Impact of relative humidity and particles size distribution on aerosol light extinction in urban area of Guangzhou

Introduction Conclusions References

model 2001 carbon analyzer, respectively; particles size distribution was measured by TSI 3321 APS; and total light scattering coefficient was measured by TSI 3565 Nephelometer.Inorganic salts that constitute PM 2.5 were recognized under an assumption of the electrical charge neutrality, while chemical components as POM, EC and water content were determined by means of hygroscopic growth calculation and chemical mass closure.As a result, (NH 4 ) 2 SO 4 , NaNO 3 , POM, EC and water content were found to be the major components.By the Mie Model, light scattering and absorption coefficient of PM 0.5−2.5 were estimated on the basis of the chemical composition of PM 2.5 and the size distribution of number concentration of PM 0.5−2.5 .This estimation was evaluated by results from Nephelometer measurement and proved to have high accuracy.With the knowledge of hygroscopic growth of some inorganic salts, it was realized that optical properties of PM 0.5−2.5 greatly depended on relative humidity, while light extinction was enhanced averagely 1.23, 1.38 and 1.75 times at 70 %, 80 % and 90 % RH, respectively.Moreover, light extinction coefficient of PM 0.5−2.5 increased averagely 1.24 to 1.28 times during wet days while merely 1.04 times in dry days.Furthermore, combined results from Nephelometer, the knowledge of relation between EC and aerosol light absorption and the Mie Model estimation, size distribution of total light extinction coefficient was determined.PM 1 contributed averagely 76 %, 85 %, 94 % and 93 % to light extinction in spring, summer, autumn and winter, respectively, while the contributions from PM 2.5 were 94 % at least.

Introduction
Atmospheric aerosol influences radiation budget of the earth-atmosphere system by scattering and absorbing solar radiation (Seinfeld and Spyros, 2006).Pollution caused by aerosol will enhance this light extinction effect.Furthermore, the aerosol impact on climate change will also be enhanced accordingly.With regard to the Mie Theory, to recognize size distribution of aerosol particle numbers and light extinction efficiency of a single particle are the keys to quantify light extinction effect (Seinfeld and Spyros, 2006).Aerosol absorbs water with an increasing relative humidity (RH).It should be taken into account in a place with a damp climate for this hygroscopic property will increase aerosol size and change optical properties of aerosol population.In the past century, scientists studied hygroscopic growth of some inorganic salts that constitute aerosol in order to quantify hygroscopic growth factor of aerosol population (Tang and Munkelwitz, 1994;Tang, 1996;Tang et al., 1997).Those studies related RH to aerosol size growth through laboratory measurement.Later on, a parallel observation with the Twin Differential Mobility Particle Sizer (TDMPS) and the Humidifying Differential Mobility Particle Sizer (H-DMPS) was used to directly measure particles size distribution and hygroscopic growth of the ambient aerosol particles smaller than 1 µm (Eichler et al., 2008).Moreover, with the combination of the Micro-Orifice Uniform Deposition Impactor (MOUDI); the Aerodynamic Particle Sizer (APS) and the knowledge of hygroscopic growth factor of some inorganic salts, it's able to determine particles size distribution and hygroscopic growth factor of the aerosol particles larger than 1 µm (Eichler et al., 2008).
Since socioeconomic developed in recent years, pollution caused by fine aerosol particles in Guangzhou and its surrounding area has attracted more and more attentions from the public and scientists.Lots of researches were carried out in order to illustrate the regional characteristic of aerosol chemical composition and the extent of visibility degradation due to aerosol light extinction (Wang et al., 2003;Bergin et al., 2004;Louie et al., 2005;Wu et al., 2005;Cao et al., 2007;Deng et al., 2008a, b;Tie Introduction

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Full and Cao, 2009).Moreover, using methods mentioned above and the numerical simulation technique, integrated experiments of the PRIDE-PRD2004 in October 2004 and the PRIDE-PRD2006 in July 2006 were conducted in both urban and rural area of Guangzhou.These experiments focused on the formation mechanism of aerosol pollution including chemical transformation and driven force from regional meteorological condition (Fan et al., 2008;Su et al., 2008;Zhang et al., 2008a, b;Fan et al., 2011), size-resolve chemical composition and pollutant sources (Gnauk et al., 2008;Liu et al., 2008a, b;Jung et al., 2009;Xiao et al., 2010;Yu et al., 2010;Yue et al., 2010), mixing state of the EC and other non-light-absorption compound in aerosol (Cheng et al., 2006(Cheng et al., , 2008b)), aerosol optical properties, radiative direct forcing and their RH dependence (Andreae et al., 2008;Cheng et al., 2008b;Eichler et al., 2008;Liu et al., 2010).
As well as in summer and autumn, it is necessary to characterize the temporal variation of aerosol light extinction effect in spring and winter regarding to the connection between meteorological condition and air pollution.To what extent, the weather during spring and winter in South China is frequently influenced by co-effects of cold air mass from the inland and warm air mass from the sea.Moreover, few studies were reported on a long term impact of RH and particles size distribution on aerosol light extinction in urban area of Guangzhou.For the purpose of an updated and complementary study on this topic, a series of experiment were carried out at the monitor site of SCIES during Introduction

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Monitoring site
The monitoring site of SCIES locates in Guangzhou city where geographical coordinates are 23 • 07 N and 113 • 21 E. It was designed for monitoring air quality influenced by pollutant regional transport and local sources emission.For this reason, instruments were installed on roof of the main building with a height of 53 m.This site was built with a clear vision over 300 • , around which there is a residential area and a park about 500 m northeast to it.There does not seem to be any big source of air pollution but mobile emissions within circumference of 3 km.A satellite photo depicting the site's location and its surroundings is illustrated in Fig. 1.So far, data on aerosol samples, gaseous pollutants and meteorological parameters have been accumulated over a long period of time at this site.Previous works on the basis of these data can be read in open literatures (Tao et al., 2009(Tao et al., , 2010(Tao et al., , 2012)).

Sampling and analysis
The measured aerosol properties and meteorological parameters during these experiments are listed in Table 1.The output from Nephelometer, APS and automatic weather station were hourly averaged before further analysis.
1. PM 2.5 sampling.During these experiments from 2009 to 2010, PM 2.5 samples were measured by an air sampler (BGI Corporation, Model PQ200) equipped with a cyclone that separates PM 2.5 particles from the aerosol population and with a vacuum pump that draws air at a rate of 16.7 l min −1 .The drawn airstream is connected to a 47 mm quartz filter (Whatman, QM-A).Before sampling, the quartz filters are baked at 800 • C for more than 3 h to remove adsorbed organic vapors, and then equilibrated in desiccators for 24 h.Prior to the measurement in ambient, the flow rate of PM 2.5 sampler is calibrated.Blank filters are collected Introduction

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Full daily quartz-filter samples with some blank ones were collected for every 23.5 h (starting at 10:00 LST each day and ending at 09:30 LST the following day).The analysis-ready samples were stored in a freezer at about −20 • C in case of particles volatilization.
2. OC/EC determination.A punch of 0.5 cm 2 from the collected quartz filter was analyzed for eight carbon fractions following the IMPROVE A thermal/optical reflectance (TOR) protocol by a DIR model 2001 carbon analyzer (Atmoslytic Inc., Calabasas, CA) (Cao et al., 2007;Chow et al., 2007).This process produced four OC fractions (OC1, OC2, OC3 and OC4) at 140  (Chow et al., 2007).Inter-laboratory sample comparisons between applying the IMPROVE TOR protocol and the TMO (thermal manganese dioxide oxidation) approach have shown the differences being lower than 5 % for TC and 10 % for OC and EC (Chow et al., 2007).Average field blanks were 1.8 and 0.1 µg m −3 for OC and EC, respectively.) and 20 mmol l −1 MSA eluent.Anions (SO 2− 4 , NO − 3 , Cl − and F − ) were separated by an AS11-HC column (Dionex Corp, Sunnyvale, CA) and 20 mmol l −1 KOH eluent.The limits of detection were less than 0.05 mg l −1 for both cations and anions.Standard reference materials produced by the National Research Centre for Certified Reference Materials in China were analyzed for quality assurance purposes.Blank values were subtracted from sample concentrations (Shen et al., 2009).
4. Nephelometer deployment.Total light scattering coefficient of aerosol was measured by an integrating Nephelometer (TSI Performance Measurement Tools, Model 3563) in wavelengths of 450 nm, 550 nm and 700 nm, respectively.Nephelometer calibration was performed by carbon dioxide (CO 2 ) as high-span gas and filtered air as low-span gas.Nephelometer draws ambient air through a temperature-controlled inlet at a flow rate of 20 l min −1 .The inner heater controls the RH of aerosol intake at a level lower than nearly 70 %.The output data were set to 1 min averaged in these experiments, and zero level data was measured continuously for 5 min after each hourly (60 min) sampling.

Chemical component determination
Recognizing chemical compounds that constitute aerosol particles is necessary for determining the thermodynamic and optical properties of aerosol population.For fine aerosol, components include inorganic salts, POM, EC and the water taken up by the salts.

Inorganic salt
Under an assumption of the electrical charge neutrality (Eichler et al., 2008), identification to associations among the water-soluble ions raised a recognition of the inorganic salts.The cations identified from PM 2.5 samples were Na + , NH In the light of previous studies (Seinfeld and Spyros, 2006;Cheng et al., 2008c;Han et al., 2009;Yue et al., 2011), it is usually considered that for fine particles: (1) NH + 4 , SO

POM and water content
As recommended in a previous study (Hand and Malm, 2006), a factor of 1.8 for urban area was adapted to "convert" the mass of OC to that of POM.Since POM, EC and other unidentified components were considered to have no hygroscopic growth, the mass of water taken up by inorganic salts can be estimated based on hygroscopic growth factors of inorganic salts following Eq.( 1) where m w and ρ w is mass concentration and density of water uptake, respectively.Moreover, V j stands for the volume of the j -th inorganic salts in aerosol, while f g,j is the hygroscopic growth factor at a specified RH.
For the PM 2.5 samples were weighted in laboratory condition of 40 % RH, the inorganic salts, POM, EC and water content were determined at this RH as well.Hence, the unidentified part in PM 2.5 , named the Residual, can be determined from the difference between the total mass and the sum mass of the identified part.Similarly, the mass of aerosol at arbitrary RH can be determined into two parts.One is the salts, POM, EC and the Residual at 40 % RH, while the other one is the water uptake at that desired RH.

Light scattering and absorption coefficient estimation
In consideration of instrumental deployment during these experiments, it is likely to introduce the internal mixing, one of simplifications about aerosol mixing status (Seinfeld and Spyros, 2006;Cheng et al., 2008c), into the Mie Model to estimate the extent of aerosol light scattering and absorption coefficient.The aerosol optical properties to discuss later are referenced to the light of 550 nm wavelength, only exceptions will be noted.Introduction

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EORI and EGF
The EORI (Effective Optical Refractive Index) represents the averaged ORI of internally mixing aerosol population, which can be calculated with the ORIs of each chemical component in aerosol following mixing rules of Volume Average (Lesins et al., 2002).This mixing rule considers relationship among each chemical component as equally weighed by their volumes, the formulas of which were written as Eq. ( 2) and Eq.(3).
In Eqs. ( 2) and ( 3), m j stands for the mass of the j -th component in aerosol, while ρ j is the density.Respectively, n j is the real part of ORI of the j -th component, k j is the imaginary part.For the EORI, n eff is the real part, and k eff is the imaginary part.
The EGF (Effective Hygroscopic Growth Factor) was the averaged hygroscopic growth factor of internally mixing aerosol, which can be calculated with f g of each chemical component in aerosol following Zdanovskii-Stokes-Robinson assumption (Stokes and Robinson, 1966).It can be depicted by Eq. ( 4).
In Eq. ( 4), f eff is the EGF. a j is the volume fraction of the j -th component in aerosol, while f g,j is the hygroscopic growth factor.The f g of some inorganic salts had been determined in previous studies (Tang and Munkelwitz, 1994;Tang, 1996;Kelly and Wexler, 2006) where related f g to ambient RH by laboratory measurements.Introduction

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In Eqs. ( 5) and ( 6), D j stands for the midpoint Stokes diameter in the j -th size range of aerosol, while N j is the number concentration of aerosol population with diameter D j .Equation ( 7) converted D a,j from APS size bin to D j required by the Mie Model, and the N j corresponding to D a,j were from APS measurement.Moreover, the f eff in Eq. ( 7) was used to estimate the hygroscopic growth of D j under an assumption that no change in particles number concentration in the process of hygroscopic growth.Q sp,j represents light scattering efficiency of a single particle with diameter D j , while Q ap,j represents light absorption efficiency.At a given light wavelength, they both are the functions of D j and the EORI.Density of dry aerosol population, ρ dry , in Eq. ( 7) can be calculated with Eq. ( 8) where ρ j is density of the j -th chemical components without water uptake.Wet aerosol density, ρ wet in Eq. ( 9), can also be calculated with Eq. ( 8) Introduction

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Full where water was included.Additionally, wet aerosol mass concentration, m wet , can be calculated with Eq. ( 9).
Practically, it's able to estimate b sp,pm0.5-2.5 by the Mie Model based on the EORI, EGF of PM 2.5 and N j ,pm0.5-2.5 from APS measurement.In order to evaluate the accuracy of this estimation, a correlation analysis was performed between temporal variations of b sp,pm0.5-2.5 from the estimation and b sp,neph (550 nm) from Nephelometer measurement.By considering an evaluation with higher temporal resolution, these two data sets should be compared with each other in hourly average.However, under the circumstance of no available data about diurnal variation of chemical composition, the required hourly EORI, EGF and ρ dry were valued to their daily averages which were determined from chemical composition of daily PM 2.5 sample.Furthermore, hourly values of b sp,neph (550 nm) were corrected with an approach stated in a previous study (Anderson and Ogren, 1998) where used a linear function of Angstrom exponent å450/700 ( å450/700 was calculated with b sp,neph,450 nm and b sp,neph,700 nm ) as correction factors for Angular Nonidealities.It is noted that the EGF adapted in this evaluation was calculated at the RH detected by Nephelometer built-in RH sensor rather than in ambient condition.

RH dependence
For convenience, it is defined that h is considered to be relative dry or without obvious hygroscopic growth.Hence, h Y is a factor that quantifies the extent of hygroscopic growth.In the light of a previous study (Cheng et al., 2008b), the RH dependence of h Y can be fitted by a few functions including Eqs. ( 10), ( 11) and ( 12).Having something in common, these three functions are monotone and ensure h Y to be unity at RH 0 .However, comparing the power part among them, Eq. ( 11) and ( 12) can better fit an RH dependence pattern where hygroscopic growth activates at high RH level.

Size distribution
For the purpose of determining the light extinction contribution from aerosol in different size, it's necessary to estimate the mass concentration of PM 0.5 , PM 0.5-1 , PM 1-2.13), ( 14) and ( 15).m pm2.5 in Eq. ( 13) was the mass concentration of daily PM 2.5 sample in ambient RH 15651 Introduction

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Mie Model estimation and its evaluation
Temporal variations of mole proportion of the identified water-soluble ions in PM 2.5 are illustrated in Fig. 2. On the whole, the cations were sufficient to neutralize the anions in every sample, while SO 2− 4 and NH + 4 had large amount but Mg 2+ was hardly any.In spring, there's hardly any Ca 2+ while NH Regarding other salts, Na 2 SO 4 and NaCl raised their amount in both summer and early autumn.For the compounds of Ca 2+ , it was in the form of CaSO 4 in summer and autumn, while Ca(NO 3 ) 2 in winter.It is for certain that the amount of water content was relative low in dry days in autumn.To what extent, there was a linear relation between POM and EC, and their proportions were higher in summer and autumn than in spring and winter.Limited to laboratory equipments, there were still 13 to 23 % in total mass, the Residual, where chemical composition can't be determined The optical parameters of chemical components for calculating the EORI were learned from some open literatures (Tang, 1996;Chazette and Louisse, 2001;Sloane, 1986;Haynes, 2011;Seinfeld and Spyros, 2006;Eichler et al., 2008) and are listed in Table 2.As illustrated in Fig. 4, those inorganic salts with determined f g including NH 4 NO 3 , NaCl, (NH 4 ) 2 SO 4 , Na 2 SO 4 , NaNO 3 , KCl were plotted in the form of their RH dependence curves.For those inorganic salts without determined f g , it's reasonable to neglect their hygroscopic growth as their volume concentration were small enough in this case.Moreover, the EGF of PM 2.5 samples were calculated and their average over a span of four months is plotted in the form of RH dependence curve in Fig. 4. Discontinuities in this curve were due to the f g considered to be unity when ambient RH dropped to a specified salt's crystallization point.Example is the one at 37 % RH which is the crystallization point of (NH 4 ) 2 SO 4 .On the basis of the methods discussed earlier, the EORI, EGF and density of dry/wet aerosol were calculated and the statistics of the results are listed in Table 3.
Regarding the Mie model evaluation mentioned above, as illustrated in Fig. 5, correlation coefficients between the b sp,pm0.5-2.5 from the estimation and b sp,neph from Nephelometer were found to be 0.81, 0.87, 0.73 and 0.80 at 0.05 significant levels in spring, summer, autumn and winter, respectively.With this implication of a coherent temporal variation between model estimation and direct measurements, it's reliable to estimate b sp,pm0.5-2.5 with high accuracy by applying the Mie Model based on APS measurement and PM 2.5 sampling.Introduction

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Full Figure 4 illustrates that f eff increased more rapidly after ambient RH over 37 %, it's reasonable to set RH 0 = 37 % in Eq. ( 10) to Eq. ( 12).By the Mie Model estimation, RH dependence patterns of h bep,pm0.5-2.5 (light extinction coefficient is denoted as b ep , it is the sum of b sp and b ap ), h bsp,pm0.5-2.5 , h bap,pm0.5-2.5 and h ω0,pm0.5-2.5 (single scattering albedo is denoted as ω 0 , it is the ratio of b sp to b ep ) were determined and plotted in Fig. 6.It was found that Eq. ( 10) best fitted RH dependence patterns of h bep,pm0.5-2.5 , h bsp,pm0.5-2.5 and h ω0,pm0.5-2.5 .Parameter "a" and the R 2 of these curve fittings by Eq. ( 10) are listed in Table 4. On the contrary, h bap,pm0.5-2.5 fluctuated around 1.00 when RH increased from 37 % to 66 %, and then began to drop.It made none of Eqs. ( 10), ( 11) and ( 12) able to fit that RH dependence pattern.Moreover, Table 4 shows hygroscopic growth enhanced b ep,pm0.5-2.5 averagely 1.23, 1.38 and 1.75 times at 70 %, 80 % and 90 % RH, respectively.Regarding temporal variation of ambient RH, autumn was a relative dry season in South China while the other three were categorized to wet seasons.Table 5 shows that b ep,pm0.5-2.5 was enhanced by hygroscopic growth averagely 1.24 to 1.28 times during wet seasons and merely 1.04 times in dry season.

Size distribution of mass concentration and light extinction coefficient
Regarding the monthly average noted in Table 6, the maximum of both m total and b ep,total was in winter while the minimum of them was in summer.Implied from a low Temporal variations of size distribution of m total and b ep,total are illustrated in Fig. 7. PM 1 contributed 58 to 79 % in total mass and 76 to 94 % in light extinction, while PM 2.5 did 78 to 88 % contributions to the total mass and 94 to 98 % to light extinction.Sharing a similarity with the low å450/700 in spring, PM 0.5-2.5 gained much weight and carried much contributions to b sp,total through hygroscopic growth.Implied from size distribution of e sp and e ap illustrated in Fig. 8, large particles were more efficient in light absorption but less in light scattering than small ones.
With a less consideration of anthropogenic emissions varying much during these experiments, a low m total and b ep,total in summer was probably due to a meteorological condition in favor of a diffusion or removal of air pollutants.As the proportion of m ec varied relative little, an aged air mass from the sea containing more internal mixing EC explained a higher light absorption during summer.This conclusion was supported by a previous study (Cheng et al., 2006).Moreover, the high mass proportion of Na + in summer was an evidence for that air mass from the sea.On the other hand, m total and b ep,total increased much in spring, autumn and winter.Aerosol population gained more weight and its light extinction was enhanced through humidification in spring.Owing to PM 0.5 having much weight and its high mass scattering efficiency, b ep,total was even higher while m total was a bit lower in autumn than in spring.It indicated that hygroscopic growth had notable contributions to the rising of light extinction effect in spring, while ultra-fine particles accumulation accounted for that rising during dry days in autumn.
Co-effects of hygroscopic growth and particles accumulation took account for the high b ep,total and m total in winter.enhanced up to nearly 2 times from 37 % to 92 % RH.Averagely, hygroscopic growth factor of b ep,pm0.5-2.5 was 1.24 to 1.28 during wet days and merely 1.04 in dry days.Combined Nephelometer measurement, mass concentration of EC and the Mie Model estimation, contributions to b ep from aerosol in different size can be estimated.PM 1 contributed averagely 76 %, 85 %, 94 % and 93 % to b ep in spring, summer, autumn and winter, respectively, while the contributions from PM 2.5 were 94 % at least.Strong atmospheric diffusion may be in favor of a low b ep in summer.Hygroscopic growth carried notable contribution to the rising of b ep during wet days in spring, while ultrafine particles accumulation was responsible for that rising during dry days in autumn.
The b ep stay high in winter as the consequence to the co-effects of the high RH level and fine aerosol pollution.Implied from the discussion in this paper, regional meteorological condition such as surface wind field pattern may govern the pollutant regional transport which resulted in the temporal variations of aerosol chemical composition.An investigation on this will be carried out in a near study.Introduction

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Sampling and analysis
The measured aerosol properties and meteorological parameters during these experiments are listed in Table -1.The output from Nephelometer, APS and automatic weather station were hourly averaged before further analysis.The optical parameters of chemical components for calculating the EORI were learned from some open literatures (Tang, 1996;Chazette and Louisse, 2001;Sloane, 1986;Haynes (Ed.), 2011;Seinfeld and Spyros, 2006;Eichler et al., 2008) and are listed in Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

3.
Water-soluble ions determination.One quarter of the collected quartz filter sample was used to determine the mass concentrations of water-soluble ions.Four anthe filter were determined by ion chromatography (Dionex Corp, Sunnyvale, CA, Model Dionex 600).For these extractions, each sample was put into a separate 20 ml vial containing 10 ml distilled-deionised water (18 MΩ resistivity), and shaken first by an ultrasonic instrument for 60 min, then by mechanical shaker for 1 h for a complete extraction.The extracts were stored at 4 • C in a Discussion Paper | Discussion Paper | Discussion Paper |

5.
APS deployment.Size distribution of number concentration of aerosol was measured by APS (TSI Aerodynamic Sizer, Model 3321) with 52 size bins in diameter range from 0.5-20 µm by determining time-of-flight of individual particle in an accelerating flow field.A drying tube was added before drawing air to capture dry aerosol.Flow rate of 5 l min −1 and 5 min data averaged were set in APS operation.6.Weather station deployment.Ambient relative humidity had been recorded by an automatic weather station (VASALA Model QMH102) installed 3 m above the roof.
(NH 4 ) 2 SO 4 , if any remains, then it associates with NO − 3 to become NH 4 NO 3 ; (2) biomass burn is responsible for K + and Cl − , K + associates with Cl − to become KCl, if any remains, then it associates with SO 2− 4 to become K 2 SO 4 ; (3) Na+ associates with Cl − in sea salt to become NaCl.However, NaCl will react with gaseous nitric acid to produce sodium nitrate in the process of atmospheric diffusion.Hence Na + associates with NO − 3 to become NaNO 3 , if any remains, then it associates with the similar case with Mg 2+ .Following these associations, inorganic salts including (NH 4 ) 2 SO 4 , NH 4 NO 3 , NaNO 3 , Na 2 SO 4 , KCl, NaCl, Ca(NO 3 ) 2 , Mg(NO 3 ) 2 , K 2 SO 4 and CaSO 4 can be recognized.Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Y (RH) = Y(RH)/Y(RH 0 ) where Y(RH) stands for a specified property of aerosol at arbitrary RH.RH 0 is a referenced RH at which aerosol 15650 Discussion Paper | Discussion Paper | Discussion Paper | 5 and PM 2.5-20 , then, the b sp and b ap of them.Regarding daily average, the b sp,pm0.5-2.5 , b ap,pm0.5-2.5 and m pm0.5-2.5 can be estimated by the Mie Model based on the chemical composition of PM 2.5 and size distribution of N pm0.5-2.5 at ambient RH.Regardless of the difference in chemical composition, it's feasible to estimate b sp,pm2.5-20, b ap,pm2.5-20and m pm2.5-20 by the Mie Model on the basis of size distribution of N pm2.5-20 .Furthermore, b sp,pm0.5 , b ap,pm0.5 and m pm0.5 can be estimated by Eqs. ( Discussion Paper | Discussion Paper | Discussion Paper |

ω 0 ,
light absorption effect in summer was strengthened.A low å450/700 indicated the size of particles that constitute the aerosol population was larger in spring than in the other seasons.In order to recognize mass efficiency of light scattering and absorption, e sp = b sp m −1 and e ap = b ap m −1 are defined.A low e sp,total and a high e ap,total supported the enhanced light absorption in summer.Moreover, the sum of e sp and e ap indicated mass efficiency of light extinction was higher in autumn and winter.Discussion Paper | Discussion Paper | Discussion Paper |

5
Summary and conclusion With regard to aerosol population in internal mixing status, b ep,pm0.5-2.5 can be estimated with high accuracy by the Mie Model.The EORI and EGF required in this estimation can be calculated on the ground of chemical composition of PM 2.5 .With a damp climate in Guangzhou, b ep,pm0.5-2.5 greatly depended on ambient RH and was Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Deng, X. J., Tie, X. X., Wu, D., Zhou, X. J., Bi, X. Y., Tan, H. B., Li, F., and Jiang, C. L.: Longterm trend of visibility and its characterizations in the Pearl River Delta (PRD) region, China, Atmos.Environ., 42, 1424-1435, 2008b.Eichler, H., Cheng, Y. F., Birmili, W., Nowak, A., Wiedensohler, A., Br üggemann, E., Guauk, T., Herrmann, H., Althausen, D., Ansmann, A., Engelmann, R., Tesche, M., Wendisch, M.Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Table 4. Parameter "a" and the R 2 of the curve fittings by Eq. (10).pm0.5-2.5 −0.0811 0.9948 1.066 (0.017) 1.098 (0.022) 1.157 (0.032) Note: the data in brackets are referenced to the standard deviation.Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Figure-1 Satellite photo of the monitoring site and surroundings (From Google Earth)

Fig. 3 .
Fig. 3. Temporal variation of the chemical components in mass proportion.Note: OIC mean the other determined inorganic salts.

Figure- 4
Figure-4 RH dependence curve of some inorganic salts and the EGF Note: the bars in the right figure stand for the standard deviation of f eff at specified RH

Fig. 4 .
Fig. 4. RH dependence curve of some inorganic salts and the EGF.Note: the bars in the right figure stand for the standard deviation of f eff at specified RH.

Figure- 5
Figure-5 Temporal variations of the b sp,pm0.5-2.5 and b sp,neph Note: standardized b sp is the ratio of individual b sp to their maximum.

Fig. 5 .
Fig. 5. Temporal variations of the b sp,pm0.5-2.5 and b sp,neph .Note: standardized b sp is the ratio of individual b sp to their maximum.

Fig. 7 .
Fig. 7. Temporal variation of size distribution of mass concentration and optical properties.

Table 1 .
Measured aerosol properties and meteorological parameters.

Table 2 .
Optical parameters of chemical components for calculating the EORI.

Table 3 .
Monthly average of the parameters required in the Mie Model.

Table 6 .
Results of the Mie Model estimation and the relevant calculations.