption coefficient (

The algorithm presented by Virkkula et al. (2007) corrects the BC mass
concentration for the loading effect by multiplying the uncorrected
concentrations by the function

Despite the weaknesses, several authors have used the function

Also, the size of particles affects the absorption coefficients calculated
from filter-based measurements. One of the reasons is that the penetration
depth of the particles into the filter depends on their size and the depth
affects the amount of light interactions with the particles (e.g., Arnott et
al., 2005; Moteki et al., 2010; Nakayama et al., 2010). Lack et al. (2009)
found that for particles larger than about 350 nm absorption measured with
the Particle Soot Absorption Photometer (PSAP), another filter-based
instrument, was significantly underestimated, and concluded that the low bias
was linked to the enhanced forward scattering from the larger particles.
Müller et al. (2014) found that the asymmetry parameter – which is a
function of the backscatter fraction – of the particles collected on the
PSAP filter has significant effects on the derived

It was mentioned above that a site-related and seasonal variation of the
value of the compensation parameter

The measurements were conducted at the SORPES station (118

Total scattering coefficients (

The raw total scattering coefficients were corrected for truncation errors
by calculating first the Ångström exponents from the non-corrected
scattering coefficients and then following the formulas presented by
Müller et al. (2011) where the tabulated factors for no cutoff at the
inlet were used. To be used in the aethalometer data processing the
truncation-corrected

The backscatter fractions (

A seven-wavelength aethalometer (AE-31) was used for measuring light
absorption at

The core of the present paper is to analyze factors affecting the
compensation parameter

The aethalometer data were first used to calculate the uncorrected
attenuation coefficients, here

Overview of the data.

In the present work the absorption coefficients were calculated according to
both the Arnott et al. (2005) and Collaud Coen et al. (2010) algorithms with
the respective mean

There is a large uncertainty in the

The hypothesis here is that all particles sampled on the filter prior to the
spot change affect the

Also, the daily or 24 h averages are non-conventional: they were calculated by averaging the compensation parameters and the corresponding filter-spot-averaged scattering and absorption coefficients that were calculated from the aethalometer filter spot changes during any given day. The number of filter changes to be used for the averaging varied according to the concentration level: during low concentrations there may have been only one or two filter spot changes, during high concentrations more. Below, in the analyses of daily averages, only those days have been used during which at least two filter spot changes occurred. During the analyzed period, the average, the median, and the maximum number of filter spot changes were 6.4, 6, and 18 during one day, respectively.

Figure 1 shows filterspot-averaged and 24 h-averaged data that fulfilled
the criteria for further analyses: the relative humidity in the nephelometer
sample volume had to be less than 50 %, both the nephelometer and the
aethalometer data had to be continuous for at least 24 h. This ruled out
summer months (June–August) and most of September. There were 2066 filter
spot changes and 342 days that were used for further analyses. In the
accepted data the average

Wavelength dependency of the

Some descriptive statistics of the optical data are given here, even though a
more detailed analysis will be presented by Shen et al. (2015). The
average

There were many several-day-long pollution episodes in November 2013–January
2014 and again the following autumn in November 2014 during which scattering
and absorption coefficients exceeded 1000 and 100 Mm

The compensation parameters (Fig. 1d) calculated for each individual
filter-spot change were noisy; for instance, for

These observations were used and two filter spot changes were picked up
during which the filter-spot-averaged

Another interesting observation can be made about Fig. 2: the range of
compensation parameters is larger the longer the wavelength is. This suggests
that the longer wavelengths are more sensitive to the factors affecting the
compensation. The near-infrared wavelength at

Selected optical properties in November–December 2013.

The slope of the wavelength dependency of the compensation
parameter (

The compensation parameter (

A shorter, 2-month time series of the data is presented in Fig. 3. In
addition to those quantities presented in Fig. 1 also the slope

As far as the compensation parameters (

Regression statistics (

Daily averaged compensation parameters of

Since the time series of

The compensation parameter medians and averages correlated positively with
the backscatter fractions, and so did the other percentiles, but their
correlation was weaker. Note that the slopes of the linear regressions of

Instead of paying much attention to the

Daily averaged slope (

No detailed theoretical explanation of this relationship is attempted to be
given here, only some qualitative discussion. The aethalometer functions in a
way similar to the PSAP. Müller et al. (2014) showed that the optical
depth of an aerosol-loaded PSAP filter is a function of the asymmetry
parameter

Also,

Daily averaged compensation parameters of blue (

Regression statistics (

The relationship of the single-scattering albedo and the compensation
parameter was analyzed analogically. The

Daily averaged slope (

A simple theoretical explanation for the decreasing

When Eq. (

Daily averaged backscatter fraction (

Compensation parameters at the aethalometer green wavelength
(

The relationship between

The above analysis showed that the compensation parameter depends both on the
single-scattering albedo and the backscatter fraction. Which of them is a
more dominant factor? It is very difficult if not impossible to show from the
data available here since

To disconnect the two effects the data were further classified into (a) a
narrow

Regression statistics (

Regression statistics (

In the individual filter-change regressions there was now a clearly more
significant dependence of

The above exercise was done to find which one, the single-scattering albedo
or the backscatter fraction affects the compensation parameter more. The

If the relationships between

An important factor missing in the present study is the mixing state of
absorbing and scattering aerosols since there was no method available to
measure it. It is likely that the same amount of absorbing aerosol such as
BC yields different compensation parameters when it is internally mixed with
scattering material and when these two are externally mixed. In these cases
the penetration depths of BC particles into the filter would be different
even if the overall backscatter fraction of the aerosol were the same.
Therefore it is not likely there will be an unambiguous relationship between

Aerosol optical properties were measured with a seven-wavelength aethalometer
and a three-wavelength nephelometer at the suburban site SORPES in Nanjing,
China, in September 2013–January 2015. The most important result obtained
from the analysis of the data is that quantities calculated from two
independent methods; i.e., the backscatter fraction measured with the
nephelometer and the compensation parameter

The interpretation of the above results is complicated by the fact that

In spite of the uncertainties, the most important conclusion is that the
backscatter fraction of the aerosol has a very clear effect on the
aethalometer data and it should be taken into account. To quantify this in
terms of

The underlying reasons for the effect of the backscatter fraction are the variations in the enhanced scattering due to variations in the asymmetry parameter and variations in the penetration depth of the particles into the filter, which depend on their size. This observation is important especially in China where anthropogenic pollution is often mixed with desert dust: the backscatter fraction is large for small particles and small for large particles such as soil dust.

Another, related conclusion is that also the multiple-scattering correction
factor

This study was conducted by analyzing data collected with the AE31 aethalometer model, which uses a different filter material and the flow setup than the new AE33 aethalometer model. Also, the compensation factor is calculated there in a slightly different way, but it was shown above that in principle the difference is not big. Therefore the results presented above will most probably not be quantitatively the same, but it is very likely that the qualitative results are the same: the larger the backscatter fraction is, the larger are the compensation parameter and the slope of the wavelength dependency on it, and the other way around when comparing with the single-scattering albedo. This can be considered as a recommendation for future research.

To obtain the data used in the paper, please contact the corresponding author.

The research was supported by the Jiangsu Provincial Natural Science Fund (no. BK20140021), National Science Foundation of China (D0510/41275129), and Academy of Finland's Centre of Excellence program (Centre of Excellence in Atmospheric Science – From Molecular and Biological processes to The Global Climate, project no. 272041). Edited by: W. Maenhaut