Effect of boundary layer dynamics on atmospheric new particle formation

In the atmosphere new aerosol particles can be formed from low-volatility gases in a process called new particle formation. These particles can impact air quality and also climate through the interaction with clouds. The precursor gases are usually emitted from the surface to the boundary layer. Various mixing and transport processes take place in the boundary layer affecting whether new particle formation occurs or not and at what intensity. However, these effects are not well-understood and observations from the atmosphere are scarce. In this work we studied the relationship between boundary layer dynamics and new particle formation by conducting and analyzing airborne measurements of aerosol particles and meteorology. Our measurements were done in a boreal forest environment in Hyytiälä, Finland using an instrumented Cessna 172 aircraft. A Zeppelin airship also measured in Hyytiälä and in Po Valley, Italy. In Hyytiälä we found that sub-3 nm particles and clusters decrease in number concentration the higher up one goes inside the mixed layer. This indicates that precursor gases are emitted by the forest, while turbulent convection is transporting the emissions to higher altitudes. This could mean that new particle formation events tend to start close to the forest canopy. From the Zeppelin we observed that a new particle formation event started within the mixed layer. We found that roll vortices (a common type of organized convection) can induce long and narrow zones of new particle formation within the mixed layer. This is likely because roll vortices can effectively transport precursors from the surface to the favorable low temperature conditions at higher altitudes. We also found that new particle formation frequently takes place at an elevated altitude decoupled from the surface in the topmost part of the residual layer. The mixing of residual layer and free troposphere air appears to be a key trigger for new particle formation in this layer. In Po Valley we observed that new particle formation started close to the surface after the mixed layer began to increase in height. The particles did not form in the residual layer and then mix down, rather different precursor gases were likely present in the residual layer (sulfuric acid) and in the surface layer (e.g. ammonia) and the mixing of these two layers started nucleation within the shallow mixed layer. Our results show that boundary layer dynamics plays an important role in new particle formation and these effects should be considered in new particle formation studies. Further work is needed to quantify and parameterize these effects.Tietyt ilmakehän hivenkaasut joilla on erityisen alhainen tasapainohöyrynpaine voivat muodostaa aerosolihiukkasia prosessissa jota kutsutaan hiukkasmuodostukseksi. Aerosolihiukkaset vaikuttavat sekä ilmanlaatuun, että ilmastoon. Hiukkasmuodostukseen osallistuvien kaasujen lähteet ovat usein maanpinnalla, mutta alailmakehän rajakerroksessa esiintyvät kuljetusprosessit ja turbulenssi vaikuttavat siihen tapahtuuko hiukkasmuodostusta vai ei. Havaintoja rajakerroksen dynamiikan vaikutuksesta hiukkasmuodostukseen on kuitenkin vähän. Tässä työssä tutkimme rajakerroksen dynamiikan vaikutusta hiukkasmuodostukseen pienellä lentokoneella (Cessna 172) sekä Zeppeliinillä tehdyillä mittauksilla. Mittauksia tehtiin havumetsäympäristössä Hyytiälässä, Suomessa, sekä Po-laaksossa Italiassa, joka ilmanlaadullisesti on Hyytiälää saasteisempi ympäristö. Mittaukset osoittivat että Hyytiälässä alle 3 nm kokoisten aerosolohiukkasten pitoisuus kasvoi lähempänä maanpintaa. Tämä viittaa siihen että havumetsä on tärkeä hiukkasia muodostavien kaasujen lähde. Zeppeliinistä havaitsimme että hiukkasmuodostus alkoi lähes samanaikaisesti kaikkialla sekoittuneessa rajakerroksessa. Hyytiälässä tehdyistä mittauksista havaitsimme että rajakerroksessa esiintyvät rullapyörteet aiheuttavat pitkittäisiä alueita joissa hiukkasmuodostus on muuta ympäristöä intensiivisempää. Tämä luultavasti liittyy rullapyörteiden kykyyn kuljettaa tehokkaasti kaasuja maanpinnalta rajakerroksen yläosiin, missä alempi lämpötila aiheuttaa suotuisammat olosuhteet hiukkasmuodostukselle. Havaitsimme myös että sekoittuneen rajakerroksen yläpuolella, ns. residuallikerroksen ja vapaan ilmakehän välisellä alueella tapahtuu hiukkasmuodostusta. Po-laaksossa havaitsimme että hiukkasmuodostus alkoi lähellä maanpintaa lähes samanaikaisesti rajakerroksen kasvun alkaessa. Tämä todennäköisesti liittyy siihen, että rajakerroksen yläpuolelta sekoittunut rikkihappoa sisältävä ilma sekoittui ammoniakkia sisältävään maanpinnan lähellä olleeseen ilmaan aiheuttaen suotuisat olosuhteet hiukkasmuodostukselle. Tämä tutkimus osoittaa, että rajakerroksen dynamiikalla on tärkeä rooli ilmakehän hiukkasmuodostuksessa. Lisätutkimuksia kuitenkin tarvitaan vaikutusten parempaan kvantifiointiin sekä parametrisointiin

Measurement report : Increasing trend of atmospheric ion concentrations in the boreal forest

The concentration of atmospheric ions affects the total aerosol particle number concentrations in the atmosphere as well as atmospheric new particle formation via ion-induced nucleation, ion-ion recombination, and effects on condensational growth. In this study, we investigate the concentrations and long-term trends of atmospheric ions in a boreal forest environment using 16 years of cluster ion (0.8-2 nm) and intermediate ion (2-7 nm) measurements and characterize the most important factors that explain those trends. We found that the median concentration of cluster ions in a boreal forest was 710 cm(-3), the median concentration of 2-4 intermediate ions was 14 cm(-3), and the median concentration of 4-7 nm intermediate ions was 9 cm(-3). The concentrations of both cluster and intermediate ions have been increasing over the 16-year measurement period, with cluster ion concentrations increasing by about 1 % yr(-1) and intermediate ion concentrations increasing 1.7 %-3.9 % yr(-1). The increase in cluster ion concentrations can be best explained by the decrease in the coagulation sink caused by larger aerosol particles. Meanwhile, the dependence of intermediate ion concentrations on meteorological conditions is evident, but ionization sources and the coagulation sink do not seem to explain the increasing trend. This is likely because the dynamics of intermediate ions are more complicated, so that ionization sources and the coagulation sink alone cannot directly explain the variation. Season-specific analysis of the ion concentrations suggests that while the coagulation sink is the limiting factor for the ion concentrations in spring and summer, the dynamics are different in autumn and winter. Based on our findings, we recommend that a more comprehensive analysis is needed to determine if the increase in ambient ion concentrations, increasing temperature, and changing abundance of condensable vapors makes ion-mediated and ion-induced nucleation pathways in the boreal forest more relevant in the future.Peer reviewe

Emerging Investigator Series: COVID-19 lockdown effects on aerosol particle size distributions in northern Italy

The lockdown measures implemented to curb the COVID-19 epidemic in Italy reduced human mobility dramatically, which resulted in a marked decline in traffic intensity. In this study, we present the effect of lockdown measures on several air pollutants, particle number size distribution as well as on regional new particle formation (NPF) frequency in the Po Valley (northern Italy). The results show that during the lockdown period, concentrations of nitrogen dioxide (NO2), nitric oxide (NO), benzene (C6H6), and toluene (C7H8) decreased, while ozone (O3) concentrations mildly increased as compared to the corresponding period in 2016–2019. Unlike gaseous pollutants, particulate matter mass concentrations (PM2.5 and PM10) showed no significant changes. The impact of lockdown measures on particle number size distributions were also quite limited. During the lockdown period, the number concentrations of 10–25 and 25–50 nm primary particles were reduced by 66% and 34%, respectively, at the regional background site (Ispra) but surprisingly there was no difference during and after lockdown at the urban background site (Modena). Conversely, the NPF frequency was exceptionally high, 70%, in Modena during the lockdown as compared to values (22–26%) observed for the same period in 2006 and 2009, while NPF frequency in Ispra only slightly increased compared to the same period in 2016–2019. The particle growth rates, however, were slightly lower during the lockdown at both sites compared to other periods. The study shows that a drastic decrease in traffic had little influence on particulate pollution levels in the Po Valley, suggesting that other sources and processes also have a prominent impact on particle number and particulate matter mass concentration in this region.Peer reviewe

Investigation of new particle formation mechanisms and aerosol processes at Marambio Station, Antarctic Peninsula

Understanding chemical processes leading to the formation of atmospheric aerosol particles is crucial to improve our capabilities in predicting the future climate. However, those mechanisms are still inadequately characterized, especially in polar regions. In this study, we report observations of neutral and charged aerosol precursor molecules and chemical cluster composition (qualitatively and quantitatively), as well as air ions and aerosol particle number concentrations and size distributions from the Marambio research station (64 degrees 15' S, 56 degrees 38' W), located north of the Antarctic Peninsula. We conducted measurements during the austral summer, between 15 January and 25 February 2018. The scope of this study is to characterize new particle formation (NPF) event parameters and connect our observations of gas-phase compounds with the formation of secondary aerosols to resolve the nucleation mechanisms at the molecular scale. NPF occurred on 40% of measurement days. All NPF events were observed during days with high solar radiation, mostly with above-freezing temperatures and with low relative humidity. The averaged formation rate for 3 nm particles (J(3)) was 0.686 cm(-3) s(-1), and the average particle growth rate (GR(3.8-12 nm)) was 4.2 nm h(-1). Analysis of neutral aerosol precursor molecules showed measurable concentrations of iodic acid (IA), sulfuric acid (SA), and methane sulfonic acid (MSA) throughout the entire measurement period with significant increase in MSA and SA concentrations during NPF events. We highlight SA as a key contributor to NPF processes, while IA and MSA likely only contribute to particle growth. Mechanistically, anion clusters containing ammonia and/or dimethylamine (DMA) and SA were identified, suggesting significant concentration of ammonia and DMA as well. Those species are likely contributing to NPF events since SA alone is not sufficient to explain observed nucleation rates. Here, we provide evidence of the marine origin of the measured chemical precursors and discuss their potential contribution to the aerosol phase.Peer reviewe

Terpene emissions from boreal wetlands can initiate stronger atmospheric new particle formation than boreal forests

Aerosols and their interaction with clouds constitute the largest uncertainty in estimating the radiative forcing affecting the climate system. Secondary aerosol formation is responsible for a large fraction of the cloud condensation nuclei in the global atmosphere. Wetlands are important to the budgets of methane and carbon dioxide, but the potential role of wetlands in aerosol formation has not been investigated. Here we use direct atmospheric sampling at the Siikaneva wetland in Finland to investigate the emission of methane and volatile organic compounds, and subsequently formed atmospheric clusters and aerosols. We find that terpenes initiate stronger atmospheric new particle formation than is typically observed over boreal forests and that, in addition to large emissions of methane which cause a warming effect, wetlands also have a cooling effect through emissions of these terpenes. We suggest that new wetlands produced by melting permafrost need to be taken into consideration as sources of secondary aerosol particles when estimating the role of increasing wetland extent in future climate change. Boreal wetlands emit terpenes which initiate atmospheric new particle formation to an even greater degree than is usually seen over boreal forests, according to direct measurements of volatile organic compounds from a Finnish wetland.Peer reviewe

Aerosol particle formation in the upper residual layer

According to current estimates, atmospheric new particle formation (NPF) produces a large fraction of aerosol particles and cloud condensation nuclei in the Earth's atmosphere, which have implications for health and climate. Despite recent advances, atmospheric NPF is still insufficiently understood in the lower troposphere, especially above the mixed layer (ML). This paper presents new results from colocated airborne and ground-based measurements in a boreal forest environment, showing that many NPF events (similar to 42 %) appear to start in the topmost part of the residual layer (RL). The freshly formed particles may be entrained into the growing mixed layer (ML) where they continue to grow in size, similar to the aerosol particles formed within the ML. The results suggest that in the boreal forest environment, NPF in the upper RL has an important contribution to the aerosol load in the boundary layer (BL).Peer reviewe

An evaluation of new particle formation events in Helsinki during a Baltic Sea cyanobacterial summer bloom

Several studies have investigated new particle formation (NPF) events from various sites ranging from pristine locations, including 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 in 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-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 on the order of 10(7) molec. cm(-3), occurred mostly when the air mass traveled over the land areas. Interestingly, when the air mass traveled 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 traveled 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 speculated to be (1) the type of phytoplankton species and intensity of bloom present in the coastal regions of Finland and the 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 methane sulfonic acid (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.Peer reviewe

Wintertime subarctic new particle formation from Kola Peninsula sulfur emissions

The metallurgical industry in the Kola Peninsula, north-west Russia, form, after Norilsk, Siberia, the second largest source of air pollution in the Arctic and subarctic domain. Sulfur dioxide (SO2/emissions from the ore smelters are transported to wide areas, including Finnish Lapland. We performed investigations on concentrations of SO2, aerosol precursor vapours, aerosol and ion cluster size distributions together with chemical composition measurements of freshly formed clusters at the SMEAR I station in Finnish Lapland relatively close (similar to 300 km) to the Kola Peninsula industrial sites during the winter 2019-2020. We show that highly concentrated SO2 from smelter emissions is converted to sulfuric acid (H2SO4/in sufficient concentrations to drive new particle formation hundreds of kilometres downwind from the emission sources, even at very low solar radiation intensities. Observed new particle formation is primarily initiated by H2SO4-ammonia (negative-)ion-induced nucleation. Particle growth to cloud condensation nuclei (CCN) sizes was concluded to result from sulfuric acid condensation. However, air mass advection had a large role in modifying aerosol size distributions, and other growth mechanisms and condensation of other compounds cannot be fully excluded. Our results demonstrate the dominance of SO2 emissions in controlling wintertime aerosol and CCN concentrations in the subarctic region with a heavily polluting industry.Peer reviewe

Diurnal evolution of negative atmospheric ions above the boreal forest : from ground level to the free troposphere

At SMEAR II research station in Hyytiala, located in the Finnish boreal forest, the process of new particle formation and the role of ions has been investigated for almost 20 years near the ground and at canopy level. However, above SMEAR II, the vertical distribution and diurnal variation of these different atmospheric ions are poorly characterized. In this study, we assess the atmospheric ion composition in the stable boundary layer, residual layer, mixing layer, and free troposphere, and the evolution of these atmospheric ions due to photochemistry and turbulent mixing through the day. To measure the vertical profile of atmospheric ions, we developed a tailored set-up for online mass spectrometric measurements, capable of being deployed in a Cessna 172 with minimal modifications. Simultaneously, instruments dedicated to aerosol properties made measurements in a second Cessna. We conducted a total of 16 measurement flights in May 2017, during the spring, which is the most active new particle formation season. A flight day typically consisted of three distinct flights through the day (dawn, morning, and afternoon) to observe the diurnal variation and at different altitudes (from 100 to 3200 m above ground), to capture the boundary layer development from the stable boundary layer, residual layer to mixing layer, and the free troposphere. Our observations showed that the ion composition is distinctly different in each layer and depends on the air mass origin and time of the day. Before sunrise, the layers are separated from each other and have their own ion chemistry. We observed that the ions present within the stable layer are of the same composition as the ions measured at the canopy level. During daytime when the mixing layer evolved and the compounds are vertically mixed, we observed that highly oxidized organic molecules are distributed to the top of the boundary layer. The ion composition in the residual layer varies with each day, showing similarities with either the stable boundary layer or the free troposphere. Finally, within the free troposphere, we detected a variety of carboxylic acids and ions that are likely containing halogens, originating from the Arctic Sea.Peer reviewe

New particle formation and its effect on cloud condensation nuclei abundance in the summer Arctic : a case study in the Fram Strait and Barents Sea

In a warming Arctic the increased occurrence of new particle formation (NPF) is believed to originate from the declining ice coverage during summertime. Understanding the physico-chemical properties of newly formed particles, as well as mechanisms that control both particle formation and growth in this pristine environment, is important for interpreting aerosol-cloud interactions, to which the Arctic climate can be highly sensitive. In this investigation, we present the analysis of NPF and growth in the high summer Arctic. The measurements were made on-board research vessel Polarstern during the PS106 Arctic expedition. Four distinctive NPF and subsequent particle growth events were observed, during which particle (diameter in a range 10-50 nm) number concentrations increased from background values of approx. 40 up to 4000 cm(-3). Based on particle formation and growth rates, as well as hygroscopicity of nucleation and the Aitken mode particles, we distinguished two different types of NPF events. First, some NPF events were favored by negative ions, resulting in more-hygroscopic nucleation mode particles and suggesting sulfuric acid as a precursor gas. Second, other NPF events resulted in less-hygroscopic particles, indicating the influence of organic vapors on particle formation and growth. To test the climatic relevance of NPF and its influence on the cloud condensation nuclei (CCN) budget in the Arctic, we applied a zero-dimensional, adiabatic cloud parcel model. At an updraft velocity of 0.1 m s(-1), the particle number size distribution (PNSD) generated during nucleation processes resulted in an increase in the CCN number concentration by a factor of 2 to 5 compared to the background CCN concentrations. This result was confirmed by the directly measured CCN number concentrations. Although particles did not grow beyond 50 nm in diameter and the activated fraction of 15-50 nm particles was on average below 10 %, it could be shown that the sheer number of particles produced by the nucleation process is enough to significantly influence the background CCN number concentration. This implies that NPF can be an important source of CCN in the Arctic. However, more studies should be conducted in the future to understand mechanisms of NPF, sources of precursor gases and condensable vapors, as well as the role of the aged nucleation mode particles in Arctic cloud formation.Peer reviewe