Measurement report: Aerosol hygroscopic properties extended to 600 nm in the urban environment

Submicron particles larger than 300 nm dominate the aerosol light extinction and mass concentration in the atmosphere. The water uptake ability of this size range greatly influences the particle mass, visibility degradation and particle chemistry. However, most of previous field measurements on aerosol hygroscopicity are limited within 350 nm. In this study, 10 the size-resolved aerosol hygroscopic properties over extended size range (50-600 nm) at 85% relative humidity were investigated in Beijing winter from 27 November 2019 to 14 January 2020 using a Humidity Tandem Differential Mobility Analyser (HTDMA) instrument. The corresponding aerosol optical properties were also analyzed using the Mie scattering theory. Results show that the averaged probability distribution of GF (GF-PDF) is generally a constitute of a more-hygroscopic (MH) group and a less-hygroscopic (LH) group (including hydrophobic). For the particles larger than 300 nm, there exist a 15 large fraction of LH group particles, resulting in an unexpected low hygroscopicity. During the development of pollution when particles are gradually aged and accumulated, the bulk hygroscopicity above 300 nm is enhanced significantly by the growth and expansion of MH group. This result is supported by previous chemical composition analysis and we give a more direct and detailed evidence from growth factor and mixing state aspects. Our calculations indicate that the optical contribution of particles larger than 300 nm constitutes about 2/3 of the total aerosol extinction. The large hygroscopic variation of aerosols 20 above 300 nm will influence the light degradation comparably with the increase of aerosol loading in the low visibility haze events. Our studies highlight that the hygroscopic properties above 300 nm are complex and vary greatly with different pollution stages, therefore more field measurements and investigations need to be done in the future.

under an elevated relative humidity (RH). Particle's water uptake property, which is mainly related to the water-soluble materials contained, determines aerosol liquid water content, affects the multiphase chemistry and local photochemistry, and facilitates particle formation and aging processes (Wu et al., 2018). Thus, a thorough understanding of aerosol hygroscopicity is crucial to quantify all these effects.

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So far, while many field measurements on aerosol hygroscopicity have been carried out worldwide using Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) (Hersey et al., 2011;Kitamori, Mochida, & Kawamura, 2009;Martin et al., 2011;Massling et al., 2009;Park, Kim, & Park, 2009), Cloud Condensation Nuclei Counter (CCNC) (Deng et al., 2013;Nan Ma et al., 2016;, and optical humidified measurements (f(RH)) (Carrico et al., 2000;Fitzgerald, Hoppel, & Vietti, 1982;Y. Kuang, Zhao, Tao, & Ma, 2015;Yan et al., 2009;Zieger et al., 2010), detailed 40 hygroscopic properties over large sizes (> 300 nm) are still very scarce. CCNC can give an estimate of bulk aerosol hygroscopicity under different supersaturations. Because of the supersaturation limits, it mostly reveals hygroscopicity property within 200 nm and cannot provide direct size-resolved or mixing state information (Roberts & Nenes, 2005). F(RH) measures the humidity-dependent scattering coefficient over the whole size range. It focuses more on the hygroscopicity of sizes that contribute more to the bulk optical properties (Y. Kuang et al., 2018), and cannot give size information or mixing 45 states, either. HTDMA is a widely used instrument to obtain the size-resolved growth factor of aerosol particles under different RH, and has been applied to many field measurements to obtain aerosol bulk hygroscopicity as well as mixing state information. However, most of these field studies were limited within the size range of 300 nm (Swietlicki et al., 2017).
For the submicron aerosols in the urban environment, especially areas suffering from severe haze pollution and affected by 50 intensive anthropogenic activities, the particles larger than 300 nm contribute significantly to the particle mass and surface concentration (Lundgren & Paulus, 2012;Sverdrup, 1977), liquid water content (Bian, Zhao, Ma, Chen, & Xu, 2014), and optical properties (Y. Kuang et al., 2018;Ouimette & Flagan, 1982). For the submicron particles, although the number distribution is dominated by particles smaller than 0.1 µm, most of the surface area is in the 0.1-0.5 µm size range (Seinfeld & Pandis, 2006). The aerosol mass distribution peaks at an even higher size range of about 0.2-1 µm. Moreover, light scattering 55 is usually approximately proportional to aerosol volume or mass (Pinnick, Jennings, & Chýlek, 1980), which means that the light degradation by aerosol particles is also concentrated at a relatively higher accumulation size. N. Ma et al. (2012) demonstrated that the aerosol particles between 200 nm and 1 um usually contribute more than 80% to light extinction at 550 nm during summer on the North China Plain (NCP). In the pollution period, the enhanced particle growth by coagulation and condensation of vapors, aerosol size distribution will shift to larger diameters. The aerosol mass and optical extinction 60 contributed by the larger accumulation mode particles will also be increased. When exposed to a high RH, these factors can be further enhanced with the addition of water. Considering the dominant contribution of this part to aerosol optical properties, it's necessary to study the detailed hygroscopic properties over extended size range and investigate its variation during different pollution conditions. https://doi.org/10.5194/acp-2020-867 Preprint. Discussion started: 28 August 2020 c Author(s) 2020. CC BY 4.0 License.
Some previous studies have tried to derive the hygroscopicity in a larger accumulation size above 300 nm. However, there is no direct evidence and measurements supporting their assumptions or parameterization schemes. Ye Kuang et al. (2017) calculated the equivalent aerosol hygroscopicity parameter κ from f(RH) curves ( ( ) ) measured at Wangdu on the NCP and the results lied between 0.06 and 0.51, presenting a very large variation range. Because the light scattering is mostly contributed by particles at a larger accumulation size, so ( ) can generally represent the hygroscopicity in this size range. 70 H. J. Liu et al. (2014) also derived the hygroscopicity over a larger size range of 30 nm-4 µm from chemical composition and found that κ values vary relatively less for particles between 250 nm and 1 μm than particles smaller than 250 nm. Chen et al. (2012) assumed that particles in each aerosol mode had the same hygroscopicity, and then estimated size-resolved κ for aerosols with diameters of 3 nm ~10 μm based on the contribution of each mode to a specific particle size. However, all these estimations are indirect methods and have some unproven assumptions. 75 Beijing, one of the biggest cities in China and the world, is a densely populated area with severe particle pollution.
Representative of the urban environment, the consumption of fossil fuels is quite considerable, and a lot of pollutants are emitted into the environment every day. Exacerbated by meteorological conditions, these pollutants are readily trapped in local region, go through a series of physical and chemical processes, and finally evolved into severe haze events (Guo et al., 2014;80 Jiang, Wang, Zhao, Li, & Che, 2015;Sun et al., 2014;Ye, Song, Cai, & Zhang, 2016). Influenced by the particle pollution and strong hygroscopic growth of aerosol particles, visibility degradation is also a frequent and urgent environmental problem.
Based on this condition, many field measurements focusing on the particles' microphysical properties including hygroscopicity were conducted in Beijing and the surrounding region. Massling et al. (2009) used a custom made HTDMA to measure the hygroscopic properties of ambient particles in Beijing and reported the corresponding measurement results. Haze in China 85 (HaChi) campaign also investigate aerosol particles' various microphysical properties including CCN activity, optical enhancement factor and hygroscopic growth properties under high humidity (Chen, Zhao, Ma, & Yan, 2014;Deng et al., 2011; H. J. Liu et al., 2014;P. F. Liu, Zhao, Göbel, et al., 2011). Wang et al. (2018) made statistical analysis and proposed parameterization of the hygroscopic growth of urban aerosol (50-350 nm) in Beijing from long-term measurement. However, there are no direct measurements focusing on the size range larger than 350 nm. 90 In this study, we deployed an HTDMA instrument to our laboratory in Beijing to conduct aerosol hygroscopicity measurements from 27 November 2019 to 14 January 2020. Other particle microphysical properties, optical properties, and meteorology parameters were also monitored simultaneously. Three questions need to be answered here: 1) what are the basic characteristics of aerosol hygroscopicity over extended size range in the urban environment? 2) how the hygroscopic properties vary with 95 pollution conditions and 3) how the variation influences the visibility degradation? In the following, section 2 will give a description of the data and methods used in this study; section 3 will present the measurement results including overall aerosol https://doi.org/10.5194/acp-2020-867 Preprint. Discussion started: 28 August 2020 c Author(s) 2020. CC BY 4.0 License. hygroscopicity and its daily variations; the hygroscopic variation under different pollution conditions and potential impacts will be shown in section 4 and the last section comes the conclusions.

measurement site
The measurement was conducted in the aerosol laboratory, which is located on the rooftop of a six-floor building in the campus of Peking University. It shares the same location with the AERONET station of BEIJING_PKU (39 ̊ 59' N, 116 18' E). The sampling site is in the northwest of Beijing, surrounded by schools, residential buildings and shopping centers. Moreover, two main streets: Zhongguancun North Street is located to the west and Chengfu Road is to the south. Except for the road traffic, 105 there are no large emission sources in the neighboring. Aerosol particles here are representative of the urban environments.
More details can be found in Zhao et al. (2018).
In the measurement, a PM10 impactor was used to remove aerosol particles with aerodynamic diameters larger than 10 µm.
Then a dryer was used to decrease the RH to less than 30%. Next, the dried poly-disperse particles were guided into a splitter 110 with different instruments located on the downstream. From 27 November 2019 to 14 January 2020, aerosol number size distributions, size-resolved hygroscopicity, and optical properties including scattering and absorption coefficients were measured. Meteorological parameters like temperature, wind speed, wind direction, and RH were also monitored in this site.

Instrumentation and data
A BMI Humidified Tandem Differential Mobility Analyser (BMI HTDMA, Model 3100) was mainly used in this study. The 115 detailed instrument description and performance evaluation has been introduced in the previous publication (Lopez-Yglesias, Yeung, Dey, Brechtel, & Chan, 2014). Similar to other conventional HTDMA systems, this instrument mainly incorporates an upstream Differential Mobility Analyzer (DMA1), a humidification system that offers humidified sheath air to the humidifier and downstream DMA to expose the particles to a prescribed RH, and a downstream DMA (DMA2) in series with a Mixing Condensation Particle Counter (BMI MCPC, Model 1720) to measure the particles' size distribution after water 120 uptake. Unique features of this BMI HTDMA include a diffusion-based particle humidifier, a DMA design allowing selection of particles up to 2 μm diameter at only 5600 volts, and the ability to study the complete deliquescence and efflorescence cycle.
It also offers a processing software to do the corrections and data inversion, which transforms the raw measured counts to the particle's growth factor distribution.
During the measurement, the RH of the second DMA was set to be 85%. The dry diameters selected by DMA1 were 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm. To calibrate the measurement system, ammonium sulfate particles were also tested to compare with the theoretical values. The calibration includes both the dry test and RH test. The temperature and RH in the humidifier and DMA2 sheath were recorded real-time for later check of system stability. For the best working performance, the room was air-conditioned at 25 C and circulated all the time. 130 The hygroscopic diameter growth factor (GF) is the ratio of the particle wet diameter at a given RH to the dry diameter, 0 : Growth factor probability distribution function (GF-PDF, c(GF)) is normalized to unity. In order to simplify parameterization and better compare the aerosol hygroscopic properties among different measurements, the hygroscopicity parameter κ was 135 also calculated in this study (Petters & Kreidenweis, 2007). It can be calculated from the following equation: where / is the droplet surface tension, is the molecular weight of water, is the density of liquid water, R is the universal gas constant, and T is the absolute temperature.

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The κ-PDF can also be derived from the GF-PDF. The volume-weighted mean growth factor is defined as the 3 rd -moment mean values of the GF-PDF: = ( 3 • ( ) ) 1/3 .The corresponding mean hygroscopic parameter can be calculated from using the equation above. To better understand the mixing state of particles and group them in terms of hygroscopic properties, we classify the particles into a less-hygroscopic group with GF lower than 1.2, and a morehygroscopic group with GF larger than 1.2. For each group, the number fraction, , and standard deviation σ can be 145 determined from the GF-PDF.
Apart from HTDMA, the aerosol light scattering coefficients at three wavelengths (450 nm, 550 nm, and 700 nm) were measured using a TSI 3563 nephelometer (Anderson & Ogren, 1998). The particle number size distribution (PNSD) was measured simultaneously by a BMI scanning electrical mobility sizer (BMI SEMS, Model 2100) and a TSI scanning mobility 150 particle sizer (SMPS). The BMI SEMS measures PNSD with a time resolution of 10 min over the size range of 10-1000 nm.
The SMPS has a time resolution of 5 min and covers the size from 10 to 600 nm. The black carbon (BC) mass concentrations were measured using an MA200 (Aethlabs, serial number 0083) at 1 min temporal resolution.

Overview of GF and distribution 155
The aerosol number size distributions during the whole measurement period are summarized in Fig. 1(a). We also summarize some previous studies on urban aerosol hygroscopicity from HTDMA measurements and present statistical results in Fig. 1(c).
The reference can be found in Table 1. We can clearly see that in Beijing winter, the peak contribution to the surface area is 200-300 nm and to the mass is 300-400 nm. For optical light extinction coefficients at 550 nm, the peak diameter is around 400-500 nm. However, the focuses of previous HTDMA measurements are only around 50-300 nm. It's this dislocation that 160 prompt us to conduct hygroscopicity measurement over extended size range and investigate the potential impacts on aerosol optical properties. Figure 2 gives an overview of the average GF-PDF and the corresponding κ-PDF during the whole measurement period. It can be seen that the mean growth factor distributions for all sizes show a distinct bi-modal pattern, consisting of a less-165 hygroscopic mode and a more-hygroscopic mode. For particles of 50 nm, the LH and MH modes cannot be clearly distinguished from each other. In general, particles of this size are not readily hygroscopic. For the size range between 100 nm and 400 nm, there exists a dominant MH group and a minor but distinct LH group. The peak of the MH group shifts significantly from about 1.2-1.4 to 1.4-1.6 as the size increases, indicating that the water-soluble or water-uptake materials in larger sizes have a relatively stronger hygroscopicity than the smaller sizes. In contrast, the peak of the LH group shifts slightly. 170 The number fraction of the LH group decreases with size when particles are smaller than 200 nm, whereas increases with size when size exceeds 200 nm. The spread (stand deviation) of the LH group has the same trend. For the particles larger than 400 nm, the LH group dominates the number fraction, resulting in the decrease of bulk hygroscopicity of large particles. Similar to the GF-PDF, κ-PDF also presents two modes corresponding to the LH and MH groups.

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In general, our results are consistent with Wang et al. (2018) for the overlap region of 50-350 nm. We can also see a stronger fraction of nearly-hydrophobic group with size in lower and moderate pollution conditions in their study. This trend is captured by our results and further extended to a larger size above 350 nm. Massling et al. (2009) also reported the similar trend of increasing hydrophobic particles with size in winter. They also gave the size-resolved chemical composition across the submicron size range. For particles larger than 300 nm, there is an increased mass ratio of Elemental Carbon and undefined 180 mass, which may correspond to this increased group of particles. The size-resolved κ in H. J. Liu et al. (2014) are different from our results in overlap region because their measurements were done in summer, in which the emissions are quite different from that in winter. An apparent difference is the missing of nearly-hydrophobic group in accumulation mode particles, which is the main cause of the decreased hygroscopicity above 300 nm in our study.
The statistics of the HTDMA measurement are summarized in Table 2. The number fraction, mean GF and mean κ, spread of GF and κ for the LH and MH groups during the whole measurement period are calculated. Corresponding standard deviations are also presented in the table. The mean GF values for 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm are 1.13±0.07, 1.25±0.06, 1.31±0.07, 1.30±0.08, 1.24±0.12, 1.16±0.14, 1.12±0.16, respectively. The number fraction of LH group particles reaches a trough at 200 nm and then increases with size. The ensemble mean κ peaks at 300 nm and then decreases 190 with size, which is related to the increase of LH group number fraction.

Time series and diurnal variation
The time series of aerosol size distribution and growth factor distribution over different sizes are shown in Fig. 3. The black line in panel (a) is the time-dependent volume concentrations calculated from particle number size distribution. This value can generally represent the particle pollution level. As elucidated by Guo et al. (2014), the PM episodes in Beijing present a 195 periodic cycle governed by meteorological conditions. This kind of cycle lasts from 3 days to 1 week, causing different degrees of pollution. Our measurement captured about 6 pollution cycles. Because the sampling was done at a fixed site, so the complete cycle may go interrupted by transport or local physical or chemical processes. Nevertheless, the measured aerosol properties from 27 November 2019 to 14 January 2020 can still be representative of the typical winter conditions in Beijing.
The blank area in the figure denotes missing data, which is caused by the instrument shutdown or measurement calibration. 200 From Fig. 3(a), we can see that the particle number concentration usually has an abrupt increase at the beginning of a pollution cycle. Then the volume or mass concentration begins to develop and increase to a high level. During the clean period, the wind is often dominated by strong northwest or northeast winds because it can bring unpolluted air masses from the northern mountainous area. Polluted days are often accompanied by weak southerly wind. Furthermore, high relative humidity tends to 205 appear at the end of each pollution cycle, which may help to push the pollution level to a peak (Fig. 8).
For each pollution cycle, we can see from Fig. 3 (b-c) that the GF-PDF of 50 nm and 100 nm particles does not have large variations. But for other sizes, an increase of growth factor along with the pollution development can be clearly seen from the GF-PDF contour. Different sizes have different lag time to respond to the evolution of pollution: the larger the size, the more 210 time needed to change the hygroscopic properties.
The average diurnal variations of GF-PDF for all the sizes are presented in Fig. 4. For better comparison, we mark the line between daytime and nighttime. During the measurement period, the time of sunrise was about 7.00 a.m. and the sunset was around 5.00 p.m. For particles of 50 nm, their hygroscopicity starts to decrease around sunrise and begins to increase around 215 sunset. So the hygroscopicity parameter κ peaks at 6.00 a.m. and reaches a minimum at 19.00 p.m. For particles larger than 200 nm, the hygroscopicity at daytime are about 10-50% larger than that of nighttime. https://doi.org/10.5194/acp-2020-867 Preprint. Discussion started: 28 August 2020 c Author(s) 2020. CC BY 4.0 License.
For accumulation sizes, the number fraction of the LH group ( ) in the daytime is smaller than that in the nighttime. On one hand, the cause could be that more hydrophobic particles are generated or emitted in the night, these particles include fresh 220 BC or organic aerosols (N. Ma et al., 2011); on the other hand, more MH group particles could be formed in the daytime, or those aged particles in the night residual layer are mixed in the planetary boundary layer again (H. J. Liu et al., 2014). No diurnal variation is found in the peak of the LH group, indicating the similar chemical compositions in the LH group. In contrast, the peak hygroscopic parameter κ for the MH group varied significantly with time of a day, indicating that the chemical compositions in the MH group varied greatly at different day time. It usually reaches a maximum around noon and 225 start to decrease in the afternoon. The black lines in Fig. 4(a-g) are the ensemble mean κ for each size. For particles larger than 50 nm, this value also peaks around noon, which may be the result of decreased and increased MH hygroscopicity. More statistical results about daytime and nighttime can be found in Table 3.

Overview of GF and distribution 230
Guo et al. (2014) reported that particle pollution in Beijing is characterized by two distinct aerosol formation processes of nucleation and growth. During different aerosol formation processes, aerosol chemical compositions and corresponding hygroscopic properties may vary greatly. In order to investigate the evolution of particle hygroscopicity with different pollution levels, we group the data into three periods: 1) clean period in which the volume concentration is lower than 30 3 3 ⁄ ; (2) transition period in which the particle volume concentration is larger than 30 but smaller than 40 have increased significantly. At the same time, the number fraction of the MH group has also been enhanced. When the air 240 quality develops from transition to polluted level, the number concentration remains similar, but the MH group is greatly enhanced, not only in the number fraction but also in the terms of the size range. The MH mode grows to the size of 600 nm, greatly squeezing the fraction of nearly-hydrophobic mode particles. From the statistical results in Table 4, the number fraction of less hygroscopic mode for 600 nm is 0.86±0.22, 0.75±0.29, 0.39±0.27 for the clean, transition and pollution stage, respectively. For smaller particles, the variation of this value is not so distinct. Taking particles of 100 nm as an example, the 245 for clean, transition and pollution stage is 0.36±0.14, 0.33±0.15, 0.30±0.13, respectively. The difference of among different stages increases with size.
It needs to be noted that the great decrease of for large particles in the pollution period doesn't necessarily mean the decrease of absolute number concentration of LH mode particles. From Fig .5(f), the number concentration of LH mode 250 particles remains similar to the transition stage. However, the total number concentration and corresponding mass concentration of large particles increases. These particles are grown from those smaller particles and are more aged in the environment. These processes are also called secondary aerosol formation. This part of particles contributes to the increase of the MH mode in a larger size and correspondingly total large particle number concentration. With the evolution of pollution level, this process will continue. Zheng et al. (2016) showed that the fraction of the secondary components in PM2.5 are 255 enhanced with increasing PM2.5 mass concentration. Our measurement results give an another direct evidence supporting this conclusion.
For each size, we summarize the average hygroscopicity under different pollution conditions (Fig. 6). The results show that for particles smaller than 300 nm, the hygroscopicity variation is not very large. But for particles larger than 300 nm, particle 260 hygroscopicity can vary significantly with different pollution levels, which correspond to different aging stages of urban aerosol particles. In conclusion, when urban aerosols in Beijing go through a series of aging processes, their hygroscopicity is enhanced, especially for larger accumulation particles.

The effects on visibility degradation 265
To better understand the meteorological, microphysical and hygroscopic evolutions during the aging processes, and the effects of hygroscopicity variation on visibility degradation, a 3-day pollution period from 27 November to 1 December 2019 is selected for the analysis. Fig. 7 shows the overall situation of this pollution event. From the noon of 27 November, the wind speed began to decrease to below 3 m/s. The synoptic situation of Beijing remained stagnant until the end of this pollution period, with an average wind speed of 1.04 m/s. This meteorological situation indicated that the local emissions were the 270 primary contribution of the surface air pollution and the aging processes were not influenced too much from transport from other areas. In this respect, our measurement can represent the temporal evolution of aerosols in this urban region.
We can see from Fig. 7(b-c) that in the first 24 hours, there was a considerable increase of aerosol number concentration for particles smaller than 100 nm, which corresponded to the aerosol nucleation. The visibility dropped down from upper limit to 275 around 30 km. In this stage, aerosol hygroscopicity was relatively low. In the transition stage, the mean particle size grew continuously and the aerosol volume concentration also had a stable increase. At the same time, the aerosol became more hygroscopic in the accumulation size range between 100-300 nm. During this period, the visibility decreased slightly to 20 km. In the polluted stage, under the combined influence from the high relative humidity and large hygroscopicity enhancement, the visibility decreased considerably to below 10 km, causing the most severe haze event, even though the aerosol volume 280 concentration remained similar to that in transition stage.
In this haze formation process, RH, aerosol loading (AL) and hygroscopicity all contribute to the visibility degradation. In order to quantify how much these factors took effect, we summarize the mean values of these factors in different stages, as shown in Fig.8(a-c). We can see that as the pollution developed, the number concentration of particles larger than 200 nm 285 increased while the small particles (<100 nm) decreased. The aerosol hygroscopicity was enhanced greatly, from a value below 0.2 to above 0.4 for particles larger than 300 nm. At the same time, the mean relative humidity also increased from 25% to 77%.
The visibility is closely related to aerosol light extinction (σ) through Koschmieder relation (Carrico, 2003;Griffing, 1980;290 Husar, Husar, & Martin, 2000). In this study, the particles are assumed to be spherical and their size-dependent extinction behaviour is computed from the Mie model using particle dry size distribution, refractive indices, size-resolved hygroscopicity, and ambient RH. The detailed calculation description can be found in Chen et al. (2012). From this method, we can not only quantify the contribution from each size bin, but can also determine the role each factor (AL, RH and κ) played in the low visibility haze event. Fig. 8(d) compares the size-resolved aerosol light extinction coefficient during clean period (blue solid 295 line) and pollution period (black solid line). The blue line is considered as the base line, which corresponds to low aerosol loading, low hygroscopicity and low RH values. When each of these factors is considered, we can obtain the following sizeresolved light extinction as shown in dotted lines. The magenta dotted line represents the condition with AL effect considered, which is calculated from the PNSD in pollution stage but with the κ and RH in the clean stage. The red dotted line represents the condition when both AL and RH are considered. The shading between lines is the difference, indicating the contribution 300 to the aerosol light extinction from each considered factor. From Fig. 8, we can see that particles larger than 300 nm contribute more than half of the total light extinction. Under this condition, if the hygroscopicity properties and variations in this size range are not known, it would be hard to fully quantify and predict the visibility impairment caused by aerosol particles in the urban environment. As we have seen from Fig.6 and 305 Fig. 8(b), the hygroscopicity variation is very large in this size range. From our calculation (pie chart in Fig. 8(d)), it would bring about 40% of the total difference between the clean and pollution period, which is similar to the contribution of added aerosol loading. It's to be noted that all these three factors are coupled together to influence the haze formation, and the hygroscopicity variation would amplify the effect of added aerosol loading and increased ambient RH.

Conclusions 310
Submicron particles larger than 300 nm dominate the aerosol light extinction and mass concentration, and constitute a large part of the aerosol surface concentration. Under the exposure to high RH, all these parameters will be enhanced with the addition of water, and then greatly influence the particle mass, visibility degradation, particle chemistry and haze formation.
However, field measurements on aerosol hygroscopicity in previous studies are focused below the size of 350 nm. For larger particles, detailed description of hygroscopicity characteristics and variations are scarce. Some studies tried to derive the 315 hygroscopicity of a larger size range from other indirect methods like chemical composition or based on assumptions that accumulation mode particles share the same hygroscopicity. Till now, no direct evidence has been provided to support these assumptions or parameterization schemes.
In this study, a comprehensive aerosol field measurement focusing on hygroscopicity properties and variations over extended 320 size range (50-600 nm) was conducted at a Beijing urban site in winter 2019. During the measurement, an HTDMA instrument was employed to measure hygroscopic growth factors of particles with dry diameter of 50, 100, 200, 300, 400, 500, and 600 nm at 85% RH from 27 November 2019 to 14 January 2020. This in-situ field measurement of atmospheric aerosols is representative of urban environment and of great importance to better understand the frequent low visibility events over North China Plain. 325 The measurement results show that the mean growth factor (GF) values for the sizes above were 1.13±0.07, 1.25±0.06, 1.31±0.07, 1.30±0.09, 1.24±0.13, 1.16±0.15, 1.12±0.15, respectively. The average GF-PDF present a bi-modal structure including a less hygroscopic group and a more hygroscopic group. For the particles larger than 300 nm, there is a large fraction of nearly-hydrophobic or less hygroscopic particles, which decrease the bulk hygroscopicity in this size range. However, 330 during the polluted episode when the aerosol particles are fully aged, the ensemble mean hygroscopicity of larger particles will be enhanced significantly because of the growth and expansion of MH group to this size range. Our measurements give a direct evidence supporting that the variation of MH group brings the biggest change and uncertainty to the bulk hygroscopicity of particles larger than 300 nm. Our results are consistent with previous HTDMA studies in overlap region (P. F. Liu, Zhao, Gobel, et al., 2011;Massling et al., 2009;Wang et al., 2018) and the chemical composition studies during the pollution 335 development (Guo et al., 2014;Wu et al., 2018;Zheng et al., 2016) The significant variation of larger particle hygroscopicity will contribute much to the variation of particle optical properties like light extinction coefficient. Size-resolved light extinction coefficients under different pollution conditions are calculated using Mie scattering theory. From our results, when coupled with increased RH, the influence of κ variation on light 340 degradation could compete with the added aerosol loading in the haze development. Based on these results, we strongly recommend simultaneous measurement of hygroscopic properties over size range larger than 300 nm in the future urban aerosol field studies.
Competing interests. The authors declare that they have no conflict of interest. 345 Data availability. The data used in this study can be obtained from this link: https://pan.baidu.com/s/1Sv9Zi3SJjBf0vhMRH2NNRA. It is also available when requesting the authors.     for the time series of particle number size distribution and the black line is the integrated volume concentration. Left axis is the log scale of particle diameters and right axis is for the volume concentration. (d) is the size-resolved hygroscopicity parameter .