Hygroscopicity and evaporation of ammonium chloride and ammonium nitrate: Relative humidity and size effects on the growth factor

Atmospheric Environment 45 (2011) 2349e2355

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Hygroscopicity and evaporation of ammonium chloride and ammonium nitrate: Relative humidity and size effects on the growth factor Dawei Hu a, Jianmin Chen a, b, *, Xingnan Ye a, Ling Li a, Xin Yang a, b, * a b

Center for Atmospheric Chemistry Study, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China Institute of Global Environment Change Research, Fudan University, Shanghai 200433, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 November 2010 Received in revised form 28 January 2011 Accepted 10 February 2011

The hygroscopicity and evaporation of ammonium chloride and ammonium nitrate in the size range of 40e200 nm are investigated from 20% to 86% RH using a self-assembled hygroscopic tandem differential mobility analyzer (H-TDMA) system. The hygroscopicity of 100 nm (NH4)2SO4 is also measured for comparison. The measured hygroscopic growth factors (GFs) of (NH4)2SO4 agree well with the theoretical Köhler curve. Great discrepancies between the measured GFs and the theoretical values are observed for NH4Cl and NH4NO3 due to their volatile properties. The evaporation of NH4Cl below the deliquescence RH (DRH) is significantly promoted while RH increases. Similar trend is also observed for NH4NO3 particle less than 50 nm. The proposed mechanism suggests that the increase of RH alters the chemical equilibrium among NH4X(s) (X represents Cl
or NO3
), NH3(g) and HX(g), i.e., NH4 XðsÞ $NH3 þ HXðgÞ , by converting NH3(g) and HX(g) into NH3$nH2O and HX$nH2O, which accelerates the evaporation of NH4X(s). When RH is higher than the DRH, the GFs of NH4X increase with initial particle size (D0) throughout the investigated size range. In this study, the iso-GF curves are also drawn to illustrate the effects of D0 and RH on the GFs. Different from (NH4)2SO4, NaCl, Na2SO4 and NaNO3, the GFs of NH4X are more sensitive to D0 than RH due to the unique volatility of NH4Cl and NH4NO3 particles. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Hygroscopicity Evaporation Ammonium chloride Ammonium nitrate H-TDMA

1. Introduction Atmospheric particles play an important role in affecting radiative forcing and climate (Charlson et al., 1992; IPCC, 2007). Ammonia (NH3) from combustion, fertilizers and biological decay is the dominant volatile base in the atmosphere (Bouwman et al., 1997). By reacting with sulfuric acid (H2SO4), nitric acid (HNO3) and hydrogen chloride (HCl), particulate ammonium salts such as (NH4)2SO4, NH4NO3 and NH4Cl are formed and account for the predominant inorganic components of atmospheric aerosols (Renard et al., 2004; Seinfeld and Pandis, 2006). Unlike (NH4)2SO4, NH4NO3 and NH4Cl are salts with higher vapor pressure. Aerosol particles containing NH4NO3 and NH4Cl are generally in equilibrium with gaseous NH3, HNO3 and HCl species, and their partition between particles and gases is commonly a strong function of ambient temperature and relative humidity (RH) (Stelson and

* Corresponding authors. Center for Atmospheric Chemistry Study, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China. Tel.: þ86 21 65642298; fax: þ86 21 65642080. E-mail addresses: [email protected] (J. Chen), [email protected] (X. Yang). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.02.024

Seinfeld, 2007; Finlayson-Pitts and Pitts Jr, 2000; Dassios and Pandis, 1999). Previous works have revealed that NH4NO3 and NH4Cl particles (especially 100 nm) deliquesced at 62% and 78% RH at 298 K, from which the solid particle spontaneously absorbed water and produced a saturated aqueous solution (Seinfeld and Pandis, 2006; Lightstone et al., 2000; Martin, 2000; Topping et al., 2005). Studies also demonstrated the hygroscopic properties of inorganic salts to be initial particle size (D0) dependent (Russell and Ming, 2002; Biskos et al., 2006a,b; Park et al., 2009; Hu et al., 2010). The hygroscopic growth factors (GFs) of sulfate ((NH4)2SO4, Na2SO4), nitrate (NaNO3) and chloride (NaCl) particles were reported to increase with D0 when the RH is above the deliquescence RH (DRH) (Russell and Ming, 2002; Biskos et al., 2006a; Park et al., 2009; Hu et al., 2010). Until now, the hygroscopicity of NH4NO3 and NH4Cl in wide-size range still not be studied and the corresponding sizeeffect is also out of discussion. The evaporation of monodisperse NH4NO3 and NH4Cl under different RH has been measured extensively by continuously and rapidly removing gaseous NH3 and HNO3 (or HCl) from aerosols (Allen et al., 1989; Tang and Munkelwitz, 1989; Harrison et al., 1990; Larson and Taylor, 1983). Larson and Taylor (1983) studied the size change of NH4NO3 aqueous droplet upon passing through

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a diffusion stripper which acted as an infinite sink for both NH3 and HNO3. Their measurements show good agreement with predictions from the theories of gaseous diffusion and concentrated solution chemistry. For aqueous particles with mass media diameter of 0.4 mm, the evaporation rate was estimated to be 6 Å s
1 by Harrison and Mackenzie (1990). In an annular denuder system, Harrison et al. (1990) measured the evaporation rate of dry and aqueous NH4Cl (and NH4NO3) aerosols, and observed the dry (30e60% RH) NH4Cl (0.65 mm) and NH4NO3 (0.6 mm) aerosols evaporated slower than the corresponding aqueous aerosols (97% RH). However, all those studies on the evaporation properties of NH4Cl and NH4NO3 were highly concentrated on the large particles (usually larger than 100 nm). For aerosol particles 100 nm or smaller, the surface curvature effect would play an important role in the vapor pressure and the size-effect should be taken into account (Bai et al., 1995). In this paper, the GFs of 40e200 nm NH4NO3 and NH4Cl aerosols are measured from 20% to 86% RH using a self-assembled hygroscopic tandem differential mobility analyzer (H-TDMA) system. The effects of RH and particle size on the hygrocopicity and evaporation of nanosize NH4NO3 and NH4Cl aerosols are discussed.

2. Experimental The experimental setup consists of an aerosol generator and a set of H-TDMA system. It has been described in detail in previous works (Ye et al., 2009; Hu et al., 2010). Briefly, aerosol particles produced by the aerosol generator (Model 3076, TSI Inc., USA), are dehydrated to w5% RH through a Nafion drier and analyzed using an H-TDMA system. In H-TDMA system, particles are neutralized with 210Po diffusion charger and sizeselected using the first DMA (DMA1, Model 3081, TSI Inc., USA). Afterward, the size-selected-particles are exposed to a predefined humid environment, i.e., a multitube Nafion humidifier (Models PD-70T-24ss, Perma Pure Inc.) where RH can regulate in the range of 5e90%. Then, a new particle size distribution is measured by the second DMA (DMA2) with combination of a condensation particle counter (CPC, Model 3771, TSI Inc., USA). And the hygroscopic growth factor (GF ¼ Dp/D0), defined as the ratio between the humidified (Dp) and initial dry particle diameter at a well-defined RH, could be deduced. The dry particle spend in the humid conditions for the hygroscopic growth before introducing into DMA2 approximately 2.5 s, which is sufficiently long to reach the hygroscopic growth equilibrium for ammonium chloride and ammonium nitrate since the equilibrium time for inorganic salts is within 0.5 s. During the experiment, the RH is monitored both on the inlet and outlet of DMA2 with RH modules (Model HMM22D Module, Vaisala Inc.) to make sure the difference between the humidified sample and the humidified DMA2 sheath gas flow within 1% RH (throughout all the RH range). In addition, to avoid RH fluctuation, all parts except CPC are installed in a thermostatic box with a temperature fluctuation lower than 0.1  C and the sheath gas will be heat exchanged to the box temperature prior to entering DMA2 so that the aerosol stream is kept at a constant temperature. All heat sources, such as pumps, power supplies, and water saturators are kept in another unit separated from the thermostatic box. The DMAs are operated with N2 as sheath gas in 6.5 LPM, and with sample in 1.0 LPM. The particle sizes mainly cover from 10 to 400 nm. In order to maintain each sample have the identical number concentration (i.e., identical dilution ratio), the dilute aqueous solutions concentration and the N2 pressure which supplied on the aerosol generator are identical for each experiment.

Before experiment, the accuracy of all DMAs is calibrated with certified polystyrene latex spheres (PSLs) and ammonium sulfate. Ye et al. (2009) have been testing our H-TDMA system using PSLs (catalog number: 3200, certificated mean particle diameter: 199  6 nm, Duke Inc.), the performance tests show that the measured average particle size and hygroscopic growth factor at 70% RH of the PSLs is 197 nm and 0.9956  8.9  10
4, respectively. It indicates that our H-TDMA system is a powerful tool for studying the hygroscopic behavior of submicron aerosols and meets the demand required for laboratory research. In this study, the predefined RH is selected as 20%, 30%, 40%, 50%, 60%, 70%, 80%, 82%, 84%, 86% and D0 is selected as 40, 60, 80, 100, 120, 140, 160, 180, 200 nm, respectively. For comparison, the hygroscopicity of 100 nm (NH4)2SO4 is also measured. Each experiment is repeated five times. In consideration of the evaporation of the ammonium salt inside the DMAs and connecting tubing and the discrepancies between the two DMAs, calibration experiments are run under the humidifier RH < 5% conditions. We found the reduction in diameter between the two DMAs only occurred in NH4NO3 aerosol. This change was then used to correct the initial diameter and hygroscopic growth factor during the main experiments. For 40, 60, 80, 100, 120, 140, 160, 180, 200 nm particles selected by DMA1, when the humidifier RH < 5%, the diameter of NH4NO3 particles determined by DMA2 is 30.7, 50.0, 69.1, 87.7, 106.4, 125.1, 143.8, 162.4, 181.0 nm, respectively. While for NH4Cl particle, the particle diameter remains unchanged between two DMAs.

3. Results and discussion 3.1. Hygroscopic growth of 100 nm ammonium salt aerosols Fig. 1(a1) presents a series of Gaussian fits to the measured size distributions of 100 nm (NH4)2SO4 as a function of RH. The fits have been normalized to the peak concentration. It shows that the particle size remains unchanged until RH reaches the DRH, from which (NH4)2SO4 deliquesces abruptly to form a saturated solution droplet. Fig. 1(a2) displays the measured hygroscopic growth curve of 100 nm (NH4)2SO4. Good agreement is observed between the measured hygroscopic GFs and the theoretical Köhler curve which derived from the AIM model (http://www.aim.env.uea.ac.uk/aim/ model2/model2a.php). And the deliquescence transition is observed at 80  1% RH, in good agreement with literature data of 79.9% (Onasch et al., 1999) and 80.3% (Tang and Munkelwitz, 1993). The hygroscopic properties of NH4Cl aerosols have been examined previously with several different techniques and were expected to deliquesce at 78% RH at 298 K (Seinfeld and Pandis, 2006). Fig. 1(b1) presents a series of Gaussian fits to the measured size distributions of 100 nm NH4Cl aerosols as a function of RH. It shows that the GFs of solid NH4Cl aerosols decrease continuously until RH reaches the DRH. Then, they deliquesce abruptly to form a saturated solution droplet by water vapor condensation, gain about 1.17 fold of D0. The droplets continue growing as RH further increases. Fig. 1(b2) displays the measured hygroscopic growth curve of 100 nm NH4Cl aerosol. It clearly shows that the measured hygroscopic growth factors are not consistent with the theoretical Köhler curve. The relationship between the measured GFs and theoretical value can be described as follows:

Below the DRH : GFtheory ¼ GFmeasured þ 0:1896RH
0:0161 Above the DRH : GHtheory ¼ GFmeasured
2:4527RH þ 2:0589 Below the DRH, the particle diameter decreases from 98 nm at 20% RH to 89 nm at 70% RH. Further analysis reveals that the

D. Hu et al. / Atmospheric Environment 45 (2011) 2349e2355

a2

2.2

3

1.0

Normalized dN(#/cm )

a1

2351

0.6 0.4 0.2 0.0

2.0

Theoretical Deliquescence Curve (NH4)2SO4 Deliquescence

1.8 1.6

GF

0.8

1.4 1.2 1.0

%RH 0.8

1.0

1.5

0.0

2.0

0.2

0.4

0.6

0.8

1.0

RH

GF

b2 2.4 3

1.0

Normalized dN(#/cm )

b1

0.6 0.4 0.2

Theoretical Deliquescence Curve NH4Cl Deliquescence

2.0

GF

0.8

1.6

0.0 1.2

%RH 0.8

0.5

1.0

1.5

2.0

0.0

2.5

0.2

0.4

c1

0.6

0.8

1.0

RH

GF

c2

2.0

3

Normalized dN(#/cm )

1.0

0.6 0.4 0.2 0.0

1.8

Theoretical Deliquescence Curve NH4NO3 Deliquescence (After Correct) NH4NO3 Deliquescence (Before Correct)

1.6

GF

0.8

1.4

1.2

1.0

%RH 0.5

1.0

1.5

2.0

2.5

GF

0.8 0.0

0.2

0.4

0.6

0.8

1.0

RH

Fig. 1. . Hygroscopic growth of D0 ¼ 100 nm: (a1) (NH4)2SO4, (b1) NH4Cl, (c1) NH4NO3 as a function of RH. The distributions displayed by Gaussian fitting and normalized to the peak concentration. Measured growth factor for D0 ¼ 100 nm: (a2) (NH4)2SO4, (b2) NH4Cl, (c2) NH4NO3 as a function of increasing RH (V). The solid line represents the theoretical deliquescence curve as calculated from Köhler theory.

particle diameter decreases approximately 2.1 Å with RH increment of 1%. Above the DRH, NH4Cl aerosol shows a continuous growth trend and the particle diameter increases to 152 nm at 86% RH. The hygroscopic behavior of 100 nm NH4NO3 aerosols is displayed in Fig. 1(c1). The continuous growth of the NH4NO3 aerosols indicates that NH4NO3 aerosols do not exhibit the deliquescence phenomenon in this experiment, which is different from (NH4)2SO4 aerosols. Literature data (Tang and Munkelwitz, 1993; Lightstone et al., 2000) reported the DRH of NH4NO3 was 61.8% at 298 K, and the efflorescence RH (ERH) was still not observed until now. The minimum RH in this experiment (w5% after the Nafion dryer) is obviously not low enough for the

spontaneous crystallization of the NH4NO3 particles. Because of the missing crystallization, NH4NO3 aerosols do not completely dry out in the DMA1 (w5% RH) and have a diameter larger than the volume equivalent diameter of the solute NH4NO3. Thus, the particle diameter measured in the DMA1 (RH ¼ 5%) should be divided by the theoretical growth factor at this RH (1.05 at 298 K and 5% RH) to deduce the volume equivalent diameter D0. As shown in Fig. 1(c2), the corrected growth factors are still not consistent with the theoretical Köhler curve, and the GFs in the RH of 20e70% are lower than the theoretical value. The valuable reason for this discrepancy is the evaporation of NH4NO3 aerosols during the RH increasing process.

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D. Hu et al. / Atmospheric Environment 45 (2011) 2349e2355

a2

a1 1.1

2.0 1.8

1.0

1.6 1.4

<5% 20% 30% 40% 50% 60% 70%

0.8

0.7

0.6

GF

GF

0.9

1.2

80% 82% 84% 86%

1.0 0.8 0.6

0.5

0.4 0

40

80

120

160

200

0

40

80

D0(nm)

160

200

b2 2.0

1.3

1.8

1.2

20% 30% 40% 50% 60%

GF

1.1

70% 80% 82% 84% 86%

1.6

GF

b1

120

D0(nm)

1.4

1.0 1.2 0.9

1.0

0.8

0.8 0

40

80

120

160

200

40

0

80

120

160

200

D0(nm)

D0(nm)

Fig. 2. The growth factor as a function of initial particle size: (a1) NH4Cl, (b1) NH4NO3 particles at the RH below deliquescence point; (a2) NH4Cl, (b2) NH4NO3 particles at the RH above deliquescence point.

in this study varies significantly with RH and D0. Typically, the growth factors of the initial 40 nm particles decrease markedly from 0.96 at 20% RH to 0.67 at 70% RH, and when D0 decreases from 200 nm to 40 nm, the growth factors decrease from 0.95 to 0.69 at 70% RH. It implies that the evaporation of NH4Cl is significantly promoted with the increases of RH and with the decrease of D0 below the DRH. When RH is higher than the DRH (Fig. 2(a2)), NH4Cl aerosols are converted to droplet solution and the GFs of NH4Cl aerosols increase with D0 throughout the whole size range investigated.

3.2. Effects of RH and size on growth factors The growth factors as a function of D0 of NH4Cl and NH4NO3 aerosols under different RH are presented in Fig. 2, and the corresponding diameter increments are summarized in Table 1. For NH4Cl aerosols, below the DRH (Fig. 2(a1)), the growth factor is approximate to 1.00 for all size-resolved aerosols when the RH is less than 5%. A small evaporation rate was observed by Harrison et al. (1990), only w1.05 Å s
1 of dry NH4Cl aerosols. However, when the RH gradually changes from 20% to 70%, the growth factor Table 1 The diameter increment of NH4Cl and NH4NO3 aerosols under different RH conditions. Compounds

D0 (nm)

Dp
D0 value (nm) 20%

30%

40%

50%

60%

70%

80%

82%

84%

86%

NH4Cl (DRH ¼ 78%)

40 60 80 100 120 140 160 180 200


1.5
1.6
1.6
1.9
2.1
2.4
2.8
3.5
4.3


3.9
3.9
3.6
3.8
4.2
4.5
4.9
5.6
6.7


6.4
6.2
6.1
6.2
6.5
6.7
7.3
8.3
9.7


8.9
8.4
8.2
8.2
8.3
8.5
9.0
9.9
11.2


11.5
11.1
11.1
10.9
10.8
10.8
11.0
11.4
12.0


12.6
11.7
11.0
10.5
10.2
10.0
10.0
10.2
10.6


16.3 2.5 16.9 35.5 53.7 71.2 89.1 108.0


18.6
7.6 4.5 26.0 46.4 66.1 84.9 102.9 123.1


17.7
1.7 20.6 39.9 59.9 83.0 102.7 121.9 140.1


17.1 2.6 27.5 51.9 75.4 96.4 119.6 142.2 170.2

NH4NO3 (no observed DRH)

30.7 50.0 69.1 87.7 106.4 125.1 143.8 162.4 181.0


2.8
1.3 0.6 2.9 5.0 7.0 8.8 10.6 12.3


4.2
2.2 0.4 3.4 6.1 8.8 11.7 14.5 17.6


4.8
2.4 0.9 4.5 7.8 11.5 14.1 17.3 20.3


5.3
2.1 2.1 6.7 11.5 15.8 19.9 24.3 28.9


4.8
0.2 5.3 11.3 17.0 22.7 28.0 33.6 39.0


4.0 3.2 10.9 19.2 26.0 33.8 41.5 49.1 57.2


3.8 7.4 18.6 31.3 43.6 55.9 67.2 79.7 93.6


2.3 11.3 27.6 42.5 58.6 71.9 88.4 104.4 121.8


2.9 10.5 25.3 42.0 56.5 72.8 88.7 109.4 130.1


1.3 14.9 32.8 51.2 66.3 87.7 104.8 127.1 148.2

D. Hu et al. / Atmospheric Environment 45 (2011) 2349e2355

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Fig. 3. Proposed evaporation mechanism of ammonium salt aerosols (X represents Cl
or NO3
).

Further analysis indicates that the GFs increase pronouncedly for particles in the range of 40e100 nm, and the increase trend relatively slows down for particles in the range of 100e200 nm. In Fig. 2 (a2), the GFs of NH4Cl increase from 0.57 for 40 nm to 1.52 for

a1 0.7

100 nm aerosols at 86% RH, corresponding to 0.95 GF-increment (i.e., ΔGF ¼ 0.95), while the GFs only increase by ΔGF ¼ 0.33 for aerosols from 100 to 200 nm. The corresponding diameter increments (Table 1) are
17.1, 51.9 and 170.2 nm for those NH4Cl

a2 0.86

0.6

0.65

0.5

0.70

0.80 0.85

0.85

0.90

1.50 0.70 1.10

0.4

RH

RH

0.84

0.75

0.95

1.60

0.83

0.90 0.82

0.3

0.81

1.20 1.30 0.2 20

0.80 40

60

80

100

120

140

160

180

200

20

40

60

80

D0(nm)

b1

120

140

160

180

200

D0(nm)

b2 0.86

0.7

1.20

0.85

1.15

0.6

100

1.70

1.10 0.84

1.10 1.301.40

RH

0.90

RH

0.5

1.05 1.00

0.4

0.83

0.82 0.3

1.001.20

1.60

1.50

0.81

0.80

0.2

40

60

80

100

120

D0(nm)

140

160

180

40

60

80

100

120

140

160

180

D0(nm)

Fig. 4. The relationship among growth factor, RH and D0 of: (a1) NH4Cl, (b1) NH4NO3 particles at the RH below deliquescence point; (a2) NH4Cl, (b2) NH4NO3 particles at the RH above deliquescence point.

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D. Hu et al. / Atmospheric Environment 45 (2011) 2349e2355

aerosols of 40, 100 and 200 nm, respectively, which indicate that the larger particles absorb more water than the smaller particles. This might be caused by the fact that the larger particles contain relatively more solute than the smaller ones. It is noteworthy that, above the DRH, the GFs are still less than 1.00 for NH4Cl particles smaller than 80 nm (Fig. 2(a2)), implying the evaporation of small particles at high RH. It is probably caused by the larger specific surface areas and the curvature effects of small particles. Fig. 2(b1 and b2) presents the growth factors as a function of D0 of NH4NO3 aerosols under different RH. Although the DRH of NH4NO3 is 61.8% at 298 K, the abrupt deliquescence cannot occur for the NH4NO3 aerosols since the RH of aerosols after dryer is not lower than its ERH. Thus, below the DRH, NH4NO3 aerosols will deliquescence gradually and eventually become droplet until RH reaches its DRH (Park et al., 2009). In Fig. 2(b1), similar to NH4Cl, the evaporation of NH4NO3 aerosols below the DRH is significantly promoted with the increase of RH for particles less than 50 nm; while the GFs increase with D0 for particles larger than 50 nm, which is different with NH4Cl. The difference might be caused by the gradual surface deliquescence of NH4NO3 aerosols with the RH which inhibited its evaporation. Above the DRH, the GFs of NH4NO3 aerosols exhibit similar increase trend with that of NH4Cl and increase with D0.

analyzer (H-TDMA) system. Below the DRH, the evaporation of NH4Cl is significantly promoted while RH increases. Similar trend is also observed for NH4NO3 particle less than 50 nm. The proposed mechanism suggests that the increase of RH alters the chemical equilibrium, i.e., NH4 XðsÞ $NH3ðgÞ þ HXðgÞ , by converting NH3(g) and HX(g) into NH3$nH2O and HX$nH2O, which accelerates the evaporation of NH4X(s) (X represent Cl
and NO3
). When RH is higher than the DRH, the GFs of NH4Cl and NH4NO3 increase with D0 throughout the whole size range investigated. The iso-GF curves with D0 and RH as the coordinates are drawn to distinguish the effects of the D0 and RH on the GFs. Different from (NH4)2SO4, NaCl, Na2SO4 and NaNO3, the GFs of NH4X are more sensitive to D0 than RH due to the unique volatility of NH4Cl and NH4NO3 aerosols.

3.3. Evaporation mechanism of ammonium salts aerosols

References

Fig. 3 displays the proposed evaporation mechanism of NH4Cl and NH4NO3 aerosols. For comparison, the change of (NH4)2SO4 particle diameters with the increase of RH is also presented. For (NH4)2SO4, as the RH increases, the particles remain solid and do not shift in diameter (Dp ¼ D0) until the DRH is reached, at which point there is a distinct and abrupt increase of the particle size (Dp > D0) as the particles undergo a solid to liquid phase transition. Further increase of the RH leads to additional water condensation onto the salt solution and the particle size becomes larger (Dp0 > Dp). While for NH4Cl and NH4NO3 aerosols, since the evaporation of NH4Cl and NH4NO3 (D0 < 50 nm) below the DRH is significantly promoted with the increase of RH. The proposed mechanism suggests that solid NH4X(s) (X represent Cl
and NO3
) equilibrates to its precursors NH3(g) and HX(g), at a stable condition. Increases of RH can alter the chemical equilibrium, i.e., NH4 XðsÞ $NH3ðgÞ þ HXðgÞ , by converting NH3(g) and HX(g) into NH3$nH2O and HX$nH2O, which will accelerate the evaporation of NH4X(s) (X represent Cl
and NO3
, Dp < D0). When RH is higher than the DRH, the NH4X(s) aerosols are converted to NH4X droplets (Dp > D0 when D0 larger than 80 and 50 nm for NH4Cl and NH4NO3 aerosol, respectively.) and the droplets continue to grow as RH further increases (Dp0 > Dp). 3.4. Relationships among RH, D0 and growth factor The GFs of aerosols are influenced markedly not only by their chemical composition, but also by D0 and ambient RH. To distinguish the dominant impact on GFs from size or RH, iso-GF curves with D0 and RH as the coordinates are put forward and the corresponding curves of NH4Cl and NH4NO3 aerosols are illustrated in Fig. 4. It can be clearly recognized that the GFs of NH4Cl and NH4NO3 particles are more sensitive to D0 than RH both at RH below and above its DRH due to their unique volatility properties, which are quite different with the performances of (NH4)2SO4, NaCl, Na2SO4 and NaNO3 aerosols. 4. Conclusions In this study, the effects of RH and particle size on hygroscopicity and evaporation of NH4Cl and NH4NO3 aerosols are investigated by a self-assembled hygroscopic tandem differential mobility

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Nos. 21077025, 40875073, 40722006, 40975075, 20937001), Science & Technology Commission of Shanghai Municipality (Nos. 09160707700, 10231203801, 10JC1401600).

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