Impact of aerosol-radiation interaction on new particle formation

10 New particle formation (NPF) is thought to contribute to 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 boundary layer because previous studies mainly focus on ground-level measurements. In this study, the developments of aerosol size distribution at different altitudes are characterized based 15 on the field measurement conducted in January 2019, in Beijing, China. We find that the partition of nucleation mode particles at the upper boundary layer is larger than that at the ground, which implies that the nucleation processing is more likely to happen in the upper boundary layer than that at the 2 ground. Results of the radiative transfer model show that the photolysis rates of the nitrogen dioxide and ozone increase with altitude within the boundary layer, which lead to a higher concentration of 20 sulfuric acid at the upper boundary layer than that at the ground. Therefore, the nucleation processing in the upper boundary 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 source of cloud condensation nuclei in global scale due to the impacts of aerosol-radiation interaction.

Details of the measurement site can refer to Wang et al. (2018). Vertical measurements were conducted from the tower-based platform, with a maximum of 350 m, on the IAP campus. All of the instruments were installed on a moving cabin of the tower, which moves up and down in altitudes between 0 and 240 m. The cabin moved around 10 meters every minute in altitude. Aerosol PNSD in the size range between 10 nm and 700 nm were measured using a scanning mobility particle size (SMPS; TSI Inc. 80 3010). Aerosol scattering coefficient ( ) at the wavelength of 450 nm, 525 nm, and 635 nm were measured by an Aurora 3000 nephelometer (Müller et al., 2011). The nitrogen dioxide (NO2) was measured based on its absorbance at 405 nm with a low-power lightweight instrument (model 405 nm, 2B Technology, USA). The nitrogen monoxide (NO) was measured by adding an excess of ozone with another power lightweight instrument (model 106-L, 2B Technology, USA). The wind speed, wind 85 direction, ambient relative humidity, and temperature were measured by a small auto meteorology station. This instrument can record the atmosphere pressure, which was used to retrieve the altitude information.

Lognormal fit of PSND
For each of the measured PNSD, it is fitted by three lognormal distribution modes by: . The three modes with geometric diameter ranges of 10 ~ 25 nm, 25 ~100 nm, and 100 ~ 700 nm correspond to the nucleation mode, Aitken mode, and accumulation mode respectively. The nucleation 95 particles mainly result from the nucleation process and the Aitken mode particles are from primary sources, such as traffic sources (Shang et al., 2018). The accumulation mode particles are correlated with secondary formation, which mainly represents the ambient pollution conditions (Wu et al., 2008).

Mie Model
Mie scattering model (Bohren and Huffman, 2007) is used to estimate the aerosol optical 100 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 kilometers away from this site. The refractive index of the non-black carbon and black carbon aerosol component are 1.64+0i, which is the measured mean aerosol refractive index measured at Beijing (paper in preparation), and 1.96 + 105 0.66i (Zhao et al., 2017) respectively. The aerosol hygroscopic growth is not considered here because the ambient relative humidity during the measurement was all the way lower than 30% as shown in fig. 1(b). With the measured different aerosol PNSD and above-mentioned information, we can calculate the corresponding aerosol optical properties, which contain the aerosol , aerosol single scattering albedo (SSA) and asymmetry factor (g).

TUV Model
The Tropospheric Ultraviolet-Visible radiation model (TUV), 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 nm and 735 nm. In this study, the photolysis frequency of the nitrogen 115 dioxide (J(NO2)) and ozone (J(O 1 D)) were used for further study. Inputs of the TUV model are the aerosol optical depth and single scattering albedo (Tao et al., 2014). The cloud aerosol optical depth is set to be zero in this study. The output of the TUV model includes the profiles of J(NO2) and J(O 1 D).
Some changes were made in the source code of the TUV model so that the model can calculate the J(NO2) and J(O 1 D) profiles with different aerosol optical profiles (including aerosol , SSA, 120 and g).

Aerosol PNSD at different altitude and time
The measured aerosol PNSD profiles in the time range between 7:00 and 18:50 on January 18 were used for analysis, which contained eight different upward movement and downward movement 125 of the cabin, respectively. Fig. 1 (a) gives detailed time-altitude information of each measurement. All On January 18, the measured ambient temperature and relative humidity ranges were -3 o C ~ 10 o C and 13% ~ 24% respectively, which implied that the ambient air in the winter of Beijing are dry and cold. Aerosol hygroscopic growth was thus not considered in this study. The wind speeds during the 130 measurement were lower than 1m/s, and thus the measurement results of aerosol microphysical properties were hardly influenced by transportation.
During the measurement, the varied between 0 and 400 Mm -1 . It ranged between 100 Mm -1 and 200 Mm -1 on 18, January. We compared the measured using the nephelometer and calculated can reach 1.5x10 4 cm -3 , and the number concentrations peaked at smaller than 100 nm. It was only 8x10 3 cm -3 with peaking aerosol diameter at around 200 nm at a higher altitude around 200 m. The solar radiation in the morning was very week, therefore, the turbulence mixing of the aerosol among different altitudes was very weak. The initial emission from the ground surface cannot be mixed up to 150 higher locations, and thus the aerosol number concentrations at the surface was larger than that at a higher level as shown in Fig. 2

(a).
With the increment of solar radiation and ambient temperature, the turbulence mixing of ambient particle 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. 2(b) and (c). However, the aerosol 155 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 boundary layer as shown in Fig. 2 (b), (c), and (d). These particles were still not well mixed at the range between 0 and 240 m until 11:20.
In the afternoon, the boundary layer was well mixed with the increment of solar radiation and 160 ambient temperature. The aerosol PNSD and PVSD were almost uniformly distributed as shown in Fig. 2 (e) 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 surface was different from that of upper levels because the initially emitted particles cannot be mixed up to higher location. The PNSD tended to be uniformly distributed 170 when the turbulence within the boundary layer was strong.

Nucleation process in the upper boundary layer
We calculated aerosol total number concentration for each measured PNSD (Ntot) and the profiles of Ntot are shown in Fig. 3 (a). All of the profiles in Fig. 3 corresponded to these cases when the cabin is moving up. The Ntot profiles varied significantly with the development of the boundary layer. In the 175 morning, the Ntot in the surface (larger than 2x10 4 cm -3 ) was larger than that at a higher level (lower than 1x10 4 cm -3 ) because the turbulence is so weak that the initially emitted particles on the surface cannot be transported to the upper level. In the afternoon around 14:00 and 16:00, the aerosol was well mixed in the boundary layer and Ntot was almost uniform with around 1.2x10 4 per cubic centimeter.
Afterward, the turbulence was weaker than that in the early afternoon and again the emitted aerosols 180 cannot reach the higher level. The profile of Ntot in the morning was similar to that in the late afternoon and night.
The number ratio profiles of nucleation mode to Aitken mode (N1/N2) for different times are shown in Fig. 3 because the temperature and turbulence increased when it came to 8:00-10:00 in the morning. However, the turbulence was not strong enough to mix all of the particles to upper levels to 240 nm. The ratio still decreased with altitude. In the afternoon, the boundary layer developed well and the ratios between 13:20 and 14:25 were almost uniformly distributed at different altitudes. However, we found that the ratio increased with altitude from 0.21 to 0.34 when it came to 16:15, which implied that more 190 nucleation mode particles were formed in the upper level in the boundary layer. The increment of the ratio was hardly influenced by transportation because the wind speed during the measurement was all the time lower than 1 m/s as shown in Fig. 1(b).
To better configure the variations of PNSD, we calculated the aerosol number concentrations with the diameter between 10 and 25 nm (N10-25nm). The N10-25nm profiles in Fig. 3(c) show almost the same 195 trends with the number ratio of N1 to N2. In the morning and later afternoon, the N10-25nm decreased with the altitude. The N10-25nm in the early afternoon were uniformly distributed due to the strong mixing in the boundary layer. When it came to 16:15, the N10-25nm at different altitudes were larger than that in the early afternoon. Most importantly, N10-25nm increases with altitude. The aerosol total volume at 16:15 does not increase with altitude because the nucleation produced particles are so small 200 that they contribute negligibly to the aerosol total volume.
Based on the discussion above, we found that the total aerosol number concentrations increased slightly with altitude at 16:15. The number ratio of N1 to N2 and the N10-25nm increased with altitude.
The total volumes of the aerosol particles were almost the same at different altitudes. The variation of https://doi.org/10.5194/acp-2020-1301 Preprint. Discussion started: 4 February 2021 c Author(s) 2021. CC BY 4.0 License. PNSD was hardly influenced by transportation. Therefore, we concluded that the nucleation process 205 was more likely to happen in the upper level of the boundary layer than the ground surface. This phenomenon was not observed in the early afternoon because the turbulence in the early afternoon is so strong that the aerosol particles are well mixed in the boundary layer.    Fig. 4(a). Thus, the [SO2] should be uniformly distributed in the afternoon within the boundary layer. The CS profiles, in Fig. 4(b), were almost uniformly distributed in the afternoon. Therefore, the [OH] is the only main factor that may result in different characteristics of https://doi.org/10.5194/acp-2020-1301 Preprint.  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 , SSA, and g profiles can be calculated by Mie theory under these assumptions (Zhao et al., 2017;Zhao et al., 2018).

Influence of Aerosol-radiation Interaction on NPF
The lines with squares in Fig. 5(a) and (b) provide the calculated photolysis rates of ( 1 ), and 240 ( 2 ) with a boundary layer altitude of 1000 m. Results show that both the ( 1 ), and ( 2 ) increase with altitude within the boundary layer. The ( 1 ) increases from 8.9x10 -3 s -1 to 14. Overall, the aerosol profiles tend to be uniformly distributed within the boundary layer due to the strong turbulence in the afternoon. The corresponding estimated ( 1 ) , and ( 2 ) values increase with altitude, which leads to higher [OH] at the top of the boundary layer than that at the 250 ground. Therefore, the [H2SO4] should increase with altitude based on equation 1. There should be more nucleation processing at the top of the boundary layer than that at the ground, which is consistent with our field measurement. The schematic graph of the influence of aerosol-radiation interaction on NPF is shown in Fig. 6. the boundary (type B). For type B, we estimated the corresponding photolysis rate for different boundary layer heights between 500 m and 1000 m, which covers the mean boundary layer altitude in the North China Plain (Zhu et al., 2018). The different aerosol optical depth (AOD), which ranges between 0.3 and 2, are used for different pollution conditions. 265

Impact of Boundary layer development on the photolysis rates 255
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 the aerosol boundary layer type A, with a boundary altitude of 1000 m and AOD of 0.3 (B1). The second aerosol profile has the same boundary altitude of 1000m and AOD of 0.3, but the boundary layer type is changed into B (B2). The third aerosol profile also corresponds to boundary layer type B, and a boundary layer altitude of 1000m, but 270 the AOD is 0.8 (B3). The last one has a boundary layer altitude of 500m, with an AOD of 0.8 and a boundary layer type of B (B4).
The ( 1 ), and ( 2 ) profiles under the above-mentioned aerosol profiles are estimated and shown in Fig. 5 (a) and (b). For each type, both the ( 1 ), and ( 2 ) increase with altitude. The increased ratio of the ( 1 ) with altitude (k 1 ) are 1.7x10 -5 , 2.0x10 -5 , 3.0x10 -5 , and 5.4x10 -5 s -275 These four profiles represent the typical ambient aerosol profiles in the early morning, late 280 morning, early afternoon, and late afternoon, respectively. In the early morning, the turbulence in the boundary layer is weak and the aerosol within the boundary 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 boundary layer altitude 285 decreased in the late afternoon (B4) because the turbulence within the boundary layer weakened compared with B3. The ambient aerosol profiles tend to change from B1 to B4 from early morning to late afternoon. The corresponding k 1 and k 2 increased with the development of the boundary layer. In the late afternoon, the difference of photolysis rate at the top of the boundary layer and ground are largest. Furthermore, the turbulence in the mixing layer is weakened and the nucleation formed 290 particles cannot be mixed down to the ground. Therefore, it is more likely to observe more nucleation mode particles at the top of the boundary layer than at the ground in the late afternoon, which is consistent with our measurement.

Conclusion
In this study, we characterized the aerosol PNSD at different times and different altitudes based 295 on field measurements at a urban site, in Beijing, China. Our measurements show that the aerosol size distribution profiles varied significantly with the development of the boundary layer.
In the morning, the turbulence in the boundary was weak and the initial 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 300 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 boundary layer were more likely to happen than that at the ground.
The TUV model was employed to estimate the profile of photolysis rate for different aerosol 305 profiles. Results showed that both the ( 1 ), and ( 2 ) values increased with altitude, which led to higher [OH] at the top of the boundary layer than that at the ground. The corresponding [H2SO4] should increase with altitude based on equation 1, when the aerosol was well mixed and uniformed in the mixed layer. Therefore, more nucleation processing at the top of the boundary layer may happen than that at the ground, which is consistent with our field measurement. 310 We also estimate the corresponding photolysis rate profile under different boundary structures.
The increasing ratio of the photolysis rate with altitude increase with the development of the boundary layer from early morning to late afternoon. In the late afternoon, the difference of the photolysis rate at the upper boundary 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 boundary layer are not able to mix 315 down to the ground. Therefore, it is a favor to observe higher nucleation mode particles concentration at the upper boundary layer than that at the ground in the afternoon. Our study reveals that the vertical https://doi.org/10.5194/acp-2020-1301 Preprint. Discussion started: 4 February 2021 c Author(s) 2021. CC BY 4.0 License.