Fine Mode Aerosol Optical Properties Related to Cloud and Fog Processing over a Cluster of Cities in Northeast China

Fine mode aerosol optical properties were retrieved from sunphotometer measurements, carried out in Shenyang, Anshan, Benxi, and Fushun in Northeast China from 2009–2013. Accumulation mode aerosol size distribution retrievals were investigated for specific situations involving extensive cloud processing during fog or haze. Large fine mode median radii (≥ 0.20 μm) frequently occurred at the four sites in July. At other times, as aerosol optical depths increased so did the fine mode median radii, reaching 0.30–0.40 μm. This was possibly due to aerosol humidification during cloud processing. Instances of large (~1.50) Alpha curvature at the sites also may be associated with cloud processing. When single scattering albedos were ≥ 0.90, the radii distributions were centered round 0.30–0.40 μm at all at sites except Benxi, perhaps due to non-absorbing material from aerosol/cloud interactions. Case studies of size distribution retrievals at each of the four sites showed that large accumulation mode radii (0.40–0.50 μm) may be linked to cloud processing.


INTRODUCTION
Aerosols play an important role in regional and global climate, by absorbing or scattering radiant energy.Both direct and indirect effects of aerosols impact the Earth's radiative balance (Charlson et al., 1992).In addition, aerosol particles can act as cloud condensation nuclei, thereby modifying the size distributions of cloud droplets, influencing their micro-physical properties, and altering the precipitation efficiency of clouds.These effects of particulate matter have significant implications for weather as well as climate (Hansen et al., 1997;Twomey et al., 1977;Koren et al., 2004).
Conversely, the physical and chemical properties of aerosol particles can be modified by clouds: for example hygroscopic growth in high relative humidity conditions (Radke and Hobbs, 1991;Kotchenruther et al., 1999).The chemical composition of cloud-processed aerosols has been shown to be modified by aqueous phase reactions, and changes in composition often remain after the bulk of the water evaporates from the particles (Lelieveld and Heintzenberg, 1992;Alkezweeny, 1995;Lu et al., 2003;Hegg et al., 2004).Changes in aerosol optical properties are also caused by cloud processing (Twohy et al., 2009;Jeong and Li, 2010).Along these lines, Eck et al. (2012) observed large fine mode aerosols using Aerosol Robotic Network (AERONET) size retrievals from sunphotometer data after the dissipation of fog or low altitude clouds.A few cases of bimodal accumulation mode distributions have been reported for cloud-processed aerosol populations (Das et al., 2008;Thornhill et al., 2008;Dall'Osto et al., 2009;Allen et al., 2011;).
In China, the researches about aerosol optical properties has been studied (Hu et al., 2010;Xin et al., 2011), especially in the condition that fog-haze events have become increasingly frequent in recent past decades, linked to atmospheric perturbations tied to rapid economic development in Asia (Che et al., 2009a;Zhang et al., 2012).Bimodal accumulation mode size distributions have been retrieved at some AERONET sites in eastern China, including Beijing, Xianghe and Lake Tai: these retrievals also exhibited large submicron radius modes (Eck et al., 2012;Li et al., 2014).The complexity of aerosol size distributions was illustrated in a study by Che et al. (2014) that showed there are bimodal peaks in the accumulation mode particle size distribution during a period of heavy pollution in 2013 over the North China Plain.
Investigations of interactions between clouds and aerosol particles in Northeast China have been extremely limited.Li et al. (2011) observed significantly larger radii for cloud residue aerosols, compared with unprocessed particles under highly polluted conditions in Northeast China.
In this paper, variations in the distributions of fine mode aerosol optical properties were analyzed to investigate interactions between aerosol populations and clouds over urban and industry region in Northeast China.Liaoning province is a heavy industry base in the economic development of Northeast China.The rapid development has caused serious particle pollution in these area (Ma et al., 2010), and caused the degradation of visibility (Zhao and Ma, 2011), especially in higher AOD loading (Zhao et al., 2012(Zhao et al., , 2013a)).The results in this region are relevant for processes occurring in other areas of world (Eck et al., 2012); in a larger sense, they contribute to our understanding of the modification of aerosol properties through cloud and fog water processing.

GEOGRAPHY, INSTRUMENTATION AND METHODOLOGY
The cluster of cities in central Liaoning Province sampled for our study comprises Shenyang, Anshan, Benxi, and Fushun (Fig. 1 and Table 1); these sites are in a heavily populated industrial region of Northeast ).The region is known to be a major source for pollution aerosols, which originate from a variety of urban, industrial, and transportation sources (Ma et al., 2012;Zhao et al., 2013a).In spring, the aerosol population can be strongly influenced by long-distance transport of dust from northern China (Wang et al., 2011).In winter, coal burning emissions for domestic heating are a well-established source of pollutants (Zhao et al., 2013b).
Standardized and calibrated Cimel sunphotometers were deployed at each of the stations (Holben et al., 1998;Che et al., 2009b;Tao et al., 2014).The fine mode median radii (r vf ), single scattering albedos (SSA), and volume-size distributions were derived from the sky radiance measurements, simultaneously with spectral aerosol optical depths (AOD) at wavelengths of 440, 675, 870, and 1020 nm when the AOD 440nm was larger than 0.40 to avoid the large inversion errors from the limited information (Dubovik and King, 2000).The latter were obtained using the retrieval method described in Dubovik and King (2000) and Dubovik et al. (2002Dubovik et al. ( , 2006)).We analyzed fine mode-(radius < 2.50   2.

Variations in Fine Mode Particle Optical Properties at the Four Sites
The AOD, SSA, volume and radius retrievals of fine mode particle made at Shenyang, Anshan, Benxi and Fushun are shown as averages for each month based on the simultaneous observation over the period 2009-2013 in Figs. 2(a)-2(d) in Figs. 2(a)-2(d).Very few continuous cases of aerosol retrievals were available from Benxi, mainly due to instrumental problems.In this paper, four seasons are divided into spring (March-May), summer (June-August), autumn (September-November) and winter (December-February) to investigate the seasonal variations of aerosol optical properties.
Fig. 2(a) illustrates the variability of fine mode AOD at 440 nm at the four sites.The fine mode AODs are higher in the summer and winter compared to spring and autumn, especially in Shenyang.Lower SSA values mainly occurred in winter (0.80), as Fig. 2(b) shows, obviously in Anshan and Fushun which may suggest an increase in the proportion of fine mode absorbent aerosols.This is possibly related to the higher black carbon aerosol emissions from residential heating.The higher summer SSA values (> 0.90) are more likely due to the hygroscopic growth of fine particles (Yan et al., 2009;Zhu et al., 2014).The cases of higher SSA in autumn indicate that there are some particles with light scattering coefficients meriting further study.
Overall, larger median fine mode radii were more common in summer and winter compared with spring or autumn with the same trend of its volume (Figs.2(c)-2(d)), though there is a weak summer peak for particle volume in Anshan.Indeed, in July, the majority of retrievals had fine mode median radii > 0.20 µm at all four sampling sites with a relatively higher fine mode median volume.The volume was higher in Shenyang compared to the other sites, perhaps due to the pollutant emission from the increased human activities expected in a central city in Northeast China.The large fine mode particles in summer may have been caused by aerosol humidification or coagulation due to the high relative humidity, particularly in the vicinity of clouds (Eck et al., 2012).In addition, parcels of air with higher aerosol concentrations tend to have larger fine mode particles, and the burning of coal for domestic heating during winter causes high particulate matter loadings in the region (Zhao et al., 2013a, b).

Relationship between AOD and Fine Mode Particles over the Four Sites
A scatter plot of the relationship between fine mode radius and aerosol optical depth at 500 nm is shown in Fig. 3.There was a trend of increasing AOD at 500 nm as the fine mode radius increased; the coefficients of determination (r 2 ) for linear regressions models were 0.15, 0.26, 0.48, 0.33 with p < 0.01 for Shenyang, Anshan, Benxi, and Fushun, respectively.This trend indicates that higher aerosol concentrations are associated with larger radius particles, especially at Benxi.In comparison, the relationship between aerosol concentrations and fine mode radius particles at    Shenyang was not significant may be solely because of higher number of samples.As a whole, when AOD was less than 1.00, the median particle radii were typically less than 0.25 µm, but with increasing AOD, the fine mode particle radii grew to 0.30-0.40µm which indicate that the aerosol concentration and associated coagulation may be one of the most important parameter to this phenomenon.This obvious increase in the fine mode radius with AOD has also been observed in Wu et al. (2015) at a semi-arid rural site in Northeast China.According the lack of limited number of data with the particle radius ranging from 0.30 to 0.40 µm in this paper, more observations are needed.

Relationship between Single Scattering Albedo and Fine Mode Particles at the Four Sites
Fig. 4 shows the retrieved aerosol single scattering albedo (SSA) at 440 nm plotted against the fine mode volume median radius.This plot includes all retrievals for each of the four sites in our study.It is likely that these retrievals include some cases where the light absorption properties of fine mode particles were modified by aerosol/cloud interactions.There was a clear trend of increasing SSA as the fine mode radius increased within the radius < 0.22 µm, and this pattern was nonlinear at all of the sites.There are fewer data points for Benxi due to the limited SSA retrievals (AOD 440 > 0.40) and instrument problems.In addition, at Benxi, when the SSA was > 0.90, the radii were concentrated in the range 0.20-0.25 µm, while the other three sites showed a much greater spread in radii for high instances of SSA.This weak tendency for SSA to increase as the fine mode median radius increases is most likely due to particle growth by coagulation and hygroscopic growth.This relationship between SSA and fine mode particle median radius implies that increases in particle size are associated with more efficient scattering especially with the radius < 0.20 µm (Eck et al., 2012) which is important in terms of optical properties.Particle growth caused by cloud-processing may also lead to more increased water and sulfate in aerosols both of which are non-absorbing (Haywood and Boucher et al., 2000).It should be mentioned here that the measurements are too limited at Benxi which need further research to prove this result.Eck et al. (1999) pointed out that relationship between α and lnλ is useful for assessing particle size).The Alpha curvature parameter (α′) is defined as dα/dlnλ, where α is the Angstrom exponent and λ represents the wavelength.In Fig. 5 we show the relationship between the volume median radius of the fine mode and the α′ values computed from the AOD (440-870 nm) obtained from the sunphotometer retrievals.Negative values for α′ occurred when the coarsemode contribution to the optical depth was large, and positive values occurred when fine mode particles gradually increased in the size distributions.Therefore, increases in α′ are indicative of increases in the fine mode radii.

Relationship between Alpha Curvature and Fine Mode Particles
As Fig. 5 shows, α′ increased at Shenyang, Anshan, Benxi and Fushun, when the particle radii also increased.
These results demonstrate a strong nonlinearity in the AOD spectra that can be attributed to large fine mode aerosols.Several α′ values exceeded 1.0 at Shenyang, Anshan, and Fushun, and those large values were likely a result of cloudprocessing.Indeed, retrieved size distributions with submicron volume median radii ≥ 0.40 µm were likely dominated by cloud-processed particles as discussed below.

Case Study of Size Distribution under Cloud/Fog Processing over the Four Sites
Residual submicron fine mode particles have been observed after a cycle of cloud/fog processing and evaporation in several ground-based sampling campaigns (Eck et al., 2012;Li et al., 2014).The accumulation mode particle size retrievals from Shenyang, Anshan, Benxi and Fushun were used here in four case studies (Fig. 6).The premise for the studies is that smaller size modes, with radii of 0.15 to 0.20 µm, may be indicative of normal, unprocessed aerosols (Eck et al., 2012;Li et al., 2014).
We used remote sensing images in our case study to infer the possible effects of clouds and fog on aerosol size distributions.Table 3 summarizes the meteorological conditions at the sites during the case studies.MODIS satellite images  show clouds or fog either over or near the Shenyang sampling region at the time of Terra satellite overpass at 10:30 local time and the Aqua satellite overpass at 13:30 local time on 23 December 2013.
The aerosol volume-size distribution retrievals had an obvious shoulder for the accumulation mode at 0.40-0.50µm for the three sets of measurements made after 10:52 in Shenyang on 23 December 2013.The MODIS images (Fig. 7) for that date showed patches of shallow clouds and fog over the area at 10:30 local time Terra overpass, and conditions appeared similar for the Aqua overpass 3 hours later.The visibilities at 10:52 and 13:51 were similar: 2.3 and 2.4 km, respectively (Table 3).There was no obvious decay or dissipation of the clouds, and the residual aerosol peak size changed little.The level 1.5 AOD at 500 nm was 0.89 and Alpha 440-870 was 1.31 at 11:51, indicating that higher AOD loading is mostly composed of fine radii particles.
The second case study showed that the volume-size distributions for the accumulation mode on 13 September 2010 at Anshan were bimodal, and at 13:50, the larger mode was centered at a radius of ~0.45 µm.Those large particles may have been associated with cloud-processing: the MODIS images (Fig. 8) showed cloud cover in the vicinity of the sampling site at that time.The accumulation mode aerosol did not show a bimodal size distribution at 09:51, and there was no cloud cover over the area according to the MODIS images.From the MODIS image for 13:30, there was clear evidence of cloud cover in the area.The bimodal volume size distribution of accumulation mode aerosol was more obvious from 13:30 to 15:22, which implicates the involvement of cloud interactions.The AOD 500nm was 1.06 and the Alpha 440-870 was 1.02 at 13:50, denoting an exit of fine mode particles in high AODs conditions.The visibilities were 13.7, 9.9, and 7.7 km at 09:51, 13:50 and 15:22, respectively, and these decreases are consistent with the occurrence of cloud cover in the MODIS images.The retrieved size distribution at Benxi showed a large aerosol mode at 0.45 µm, and while this may have been due to the hygroscopic growth of cloud-processed aerosols, there were no MODIS images available to judge cloud cover.The level 1.5 AOD at 500 nm and Alpha 440-870 at Benxi were 1.88 and 0.88 respectively: these values are indicative of a strong pollution event.The low visibilities of ~5.5 km at 06:37 also indicated polluted conditions.
In contrast to the other areas, an increase in the width of the fine mode particle size distribution was recorded at Fushun on 3 December 2009.The MODIS image shows some cloud cover at 10:30 and 13:30 (Fig. 9).Fig. 6 shows a bimodal submicron size distribution of accumulation mode aerosol, with the larger of the two modes having a volume-radius peak of 0.48 µm at 09:40.The 500 nm AOD and Alpha 440-870 at that time averaged about 0.84 and 1.30, respectively, indicating the formation of fine mode particles.The visibility at 09:40 was very low, ~1.9 km.
Selected meteorological data for the four case studies are shown in Table 3.When the accumulation mode aerosol  size distributions were bimodal at Shenyang and Anshan, the winds were from the northeast or east-northeast and around 2.0 and 2.4 m s -1 which may be conducive to cloud interactions to lead an increase in the accumulation mode aerosol size distribution.In contrast, at Fushun, the winds were westerly or west-southwesterly with larger wind speed > 2.5 m s -1 which would have been less conducive to cloud processing, and hence a smaller accumulation mode aerosol size distribution.
In the four case studies, the relative humidity gradually reduced with the time series.The average relative humidity values were about 75%, 73%, 75%, 76% and the average visibilities were about 1.9, 7.9, 4.8, and 2.4 km at Shenyang, Anshan, Benxi, and Fushun, respectively.In China, haze is usually defined as conditions when the visibility is less than 10 km and relative humidity is below 90% (CMA, 2003).The above particle size distribution may be mainly attributed to cloud processing during fog-haze transition processing during the development of the case; while, the regional transport of aged aerosol particles may also provide a larger size particle else which need more study.

SUMMARY AND CONCLUSIONS
Variations in the particle size distributions and optical properties of fine mode aerosols were investigated at Shenyang, Anshan, Benxi, and Fushun in Northeast China from 2009 to 2013.Larger fine mode median radii were more common in summer, presumably due to aerosol humidification and coagulation; in winter when high aerosol loadings resulted from the burning of large quantities of coal.There was strong correlation between aerosol concentrations and particle radius at Benxi; that is, when the AOD increased, the fine mode particle radii also increased to 0.30-0.40µm, presumably due to humidification during cloud processing.In addition, the α′ values at the other three sites reached 1.50 at times, again possibly as a consequence of cloudprocessing.
When SSAs were more than 0.90 at the three sites other than Benxi, the radii distributions were concentrated in the range 0.30-0.40µm, possibly due to large particles with nonabsorbing materials accumulating due interactions with fog water and clouds.Several special case studies showed that large accumulation mode radii (0.40-0.50 µm) were synchronous with cloud cover evident in MODIS images.
The results of this paper may provide the distributions of fine mode aerosol optical properties in Northeast China and what is more important, it also investigated the interactions between aerosol populations and clouds when there exist cloud processing during fog or haze which may allow us better understand the modification of aerosol properties through cloud and fog water processing.

Fig. 1 .
Fig. 1.Site distribution of this cities cluster in Northeast China.

Fig. 4 .
Fig. 4. Single scattering albedo (SSA) and fine mode particles over the four sites.
Fig. 5. Alpha curvature and fine mode particles over the four sites.

Fig. 6 .
Fig. 6.Fine mode aerosol optical properties under cloud processing over the four sites.

Table 1 .
Site information of four urban sites in this study.

Table 2 .
Statistic of available samples in this study.

Table 3 .
Meteorological data for the four case studies.