2.3.1 Backward trajectory analysis

The 48 h backward trajectories of the air mass were computed at 4000 m a.s.l. (600 m above the ground at the Mt. Yulong site) by the Weather Research and Forecasting (WRF) model (version 3.61) to identify the impacts from southern Asia. The fire spots were obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite map (https://firms.modaps.eosdis.nasa.gov/firemap/; last 26 August 2016). In order to characterize the air mass origin during the NPF events, the 48 h backward trajectories at 600 m above the ground were calculated using the NOAA HYSPLIT 4 (Hybrid Single-Particle Lagrangian Integrated Trajectory) model (Draxier and Hess, 1998).

2.3.2 Parameterization of NPF

The data of each PNSD during NPF were fitted as the sum of two-mode or three-mode lognormal distribution (Hussein et al., 2005), including the geometric mean diameter D_m, geometric standard deviation σ_m, and total number concentration of each mode. During the NPF events, the growth rate (GR) was calculated as the variation of the mean diameter D_m of newly formed mode in unit internal:

The formation rate (FR) was calculated for the nucleation fraction of the particles (3–25 nm), using the following formula:

where N₃₋₂₅ is number concentration of particles within the 3–25 nm size range, and CoagS₈ is the coagulation rate of particles with diameter of 8 nm, which is the geometric mean of 3–25 nm. The coagulation rate was calculated as follows:

where n(D_p^′) is number concentration of particles with a size of D_p^′, and $K\left(D_{\mathrm{p}},D_{\mathrm{p}}{}^{\prime }\right)$ is the coagulation coefficient between D_p and D_p^′ particles. During nucleation events, there were a negligible number of particles that grew beyond 25 nm, so the last term in formula of was not included (Dal Maso et al., 2005). To quantify the limitation of NPF from preexisting particles, the condensation sink (CS) was calculated as follows:

where D is the diffusion coefficient of the condensational vapor (e.g., sulfuric acid), β is the transitional regime correction factor, and D_i and N_i are the diameter and number concentration of particles in class i, respectively. In the calculation described above, all diameters were dry diameters directly measured from the SMPS instruments, so the coagulation and condensation sinks could be underestimated.

Figure 1Time series of (a) the particle number size distribution and the geometric mean diameter, (b) the ambient temperature and the relative humidity, (c) the PM_2.5 mass concentration and the number concentration of nucleation mode (3–25 nm) particles (PN₃₋₂₅), (d) the SO₂ and CO concentration, and (e) the black carbon concentration and fraction of f60 (organic fragment ions with $m/z=\mathrm{60}$) during the monitoring campaign. Periods influenced by biomass burning (BB1, BB2, BB3) are marked using grey shades, and the period representing background conditions (BG) is marked using blue shade.

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Sulfuric acid was thought to be the most important precursor of NPF events (Sipilä et al., 2010), and could contribute to particle growth by condensation (Yue et al., 2010; Zhang et al., 2012). In this study, the content of H₂SO₄ was calculated using a pseudo-steady state method (Kulmala et al., 2001):

where [OH] and [SO₂] are the number concentrations of OH radicals and SO₂, respectively, and the value of k is 10⁻¹² cm³ s⁻¹. [OH] was estimated using

where α=0.83, β=0.19, $a=\mathrm{4.1}\times {\mathrm{10}}^{\mathrm{9}}$, b=140, c=0.41, and d=1.7 (Ehhalt and Rohrer, 2000). The contribution of sulfuric acid condensation to particle growth was calculated following Yue et al. (2010).

Table 1Densities and hygroscopic parameters of the compounds used in the CCN calculation.

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2.3.3 Calculation of CCN concentration

In order to evaluate the variation of the indirect climate effects of the particles at Mt. Yulong, the CCN number concentration was estimated from data of PNSD and particle chemical composition. Firstly, SNA (sulfate, nitrate, ammonium) was ion-coupled to establish the exact chemical compounds of the inorganic salts in the particles. NH₄NO₃, H₂SO₄, NH₄HSO₄, and (NH₄)₂SO₄ were calculated using the following formula:

where n is the mole number of the specific compounds (Gysel et al., 2007). Based on κ-Köhler theory and the Zdanovskii–Stokes–Robinson (ZSR) mixing rule, the hygroscopic parameter of mixed particles can be calculated as follows (Petters and Kreidenweis, 2007):

where ε_m is the volume fraction of the composition m in particles, and κ_m is the hygroscopic parameter of pure composition m. In this research, we consider secondary inorganic ions, organics, and BC as the major components of particles, and enter them into the ZSR mixing formula. The correlated parameters for the compounds we used are given in Table 1.

Table 2Particle number concentration (PN) of high altitude sites around the world compared with this study.

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Based on κ-Köhler theory, the relationship between κ and D_c under certain critical supersaturation (S_c) is:

where σ_s∕a is the surface tension of water, M_w and ρ_W are the molecular weight and density of water, respectively, R is 8.317 J mol⁻¹ K⁻¹, and T is the ambient temperature. With the κ of the particles, the critical diameter D_c of the CCN activation can be achieved with this formula. Then the number concentration of CCN can be calculated as the number concentration of particles larger than D_c.

3.1.1 Particle and meteorology parameters

Figure 1 shows the time series of the PNSD and correlated meteorological parameters. The temperature and relative humidity were 6.1±3.5 ^∘C and 54.9±19.7 %, respectively (Fig. 1b). Southeasterly winds were dominant during the campaign, followed by southerly and southwesterly winds. The average wind speed was 2.9±1.8 m s⁻¹ (Fig. S1 in the Supplement). Most of the monitoring days were sunny, which favored the nucleation process, while short periods of rainfall occurred on 24 and 26 March and 4, 6, 7, 8, 10, and 11 April. On 12 April there was heavy snow, and the RH was over 90 %.

As a background high altitude site on the Tibetan Plateau, the Mt. Yulong site displayed a low particle concentration and strong oxidation capacity. On average PM_2.5 was 10.51±9.16 µg m⁻³, which is similar to the results from the northeastern slope of the Tibetan Plateau (Xu et al., 2014; Du et al., 2015). This result was only 1∕10–1∕3 of that in the atmosphere of respective urban and rural regions in China, indicating a background situation in southwestern China (Zheng et al., 2016). However, the PM_2.5 concentration at the Mt. Yulong background site during the monsoon season was around 3 times the concentration observed Qilian Shan Station (4180 m a.s.l.), in the northeast of the Tibetan Plateau (Xu et al., 2015), and at Jungfraujoch (3580 m a.s.l.), Switzerland (Bukowiecki et al., 2016), both of which have a similar altitude. This indicates relatively stronger anthropogenic influences at Mt. Yulong during this period. From 22 to 30 March, 4 to 5 April, and 11 to 12 April, the particle mass concentration exceeded 10 µg m⁻³, which constituted a pollution episode.

During the measurement period, the ozone level was 50.1±7.0 ppbv, which is similar to the results from high mountain sites in Europe (Cristofanelli et al., 2016; Okamoto and Tanimoto, 2016) and higher than the results from Beijing during spring. This indicates that the atmosphere at the Mt. Yulong site had a higher oxidation capacity for the secondary transformation of pollutants. The concentrations of NO_x and NO were 0.94±0.62 and 0.07±0.05 ppbv, respectively. The SO₂ concentration was 0.06±0.05 ppbv, which is around the detection limit, showing no strong primary pollution. The CO concentration was 0.22±0.07 ppmv, and showed a higher level at the beginning of the campaign (24 to 30 March), which may have been the result of the influence of BB (Fig. 1d).

Figure 2Particle size distribution in the atmosphere at Mt. Yulong. (a) Mean size distribution of the particle number (hollow circle) and volume (filled circle) concentrations; the contribution of different fractions to the total particle (b) number concentration and (c) volume concentration. Different colors represent different size ranges: yellow (3–25 nm), blue (25–100 nm), green (100–1000 nm), and red (1–10 µm).

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Although the particles we measured in this study had a larger size range than most other studies, the results are still comparable, considering that Aitken and accumulation mode particles, which all measurements included, constitute most of the particle number concentration (PN). Table 2 shows the particle number concentrations in atmosphere at Mt. Yulong and other high altitude stations. The total number concentration of PM₁₀ was 1600±1290 cm⁻³ during the monsoon season at Mt. Yulong, which is slightly lower than concentrations measured at other sites throughout the Tibetan Plateau, e.g., Waliguan and Mukteshwar, and Mt. Huang. However, this result is several times higher than those from areas with scarce emission sources, e.g., the Alps and Antarctica. Conversely, the PN did not show a clear trend as the altitude increased, which means that regional emission and transport had larger impact on aerosols in the upper troposphere than vertical distribution did. We define N₃₋₂₅, N₂₅₋₁₀₀, N_100−1000, and N₁₀₀₀₊ as number concentrations of particles with diameters of 3–25, 25–100, 100–1000 nm, and 1–10 µm, respectively. There were bursts of N₃₋₂₅ on midday on 29 March, 4 April, and 13 April, with the peak values at 9900, 11 700, and 5400 cm⁻³, respectively (Fig. 1c). During those periods, the geometric mean diameter of the particles was lower than 25 nm. These events may have resulted from local or regional NPF, which is discussed later.

Figure 3Lognormal fit (three modes) of the average particle number size distribution during the campaign at Mt. Yulong. Black circles mark the measured PNSD, colored dashed lines represent the PNSD of fitting modes, and the red solid line marks the sum of the PNSD of all fitting modes. Mode 1, 2, and 3 are nucleation mode, Aitken mode, and accumulation mode, respectively.

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3.1.2 Analysis of PNSD and PVSD

The average PNSD during the measurement is shown in Fig. 2a. In this study, we sorted the particles by their sizes (Dal Maso et al., 2005). We consider that N₂₅₋₁₀₀ correlates to primary emissions, whilst N_100−1000 has stronger connection with secondary formation. According to the results from bench tests and road tests, gasoline and diesel vehicles emit particles within the 10–100 nm range (Harris and Maricq, 2001; Kittelson et al., 2006; Benajes et al., 2017; Tan et al., 2017). While studies on source apportionment of the atmospheric PNSD have found that secondary formation or long-range transport mainly contributed particles within the 100–1000 nm range (Wang et al., 2013b; Vu et al., 2015). The diameter with highest particle number concentration in this study (D_p-max) was 107 nm. The number concentration (dN∕dlogD_mp) was larger than 1000 cm⁻³ between 40 and 200 nm, which was the area adjacent to N₂₅₋₁₀₀ and N_100−1000. This indicates that both primary emission sources and secondary formation processes were influencing the Mt. Yulong site. N₃₋₂₅, N₂₅₋₁₀₀, and N_100−1000 were 244, 676, and 638 cm⁻³, constituting 16 %, 43 %, and 41 % of total concentration, respectively.

Table 3Fitted parameters of the lognormal modes for the different periods. μ, σ, and N represent the mean diameter, standard deviation, and total number concentration of each mode, respectively. “Total” represents the mean result of all data achieved from the campaign.

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In contrast to PNSD, the particle volume (PV) exhibited a bimodal distribution (Fig. 2a). The first peak had an extreme value at 340 nm, which represented the contribution of primary emission and aging processes. This mass peak constituted 66 % of the total PV, including PV₂₅₋₁₀₀ (2 %) and PV_100−1000 (64 %). The 3–25 nm particles had a negligible influence on the PV. The other mode in the PV size distribution was within the 1–10 µm range, with the D_p-max at 2.2 µm. This mode could be attributed to suspended soil. The volume of 1–10 µm particles constituted 34 % of the total PV, which was similar to Qilian Shan Station (38 %) in the northeast of the Tibetan Plateau (Xu et al., 2015) but higher than that urban Beijing (25 %) (Wu et al., 2008). This was due to the fact that the Mt. Yulong site had significantly fewer emission sources and stronger wind compared to urban Beijing.

Figure 4Diurnal variations of N₃₋₂₅, $N_{\mathrm{25}-\mathrm{100},}$ N_100−1000, CO∕NO_x, wind speed, and absolute humidity at Mt. Yulong during the monitoring campaign. Red lines with circles and blue lines represent the mean and median results, respectively. The light blue area marks the range between the 25th and 75th percentiles of the data.

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To better characterize the contribution from different process, the mean PNSD was fitted to three lognormal modes (Fig. 3, Table 3). We define the three fitted modes as the nucleation mode, the Aitken mode, and the accumulation mode, based on their geometric mean diameters, which were within 3–25, 25–100, and 100–1000 nm, respectively. The nucleation mode can be derived from the nucleation process. Nucleation mode particles contributed 15 % to the total PN, which was half the proportion of nucleation mode particles found at Mt. Tai; this indicates a lower impact from nucleation events at Mt. Yulong. The median diameter of Aitken and accumulation mode particles are 52 and 130 nm, respectively. These mean diameters are similar to results from Jungfraujoch (Bukowiecki et al., 2016) and Beijing (Wu et al., 2008). Accumulation mode particles, which are correlated with secondary formation (mode 3), contributed 54 % of total PN, which is twice as high as the result from urban Beijing (Wu et al., 2008), and similar to the results from the pristine atmosphere of Jungfraujoch (Bukowiecki et al., 2016). This indicates that the aerosols that arrived at Mt. Yulong were aged during transport.

3.3.1 Identification of BB episodes

The background condition (BG) was chosen to be from 1 to 4 April, when the concentration of BC was 85 ng m⁻³ on average. During this period, the wind was relatively strong, and fire spots were barely visible on the westward path of the air mass (Fig. S3). Based on the background condition, three BB events were identified using the following criteria: (1) BC was higher than the background level (85 ng m⁻³); (2) there was a higher fraction of organic fragment ions with $m/z=\mathrm{60}$ (f60) than during BG conditions (0.4 %); and (3) fire spots appeared in the source regions of the air masses or in the areas surrounding the study site.

During the first BB event (BB1, 22 to 30 March), dense fire spots were found in the source region in northern Burma. The BC concentration (1.2 µg m⁻³), PV, and f60 signal displayed their highest values during BB1. The trajectories of BB2 (5 to 6 April) passed fewer fire spots in southern Asia than those of BB1, and the BC concentration was also lower (0.3 µg m⁻³). The concentration of organic components during BB2 was 4 times higher than the background period. According to an AMS study from the same campaign, organic particles during BB2 were mainly influenced by transported oxidized organics that originated from biomass burning (OOA-BB), as opposed to fresh BB organic aerosols (Zheng et al., 2017). The proportion of f60 was highest (1.4 %) during BB3 (11 to 12 April), showing a strong BB influence. However, few fire spots were observed along the path of the air mass, indicating that BB particles could have been derived from nearby domestic heating. The study from Zheng (2017) also indicated that organic aerosols showed characteristics closer to those of fresh BBOA compared with BB1.

3.3.2 Influences of BB on PNSD and CCN

The average PNSDs during BB events and under BG conditions are plotted in Fig. 5, and fitted by three-mode lognormal distributions. The fitting results are displayed in Table 3. For the BG condition, only two modes were obtained (Fig. 5d), including the nucleation mode (mode_1, D_mean=15 nm), which originated from nucleation events, and the Aitken mode (mode_2, D_mean=79 nm). The total number concentration of these modes was 669 cm⁻³, which was similar to the level found at the Swiss Jungfraujoch site (Herrmann et al., 2015); therefore, our study site was found to exhibit a Eurasian background character.

The averaged PNSD during the abovementioned BB episodes showed discrepancies, indicating the variant influences of transported and local BB on particles in atmosphere at Mt. Yulong. During BB2, the Aitken mode number concentration was similar to that of the BG condition. However, an accumulation mode (mode 3, D_mean=106 nm) with a higher PN (775 cm⁻³) appeared (Fig. 5b). This mode (with a larger particle size) could be aged particles transported from BB source regions in southern Asia. In comparison to BB2, Aitken mode particles increased by a factor of 3 and became dominant in the PNSD during BB3 – the local event (Fig. 5c). The number concentration of this mode was 1309 cm⁻³, 2 times higher than the number concentration of accumulation mode particles. The reason for this could have been that the particles during BB3 were freshly emitted from sources near the monitoring site. The study from Zheng et al. (2017) also showed that the OOA fraction in organic aerosols was relatively lower during this period, while the biomass burning organic aerosol (BBOA) fraction was higher, which indicated impacts from more local BB sources. The geometric mean diameters of the accumulation mode were 169, 106, and 130 nm during BB1, BB2, and BB3, which was smaller than that of aged biomass burning particles at Mt. Bachelor, USA (Laing et al., 2016); this indicates the particles at Mt. Yulong were fresher. The nucleation mode had lower PN during BB2 and BB3, as the higher number of larger particles acted as strong coagulation sink for nucleation mode particles. The Aitken mode and accumulation mode were comparable during BB1 (Fig. 5a), indicating that the fresh aerosols from sources surrounding the site had a comparable influence to the transported, aged BB aerosols.

Figure 6Mean number concentration of CCN under a supersaturation (SS) of 0.6 % and 1.2 % during the whole monitoring period (labeled as “total”) and during the BB1, BB2, and BB3 events (marked as previously described in Fig. 1). PBL (10:00–14:00) and FT (01:00–06:00) conditions were separated.

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In short, the BG condition at Mt. Yulong could represent the background level of particles on the Tibetan Plateau or even for Eurasia. The local and long-range transported BB emissions increase the level of Aitken mode and accumulation mode particles in this region, respectively.

The concentration of CCN was calculated following the method described in Sect. 2.3.3. The κ value during the sampling period was 0.12±0.01, which is only one-third of the value found in urban Beijing (Wu et al., 2016) and at a rural site in Thuringia, Germany (Wu et al., 2013); however, this value is consistent with values observed in Alberta, Canada (0.11±0.04) during BB events (Lathem et al., 2013). Pierce et al. (2012) reported that κ was around 0.1 for > 100 nm particles in a forest mountain valley during biogenic secondary organic aerosol formation and growth events. Similarly, the organic volume fraction was 0.73 in particles at Mt. Yulong, explaining the low value of κ. As a result, the D_c at supersaturation levels of 0.6 % and 1.2 % was 72.0±2.2 and 45.4±1.4 nm, respectively. There could be uncertainties regarding the values of κ and D_c, as we used a manually set hygroscopicity of organics, which may have varied with the oxidation level or with other factors (Wu et al., 2016). Considering that the variation in the range of D_c was small, the CCN concentration was mainly controlled by the size distribution of the particle number.

BB events raised the CCN level in atmosphere by influencing the PNSD. An increase in the PN was observed during BB events, i.e., 2207±1388, 1214±638, and 2062±1112 cm⁻³ during BB1, BB2, and BB3, respectively. As a consequence, the increased particles acted as CCN in the atmosphere at Mt. Yulong, resulting in a rapid increase in the CCN concentration during BB events. Figure 6 shows the mean number concentration of CCN in periods influenced by the PBL (10:00–14:00, as discussed in Sect. 3.2) and FT conditions (01:00–06:00) during BG and BB events. The mean number concentration of CCN under a supersaturation level of 0.6 % was 936±754 and 807±705 cm⁻³ for the respective PBL and the FT periods at Mt. Yulong, which is comparable to a boreal forest station in Finland (Cerully et al., 2011). The concentration of CCN in periods influenced by the PBL during BB1, BB2, and BB3 was 5, 2, and 2 times higher than under BG conditions. Promotions of CCN during BB1, BB2, BB3 were more remarkable for the FT, i.e., 9, 3, and 8 times that of the BG (Fig. 6). This result indicates that the BB particles from southern Asia could have strong influences on the climate parameters. For the data under a supersaturation of 1.2 %, the ratios between the CCN concentration of BB and that of the BG were less, i.e., 2–4 times less for periods influenced by the PBL, and 2–7 times less for FT conditions. This is due to the fact that the critical diameters under supersaturation values of 0.6 % and 1.2 % were around 72 and 45 nm, respectively. Furthermore, the PN within 45–72 nm was relatively stable compared to the larger particles, due to the daily input of anthropogenic primary aerosols from urban air masses.

3.4.1 NPF events at Mt. Yulong under anthropogenic influences

Following the method from Tröstl et al. (2016b) and Yli-Juuti et al. (2009) used, we define three NPF events on 29 March, 4 April, and 13 April as follows:

• a.

  The “type A” event on 29 March comprised the appearance of newly formed particles (3 nm, the first bin of nano-SMPS) and the continuous growth of those particles, which reached the upper limit of the nucleation mode (25 nm). This NPF event was within the BB1 period, when the air mass arriving at Mt. Yulong was from the north of Burma. This air mass displayed slow movement before 28 March and transported abundant pollutants to the study area. From 28 March 12:00 to 29 March 06:00, the air mass was from the western upper troposphere (Fig. S4), which cleaned out the preexisting particles built up by BB events. The condensation sink (CS) at Mt. Yulong decreased from 0.006 to 0.002 s⁻¹ on the morning of 29 March. Conversely, the concentration of SO₂ was stable (around 0.04 ppb) before the occurrence of nucleation. The calculated H₂SO₄ increased before nucleation, reaching 5×10⁶ cm⁻³ (Fig. S4). The nucleation rate was 1.43 cm⁻³s⁻¹, which increased nucleation mode particles to around 10⁴ cm⁻³. Benefited by the increase of α-pinene (from 0.02 to 0.10 ppb), β-pinene (from 0.03 to 0.20 ppb), and SO₂ during the day on 29 March, the formation of secondary aerosol continued, and the newly formed particles grew to over 30 nm before nightfall. This process may have been due to gaseous oxidation and condensation. Particle growth stopped during night on 29 March, when the gaseous reaction was inhibited by the absence of sunlight. After sunrise on 30 March, the particles continued growing and reached 40 nm. The concentration of toluene was lower than 0.1 ppb, which indicated a small contribution from anthropogenic volatile organic compounds (VOCs). The GR was 3.48 nm h⁻¹ within the 3–25 nm size range. In short, under the influence of transported pollutants, nucleation was triggered by the upper clean air mass, which reduced the level of preexisting particles. Furthermore, the growth of particles was favored by the photochemical reaction and condensation process.

• b.

  The “type B” event on 4 April involved a newly formed mode that occurred with a growing trend, but growth stopped at early stage (< 15 nm). There were also temporal low values of N₃₋₂₅ during the event. The event on 4 April took place under BG conditions, during which the concentration of SO₂ was lower (around 0.02 ppb). Beginning at 02:00 on 4 April, the air mass that arrived at Mt. Yulong was transported by an upslope flow from the western lower troposphere. However, from 09:00 to 12:00, the air mass that arrived at Mt. Yulong passed northeastern India where fire spots had been observed on the MODIS map on 2 April. Thus, the gaseous pollutants and particles from nearby anthropogenic sources or from BB sources in northeastern India were transported to this site on morning of 4 April. The concentration of SO₂ and H₂SO₄ increased to 0.07 ppb and 6×10⁶ cm⁻³ at 11:00, respectively, corresponding to the occurrence of nucleation (Fig. S5). SO₂ shared a similar time series to black carbon, indicating a combustion source. The formation rate and growth rate were 0.93 cm⁻³ s⁻¹ and 3.2 nm h⁻¹, respectively. N₃₋₂₅ fluctuated during the NPF events, showing low values when there were temporary changes in cloud condition (influencing radiation) and wind direction. The concentrations of β-pinene and toluene were stable and lower than 0.10 and 0.65 ppb, respectively, throughout the NPF event, which may have been the reason for the smaller growth rate. At around 14:00, the source region of the air mass changed to the western upper troposphere, and stronger winds cleaned out both the nucleation mode particles and the gaseous precursors, terminating the NPF event. In summary, the type B event was triggered by the injection of gaseous pollutants from an elevated PBL and the short-term transport of BB pollutants under background condition.

• c.

  An “off-site” NPF event occurred on 13 April, during which a narrow particle size distribution band (8–50 nm) was observed from 13 to 14 April. Primary particles should have wider range and larger size. These particles were mostly likely nucleated off-site, and transported to Mt. Yulong by an uplifting air mass. On the afternoon of 13 April, the air mass arrived at Mt. Yulong, passing the local ground layer (yellow trajectory in Fig. S6). The SO₂ increased from 0.06 to 0.13 ppb, and toluene reached its highest level at 0.098 ppb (Fig. S6), which indicated an anthropogenic impact. As a result, particles formed from the ground level were transported to the site and a burst of N₃₋₂₅ occurred at around 18:00. β-pinene also showed a higher value at dawn on 13 April. These nanoparticles showed a growth trend, with a GR of 2.99 nm h⁻¹. To summarize, this NPF event resulted in the occurrence of nucleation mode particles that were nucleated off-site in the PBL and transported to the study site.

These new particle formation events had strong influences on the PNSD. Particles showed a unimodal distribution during non-NPF event periods, with a peak diameter of around 125 nm (Fig. S7). This is mainly due to the impact from aged biomass burning particles. During the NPF events, the PNSD showed a clear bimodal characteristic. The smaller mode originated from nucleation and the growth of nanoparticles. This mode had a higher peak than the larger mode, indicating that NPF significantly contributed to the particle number concentration.

Figure 7Diurnal variation of J(O¹D), SO₂, CS, benzene, and β-pinene during days with NPF events (labeled as “NPF”, red lines), and without NPF events (labeled as “non-NPF”, blue lines). The shaded area represents the time period during which nucleation occurred.

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3.4.2 Limiting factors of NPF events

The frequency of NPF was 14 % during our measurement. This NPF frequency is clearly lower than in the polluted atmosphere of the North China Plain (40 %–65 %) in March and April (Wang et al., 2013a; Shen et al., 2011), the top of Mt. Huang (38 %) during April (Zhang et al., 2016), a background site on the northeastern Tibetan Plateau (79 %) in autumn (Du et al., 2015), and a remote rural site in the Sierra Nevada Mountains (47 %) in spring (Creamean et al., 2011). It is common knowledge that the CS is the limiting factor that controls NPF (Cai et al., 2017). Thus, preexisting particle levels on event days should be less than non-event days, at high altitude mountain sites (Shen et al., 2016; Guo et al., 2012) and urban sites (Wang et al., 2011, 2017). The low NPF frequency was unexpected in the clean atmosphere of Mt. Yulong, as the mean CS at Mt. Yulong was 0.0038 s⁻¹. Conversely, a similar low frequency of NPF events has also been observed in other locations with pristine atmospheres, e.g., 24 % at the Antarctic site “Neumayer” (Weller et al., 2015) and 12 %–17 % at Dome C, Antarctica (Järvinen et al., 2013).

During the first 5 days of the campaign (22 to 27 March), the nucleation events may have been prevented by large amount of preexisting particles acting as big condensation sink. The CS was more than 0.005 s⁻¹, which was similar to the polluted city of Beijing on days with NPF events (Wu et al., 2007). However, on the remaining days when the CS was even lower than 0.002 s⁻¹, NPF events were still scarce. Considering that the content of condensable vapor that participates in nucleation is determined by competition between formation from precursor oxidation and condensation on surface of preexisting particles (Zhang et al., 2012), the lower NPF frequency at pristine sites could have resulted from a lack of precursors, e.g., VOCs and SO₂ from fossil fuel and biomass burning sources.

To further evaluate the effect of different parameters on NPF, the daily variations of SO₂, CS, J(O¹D), benzene, and β-pinene during the period from 28 March to 14 April were calculated and are plotted in Fig. 7. The results during the period from 10:00 to 14:00 are compared between NPF days and non-event days, as this time period is pinpointed as being when nucleation occurred. As shown in Fig. 7, NPF days and non-NPF days shared the same level of J(O¹D), and there was also a 15 % difference in the CS when nucleation happened, indicating a small influence of solar radiation and preexisting particles on NPF. In addition to this, if we removed the data with biomass burning influences from non-NPF days, the averaged PNSD (Fig. S7) would be similar to the PNSD during NPF days in a larger size range (> 80 nm). Considering that larger particles are the main contributor to the condensation and coagulation sink, we can conclude that the CS is not the decisive factor controlling NPF in the pristine atmosphere of Mt. Yulong.

Table 4Comparisons of NPF parameters (FR, GR, CS) with the other studies.

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The concentration of SO₂ showed an increase on NPF days, and was 60 % higher than on non-event days, indicating that anthropogenic SO₂ is the controlling factor of NPF at Mt. Yulong. Studies at Jungfraujoch (Bianchi et al., 2016; Tröstl et al., 2016b), Izaña (García et al., 2014), and Mukteshwar (Neitola et al., 2011) have also reported that nucleation events in the upper troposphere correspond to an increase of anthropogenic gas pollutants due to an elevated PBL. At Daban Mountain on the northern slope of the Tibetan Plateau, the PM_2.5 level was similar to Mt. Yulong, but NPF was observed nearly every day. This may have been caused by the fact that concentration of SO₂ was around 2 ppb on average, which was 2 orders of magnitude higher than Mt. Yulong.

Organics may have also been a driving factor of NPF. The concentration of β-pinene showed a higher value (40 %) in the afternoon on NPF days, while there was little difference (9 %) between NPF days and non-event days with regards to the anthropogenic benzene concentration. Recent studies have found that in addition to sulfuric acid, the highly oxidized multifunctional organics from biogenic VOCs can also take part in nucleation and growth (Huang et al., 2016; Tröstl et al., 2016a): in free troposphere, pure organic nucleation without sulfuric acid may even be dominant (Bianchi et al., 2016; Gordon et al., 2016). According to the study from Du et al. (2015), the fraction of oxidized organics in the particle phase have a positive relationship with the particle growth rate, which indicates a contribution from organics to particle growth. Therefore, the increase of biogenic VOCs could benefit nucleation and the growth of nucleation mode particles at Mt. Yulong.

3.4.3 Parameters of NPF events in this study

The formation rate, growth rate, and condensation sink of NPF events at Mt. Yulong are summarized in Table 4. Compared with other high mountain measurements, this study reported a higher FR, e.g., 3 times the rate recorded at Storm Peak Laboratory (Hallar et al., 2011). However, the GR at Mt. Yulong was within average levels, which indicates that different precursors were participating in the nucleation and growth processes. Even though SO₂ showed a good correlated with nucleation events, the calculated growth rate from condensation of H₂SO₄ can only explain 5 % of the measured GR. This result indicates the participation of some other precursors in particle growth at this site, e.g., organics.