Particle number size distribution and new particle formation (NPF) in Lanzhou, Western China

Particuology 9 (2011) 611–618

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Particle number size distribution and new particle formation (NPF) in Lanzhou, Western China Jian Gao a,∗ , Fahe Chai a , Tao Wang a,b , Wenxing Wang a a b

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Science, Beijing 100012, China Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, HongKong, China

a r t i c l e

i n f o

Article history: Received 20 September 2010 Received in revised form 31 May 2011 Accepted 9 June 2011 Keywords: Lanzhou New particle formation (NPF) Number size distribution Particle growth rate Sulphuric acid

a b s t r a c t Particle number size distribution from 10 to 10,000 nm was measured by a wide-range particle spectrometer (WPS-1000XP) at a downwind site north of downtown Lanzhou, western China, from 25 June to 19 July 2006. We first report the pollution level, diurnal variation of particle concentration in different size ranges and then introduce the characteristics of the particle formation processes, to show that the number concentration of ultrafine particles was lower than the values measured in other urban or suburban areas in previous studies. However, the fraction of ultrafine particles in total aerosol number concentration was found to be much higher. Furthermore, sharp increase of ultrafine particle concentration was frequently observed at noon. An examination of the diurnal pattern suggests that the burst of the ultrafine particles was mainly due to nucleation process. During the 25-day observation, new particle formation (NPF) from homogeneous nucleation was observed during 33% of the study period. The average growth rate of the newly formed particles was 4.4 nm/h, varying from 1.3 to 16.9 nm/h. The needed concentration of condensable vapor was 6.1 × 107 cm−3 , and its source rate was 1.1 × 106 cm−3 s−1 . Further calculation on the source rate of sulphuric acid vapor indicated that the average participation of sulphuric acid to particle growth rate was 68.3%. © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction The size distribution of atmospheric aerosols, together with their composition, sources, and sinks, is a key element in understanding and managing aerosol effects on health, visibility and climate (Harrison et al., 2000; Yu, Wang, Luo, & Turco, 2008). Given the increased toxicity of ultrafine particles and the role of ultrafine particles in particle related premature deaths and morbidity (Donaldson et al., 2002), the abundance of these ultrafine particles after nucleation is considered a potential human health hazard. In addition, the growth of nuclei from a detectable size of a few nanometers into particles that are optically active and efficient as cloud condensation nuclei has important implications for visibility and climate (Kulmala et al., 2004). The importance of NPF (new particle formation) has motivated a number of studies around the world (Kulmala et al., 2004), which span from the remote boreal forest to polluted urban area, and to agricultural regions. Some recent continental sampling campaigns that measured size distributions include the sampling

∗ Corresponding author. E-mail address: [email protected] (J. Gao).

campaigns in Europe (Birmili & Wiedensohler, 2000; Ruuskanen et al., 2001), North America (Stanier, Khlystov, & Pandis, 2004a), Australia (Morawska, Jayarantne, Mengersen, Jamriska, & Thomas, 2002) and China (Gao et al., 2007; Gao, Wang, Zhou, Wu, & Wang, 2009; Wu et al., 2007; Yue et al., 2009). These studies found that NPF was a common phenomenon which occurred in clean and polluted environments, but the nucleation and growth property of the process varied by a great margin due to the diversity of the precursors and the complexity of the meteorological conditions. Rapid urbanization and industrial development in China in the past two decades have led to a significant increase of emission in both particle mass concentrations and its gas precursors (Akimoto & Narita, 1994). Rapid increase in the use of vehicles and energy consumption and high concentration of particle precursors gave rise to nucleation and NPF, which will likely foster severe aerosol pollution in major urban centers and their surrounding regions. As a mega city in Western China, Lanzhou has played an important role in economic development for two decades. At the same time, Lanzhou is one of the ten most polluted cities in China, the place where serious photochemical smog was first reported in the 1980s (Zhang, Tang, Bi, Tang, & Zhao, 1987). There are several important factors which affect the air quality in Lanzhou, including considerable emission from local industry, poor atmospheric diffusion

1674-2001/$ – see front matter © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.partic.2011.06.008

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J. Gao et al. / Particuology 9 (2011) 611–618 Table 1 Statistics of particle number concentration in different size bins.

Fig. 1. Location and topography of study site and surrounding regions, with urban and industrial areas of Lanzhou shown in red color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

conditions due to special landform (Wang, Jiang, Yang, Shang, & Qi, 2000; Wang, Feng, Zeng, Ma, & Shang, 2009; Zhang, 2001), and dust intrusions from upstream regions (Liu, Tian, & Zhang, 2004; Ta, Wang, Xiao, Zhu, & Xiao, 2004). It should be noticed that previous research mainly focused on TSP (total suspended particulates) and PM10 (particles with diameters less than 10 ␮m) concentrations in Lanzhou, but very little has been done to detect the variation of sub-micrometer PM concentrations, not to mention the ultrafine particles. Specifically, no work has been done to measure the NPF process over this region. In this report, we describe the particle number size distribution, their diurnal variation, the relationship between NPF process and meteorological/photochemical conditions, and then evaluate the growth characteristics of newly formed particles, including the contribution of sulphuric acid to particle growth. 2. Methodology 2.1. Site information Field study was conducted at a suburban site in the north of Lanzhou, the capital of Gansu province, which has an area of 13,086 km2 and a population of 2.83 million including 1.48 million in the city zone. Our study site is located at the southern edge of the mountains (36.13◦ N, 103.69◦ E, 1631 m above sea level); as shown in Fig. 1, and with details in Zhang et al. (2009), and is about 5 km northeast to Xigu Petrochemical District (large industrial area) and 15 km northwest to the city centre.

Size range (nm)

Mean (cm−3 )

Median (cm−3 )

Max (cm−3 )

Min (cm−3 )

SD

10–20 20–50 50–100 100–200 200–500 500–1000 1000–2500

4540 2661 889 329 152 20 8

1269 1431 710 265 119 16 5

55,066 22,567 8465 2357 1183 110 63

32 46 14 4 3 2 0

7798 3266 743 254 130 12 7

particle size from 0.35 to 10 ␮m. Before and after the field campaigns, the DMA was calibrated with NIST SRM 1691 and SRM 1963 PSL spheres (0.269 and 0.1007 ␮m mean diameter) to verify proper DMA transfer function and accurate particle sizing traceable to NIST. The LPS was calibrated with four NIST traceable sizes of PSL (0.701, 1.36, 1.6, and 4.0 ␮m mean diameter). The DMA and the CPC can measure the aerosol size distribution in the 10–500 nm range in up to 96 channels. The LPS covers the 350–10,000 nm range in 24 additional channels. In the present study we chose the sample mode with 48 channels in DMA and 24 in LPS. Thus it takes about 8 min for one complete scanning of the entire size range and 5 s of scanning period for each channel. The sampling loss of the particles was calculated based on the theory of Hinds (1982). Ozone was measured by a UV photometric analyzer (TEI 49i), CO with a non-dispersive infrared analyzer (API model 300EU or API model 300E), and SO2 with a pulsed UV fluorescence analyzer (TEI model 43C). Nitric oxide (NO) and total odd reactive nitrogen (NOy ) were measured with a commercial chemiluminescence analyzer fitted with an externally placed molybdenum oxide (MoO) catalytic converter. The methods used to calibrate these instruments were the same as those reported by Wang, Ding, Gao, and Wu (2006). The mass concentrations of PM2.5 were continuously measured using a TEOM 1400a ambient particulate monitor (Rupprecht & Patashnick Co., Inc., Albany, NY) with instrument temperature at 50 ◦ C in order to minimize thermal expansion of the tapered element. Black carbon was measured using an Aethalometer (Magee AE21) with a time resolution of 5 min. Meteorological parameters such as air temperature, humidity, solar-radiation, TUV and wind were also measured continuously. 3. Result and discussion

2.2. Instruments

3.1. Number concentration and size distribution

Instruments were placed in an air-conditioned shelter. Ambient air samples were drawn through a Tygon tube (1/4 inch diameter inside) to the particle size distribution analysis system. A wide-range particle spectrometer (WPSTM , MSP Corporation Model 1000XP) was used to measure particle concentrations in the range of 0.01–10 ␮m. The instrument is an aerosol spectrometer that combines the principles of differential mobility analysis (DMA), condensation particle concentration (CPC) and laser light scattering (LPS). The DMA in the WPSTM has a cylindrical geometry with an annular space for the laminar aerosol and sheath air flows. These critical dimensions were optimized to obtain size classification of particles between 10 and 500 nm with a maximum voltage smaller than 10,000 V and a sheath flow rate of 3 L/min. The CPC is of the thermal diffusion type, with a saturator maintained at 35 ◦ C. A feedback flow control system maintains a constant CPC flow rate at 0.30 L/min. It has a dual reservoir design to eliminate contamination of the working fluid with condensed sampling-air humidity. The LPS is a single-particle, wide-angle optical sensor used for measuring

The results discussed in this paper include the data from 25 June to 19 July 2006. Table 1 gives the statistics of number concentrations in 7 size ranges from 10 nm to 2.5 ␮m, which are divided into 4 modes: 10–20 nm (nucleation mode), 20–50 and 50–100 nm (Aitken mode), 100–200, 200–500 and 500–1000 nm (accumulation mode) and 1000–2500 nm (coarse mode). In this study, the moderate concentration of ultrafine particles (10 nm < D < 20 nm) was observed. And in the comparison shown in Table 2, the particle concentrations in size ranges of 10–100 and 100–500 nm for Lanzhou are only comparable with the values measured in rural area of Pittsburgh, much lower than the previously reported results measured in other urban or suburban areas. Nevertheless, as seen from Fig. 2, the particle concentration exhibits large temporal variability with occasional spikes as high as 85,000 cm−3 that exceed the mean by a factor of five to seven, implying that the site may have experienced very serious aerosol pollution. The sharp increases in nucleation mode (10–20 nm) particles dominated most of the particle bursts, which frequently occurred at noon on sunny days. Note that the high temperature and strong solar radiation intensity

J. Gao et al. / Particuology 9 (2011) 611–618

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Fig. 2. Time series of particle number size distribution (in dN/d Log Dp ) of ultrafine particles (D < 100 nm) and particle number concentrations in 7 sub-size ranges.

Table 2 Comparison of number concentration results in other continental sites. Observation sites

Number concentrations (cm−3 )

References

10–100 nm 100–500 nm Alkmaar, Netherlands Erfurt, Germany Helsinki, Finland Pittsburgh (urban), USA Pittsburgh (rural), USA Taicang, China (summer) Jinan, China (summer) Jinan, China (winter) Lanzhou, China

of sub-size bins from 10 nm to 2.5 ␮m. The volume concentration was dominated by particles larger than 100 nm (98.5% of the total volume). For the particles in ultrafine mode (D < 100 nm), they were characterized by not only the high number per unit of air volume but also the preponderance of the number of all particles (up to 94% of the number concentrations). It has been well known that most of the ambient particles in urban atmospheres are in the ultrafine size range (<100 nm) (Peters, Wichmann, Tuch, Heinrich, & Heyder, 1997; Woo, Chen, Pui, & McMurry, 2001). However, for the percentage of the particles between 10 and 100 nm to total particle concentration, the values in this study are much higher than those reported in Europe and north America (Tuch, Brand, Wichmann, and Heyder (1997) for Eastern Germany (72%) and Woo (2003) for Atlanta (61%)).

18,300

2120

Ruuskanen et al. (2001)

17,700 16,200 14,300

2270 973 2170

Wichmann and Peters (2000) Ruuskanen et al. (2001) Stanier et al. (2004a)

6500

1900

Stanier et al. (2004a)

28,511

1676

Gao et al. (2009)

10,300

385

Gao et al. (2007)

15,591

1796

Gao et al. (2007)

3.2. Diurnal variation

8034

480

This work

Fig. 5 shows the diurnal patterns of the average particle concentrations in the size ranges of 10–20 nm (nucleation mode), 20–50 nm (Aitken nuclei), 50–100 nm (Aitken nuclei), and 100–1000 nm (accumulation mode). Figs. 6 and 7 show respectively the diurnal patterns of O3 , SO2 , CO, NOy , BC, PM2.5 , and the meteorological factors. Number concentration of nucleation mode (10–20 nm) and Aitken nuclei (20–50 nm) particles reached their daily peaks at noon (Fig. 5), together with the daily peaks of O3 concentration (Fig. 6), the solar radiation level and temperature (Fig. 7).

during summer months and the favorite meteorological condition may have strongly promoted the photochemical production and homogeneous nucleation of particles. Fig. 3 shows the average number size distribution in dN/d Log Dp during the observation. It is evident that the number concentration was dominated by particles smaller than 100 nm. With the consideration of the detection limit of WPS system (∼10 nm), it can be inferred that the particles might have dominant percentage in the size of 10 nm or smaller. Fig. 4 shows the average fractions of the particle number, surface and volume concentrations in the series

Fig. 3. Averaged number size distribution (in dN/d Log Dp ) of particles during the observation.

Fig. 4. Average fractions of particle number (N), surface (S) and volume (V) concentrations for different size ranges.

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J. Gao et al. / Particuology 9 (2011) 611–618

Fig. 7. Diurnal variation of meteorological factors: wind direction and wind speed, intensity of solar radiation, temperature and relative humidity.

Fig. 5. Average diurnal variation of number concentration in 10–20, 20–50, 50–100 and 100–1000 nm size ranges.

These profiles are in sharp contrast to urban pollutants such as CO, NOy , and BC, which exhibited typical morning maxima and afternoon minima (Fig. 6). These results suggest that the noon peaks for particle concentrations in 10–50 nm was a result of the homogeneous nucleation promoted by photochemical process, rather than direct emission from urban sources. Particles in the size range of 50–100 nm show a different diurnal pattern, that is, two additional peaks flanking the one at noon. A morning peak appears together with the trace pollutants of combustion (CO and BC) at 0800 local time (LT), implying the advection of urban plumes to the site in the morning. The noontime maximum (30 min after the peak of 10–50 nm mode particles) and an afternoon peak indicate that the diurnal variation of the particles in this size range was affected by not only direct emission but also NPF and the subsequent growth process. For particles in size range of 100–1000 nm, the unique morning peak together with the primary pollutants suggests significant contribution from the urban plume. 3.3. New particle formation (NPF) process

Fig. 6. Average diurnal variation of gaseous species (O3 , CO, SO2 and NOy ) and particle mass concentration (PM2.5 and BC).

3.3.1. Statistics of the NPF events The previous section showed initial evidences that the increase of the nucleation mode particles was due to nucleation processes rather than local emissions. Examination of the data revealed that in some cases there was only a burst of nucleation particles without subsequent growth into larger particles, while in other cases both particle number increase and growth occurred. Here we adopt the criteria for NPF which have been used in previous studies (Dal Maso et al., 2005; Kulmala et al., 2004; Wu et al., 2007). Each day, the contour plots of particle size distribution, meteorological variables, trace gases, and particle properties were examined. An event of NPF was identified if (1) there was a spontaneous burst of particle number in the nucleation mode (10–20 nm, the smallest channel of the WPS system) followed by subsequent particle growth at the rate of a few nanometers per hour over a time span of hours, and (2) the nucleation mode particle concentrations were not correlated with traffic or power plant related gases such as CO and SO2 . Fig. 8 shows an example on 9 July. During the 24-day study period there were

J. Gao et al. / Particuology 9 (2011) 611–618

Fig. 8. Particle formation and growth process on 9 July 2006.

8 days indicating NPF. This frequency is comparable to that of urban areas in the US (Qian, Sakurai, & McMurry, 2007; Stanier, Khlystov, & Pandis, 2004b) and suburban Shanghai (Gao et al., 2009), but lower than that of urban Beijing (Wu et al., 2007). In these events, the number concentration of ultrafine particles was found to increase significantly at the beginning and then grow at a rate of a few nanometer per hour from 10 to 40–50 nm in a few hours. To study the favorable condition for NPF process, we compared the atmospheric chemical and meteorological factors on event days and non-event days, as shown in Table 3. It is obvious that the primary pollutants such as CO and BC were a little lower on the event days, but the values of SO2 , UV*SO2 (a parameter to evaluate the production of H2 SO4 ) and O3 were much higher. Thus, the strong photochemical process and production of H2 SO4 could be considered as the major contributor in triggering the NPF process. Table 4 shows the statistics of the NFP events, including the beginning/ending time and the sample number. The observed particle GR (growth rate) can be expressed as: GR =

Dm , t

(1)

where Dm is a geometric number mean diameter (NMD) of the nucleation mode particles during the event by the method proposed by Dal Maso et al. (2005). The coefficient R in Table 4, representing the correlation between the NMD and time, was as high as 0.94 which indicates good stability of the growth processes. The GR ranged from 1.28 to 16.97 nm/h, with an average of 4.4 nm/h, which is within the range of reported values in previous studies (Holmes, 2007; Kulmala et al., 2004). It is interesting that the GR in event of 9 July was found to be much higher than those on the other event days, which made the newly formed particles

615

grow to larger size in much shorter time. Table 5 shows that north wind dominated the site during the event on 9 July. Other events happened with the shift of wind from clean area (north) to the polluted urban area (southwest), indicating the role which urban plume played in the nucleation process as a trigger. The higher concentrations of both the primary and secondary pollutants (CO and O3 ) (Table 4) also prove the different chemical condition from the event on 9 July. Particle growth rate depends not only on the concentration of condensable vapor but also on the meteorological factors (Kulmala et al., 2004). Lower solar radiation and temperature were not favorable for the production of H2 SO4 , although the SO2 concentration was high in the event of 9 July. It has been suggested that H2 SO4 condensation typically accounts for only 10% to 30% of the observed growth (Boy et al., 2005) compared to the VOCs which accounts for more than 70% of the material for the particle growth. Thus, further analysis on the growth property of the events is needed to clarify the dominant species in the processes.

3.3.2. Intensity of the NPF events The most important marker for nucleation was a significant increase in the nuclei mode particle count, defined as particles varying in size from the lower detection limit of 3 nm up to 10 nm (Stanier et al., 2004b). It should be noted that dN10 /dt is not the nucleation rate, but is a rough measure of the intensity of the event and also of its impact on the particle number and size distribution in the region. Considering the detection limit of WPS system, the net rate of increase in number concentration of particles in 10–20 nm (dN10–20 /dt) was calculated to evaluate the intensity of the NPF events. Sharing increase of the particle number concentration in nuclei mode (10 nm < D < 20 nm) was frequently observed in the beginning of the NPF events, suggesting that the size of the ultrafine mode reached the lower detection limit of WPS system and at least part of the ultrafine particles were newly formed close to the observation area. dN10–20 /dt was found to be as high as 15,477 cm−3 h−1 in average, as shown in Table 6. Considering that the predictable high number concentration of particles smaller than 10 nm (Fig. 3), dN10 /dt can be inferred to be much higher than this value. Thus the nucleation process before the new particle formation can be defined rather strong compared with the process which has been observed in rural Pittsburgh (Stanier et al., 2004b).

Table 3 Comparison of average values of chemical and meteorological factors on event and non-event days.

Event days Non-event days

PM2.5 (␮g/m3 )

BC (␮g/m3 )

O3 (ppb)

CO (ppb)

SO2 (ppb)

NOy (ppb)

RH (%)

T (◦ C)

Solar radiation (W m2 )

UV*SO2 (W m2 ppb)

54.2 59.1

1.6 1.9

78.2 70.1

360.7 381.2

6.1 4.5

4.2 4.9

37.0 37.9

27.4 28.7

449.1 435.2

66.4 48.4

Table 4 Statistics of the NPF events. Date

Start

End

n

R2

GR (nm/h)

2006-6-28 2006-6-29 2006-7-2 2006-7-6 2006-7-7 2006-7-8 2006-7-9 2006-7-15

15:00 13:00 13:00 14:30 11:00 12:30 14:30 12:30

20:30 19:00 16:30 21:30 16:30 16:30 17:00 16:30

41 44 25 51 41 28 17 31

0.94 0.95 0.95 0.95 0.96 0.92 0.96 0.83

1.96 2.51 3.72 3.35 2.86 2.60 16.97 1.28

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J. Gao et al. / Particuology 9 (2011) 611–618

Table 5 Related pollutants and meteorological factors in the NPF events. Date

O3 (ppb)

CO (ppb)

SO2 (ppb)

RH (%)

T (◦ C)

Solar (W/m2 )

WD Begin

Mid

After

2006-6-28 2006-6-29 2006-7-2 2006-7-6 2006-7-7 2006-7-8 2006-7-9 2006-7-15

93.38 63.43 113.82 64.60 83.99 80.46 64.03 70.08

317.98 268.67 526.08 310.95 509.05 234.51 164.14 546.29

2.85 3.67 6.47 8.64 11.26 2.65 8.11 4.79

22.03 39.44 31.20 38.75 41.46 27.16 55.92 46.84

29.43 24.32 31.16 26.34 26.10 29.70 21.51 30.92

365.56 139.35 648.57 330.21 438.65 878.37 295.64 716.13

NW NE NE NW NE N N SW

SW SW S W SW SW NE SW

SE NW E SE NE NE NE SE

Note: N denotes north, E east, W west and S south for wind direction. Table 6 Intensity of the NPF events (dN10–20 /dt). Date

6-28

6-29

7-2

7-6

7-7

7-8

7-9

7-15

dN10–20 /dt (cm−3 h−1 )

6585

13,521

14,383

18,473

17,497

16,689

10,982

25,691

3.3.3. Growth property of the NPF events To provide a better understanding of the NPF process, this study calculated the condensation sink, source rate, and concentration of condensable vapors in the NPF events using the method reported by Dal Maso et al. (2005). The condensation sink (CS) is a measure of the scavenging speed of gaseous molecules caused by condensation onto preexisting particles, depending strongly on the particle size distribution and calculated using the following equation:



∞

CS = 2Dv

Dp ˇm N(Dp )dDp = 2Dv 0



Dp,i ˇm,i Ni .

(2)

i

The transitional correction factor ˇm is calculated with the expression by Fuchs and Sutugin (1971). Dp,i and Ni denote respectively the particle diameter and number concentration of size class i. The particle growth rate dDp /dt can be expressed as follows (Kulmala et al., 2001): dDp 4mv ˇm Dv Cv , = D dt

(3)

where mv is the molecular mass of the condensing vapor, Cv is the condensable vapor’s concentration. Here we used the observed growth rates and took the diffusion coefficient (Dv ) of H2 SO4 to accomplish the calculation. The above equation can be integrated from Dp,0 to Dp : Cv =

 tDm



2 Dp2 − Dp,0

+0.6232 ln

8 2 + Dp 2 + Dp,0

+

2 3˛



− 0.312 (Dp − Dp,0 )

 .

(4)

A balance equation for condensable vapor is: dCv = Q − CSCv , dt

(5)

where Q is the source rate of the condensable vapor. By assuming pseudo steady state, Q can be estimated using the following equation: Q = CSCv .

(6)

Here we also made a rough estimation for the contribution of H2 SO4 (SA, sulphuric acid) in the particle growth process. A zerodimensional pseudo steady-state model similar to Boy et al. (2005) and Weber et al. (1997) was adopted. The budget equation of SA is shown as follows (Kulmala et al., 2001): dCSA ≈ 0 = Q − CSCSA , dt

(7)

where CSA is the SA concentration. In the lower troposphere, gaseous SA is produced mainly by the reaction of sulphur dioxide (SO2 ) with the OH radical. The following QSA is the SA source rate via reaction of SO2 and OH: QSA = [OH][SO2 ]k,

(8)

where k is calculated using the temperature dependent rate equations from DeMore et al. (1997). We acquired the OH radical concentration for different latitudes and seasons from the global model of Bahm and Khalil (2004) and then calibrated with the intensity of solar radiation measured in situ. From QSA and CSA , we calculated the contribution of SA to particle growth. Table 7 shows the growth properties of the NPF events. Because of uncertainty of OH values, we could not get accurate concentrations of SA, but we could evaluate and compare the contribution

Table 7 Growth properties of the NPF events. Date

GR (nm/h)

CS (s−1 )

C (cm−3 )

Q (s−1 cm−3 )

QSA (s−1 cm−3 )

CSA (cm−3 )

GRSA (nm/h)

Contribution of H2 SO4 (%)

2006-6-28 2006-6-29 2006-7-2 2006-7-6 2006-7-7 2006-7-8 2006-7-9 2006-7-15

1.96 2.51 3.72 3.35 2.86 2.60 16.97 1.39

0.012 0.009 0.022 0.019 0.024 0.011 0.019 0.012

2.69E+07 3.44E+07 5.10E+07 4.59E+07 3.92E+07 3.56E+07 2.32E+08 1.90E+07

3.16E+05 2.96E+05 1.12E+06 8.53E+05 9.51E+05 3.89E+05 4.37E+06 2.36E+05

2.90E+05 2.64E+05 7.53E+05 5.37E+05 7.15E+05 2.11E+05 3.86E+05 2.28E+05

2.47E+07 3.07E+07 3.44E+07 2.89E+07 2.94E+07 1.93E+07 2.05E+07 1.84E+07

1.80 2.24 2.51 2.11 2.15 1.41 1.50 1.34

91.9 89.3 67.5 62.9 75.1 54.2 8.8 96.5

J. Gao et al. / Particuology 9 (2011) 611–618

of SA qualitatively, estimated to be 68.3%. In most cases, particle growth depends mainly on SA, except for the event on 9 July. With comparable CS value and concentration of SA, the total concentration of condensable vapors (C in Table 7) on 9 July was 4–10 times higher than others, indicating that there must be some other species that played more significant roles than SA. Except SA, biogenic VOCs were considered important precursors taking part in the nanoparticle growth process (Kulmala, Toivonen, Mäkelä, & Laaksonen, 1998). As to wind direction of the events (Table 5), 9 July was dominated by northward air mass originating from the vegetation-covered mountain regions. As such, biogenic volatile components were not only dominant contributors to particle growth in this event, but also played a major role in driving the newly formed particles to increase in size with a rapid rate.

4. Conclusions The number concentration of ultrafine (10–100 nm) particles in the total particle concentration at a chosen site in Lanzhou was found to be much lower than those reported for urban/suburban areas in North America and in Europe. The burst of concentrations of particles in the 10–50 nm range was proved to be due to the NPF process promoted by photochemical processes, rather than from direct emission. The formation of the nucleation-mode particles and their subsequent growth were distinctly observed for 8 days. The average particle growth rate was found to be 4.4 nm/h, which is comparable to values reported in previous investigations. Sulphuric acid was proved to be the major contributor to triggering off the NPF process in an urban plume. When the atmospheric environment was clean and biogenic volatile components were supposed to be major participants in the NPF process, the growth rate of the newly formed particles was found to be much larger.

Acknowledgements We thank Mr. C.N. Poon and Mr. L.K. Xue for their contribution to our field study. This research was funded by the National Basic Research Program of China (2005CB422203, 2005CB422208), National Department Public Benefit Research Foundation (No. 201009001) and National Natural Science Foundation of China (Grant No. 41005065).

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