New particle formation (NPF) is thought to contribute half of the global cloud condensation nuclei. A better understanding of the NPF at different altitudes can help assess the impact of NPF on cloud formation and corresponding physical properties. However, NPF is not sufficiently understood in the upper mixing layer because previous studies mainly focused on ground-level measurements. In this study, the developments of aerosol size distribution at different altitudes are characterized based on the field measurement conducted in January 2019 in Beijing, China. We find that the partition of nucleation-mode particles in the upper mixing layer is larger than that at the ground, which implies that the nucleation processing is more likely to happen in the upper mixing layer than that at the ground. Results of the radiative transfer model show that the photolysis rates of the nitrogen dioxide and ozone increase with altitude within the mixing layer, which leads to a higher concentration of sulfuric acid in the upper mixing layer than that at the ground. Therefore, the nucleation processing in the upper mixing layer should be stronger than that at the ground, which is consistent with our measurement results. Our study emphasizes the influence of aerosol–radiation interaction on the NPF. These results have the potential to improve our understanding of the source of cloud condensation nuclei on a global scale due to the impacts of aerosol–radiation interaction.
Atmospheric particles influence the earth's energy balance by directly interacting with the solar radiation and indirectly being activated as cloud condensation nucleation (CCN) (Ghan and Schwartz, 2007). New particle formation (NPF) in the atmosphere and the coagulation herein may enable particles to grow larger than 60 nm, at which point aerosols can exert radiative effects on the solar radiation and act as CCN (Williamson et al., 2019; Shang et al., 2021). Some researchers find that the NPF is responsible for around half of the global CCN (Merikanto et al., 2009; Du et al., 2017; Kulmala et al., 2014). However, there is still considerable uncertainty about the magnitude that the NPF attributes to CCN (Kulmala et al., 2004; Merikanto et al., 2009; Zhang et al., 2012). A better understanding of the NPF at different altitudes can help assess the impact of NPF on cloud formation and corresponding radiative effects. However, the underlying mechanism of NPF at different altitudes has not been well studied yet.
Nucleation requires sufficient amounts of precursor gases (Kulmala et
al., 2004). Sulfuric acid (H
The content of
In the past few decades, extensive measurements have been conducted at ground level to characterize the ambient aerosol particle number size distribution (PNSD) and then NPF events (Bullard et al., 2017; Du et al., 2018; Peng et al., 2017; Malinina et al., 2018). Some studies suggest that the nucleation of fine particles can be altitude-dependent (Shang et al., 2018). High concentrations of nucleation-mode particles were found in the upper parts of the mixing layer (Schobesberger et al., 2013). It is observed that the particle growth rate in the upper mixing layer is larger than that on the ground (Du et al., 2017). Measurements from the tethered balloon also show that a large partition of 11–16 nm particles was generated from the top region of the mixing layer and was then rapidly mixed down throughout the mixing layer (Chen et al., 2018; Platis et al., 2016). Aircraft measurements (Wang et al., 2016; Zhao et al., 2020) also found that the free troposphere favors the NPF. Most of these studies, to the best of our best knowledge, focus on the concentration of precursor gases but not on the aerosol–radiation interaction.
In this study, we first demonstrate that the NPF is more likely to happen in the upper mixing layer than in the near-ground surface layer based on field measurement of the aerosol PNSD profiles. We find that the tendency of NPF is well related to ultraviolet radiation, implying that the aerosol–radiation interaction is an important factor that influences the NPF.
The field measurements were carried from 17 to 19 January 2019 at the
Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences
(39
For each of the measured PSNDs, it is fitted by three lognormal distribution modes by
The Mie scattering model (Bohren and Huffman, 2007) is used to estimate the aerosol optical properties. When running the Mie model, aerosol
PNSD, aerosol black carbon mass size distribution, and refractive index are essential. The measured mean black carbon mass size distribution from
Zhao et al. (2019) is adopted in this study, which is
measured around 3 km away from this site. The refractive index of
the non-black carbon and black carbon aerosol components is
Time series of the
The Tropospheric Ultraviolet-Visible radiation (TUV) model, developed by Madronich and Flocke (1997), is an advanced transfer model with an
eight-stream, discrete ordinate solver. This model can calculate the
spectral irradiance, spectral actinic flux, and photo-dissociation
frequencies in the wavelength range between 121 and 735 nm. In this
study, the photolysis frequency of the nitrogen dioxide (J(NO
In the TUV model, the inputs of the aerosol optical properties are the
aerosol optical depths at the wavelength of 550 nm and the column-averaged
SSA. The profiles of the
The content of H
The measured aerosol PNSD profiles in the time range between 07:00 and 18:50 LT on 18 January were used for analysis, which contained eight different upward and downward movements of the cabin, respectively. Figure 1a gives detailed time–altitude information of each measurement. All of the time mentioned in the research corresponds to the local time zone.
On 18 January, the measured ambient temperature and relative humidity ranges
were
During the measurement, the
The measured aerosol PSND (dashed line) and the PVSD (dashed line
with star) at
The measured aerosol PNSD varied significantly for different altitudes and
times. PNSD profiles in Fig. 2 corresponded to these periods when the cabin moved upward. The corresponding downward PNSD profiles are shown in Fig. S2.
In the early morning, the PNSD on the ground surface is substantially
different for different altitudes. Particle number concentration on the
ground surface can reach
With the increment of solar radiation and ambient temperature, the turbulence mixing of ambient particles became stronger. The aerosol PNSD at the surface decreased with time because the near-ground particles were mixed up to a higher location as shown in Fig. 2b and c. However, the aerosol PNSD at higher altitude increased with time due to the upcoming mixed aerosol particles from lower altitude. Therefore, the difference between the aerosol PNSD at different altitudes became smaller with the development of the mixing layer as shown in Fig. 2b, c, and d. These particles were still not well mixed in the range between 0 and 240 m until 10:20.
In the afternoon, the mixing layer was well mixed with the increment of solar radiation and ambient temperature. The aerosol PNSD and PVSD were almost uniformly distributed as shown in Fig. 2e and f. However, the turbulence was relatively weak after 15:00 as the measured PNSD and PVSD on the ground surface were slightly larger than that of a higher place. After 16:00, the turbulence was weaker because a larger difference between the PNSD at the ground surface and the higher level existed. The ambient particles were hardly mixed after the sunset. The measured aerosol PNSD profiles showed almost the same properties as that in the morning, with more aerosol particles located on the ground surface from emissions.
Overall, the measured PNSD profiles were highly related to the intensity of turbulence. When the turbulence was weak, the PNSD at the surface was different from that of upper levels because the initially emitted particles cannot be mixed up to a higher location. The PNSD tended to be uniformly distributed when the turbulence within the mixing layer was strong.
The measured
We calculated aerosol total number concentration for each measured PNSD
(
The number ratio of nucleation mode to Aitken mode.
The number ratio profiles of nucleation mode to Aitken mode
(
To better configure the variations of PNSD, we calculated the aerosol number
concentrations with the diameter between 10 and 25 nm (
Based on the discussion above, we found that the total aerosol number
concentrations increased slightly with altitude at 16:15. The number ratio
of
Many previous studies have reported the NPF events in the upper mixing layer. The study in Platis et al. (2016) reported that the NPF originated at elevated altitude and then was mixed down to the ground in Germany. The higher nucleation-mode particle number concentrations were observed in the top region of the mixing layer and were then rapidly mixed throughout the mixing layer in South America (Chen et al., 2018). Qi et al. (2019) also found the NPF at the top of the mixing layer based on tethered airship measurements in eastern China. The NPF events were also observed at different altitudes in the North China Plain (Zhu et al., 2019).
The measured
Based on Eq. (2), the nucleation rate mainly depends on [OH], [SO
The input of the TUV needs the aerosol optical properties in the altitude
range between 0 and 20 km. The parameterization of aerosol number concentration profiles by Liu et al. (2009) with aircraft measurement in Beijing is
used in this study. Liu et al. (2009) found that number concentration
constant within the mixing layer, linearly decreasing within the transition
layer and exponentially decreasing above the transition layer when the particles within the boundary are well mixed. The normalized aerosol PNSD
(PNSD divided by total aerosol number concentration) was assumed to be the
same at different altitudes. The BC to total aerosol mass concentration
ratio was also assumed to be the same at different altitudes
(Ferrero et al., 2011). The
The estimated
The lines with squares in Fig. 5a and b provide the calculated
photolysis rates of
Details of the aerosol optical profiles and estimated photolysis values.
Overall, the aerosol profiles tend to be uniformly distributed within the
mixing layer due to the strong turbulence in the afternoon. The
corresponding estimated
For a better understanding of the aerosol–radiation interaction on NPF, we estimated the photolysis rates under different aerosol vertical profiles. Based on the work of Liu et al. (2009), two typical types of aerosol profiles exist under different mixing layers as shown in Fig. S4. For the first type of mixing layer, aerosols were not well mixed within the mixing layer, and the aerosol number concentrations decrease with altitude exponentially (type A). Another type of mixing layer has aerosol number concentration constant in the mixing layer and then decreasing with altitude above the boundary (type B). For type B, we estimated the corresponding photolysis rate for different mixing layer heights between 500 and 1000 m, which covers the mean mixing layer altitude in the North China Plain (Zhu et al., 2018). The different aerosol optical depth (AOD), which ranges between 0.3 and 2, is used for different pollution conditions.
Four different aerosol profiles are used in this study. Details of the four different aerosol profiles are summarized in Table 2. The first one corresponds to aerosol mixing layer type A, with an exact boundary altitude of 1000 m and AOD of 0.3 (B1). The second aerosol profile has the same boundary altitude of 1000 m and AOD of 0.3, but the mixing layer type is changed to B (B2). The third aerosol profile also corresponds to mixing layer type B and a mixing layer altitude of 1000 m, but the AOD is 0.8 (B3). The last one has a mixing layer altitude of 500 m, with an AOD of 0.8 and a mixing layer type of B (B4).
The
These four profiles represent the typical ambient aerosol profiles in the
early morning, late morning, early afternoon, and late afternoon,
respectively. In the early morning, the turbulence in the mixing layer is
weak and the aerosol within the mixing layer is not well mixed (B1). In the
late morning, the aerosol in the boundary is well mixed and uniformly
distributed due to the increasing turbulence (B2). The early afternoon (B3)
should have higher AOD when compared with that in the late morning due to
the formation of the secondary aerosol. However, the mixing layer altitude
decreased in the late afternoon (B4) because the turbulence within the
mixing layer weakened compared with B3. The ambient aerosol profiles tend to
change from B1 to B4 from early morning to late afternoon. The corresponding
In this study, we characterized the aerosol PNSD at different times and different altitudes based on field measurements at an urban site in Beijing, China. Our measurements show that the aerosol size distribution profiles varied significantly with the development of the mixing layer.
In the morning, the turbulence in the boundary was weak, and the initially emitted particles cannot be mixed to a higher layer. The corresponding aerosol PNSD at the surface was larger than that at higher locations. At noon, the particles within the boundary were well mixed and tend to be uniformly distributed at different altitudes. In the late afternoon, we found more nucleation-mode particles at a higher altitude than that at the ground. The larger partitions of nucleation-mode particles do not result from transformation. We concluded that the nucleation processing in the upper mixing layer was more likely to happen than that at the ground.
The TUV model was employed to estimate the profiles of photolysis rates for different aerosol profiles. Results showed that both the
We also estimate the corresponding photolysis rate profile under different boundary structures. The increasing ratio of the photolysis rate with altitude increases with the development of the mixing layer from early morning to late afternoon. In the late afternoon, the difference of the photolysis rate at the upper mixing layer and that at the ground are the largest. At the same time, the turbulence is not so strong that the nucleation-mode particles formed in the upper mixing layer are not able to mix down to the ground. Therefore, it is a favor to observe a higher nucleation-mode particle concentration at the upper mixing layer than that at the ground in the afternoon. Our study reveals that the vertical distribution of ambient aerosols would first influence the vertical profile of the photolysis rate. Then the NPF for different altitudes is tuned due to the different photolysis rates.
The data are available upon request to the corresponding author.
The supplement related to this article is available online at:
GZ and YZ did the analysis and wrote the manuscript. MH, CZ, ZW, XF, and GZ discussed the results. YZ, JC, TZ, TT, KL, and HW conducted the measurements.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This research has been supported by the National Natural Science Foundation of China (grant no. 91844301) and the National Key Research and Development Program of China (grant no. 2016YFC0202000, tasks 3 and 5).
This paper was edited by Aijun Ding and reviewed by two anonymous referees.