Comment on acp-2022-436

Aerosol size distribution and new particle formation events in the suburb of Xi'an, northwest China

Abstract. Understanding the particle number size distributions in diversified atmospheric environments is important in order to design mitigation strategies related to submicron particles and their effects on regional air quality, haze and human health. In this study, we conducted 15 different field measurement campaigns between 2007 and 2011 at 13 individual sites in China, including five urban sites, four regional sites, three coastal/background sites and one ship cruise measurement along eastern coastline of China. Size resolved particles were measured in the 15–600 nm size range. The median particle number concentrations (PNCs) were found to vary in the range of 1.1−2.2 × 104 cm−3 at urban sites, 0.8−1.5 × 104 cm−3 at regional sites, 0.4−0.6 × 104 cm−3 at coastal/background sites, and 0.5 × 104 cm−3 during cruise measurement. Peak diameters at each of these sites varied greatly from 24 to 115 nm. Particles in the 15–25 nm (nucleation mode), 25–100 nm (Aitken mode) and 100–600 nm (accumulation mode) range showed different characteristics at each sites, indicating the features of primary emissions and secondary formation in these diversified atmospheric environments. Diurnal variations show a build-up of accumulation mode particles belt at regional sites, suggesting the contribution of regional secondary aerosol pollution. Frequencies of new particle formation (NPF) events were much higher at urban and regional sites than at coastal sites and during cruise measurement. The average growth rates (GRs) of nucleation mode particles were 8.0–10.9 nm h−1 at urban sites, 7.4–13.6 nm h−1 at regional sites and 2.8–7.5 nm h−1 at coastal sites and during cruise measurement. The high gaseous precursors and strong oxidation at urban and regional sites not only favored the formation of particles, but also accelerated the growth rate of the nucleation mode particles. No significant difference in condensation sink (CS) during NPF days were observed among different site types, suggesting that the NPF events in background areas were more influenced by the pollutant transport. In addition, average contributions of NPF events to potential cloud condensation nuclei (CCN) at 0.2% super-saturation in the afternoon of all sampling days were calculated as 11% and 6% at urban sites and regional sites, respectively. On the other hand, NPF events at coastal sites and during cruise measurement had little impact on potential production of CCN. This study provides a large data set of particle size distribution in diversified atmosphere of China, improving our general understanding of emission, secondary formation, new particle formation and corresponding CCN activity of submicron aerosols in Chinese environments.

The formation of new atmospheric aerosol particles and their subsequent growth have been observed under different environmental conditions globally; such observations are few over northwest China. Here, we present an analysis of some case studies for new particle formation (NPF) events from two distinct suburban locations in northern China during May and June of two consecutive years, and provide more information to understand the characteristics of NPF events in North China. Particle number size distribution was measured at suburbs of Beijing (39.75° N, 116.96° E) during 1 June to 2 July 2013 and at suburbs of Xi’an (34.09° N, 108.55° E) during 1 to 25 May 2014. The average of total particle number concentration in the similar size range of 10–487 nm at the suburbs of Beijing (9.0 × 103 cm−3) was about two times higher than those observed at Xi’an (4.7 × 103 cm−3), and the mean particle mode diameter at Beijing was 1.4-fold higher than that at Xi’an. The estimated total condensation sink (CS) at Beijing (3.11 × 10−2 s−1) was also higher than at Xi’an (1.13 × 10−2 s−1). The frequency of NPF events at suburb of Beijing was 24%, lower than that in Xi’an (50%), and also lower than urban site of Beijing (35% in June) and another suburb of Beijing (over 50% in June). The NPF events with (Class I) or without (Class II) subsequent growth were both observed at the two suburb sites. The derived GR at the suburb of Beijing (range from 4.6 to 8.6 nm h−1) was a little higher than that at Xi’an (range from 3.3 to 6.7 nm h−1), which are generally comparable to typical values in mid-latitude reported in previous studies. The air masses coming from north or northwest China favor the occurrence of NPF event under low condensation sink and clear days. The number size distributions of freshly nucleated particles showed clear bimodal distributions on both sites. Additionally, Mode Dp of nucleated particles at the two sites was 17 ± 1 nm and 22 ± 4 nm, respectively during the periods with NPF events. The case study of NPF events at the two suburb sites shows that the surface area concentration and total scattering coefficient (SC) was significant decreased during the NPF events at both sites. High temperature, low condensation sink and low relative humidity furthered the occurrence of NPF events, and wind direction shifts were important for the subsequent growth of particles. NPF events in the suburbs of Beijing usually occurred when relative humidity (RH) < 55%, CS < 0.02 s−1, or 55% < RH < 68%, CS < 0.01 s−1. However, there is no clear range for Xi’an. Furthermore, we observed that some NPF events occurred at higher RH and very low CS in this study on both sites, which means that low CS may be more important than low RH for the particle formation on clear days.

The aerosol number spectrum and gas pollutants were measured and the new particle formation (NPF) events were discussed in Nanjing. The results showed that the size distributions of aerosol number concentrations exhibited distinct seasonal variations, implying the relations of particle sizes and their sources and sinks. The number concentrations of particles in the nuclei mode (10–30 nm), Aitken mode (30–100 nm), accumulation mode (100–1000 nm) and coarse mode (>1 μm) varied in the order of summer > spring > autumn, summer > autumn > spring, autumn > summer > spring, and spring > autumn >summer, respectively. The diurnal variation of total aerosol number concentrations showed three peaks in all observed periods, which corresponded to two rush hours and the photochemistry period at noon. In general, the NPF in summer occurred under the conditions of east winds and dominant air masses originating from marine areas with high relative humidity (50%–70%) and strong solar radiations (400–700 W m−2). In spring, the NPF were generally accompanied by low relative humidity (14%–30%) and strong solar radiations (400–600 W m−2). The new particle growth rates (GR) were higher in the summertime in the range of 10–16 nm h−1. In spring, the GR were 6.8–8.3 nm h−1. Under polluted air conditions, NPF events were seldom captured in autumn in Nanjing. During NPF periods, positive correlations between 10–30 nm particles and O3 were detected, particularly in spring, indicating that NPF can be attributed to photochemical reactions.

Abstract. The spatial and temporal variability of the number size distribution of aerosol particles is an indicator of the dynamic behavior of Beijing's atmospheric pollution cocktail. This variation reflects the strength of different primary and secondary sources, such as traffic and new particle formation, as well as the main processes affecting the particle population. In this paper, we report size-segregated particle number concentrations observed at a newly developed Beijing station during the winter of 2018. Our measurements covered particle number size distributions over the diameter range of 1.5 nm–1 µm (cluster mode, nucleation mode, Aitken mode and accumulation mode), thus being descriptive of a major fraction of the processes taking place in the atmosphere of Beijing. Here we focus on explaining the concentration variations in the observed particle modes, by relating them to the potential aerosol sources and sinks, and on understanding the connections between these modes. We considered haze days and new particle formation event days separately. Our results show that during the new particle formation (NPF) event days increases in cluster mode particle number concentration were observed, whereas during the haze days high concentrations of accumulation mode particles were present. There was a tight connection between the cluster mode and nucleation mode on both NPF event and haze days. In addition, we correlated the particle number concentrations in different modes with concentrations of trace gases and other parameters measured at our station. Our results show that the particle number concentration in all the modes correlated with NOx, which reflects the contribution of traffic to the whole submicron size range. We also estimated the contribution of ion-induced nucleation in Beijing, and we found this contribution to be negligible.

Significant effects of transport on nanoparticles during new particle formation events in the atmosphere of Beijing

Relatively high concentrations of preexisting particles, acting as a condensation sink (CS) of gaseous precursors, have been thought to suppress the occurrence of new particle formation (NPF) in urban environments, yet NPF still occurs frequently. Here, we aim to understand the factors promoting and inhibiting NPF events in urban Beijing by combining one-year-long measurements of particle number size distributions and PM2.5 chemical composition. Our results show that indeed the CS is an important factor controlling the occurrence of NPF events, with its chemical composition affecting the efficiency of the background particles in removing gaseous H2SO4 (effectiveness of the CS) driving NPF. During our observation period, the CS was found to be more effective for ammonium nitrate-rich (NH4NO3-rich) fine particles. On non-NPF event days, particles acting as CS contained a larger fraction of NH4NO3 compared to NPF event days under comparable CS levels. In particular, in the CS range from 0.02 to 0.03 s–1, the nitrate fraction was 17% on NPF event days and 26% on non-NPF event days. Overall, our results highlight the importance of considering the chemical composition of preexisting particles when estimating the CS and their role in inhibiting NPF events, especially in urban environments.

In urban and remote atmospheres, new particle formation (NPF) events often occur after dust storms, which can have further impacts on regional and global climate. Additionally, the levels of gaseous precursors and condensation sink (CS) resulting from anthropogenic emissions can also influence the occurrence and intensity of NPF events. Over the past decade, China has implemented various air quality improvement actions, leading to significant changes in air pollutant emissions. However, there have been limited studies on the long-term evolution of NPF events caused by the synergy of dust storms and anthropogenic emissions. This study aims to analyze the characteristics of dust-related NPF events in Beijing from 2009 to 2017 and compare them with non-dust NPF events. The results show that the formation rates of dust-related NPF events decreased due to the reduction of SO2 emissions. However, the growth rates remained stable, possibly due to the increasing oxidation capacity in Beijing. These findings contribute to a deeper understanding of the climate effects of natural dust under the influence of anthropogenic emission control measures.

<p>The Cloud Condensation Nuclei (CCN) budget, the aerosol particles population that could become cloud droplets, can be influenced by primary aerosol particles emitted by different sources (anthropogenic or biogenic) or by secondary particles that have undergone growth processes or chemical transformations. Aerosol particles originated by nucleation of precursor gases in the atmosphere have been identified as an important source of CCN particles. The influence of New Particle Formation (NPF) events to CCN concentrations is highly dependent on the environment where it takes place. Specifically, the study of the influence of NPF events on CCN concentration at high-altitude sites, where atmospheric conditions favor the formation of clouds, is a very interesting scientific goal.</p><p> </p><p>This study presents CCN measurements combined with aerosol size distribution at a high-altitude station in the South East of Spain: a remote high mountain site (Sierra Nevada; SNS, 2500 m a.s.l.). Due to its high altitude, the aerosol particles over SNS station are often representative of pristine free troposphere conditions, especially in winter and nighttime. During summer, SNS station is frequently influenced by transport of pollutants from Granada city to Sierra Nevada station as a result of mixing layer growth and the activation of the mountain-valley breeze phenomenon as well as by NPF events at midday (De Arruda Moreira et al., 2019; Casquero-Vera et al., 2020).</p><p> </p><p>In this study, we analyze the influence of NPF events to CCN concentrations during summer 2019 at the SNS high-altitude station. The study period (from June to August of 2019) was characterized by 67 NPF events, 16 undefined events and 13 non-events days. Following Rose et al. (2017) criteria, only those NPF events referred as type I, i.e. with clear particle growth from smallest sizes, were selected to investigate the contribution of NPF events on CCN concentrations. In this sense, we selected the 15 clearest NPF events for this analysis.</p><p> </p><p>Results show clear differences in the diurnal evolution of CCN concentration between NPF event and non-event days, demonstrating the large influence of NPF to CCN concentrations, especially at high supersaturations (Rejano et al., 2021). NPF events have been estimated to increase the CCN concentrations by 175% at SS=0.5%, evidencing NPF events as one of the major CCN source at this mountain site</p><p> </p><p> </p><p><strong>Acknowledgments</strong>: This work was supported by the European Union's Horizon 2020 research and innovation programme through project ACTRIS 2 (grant agreement No 654109), by the Spanish Ministry of Economy and Competitiveness through projects CGL2016-81092-R, CGL2017-90884-REDT and RTI2018-101154-A-I00 and by University of Granada Plan Propio through Visiting Scholars program. The Spanish Ministry of Universities funds Fernando Rejano under the predoctoral program FPU (FPU19/05340).</p><p> </p><p><strong>References</strong></p><p>Casquero-Vera, et al. (2020) Atmos. Chem. Phys. <strong>20, </strong>14253–14271.</p><p>De Arruda Moreira et al. (2019) Atmos. Chem. Phys. <strong>19</strong>, 1263-1280.</p><p>Rejano et al. (2021) Sci. Tot Envi., <strong>762</strong>, 143100.</p><p>Rose et al. (2017) Atmos. Chem. Phys. <strong>17</strong>, 1529-1541.</p><p>tract HTML here.</p>

We present a supervised machine learning (ML) framework to automatically identify new particle formation (NPF) events and analyze key atmospheric factors associated with their occurrence and growth. We applied ML to detect NPF events using start time and particle concentrations across size ranges, while identifying atmospheric variables including ambient temperature, relative humidity, solar radiation intensity (SRI), wind speed, wind direction, boundary layer height, total organics, sulfate, nitrate, total surface area concentration, sulfur dioxide, and turbulent kinetic energy (TKE). We analyzed a 6‐year data set from the Atmospheric Radiation Measurement at the Southern Great Plains (SGP) site in Oklahoma, USA. Using long‐term ground‐based measurements, we identified NPF events and applied Random Forest Classifiers, which achieved 90%–95% prediction accuracy. Feature importance analysis highlighted SRI, relative humidity, and ambient temperature as the most influential variables, contributing normalized importances of 28%, 17%, and 10%. Partial Dependence Plots (PDPs) indicated that higher SRI and lower relative humidity were critical in promoting NPF formation at SGP. Seasonally, NPF events were more frequent in winter (42.1%) and spring (35.5%), and least in summer (4.0%). Particle growth rates also exhibited a seasonal variation, with the lowest in winter (below 2 nm hr −1 ) and highest in late spring and early summer (exceeding 5 nm hr −1 ). Temperature, turbulent kinetic energy, and aerosol properties were the primary factors of growth rate variability. This study advances predictive modeling of NPF, offers insights for future campaign deployments, and demonstrates the effectiveness of ML in understanding the formation and growth of atmospheric aerosols.

Personal care products (PCPs) contain diverse volatile organic compounds (VOCs) and routine use of PCPs indoors has important implications for indoor air quality and human chemical exposures. This chamber study deployed aerosol instrumentation and two online mass spectrometers to quantify VOC emissions from the indoor use of five fragranced PCPs and examined the formation of gas-phase oxidation products and particles upon ozone-initiated oxidation of reactive VOCs. The tested PCPs include a perfume, a roll-on deodorant, a body spray, a hair spray, and a hand lotion. Indoor use of these PCPs emitted over 200 VOCs and resulted in indoor VOC mixing ratios of several parts per million. The VOC emission factors for the PCPs varied from 2 to 964 mg g-1. We identified strong emissions of terpenes and their derivatives, which are likely used as fragrant additives in the PCPs. When using the PCPs in the presence of indoor ozone, these reactive VOCs underwent oxidation reactions to form a variety of gas-phase oxidized vapors and led to rapid new particle formation (NPF) events with particle growth rates up to ten times higher than outdoor atmospheric NPF events. The resulting ultrafine particle concentrations reach ∼34000 to ∼200000 cm-3 during the NPF events.

Abstract. The predominating role of aerosol Fuchs surface area, AFuchs, in determining the occurrence of new particle formation (NPF) events in Beijing was elucidated in this study. The analysis was based on a field campaign from 12 March to 6 April 2016 in Beijing, during which aerosol size distributions down to ∼ 1 nm and sulfuric acid concentrations were simultaneously monitored. The 26 days were classified into 11 typical NPF days, 2 undefined days, and 13 non-event days. A dimensionless factor, LΓ, characterized by the relative ratio of the coagulation scavenging rate over the condensational growth rate (Kuang et al., 2010), was applied in this work to reveal the governing factors for NPF events in Beijing. The three parameters determining LΓ are sulfuric acid concentration, the growth enhancement factor characterized by contribution of other gaseous precursors to particle growth, Γ, and AFuchs. Different from other atmospheric environments, such as in Boulder and Hyytiälä, the daily-maximum sulfuric acid concentration and Γ in Beijing varied in a narrow range with geometric standard deviations of 1.40 and 1.31, respectively. A positive correlation between the estimated new particle formation rate, J1.5, and sulfuric acid concentration was found with a mean fitted exponent of 2.4. However, the maximum sulfuric acid concentrations on NPF days were not significantly higher (even lower, sometimes) than those on non-event days, indicating that the abundance of sulfuric acid in Beijing was high enough to initiate nucleation, but may not necessarily lead to NPF events. Instead, AFuchs in Beijing varied greatly among days with a geometric standard deviation of 2.56, whereas the variabilities of AFuchs in Tecamac, Atlanta, and Boulder were reported to be much smaller. In addition, there was a good correlation between AFuchs and LΓ in Beijing (R2 = 0.88). Therefore, it was AFuchs that fundamentally determined the occurrence of NPF events. Among 11 observed NPF events, 10 events occurred when AFuchs was smaller than 200 µm2 cm−3. NPF events were suppressed due to the coagulation scavenging when AFuchs was greater than 200 µm2 cm−3. Measured AFuchs in Beijing had a good correlation with its PM2.5 mass concentration (R2 = 0.85) since AFuchs in Beijing was mainly determined by particles in the size range of 50–500 nm that also contribute to the PM2.5 mass concentration.

Abstract. The strictest ever clean air action (CAA) plan has been implemented by the Chinese government since 2013 to alleviate the severe haze pollution. The PM2.5 mass concentration was found to largely be reduced in response to emission mitigation policies, but the response of particle number concentrations (PNCs) to CAA was less evaluated in the previous studies, which may be significantly different from PM2.5 mass due to newly formed particle impacts. In this work, the first in situ observation of particle number size distributions (PNSDs) during 2012–2019 in urban Lanzhou was used to analyze long-term PNC variations and CAA impacts. The average number of particles in nucleation (N13−25, particle number in the size range of 13–25 nm), Aitken (N25−100, particle number in the size range of 25–100 nm) and accumulation (N100−800, particle number in the size range of 100–800 nm) modes were respectively 2514.0, 10 768.7 and 3258.4 cm−3, and N25−100 accounted for about 65.1 % of total PNCs during the campaign. The k-means clustering technique was used to classify the hourly mean PNSDs into six clusters, and each cluster corresponded to a specific source and influencing factor. The polluted clusters governed the winter PNCs before 2016, and their occurrence was less and less frequent after 2016, which was largely dominated by reduction in primary emissions. However, the contribution of new particle formation (NPF) events to summer N13−25 decreased from 50 % to about 10 % during 2013 to 2015 and then increased to reach around 60 % in 2019. The trends of size-resolved PNCs for each cluster were quantified by Theil–Sen regression. The size-segregated PNCs exhibited downward trends for all clusters during 2012–2015, especially in spring. The annual relative slopes of spring PNCs varied from −54.7 % to −17.2 %, −42.6 % to −14.1 %, and −40.7 % to −17.5 % per year for 13–25, 25–100, and 100–800 nm size ranges, and the reduction in the polluted clusters was much larger than NPF clusters. The ultrafine particle number was increased, and the amplitude was much greater during 2016–2019. The annual relative slopes of N13−25 varied between 8.0 % in fall and 135.5 % in spring for the NPF cluster. In response to CAA, the increased daytime net radiation, higher ambient temperature and lower relative humidity at noon for NPF events also could partly explain the higher N13−25 induced by the more frequent nucleation events after 2016, especially in spring. The air masses were mainly from the adjacent regions of urban Lanzhou and less affected by long-range transport for NPF events, and thus the particles were not easily grown by coagulation during transport processes, which was helpful for the occurrence of NPF events. Therefore, some effective measures to cooperatively control particle number concentration and mass should be taken for the Chinese megacities.

Abstract. New particle formation (NPF) and growth are a major source of atmospheric fine particles. In polluted urban environments, NPF events are frequently observed with characteristics distinct from those in clean environments. Here we simulate NPF events in urban Beijing with a discrete-sectional model that couples cluster dynamics and multicomponent particle growth. In the model, new particles are formed by sulfuric acid–dimethylamine nucleation, while particle growth is driven by particle coagulation and the condensation of sulfuric acid, its clusters, and oxygenated organic molecules (OOMs). A variable simulation domain in the particle size space is applied to isolate newly formed particles from preexisting ones, which allows us to focus on new particle formation and growth rather than the evolution of particles of non-NPF origin. The simulation yields a rich set of information including the time-dependent NPF rates, the cluster concentrations, the particle size distributions, and the time- and size-specific particle chemical compositions. These can be compared with the field observations to comprehensively assess the simulation–observation agreement. Sensitivity analysis with the model further quantifies how metrics of NPF events (e.g., particle survival probability) respond to model input variations and serves as a diagnostic tool to pinpoint the key parameter that leads to simulation–observation discrepancies. Seven typical NPF events in urban Beijing were analyzed. We found that with the observed gaseous precursor concentrations and coagulation sink as model inputs, the simulations roughly captured the evolution of the observed particle size distributions; however, the simulated particle growth rate was insufficient to yield the observed particle number concentrations, survival probability, and mode diameter. With the aid of sensitivity analysis, we identified under-detected OOMs as a likely cause for the discrepancy, and the agreement between the simulation and the observation was improved after we modulated particle growth rates in the simulation by adjusting the abundance of OOMs.

Several studies have investigated New Particle Formation (NPF) events from various sites ranging from pristine locations, including (boreal) forest sites to urban areas. However, there is still a dearth of studies investigating NPF processes and subsequent aerosol growth in coastal yet semi-urban sites, where the tropospheric layer is a concoction of biogenic and anthropogenic gases and particles. The investigation of factors leading to NPF becomes extremely complex due to the highly dynamic meteorological conditions at the coastline especially when combined with both continental and oceanic weather conditions. Herein, we engage a comprehensive study of particle number size distributions and aerosol-forming precursor vapors at the coastal semi-urban site in Helsinki, Finland. The measurement period, 25 June 2019–18 August 2019, was timed with the recurring cyanobacterial summer bloom in the Baltic Sea region and coastal regions of Finland. Our study recorded several regional/local NPF and aerosol burst events during this period. Although the overall anthropogenic influence on Sulfuric Acid (SA) concentrations was low during the measurement period, we observed that the regional or local NPF events, characterized by SA concentrations in the order of 107 molecules per cm−3 occurred mostly when the air mass travelled over the land areas. Interestingly, when the air mass travelled over the Baltic Sea, an area enriched with Algae and cyanobacterial blooms, high Iodic Acid (IA) concentration coincided with an aerosol burst or a spike event at the measurement site. Further, SA-rich bursts were seen when the air mass travelled over the Gulf of Bothnia, enriched with cyanobacterial blooms. The two most important factors affecting aerosol precursor vapor concentrations, and thus the aerosol formation, were (1) the type of phytoplankton species and intensity of bloom present in the coastal regions of Finland/ Baltic Sea and (2) the wind direction. During the events, most of the growth of sub-3 nm particles was probably due to SA, rather than IA or MSA, however much of the particle growth remained unexplained indicative of the strong role of organics in the growth of particles, especially in the 3–7 nm particle size range. Further studies are needed to explore the role of organics in NPF events and the potential influence of cyanobacterial blooms in coastal locations.