Growth Of New Particles Research Articles

What chemical species are responsible for new particle formation and growth in the Netherlands? A hybrid positive matrix factorization (PMF) analysis using aerosol composition (ACSM) and size (SMPS)

Abstract. Aerosol formation acts as a sink for gas-phase atmospheric species that controls their atmospheric lifetime and environmental effects. To investigate aerosol formation and evolution in the Netherlands, a hybrid positive matrix factorization (PMF) analysis was conducted using observations from May, June, and September 2021 collected in the rural site of Cabauw in the central part of the Netherlands. The hybrid input matrix consists of the full organic mass spectrum acquired from a time-of-flight aerosol chemical speciation monitor (ToF-ACSM), ACSM inorganic species concentrations, and binned particle size distribution concentrations from a scanning mobility particle sizer (SMPS). These hybrid PMF analyses discerned four factors that describe aerosol composition variations: two size-driven factors that are related to new particle formation (NPF) and growth (F4 and F3), as well as two bulk factors driven by composition, not size (F2 and F1). The distribution of chemical species across these factors shows that different compounds are responsible for nucleation and growth of new particles. The smallest-diameter size factor (F4) contains ammonium sulfate and organics and typically peaks during the daytime. Newly formed particles, represented by F4, are mainly correlated with wind from the southwesterly–westerly and easterly sectors that transport sulfur oxides (SOx), ammonia (NH3), and organic precursors to Cabauw. As the particles grow from F4 to F3 and to bulk factors, nitrate and organics play an increasing role, and the particle loading diurnal cycle shifts from daytime to a nighttime maximum. Greater organics availability makes secondary organic aerosol (SOA) more influential in summertime aerosol growth, principally due to volatility differences produced by seasonal variation in photooxidation and temperature.

New particle formation in the tropical free troposphere during CAMP2Ex: statistics and impact of emission sources, convective activity, and synoptic conditions

Abstract. Nucleation in the free troposphere (FT) and subsequent growth of new particles represent a globally important source of cloud condensation nuclei (CCN). Whereas new particle formation (NPF) has been shown to occur frequently in the upper troposphere over tropical oceans, there have been few studies of NPF at lower altitudes. In addition, the impact of urban emissions and biomass burning on the NPF in tropical marine FT remains poorly understood. In this study, we examine NPF in the lower and mid-troposphere (3–8.5 km) over the tropical ocean and coastal region using airborne measurements during the recent Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex). NPF was mostly observed above 5.5 km and coincided with elevated relative humidity (RH) and reduced condensation sink (CS), suggesting that NPF occurs in convective cloud outflow. The frequency of NPF increases with altitude, reaching ∼ 50 % above 8 km. An abrupt decrease in NPF frequency coincides with early monsoon transition and is attributed to increased CS resulting from reduced convective activity and more frequent transport of aged urban plumes. Surprisingly, a large fraction of NPF events in background air were observed in the early morning, and the NPF is likely made possible by very low CS despite low actinic flux. Convectively detrained biomass-burning plumes and fresh urban emissions enhance NPF as a result of elevated precursor concentrations and scavenging of pre-existing particles. In contrast, NPF is suppressed in aged urban plumes where the reactive precursors are mostly consumed, while CS remains relatively high. This study shows a strong impact of urban and biomass-burning emissions on the NPF in tropical marine FT. The results also illustrate the competing influences of different variables and interactions among anthropogenic emissions, convective clouds, and meteorology, which lead to NPF under a variety of conditions in tropical marine environments.

Emissions of volatile organic compounds from Norway spruce and potential atmospheric impacts

Published biogenic volatile organic compound (BVOC) emission rates of Norway spruces vary a lot. In this study we combined published Norway spruce emission rates measured in boreal forests and added our new, unpublished emission data from Southern (SF) and Northern Finland (NF). Standardized summer monthly mean emission potentials of isoprene vary from below the detection limit to 7 μg g–1(dw) h–1, and monoterpene (MT) and sesquiterpene (SQT) emission potentials 0.01–3 μg g–1(dw) h–1 and 0.03–2.7 μg g–1(dw) h–1, respectively. In this study, we found much higher SQT emissions from Norway spruces than previously measured, and on average SQTs had higher emission potentials than isoprene or MTs. The highest monthly mean SQT emission potential of 13.6 μg g–1(dw) h–1 was observed in September in Southern Finland. We found that none of the younger (33–40 years) trees in Hyytiälä, Southern Finland, emitted isoprene, while one 50-year-old tree was a strong isoprene emitter. The difference due to age could not be confirmed, since all measured small trees were growing in Hyytiälä, so this could also be due to the same genetic origin. On average, older trees (>80 years) emitted about ten times more isoprene and MTs than younger ones (<80 years), but no clear difference was seen in SQT emissions. SQT emissions can be more related to stress effects. As shown here for Norway spruce, it is possible that the emission factor of SQTs is significantly higher than what is currently used in models, which may have significant effects on the prediction of formation and growth of new particles, since the secondary organic aerosol (SOA) formation potential of SQTs is high, and this may have significant effects on the formation and growth of new particles. Due to the high secondary organic aerosol (SOA) formation potentials of SQTs, the impact on SOA formation and mass could be even higher.

Collective geographical ecoregions and precursor sources driving Arctic new particle formation

Abstract. The Arctic is a rapidly changing ecosystem, with complex ice–ocean–atmosphere feedbacks. An important process is new particle formation (NPF), from gas-phase precursors, which provides a climate forcing effect. NPF has been studied comprehensively at different sites in the Arctic, ranging from those in the High Arctic and those at Svalbard to those in the continental Arctic, but no harmonised analysis has been performed on all sites simultaneously, with no calculations of key NPF parameters available for some sites. Here, we analyse the formation and growth of new particles from six long-term ground-based stations in the Arctic (Alert, Villum, Tiksi, Zeppelin Mountain, Gruvebadet, and Utqiaġvik). Our analysis of particle formation and growth rates in addition to back-trajectory analysis shows a summertime maxima in the frequency of NPF and particle formation rate at all sites, although the mean frequency and particle formation rates themselves vary greatly between sites, with the highest at Svalbard and lowest in the High Arctic. The summertime growth rate, condensational sinks, and vapour source rates show a slight bias towards the southernmost sites, with vapour source rates varying by around an order of magnitude between the northernmost and southernmost sites. Air masses back-trajectories during NPF at these northernmost sites are associated with large areas of sea ice and snow, whereas events at Svalbard are associated with more sea ice and ocean regions. Events at the southernmost sites are associated with large areas of land and sea ice. These results emphasise how understanding the geographical variation in surface type across the Arctic is key to understanding secondary aerosol sources and providing a harmonised analysis of NPF across the Arctic.

On the influence of VOCs on new particle growth in a Continental-Mediterranean region

A field campaign has been performed in the Madrid region to study the VOC influence in the growth of new particles in ambient air. A number of instruments have been deployed to characterize the main pollutant gases and particle properties and composition. The measurements were performed simultaneously at three sites (rural, urban background and urban traffic influenced) in the period 1–17 July 2019. The sites: Tres Cantos (rural), CIEMAT (urban background) and Leganés (urban traffic) were located within the Madrid airshed. Particle size distributions, mass concentrations at fractions PM10, PM2.5 and PM1, black carbon, VOCs species and gaseous pollutants (NOx and O3) were obtained in the sites. Some supplementary measurements were obtained in at least one of the sites: meteorological parameters, non-refractory submicron aerosol species and vertical profiles of aerosol optical properties. It has been observed that the new particle formation (NPF) events, nucleation and subsequent growth, happened at a regional scale, although differently among the sites. In the rural site, fewer events than expected were observed because of the high temperatures that affected the BVOC emissions. In the urban background site, the highest number of events was reached. In this station, it is common to receive air masses from the nearby forest and from the urban area, producing a mix of conditions with high BVOC and AVOC concentrations. In the urban traffic site, several NPF cases appeared, being a site dominated by AVOCs. Among the BVOCs measured in the three stations, the most common were α-Pinene and Limonene. Among the AVOCs measured, aromatics and linear hydrocarbon compounds for C10 and above were found. The linear group was found to be predominant during the NPF event days in the urban background site. This work provides new insights about the aerosol-forming precursors and growth of new particles in the Madrid region.

The effectiveness of the coagulation sink of 3–10 nm atmospheric particles

Abstract. As a major source of ultrafine particles, new particle formation (NPF) occurs frequently in various environments. However, the survival of new particles and the frequent occurrence of NPF events in polluted environments have long been perplexing, since new particles are expected to be scavenged by high coagulation sinks. Towards solving these problems, we establish an experimental method and directly measure the effectiveness of the size-dependent coagulation sink of monodisperse 3–10 nm particles in well-controlled chamber experiments. Based on the chamber experiments and long-term atmospheric measurements from Beijing, we then discuss the survival of new particles in polluted environments. In the chamber experiments, the measured coagulation sink of 3–10 nm particles increases significantly with a decreasing particle size, whereas it is not sensitive to the compositions of test particles. Comparison between the measured coagulation coefficient with theoretical predictions shows that almost every coagulation leads to the scavenging of one particle, and the coagulation sink exceeds the hard-sphere kinetic limit due to van der Waals attractive force. For urban Beijing, the effectiveness of the coagulation sink and a moderate or high (e.g., > 3 nm h−1) growth rate of new particles can explain the occurrence of measured NPF events; the moderate growth rate further implies that, in addition to gaseous sulfuric acid, other gaseous precursors also contribute to the growth of new particles.

The missing base molecules in atmospheric acid–base nucleation

Transformation of low-volatility gaseous precursors to new particles affects aerosol number concentration, cloud formation and hence the climate. The clustering of acid and base molecules is a major mechanism driving fast nucleation and initial growth of new particles in the atmosphere. However, the acid–base cluster composition, measured using state-of-the-art mass spectrometers, cannot explain the measured high formation rate of new particles. Here we present strong evidence for the existence of base molecules such as amines in the smallest atmospheric sulfuric acid clusters prior to their detection by mass spectrometers. We demonstrate that forming (H2SO4)1(amine)1 is the rate-limiting step in atmospheric H2SO4-amine nucleation and the uptake of (H2SO4)1(amine)1 is a major pathway for the initial growth of H2SO4 clusters. The proposed mechanism is very consistent with measured new particle formation in urban Beijing, in which dimethylamine is the key base for H2SO4 nucleation while other bases such as ammonia may contribute to the growth of larger clusters. Our findings further underline the fact that strong amines, even at low concentrations and when undetected in the smallest clusters, can be crucial to particle formation in the planetary boundary layer.

Insufficient Condensable Organic Vapors Lead to Slow Growth of New Particles in an Urban Environment.

Atmospheric new particle formation significantly affects global climate and air quality after newly formed particles grow above ∼50 nm. In polluted urban atmospheres with 1-3 orders of magnitude higher new particle formation rates than those in clean atmospheres, particle growth rates are comparable or even lower for reasons that were previously unclear. Here, we address the slow growth in urban Beijing with advanced measurements of the size-resolved molecular composition of nanoparticles using the thermal desorption chemical ionization mass spectrometer and the gas precursors using the nitrate CI-APi-ToF. A particle growth model combining condensational growth and particle-phase acid-base chemistry was developed to explore the growth mechanisms. The composition of 8-40 nm particles during new particle formation events in urban Beijing is dominated by organics (∼80%) and sulfate (∼13%), and the remainder is from base compounds, nitrate, and chloride. With the increase in particle sizes, the fraction of sulfate decreases, while that of the slow-desorbed organics, organic acids, and nitrate increases. The simulated size-resolved composition and growth rates are consistent with the measured results in most cases, and they both indicate that the condensational growth of organic vapors and H2SO4 is the major growth pathway and the particle-phase acid-base reactions play a minor role. In comparison to the high concentrations of gaseous sulfuric acid and amines that cause high formation rates, the concentration of condensable organic vapors is comparably lower under the high NOx levels, while those of the relatively high-volatility nitrogen-containing oxidation products are higher. The insufficient condensable organic vapors lead to slow growth, which further causes low survival of the newly formed particles in urban environments. Thus, the low growth rates, to some extent, counteract the impact of the high formation rates on air quality and global climate in urban environments.

Measurement of atmospheric nanoparticles: Bridging the gap between gas-phase molecules and larger particles

Atmospheric nanoparticles are crucial components contributing to fine particulate matter (PM2.5), and therefore have significant effects on visibility, climate, and human health. Due to the unique role of atmospheric nanoparticles during the evolution process from gas-phase molecules to larger particles, a number of sophisticated experimental techniques have been developed and employed for online monitoring and characterization of the physical and chemical properties of atmospheric nanoparticles, helping us to better understand the formation and growth of new particles. In this paper, we firstly review these state-of-the-art techniques for investigating the formation and growth of atmospheric nanoparticles (e.g., the gas-phase precursor species, molecular clusters, physicochemical properties, and chemical composition). Secondly, we present findings from recent field studies on the formation and growth of atmospheric nanoparticles, utilizing several advanced techniques. Furthermore, perspectives are proposed for technique development and improvements in measuring atmospheric nanoparticles.

Growth of New Particles into Cloud Condensation Nuclei at the Summit of Mt. Fuji during Summer

Growth of New Particles into Cloud Condensation Nuclei at the Summit of Mt. Fuji during Summer

The NPF Effect on CCN Number Concentrations: A Review and Re‐Evaluation of Observations From 35 Sites Worldwide

AbstractThe new particle formation (NPF) effect on cloud condensation nuclei (CCN) differs from region to region and remains highly uncertain. Here, we re‐evaluate the NPF‐initiated enhancements in CCN number concentrations (E_NCCN) by compiling results from 35 field sites worldwide and five global models. We show the E_NCCN at sites close to the anthropogenic sources are more than 2‐fold as large as that at remote sites with supersaturations of 0.1%–1.0%. The enhancements anti‐correlate with the time scales for the new particles to grow to CCN size, which are 5–10 and 20–30 hr at polluted and remote sites, respectively. We further reveal that under polluted environments the condensational growth of new particles may be accelerated, leading to larger E_NCCN. The models fail to simulate the E_NCCN in polluted regions. Our study highlights the importance of parameterizing the time scale of NPF to affect CCN differently for remote and polluted regions in models.

Contrasting aerosol growth potential in the northern and central-southern regions of the North China Plain: Implications for combating regional pollution

Aerosol growth potentials were investigated based on measurements of aerosol properties from three field campaigns carried out at sites in urban Beijing (BJ) and suburban Xingtai (XT) located in the North China Plain (NCP). BJ and XT were located in a northern megalopolis area and a central-southern industrial area, respectively, in the NCP. Results suggest new particle formation (NPF) events occurred more frequently, and the particle growth potential was greater in the central-southern NCP region than in the northern NCP region in summer. It is found that differences in primary emissions and meteorological conditions can cause regional and seasonal variations in aerosol growth potential in the NCP. Photochemical reactions were more important for particle growth in summer, while nocturnal aqueous chemical reactions were more important in winter. The large nocturnal particle growth rate (7.5 nm h−1) was the reason inducing the severe aerosol pollution in winter BJ. Further analyses of submicron aerosol chemical species indicate more sulfate in submicron aerosols at XT, suggesting that the growth of new particles in the central-southern NCP in summer may be mainly dominated by the secondary formation of inorganics. Diurnal variations in aerosol chemical species during the three field campaigns suggest that the downward vertical mixing of nitrate over the nocturnal boundary layer played an important role in aerosol growth in the morning in summer. Moreover, the enhanced anthropogenic emissions during the late-day rush hours can cause clear increases in nitrate and organics. Our results highlight the distinct aerosol growth potentials in the northern and central-southern areas of NCP, implying that different emission control measures are needed in two regions to further improve air quality in the future.

Technical note: The enhancement limit of coagulation scavenging of small charged particles

Abstract. We show that the limit of the enhancement of coagulation scavenging of charged particles is 2, that is, doubled compared to the neutral case. Because the particle survival probability decreases exponentially as the coagulation sink increases, everything else being equal, the doubling of the coagulation sink can amount to a dramatic drop in survival probability – squaring the survival probability, p2, where p≤1 is the survival probability in the neutral case. Thus, it is imperative to consider this counterbalancing effect when studying ion-induced new-particle formation and ion-enhanced new-particle growth in the atmosphere.

Investigating three patterns of new particles growing to the size of cloud condensation nuclei in Beijing's urban atmosphere

Abstract. The growth of newly formed particles with diameters from ∼ 10 nm to larger sizes was investigated in Beijing's urban atmosphere during 10–23 December 2011, 12–27 April 2012, and June–August 2014. In 11 out of 27 new particle formation (NPF) events during June–August, the maximum geometric median diameter (Dpgmax) of newly formed particles exceeded 75 nm, and the grown new particles may contribute to the population of cloud condensation nuclei. In contrast, no apparent growth in new particles with Dpgmax < 20 nm was observed in all of the events in December, in approximately half of the NPF events in April, and in only two events during June–August. New particles observed in the latter NPF events were too small to be activated as cloud condensation nuclei. Apparent new particle growth with Dpgmax ≤ 50 nm was observed in the remaining 18 NPF events. The 11 NPF events during June–August with Dpgmax exceeding 75 nm were analyzed in detail. The particle growth patterns can be clearly classified into three types: one-stage growth and two-stage growth-A and growth-B patterns. The one-stage growth pattern is characterized by a continuous increase in Dpg with Dpgmax ≥ 80 nm (4 out of 11 NPF events), and the two-stage growth-A and growth-B patterns are characterized by no apparent growth and shrinkage of particles, respectively, in the middle 2–4 h of the growth period (7 out of 11 NPF events). Combining the observations of gaseous pollutants and measured (or modeled) concentrations of particulate chemical species, the three growth patterns were discussed in terms of the spatial heterogeneity of NPF, formation of secondary aerosols, and evaporation of semivolatile particulates. Secondary organic species and NH4NO3 were argued to be two major contributors to the growth of new particles, but NH4NO3 likely contributed to growth only in the late afternoon and/or at nighttime.

Size-dependent influence of NOx on the growth rates of organic aerosol particles.

Atmospheric new-particle formation (NPF) affects climate by contributing to a large fraction of the cloud condensation nuclei (CCN). Highly oxygenated organic molecules (HOMs) drive the early particle growth and therefore substantially influence the survival of newly formed particles to CCN. Nitrogen oxide (NOx) is known to suppress the NPF driven by HOMs, but the underlying mechanism remains largely unclear. Here, we examine the response of particle growth to the changes of HOM formation caused by NOx. We show that NOx suppresses particle growth in general, but the suppression is rather nonuniform and size dependent, which can be quantitatively explained by the shifted HOM volatility after adding NOx. By illustrating how NOx affects the early growth of new particles, a critical step of CCN formation, our results help provide a refined assessment of the potential climatic effects caused by the diverse changes of NOx level in forest regions around the globe.

Simultaneous determination of nine atmospheric amines and six inorganic ions by non-suppressed ion chromatography using acetonitrile and 18-crown-6 as eluent additive

Atmospheric amines contribute to the nucleation and initial growth of new particles as well as secondary organic aerosol formation, influencing the radiative balance of the Earth's atmosphere. In this study, we develop an ion chromatography (IC) method for separating and quantifying the nine most abundant amines (methylaminium (MMAH+), dimethylaminium (DMAH+), trimethylaminium (TMAH+), ethylaminium (MEAH+), diethylaminium (DEAH+), propylaminium (MPAH+), butylaminium (MBAH+), ethanolaminium (MEOH+), and triethanolaminium (TEOH+)) from six common inorganic species in atmospheric aerosols. The retention times of the amines were altered by the addition of acetonitrile to the eluent because acetonitrile can reduce the adsorption of hydrophobic amines on the stationary phase. The developed method achieved the successful separation of DEAH+ and TMAH+ from inorganic cations, which often coelute with each other in established IC methods. The interference of K+ on the determination of MEAH+ was eliminated by the complexation of K+ with 18-crown-6, which prolonged the retention time of K+. Finally, 9 target amines and 6 common inorganic cations were separated, with a resolution Rs ≥ 1.2 for DEAH+ and MPAH+ and Rs > 1.5 for other species. The detection limits varied in the range of 0.34-1.48 ng for the 9 amines and 0.19-0.56 ng for the inorganic cations. The developed method was successfully applied for the determination of low molecular weight amines and inorganic cations in PM2.5 collected from an urban site in Shanghai and an isolated coast of Chongming Island. Eight amines were detected in the urban samples, in which MMAH+ and DMAH+ dominated. The average amine concentration in the urban aerosols was 76.3 ± 38.4 ng m−3, which is approximately 4-fold higher than those in the marine samples collected from the coast.

Chemical Characterization of Sub-micron Aerosols during New Particle Formation in an Urban Atmosphere

ABSTRACT While high concentrations of pre-existing particles tend to inhibit new particle formation (NPF) in the atmosphere, severely polluted megacities around the world are becoming hot spots for the latter. We measured the particle number-size distributions with a Scanning Mobility Particle Sizer (SMPS) in the urban environment of Kanpur, India, and discovered that particle bursts occurred on 82% of the observation days, indicating that new particles frequently formed from gaseous precursors despite the relatively high concentrations of pre-existing particles. During such events, Aitken-mode particles contributed more than 50% of the total particle mass. Additionally, we used a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) to assess chemical changes in the sub-micron particles during NPF events. Because the HR-ToF-AMS can not detect particles smaller than 40 nm in diameter, however, it was not possible to investigate the chemistry driving the NPF. Our results indicated that oxygenated organic aerosols (OAs) constituted almost 77%—the largest fraction—of the sub-micron particles. The m/z 57 ion (C4H9+), a tracer of hydrocarbon-like OA (HOA), displayed significantly enhanced signal intensity during all of the NPF event days. Moreover, the increased proportion of organic ions, m/z 44 (CO2+), on these days suggested the presence of less volatile, highly oxidized OAs (LV-OOAs), revealing that the growth of new particles was mainly due to the condensation of low-volatility organic species. The substantially elevated signal intensity of amines (viz., CHN+, CH4N+, C2H4N+, C3H8N+, and C5H12N+) in the sub-micron aerosols during NPF further demonstrated that these nitrogen-containing organic compounds may have played a critical role in these events. Thus, our findings emphasize the relevance of amines to secondary aerosol formation in severely polluted urban environments.

Measurement of Formation Rates of Secondary Aerosol in the Ambient Urban Atmosphere Using a Dual Smog Chamber System.

A dual smog chamber system was used to quantify the formation rates of secondary organic and inorganic aerosol in an urban environment (Pittsburgh, US). Ambient air was introduced in both chambers, and HONO photolysis was used to produce hydroxyl radicals (OH) in the perturbed chamber. The second chamber was used as a reference. The production rate of secondary organic aerosol (SOA) under typical noon-time OH concentrations ranged from 0.2 to 0.8 μg m-3 h-1. The production rate of sulfate was approximately five times less than that of the SOA. Nucleation and growth of new particles were observed in the perturbation chamber. The produced SOA had a similar composition with the preexisting oxygenated ambient OA. The reacted amounts of the measured VOCs were able to explain 5-50% of the formed SOA in the perturbed chamber. Intermediate volatility organic compounds could be responsible for the rest. The oxygen to carbon ratio (O:C) in the perturbed chamber remained approximately the same during SOA production, while an increase was observed in the control chamber. A possible explanation could be the loss of less oxidized species to the chamber walls. After 2 h, the OA increased by 70% on average and the sulfate by 40%.

First Long-Term Study of Atmospheric New Particle Formation in the Suburb of Shanghai from 2015 to 2017

In this study, long-term continuous monitoring of atmospheric new particle formation was conducted from 2015 to 2017 in the Shanghai suburbs using a scanning mobility particle sizer (SMPS). Combined with meteorological parameters, gaseous pollutants, and PM2.5 chemical composition data, the characterization of new particle formation was analyzed. The results of data analysis showed there were 172 new particle formation (NPF) days in the Shanghai suburbs, accounting for 18.3% of the total effective days (942 d). Typical new particle formation days (Event) and new particle growth-shrinkage (Shrinkage) days were 150 d and 32 d, respectively. The frequency of NPF occurrence was the highest in spring and summer, followed by autumn and winter. Compared with non-new particle formation (Non-NPF) days, Event and Shrinkage days had higher temperature and wind speed, lower humidity, less rainfall, and stronger solar radiation. The ratio of Event days was the highest when the prevailing wind was southerly, southwesterly, or westerly, and when the air masses were mainly from the vegetation cover and agricultural planting areas in the Taihu Lake Basin. The prevailing wind directions for Non-NPF and Shrinkage days were northeasterly and easterly to southeasterly. On the Event days, SO2 and O3 were higher than that on the Non-NPF days, indicating gaseous sulfuric acid and photochemical reactions were key contributors to new particle formation. Higher PM10 concentration was detected on the Event days than on the Non-NPF days, which may be attributed to the photocatalytic reaction. All the pollutant concentrations were the lowest on Shrinkage days, except that of O3. The average concentrations of inorganic components of PM2.5, such as NH4+, SO42-, and NO3- were higher on Event than on Non-NPF days in fall, whereas the opposite results were observed in other seasons. The average concentration of organic carbon on Event days was higher than that on Non-NPF days in each season. The concentrations of PM2.5 components on Shrinkage days were the lowest. However, the ratio of organic carbon on Shrinkage days was higher than that on Non-NPF days in spring, summer, and winter. The higher ratio of organic carbon on the NPF days than on the Non-NPF days suggested an important role of organic matter in the formation and growth of new particles in the suburbs of Shanghai.

A large source of cloud condensation nuclei from new particle formation in the tropics.

Cloud condensation nuclei (CCN) can affect cloud properties and therefore Earth's radiative balance1-3. New particle formation (NPF) from condensable vapours in the free troposphere has been suggested to contribute to CCN, especially in remote, pristine atmospheric regions4, but direct evidence is sparse, and the magnitude of this contribution is uncertain5-7. Here we use in situ aircraft measurements of vertical profiles of aerosol size distributions to present a global-scale survey of NPF occurrence. We observe intense NPF at high altitudes in tropical convective regions over both Pacific and Atlantic oceans. Together with the results of chemical-transport models, our findings indicate that NPF persists at all longitudes as a global-scale band in the tropical upper troposphere, covering about 40 per cent of Earth's surface. Furthermore, we find that this NPF in the tropical upper troposphere is a globally important source of CCN in the lower troposphere, where CCN can affect cloud properties. Our findings suggest that the production of CCN as new particles descend towards the surface is not adequately captured in global models, which tend to underestimate both the magnitude of tropical upper tropospheric NPF and the subsequent growth of new particles to CCN sizes.