Q1. What have the authors contributed in "Atmospheric new particle formation from sulfuric acid and amines in a chinese megacity" ?
According to both observation and theoretical arguments, NPF usually requires a relatively high H2SO4 concentration to promote the formation of new particles and a low pre-existing aerosol loading to minimize the sink of new particles. Here, the authors investigated NPF in Shanghai and were able to observe both precursor vapors ( H2SO4 ) and initial clusters at a molecular level in a megacity. One Sentence Summary: New particle formation in a Chinese megacity proceeds via the sulfuric aciddimethylamine-water, acid-base nucleation mechanism and it is an important source of urban aerosol particles in terms of number concentration. During the past several years, their knowledge on new particle formation ( NPF ) has dramatically increased through laboratory experiments, especially those carried out in the Cosmics Leaving OUtdoor Droplets ( CLOUD ) chamber at CERN ( 6-11 ). However, even in highly polluted areas like in Nanjing in Eastern China, the contribution of secondary aerosol production to the total aerosol number load is dominating and even more than half of accumulation mode aerosol particles have been estimated to be of secondary origin ( 19 ). Here the authors performed measurements in a Chinese megacity, Shanghai ( fig. S1 ), in order to investigate the mechanisms and impacts of atmospheric NPF. The first data set includes long-term continuous observations between March 2014 and February 2016 of particle number size distributions down to about 1. 2 nm and atmospheric trace gas concentrations. The instruments used during this period were one particle size magnifier ( PSM ), one nano scanning mobility particle sizer ( nano-SMPS ) and one longSMPS. During the long-term measurements, the authors identified 114 strong NPF events, with maximum-to-background concentration ratios > 20 for sub-3 nm particles, corresponding to a NPF frequency of 15. Their measured dimer-to-monomer ratio is much larger than the corresponding theoretical maximum ratio due to ion-induced clustering ( IIC ) of sulfuric acid within the CI-APi-TOF ion reaction zone ( 29 ), being about one order of magnitude larger than previously reported ambient values when sulfuric acid monomer concentration reached 1 × 10 molecule cm ( 27,29 ). On the other hand, the fast particle growth rates in Shanghai corresponded to an almost simultaneous appearance of sulfuric acid clusters and sulfuric acid-DMA clusters. The measured particle formation rates give further support for the involvement of DMA, instead of any other stabilizer, in the observed NPF events. The average temperature ( 278±8 K ) and relative humidity ( 36±7 % ) on the NPF days during the intensive campaign are close to the CLOUD experimental conditions ( 278 K and 38 % RH ) and hence temperature and relative humidity are not expected to significantly enhance the particle formation rates during this period in Shanghai ( 32 ). Figure 3 shows that the GR of clusters and nanoparticles during intensive campaign increased steeply with their increasing size up to 25 nm, which is consistent with the observations from long-term measurements ( Fig. S9 ). The authors performed cluster kinetics simulations for a collision-limited H2SO4-DMA system ( 34 ) using median sulfuric acid concentration and CS for the NPF events observed during the intensive campaign. The sub-3 nm GR determined from simulations is on average higher than the measured GR, which means that the sulfuric acid concentrations are sufficient to explain the observed growth of sub-3 nm particles considering that there is always a neutralizing base to stabilize sulfuric acid clusters ( for instance, DMA ). With the typical values of GR observed in Shanghai, newly formed particles reach cloud condensation nuclei ( CCN ) sizes within a day. Based on their calculations, the number of particles produced in NPF events is ca. However, it is notable that, according to Paasonen et al. ( 2016 ) ( 36 ), also the anthropogenic particle number emissions are highly uncertain. The authors estimated that anthropogenic emissions of nucleation mode particles are in general underestimated due to the incomplete representation of the volatile primary particles in some of observational data applied for deriving the emission factors. Due to the mentioned probable underestimation in and uncertainties related to both the NPF and the anthropogenic particle number sources, this comparison should not be taken as an estimate of the exact shares of the sources, but as an indication that neither of them clearly dominates over the other. In summary, the authors have performed a molecular level study of NPF events in a Chinese megacity. Within experimental uncertainties, the sulfuric acid concentrations were high enough to result in the observed growth of sub-3 nm particles, provided that neutralizing bases, such as DMA, stabilized the sulfuric acid clusters. NPF events in turn lead to the formation of large concentrations of new atmospheric particles that have an impact on regional air quality and potentially also the regional and global climate. In addition, the concentrations of amines ( 33,38 ) are sufficient to allow sulfuric acid particles to form at their maximum ( kinetically-limited ) rate. Hence, in order to reduce secondary aerosol formation in China, it is crucial to control the emissions of precursor compounds for new particle formation. On the formation and growth of atmospheric nanoparticles. On the mode-segregated aerosol particle number concentration load: contributions of primary and secondary particles in Hyytiälä and Nanjing. The role of relative humidity in continental new particle formation, J. Geophys. On the formation of sulphuric acid amine clusters in varying atmospheric conditions and its influence on atmospheric new particle formation. The effect of acid-base clustering and ions on the growth of atmospheric nano-particles. The role of low-volatility organic compounds in initial particle growth in the atmosphere. 547889 43. K. Lehtipalo, J. Leppä, J. Kontkanen, J. Kangasluoma, A. Franchin, D. Wimmer, S. Schobesberger, H. Junninen, T. Petäjä, M. Sipilä, J. Mikkilä, J. Vanhanen, D. R. Worsnop, M. Kulmala, Methods for determining particle size distribution and growth rates between 1 and 3 nm using the Particle Size Magnifier. 5194/acp-12-2345-2012 55. M. Kulmala, T. Petäjä, T. Nieminen, M. Sipilä, H. E. Manninen, K. Lehtipalo, M. Dal Maso, P. P. Aalto, H. Junninen, P. Paasonen, I. Riipinen, K. E. Lehtinen, A. Laaksonen, V. M. Kerminen, Measurement of the nucleation of atmospheric aerosol particles. High new particle formation rates were observed to coincide with several familiar markers suggestive of H2SO4-dimethylamine ( DMA ) H2O nucleation, including sulfuric acid dimers and H2SO4-DMA clusters. This suggests that the formation of atmospheric clusters giving a strong sulfuric acid dimer signal in the CI-APi-TOF was crucial for the observed nucleation processes. As suggested by other neutral clusters ( Fig. 1C ), the identity of the stabilizer for sulfuric acid dimer is most likely DMA. Nevertheless, the presence of sulfuric acid clusters and sulfuric acid-DMA clusters suggest that the initial growth of neutral clusters proceeded by addition of precursor gases or preformed clusters ( 6 ). Direct comparison of their results with these estimates of primary anthropogenic particle number emissions suggests that the NPF contribution to the total aerosol number production is about 20 % in the most polluted area and 50 % in a wider urban environment. 16. Z. Wang, Z. Wu, D. Yue, D. Shang, S. Guo, J. Sun, A. Ding, L. Wang, J. Jiang, H. Guo, J. Gao, H. C. Cheung, L. Morawska, M. Keywood, M. Hu, New particle formation in China: Current knowledge and further directions, Sci. Total Environ.