Assessing the importance of nitric acid and ammonia for particle growth in the polluted boundary layer.

Aerosols formed and grown by gas-to-particle processes are a major contributor to smog and haze in megacities, despite the competition between growth and loss rates. Rapid growth rates from ammonium nitrate formation have the potential to sustain particle number in typical urban polluted conditions. This process requires supersaturation of gas-phase ammonia and nitric acid with respect to ammonium nitrate saturation ratios. Urban environments are inhomogeneous. In the troposphere, vertical mixing is fast, and aerosols may experience rapidly changing temperatures. In areas close to sources of pollution, gas-phase concentrations can also be highly variable. In this work we present results from nucleation experiments at -10 °C and 5 °C in the CLOUD chamber at CERN. We verify, using a kinetic model, how long supersaturation is likely to be sustained under urban conditions with temperature and concentration inhomogeneities, and the impact it may have on the particle size distribution. We show that rapid and strong temperature changes of 1 °C min-1 are needed to cause rapid growth of nanoparticles through ammonium nitrate formation. Furthermore, inhomogeneous emissions of ammonia in cities may also cause rapid growth of particles.

Nitrate Radicals Suppress Biogenic New Particle Formation from Monoterpene Oxidation.

Highly oxygenated organic molecules (HOMs) are a major source of new particles that affect the Earth's climate. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both the day and night and can lead to new particle formation (NPF). However, NPF involving organic vapors has been reported much more often during the daytime than during nighttime. Here, we show that the nitrate radicals (NO3), which arise predominantly at night, inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN (European Organization for Nuclear Research), radical chemistry experiments using an oxidation flow reactor, and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound peroxy radical (RO2) dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub-parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments.

Iodine oxoacids enhance nucleation of sulfuric acid particles in the atmosphere.

The main nucleating vapor in the atmosphere is thought to be sulfuric acid (H2SO4), stabilized by ammonia (NH3). However, in marine and polar regions, NH3 is generally low, and H2SO4 is frequently found together with iodine oxoacids [HIOx, i.e., iodic acid (HIO3) and iodous acid (HIO2)]. In experiments performed with the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we investigated the interplay of H2SO4 and HIOx during atmospheric particle nucleation. We found that HIOx greatly enhances H2SO4(-NH3) nucleation through two different interactions. First, HIO3 strongly binds with H2SO4 in charged clusters so they drive particle nucleation synergistically. Second, HIO2 substitutes for NH3, forming strongly bound H2SO4-HIO2 acid-base pairs in molecular clusters. Global observations imply that HIOx is enhancing H2SO4(-NH3) nucleation rates 10- to 10,000-fold in marine and polar regions.

The synergistic role of sulfuric acid, ammonia and organics in particle formation over an agricultural land.

Agriculture provides people with food, but poses environmental challenges. Via comprehensive observations on an agricultural land at Qvidja in Southern Finland, we were able to show that soil-emitted compounds (mainly ammonia and amines), together with available sulfuric acid, form new aerosol particles which then grow to climate-relevant sizes by the condensation of extremely low volatile organic compounds originating from a side production of photosynthesis (compounds emitted by ground and surrounding vegetation). We found that intensive local clustering events, with particle formation rates at 3 nm about 5-10 times higher than typical rates in boreal forest environments, occur on around 30% of all days. The requirements for these clustering events to occur were found to be clear sky, a low wind speed to accumulate the emissions from local agricultural land, particularly ammonia, the presence of low volatile organic compounds, and sufficient gaseous sulfuric acid. The local clustering will then contribute to regional new particle formation. Since the agricultural land is much more effective per surface area than the boreal forest in producing aerosol particles, these findings provide insight into the participation of agricultural lands in climatic cooling, counteracting the climatic warming effects of farming.

Measurements of aerosol microphysical and chemical properties in the central Arctic atmosphere during MOSAiC.

The Arctic environment is transforming rapidly due to climate change. Aerosols' abundance and physicochemical characteristics play a crucial, yet uncertain, role in these changes due to their influence on the surface energy budget through direct interaction with solar radiation and indirectly via cloud formation. Importantly, Arctic aerosol properties are also changing in response to climate change. Despite their importance, year-round measurements of their characteristics are sparse in the Arctic and often confined to lower latitudes at Arctic land-based stations and/or short high-latitude summertime campaigns. Here, we present unique aerosol microphysics and chemical composition datasets collected during the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, in the central Arctic. These datasets, which include aerosol particle number concentrations, size distributions, cloud condensation nuclei concentrations, fluorescent aerosol concentrations and properties, and aerosol bulk chemical composition (black carbon, sulfate, nitrate, ammonium, chloride, and organics) will serve to improve our understanding of high-Arctic aerosol processes, with relevance towards improved modelling of the future Arctic (and global) climate.

Role of sesquiterpenes in biogenic new particle formation.

Biogenic vapors form new particles in the atmosphere, affecting global climate. The contributions of monoterpenes and isoprene to new particle formation (NPF) have been extensively studied. However, sesquiterpenes have received little attention despite a potentially important role due to their high molecular weight. Via chamber experiments performed under atmospheric conditions, we report biogenic NPF resulting from the oxidation of pure mixtures of ß-caryophyllene, α-pinene, and isoprene, which produces oxygenated compounds over a wide range of volatilities. We find that a class of vapors termed ultralow-volatility organic compounds (ULVOCs) are highly efficient nucleators and quantitatively determine NPF efficiency. When compared with a mixture of isoprene and monoterpene alone, adding only 2% sesquiterpene increases the ULVOC yield and doubles the formation rate. Thus, sesquiterpene emissions need to be included in assessments of global aerosol concentrations in pristine climates where biogenic NPF is expected to be a major source of cloud condensation nuclei.

NO at low concentration can enhance the formation of highly oxygenated biogenic molecules in the atmosphere.

The interaction between nitrogen monoxide (NO) and organic peroxy radicals (RO2) greatly impacts the formation of highly oxygenated organic molecules (HOM), the key precursors of secondary organic aerosols. It has been thought that HOM production can be significantly suppressed by NO even at low concentrations. Here, we perform dedicated experiments focusing on HOM formation from monoterpenes at low NO concentrations (0 - 82 pptv). We demonstrate that such low NO can enhance HOM production by modulating the RO2 loss and favoring the formation of alkoxy radicals that can continue to autoxidize through isomerization. These insights suggest that HOM yields from typical boreal forest emissions can vary between 2.5%-6.5%, and HOM formation will not be completely inhibited even at high NO concentrations. Our findings challenge the notion that NO monotonically reduces HOM yields by extending the knowledge of RO2-NO interactions to the low-NO regime. This represents a major advance towards an accurate assessment of HOM budgets, especially in low-NO environments, which prevails in the pre-industrial atmosphere, pristine areas, and the upper boundary layer.

Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity.

The mechanistic pathway by which high relative humidity (RH) affects gas-particle partitioning remains poorly understood, although many studies report increased secondary organic aerosol (SOA) yields at high RH. Here, we use real-time, molecular measurements of both the gas and particle phase to provide a mechanistic understanding of the effect of RH on the partitioning of biogenic oxidized organic molecules (from α-pinene and isoprene) at low temperatures (243 and 263 K) at the CLOUD chamber at CERN. We observe increases in SOA mass of 45 and 85% with increasing RH from 10-20 to 60-80% at 243 and 263 K, respectively, and attribute it to the increased partitioning of semi-volatile compounds. At 263 K, we measure an increase of a factor 2-4 in the concentration of C10H16O2-3, while the particle-phase concentrations of low-volatility species, such as C10H16O6-8, remain almost constant. This results in a substantial shift in the chemical composition and volatility distribution toward less oxygenated and more volatile species at higher RH (e.g., at 263 K, O/C ratio = 0.55 and 0.40, at RH = 10 and 80%, respectively). By modeling particle growth using an aerosol growth model, which accounts for kinetic limitations, we can explain the enhancement in the semi-volatile fraction through the complementary effect of decreased compound activity and increased bulk-phase diffusivity. Our results highlight the importance of particle water content as a diluting agent and a plasticizer for organic aerosol growth.

The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source.

Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O3 surface concentrations. Although iodic acid (HIO3) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO3 via reactions (R1) IOIO + O3 → IOIO4 and (R2) IOIO4 + H2O → HIO3 + HOI + (1)O2. The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO3 in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.

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.

A central arctic extreme aerosol event triggered by a warm air-mass intrusion.

Frequency and intensity of warm and moist air-mass intrusions into the Arctic have increased over the past decades and have been related to sea ice melt. During our year-long expedition in the remote central Arctic Ocean, a record-breaking increase in temperature, moisture and downwelling-longwave radiation was observed in mid-April 2020, during an air-mass intrusion carrying air pollutants from northern Eurasia. The two-day intrusion, caused drastic changes in the aerosol size distribution, chemical composition and particle hygroscopicity. Here we show how the intrusion transformed the Arctic from a remote low-particle environment to an area comparable to a central-European urban setting. Additionally, the intrusion resulted in an explosive increase in cloud condensation nuclei, which can have direct effects on Arctic clouds' radiation, their precipitation patterns, and their lifetime. Thus, unless prompt actions to significantly reduce emissions in the source regions are taken, such intrusion events are expected to continue to affect the Arctic climate.

High Gas-Phase Methanesulfonic Acid Production in the OH-Initiated Oxidation of Dimethyl Sulfide at Low Temperatures.

Dimethyl sulfide (DMS) influences climate via cloud condensation nuclei (CCN) formation resulting from its oxidation products (mainly methanesulfonic acid, MSA, and sulfuric acid, H2SO4). Despite their importance, accurate prediction of MSA and H2SO4 from DMS oxidation remains challenging. With comprehensive experiments carried out in the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN, we show that decreasing the temperature from +25 to -10 °C enhances the gas-phase MSA production by an order of magnitude from OH-initiated DMS oxidation, while H2SO4 production is modestly affected. This leads to a gas-phase H2SO4-to-MSA ratio (H2SO4/MSA) smaller than one at low temperatures, consistent with field observations in polar regions. With an updated DMS oxidation mechanism, we find that methanesulfinic acid, CH3S(O)OH, MSIA, forms large amounts of MSA. Overall, our results reveal that MSA yields are a factor of 2-10 higher than those predicted by the widely used Master Chemical Mechanism (MCMv3.3.1), and the NOx effect is less significant than that of temperature. Our updated mechanism explains the high MSA production rates observed in field observations, especially at low temperatures, thus, substantiating the greater importance of MSA in the natural sulfur cycle and natural CCN formation. Our mechanism will improve the interpretation of present-day and historical gas-phase H2SO4/MSA measurements.

Using Novel Molecular-Level Chemical Composition Observations of High Arctic Organic Aerosol for Predictions of Cloud Condensation Nuclei.

Predictions of cloud droplet activation in the late summertime (September) central Arctic Ocean are made using κ-Köhler theory with novel observations of the aerosol chemical composition from a high-resolution time-of-flight chemical ionization mass spectrometer with a filter inlet for gases and aerosols (FIGAERO-CIMS) and an aerosol mass spectrometer (AMS), deployed during the Arctic Ocean 2018 expedition onboard the Swedish icebreaker Oden. We find that the hygroscopicity parameter κ of the total aerosol is 0.39 ± 0.19 (mean ± std). The predicted activation diameter of ∼25 to 130 nm particles is overestimated by 5%, leading to an underestimation of the cloud condensation nuclei (CCN) number concentration by 4-8%. From this, we conclude that the aerosol in the High Arctic late summer is acidic and therefore highly cloud active, with a substantial CCN contribution from Aitken mode particles. Variability in the predicted activation diameter is addressed mainly as a result of uncertainties in the aerosol size distribution measurements. The organic κ was on average 0.13, close to the commonly assumed κ of 0.1, and therefore did not significantly influence the predictions. These conclusions are supported by laboratory experiments of the activation potential of seven organic compounds selected as representative of the measured aerosol.

Deposition potential of 0.003-10 µm ambient particles in the humidified human respiratory tract: Contribution of new particle formation events in Beijing.

Ultrafine particles (UFPs) usually explosive growth during new particle formation (NPF) events. However, the risk of exposure to UFPs on NPF days has been ignored due to the prevalence of mass-based air quality standards. In this study, the daily deposited doses, i.e., the daily deposited particle number dose (DPNd), mass dose (DPMd), and surface area dose (DPSd), of ambient particles in the human respiratory tract in Beijing were evaluated based on the particle number size distribution (3 nm-10 µm) from June 2018 to May 2019 utilizing a Multiple-Path Particle Dosimetry Model (MPPD) after the hygroscopic growth of particles in the respiratory tract had been accounted for. Our observations showed a high frequency (72.6%) of NPF on excellent air quality days, with daily mean PM2.5 concentrations less than 35 µg m-3. The daily DPNd on excellent air quality days was comparable with that on polluted days, although the DPMd on excellent air quality days was as low as 15.6% of that on polluted days. The DPNd on NPF days was ~1.3 times that on non-NPF days. The DPNd in respiratory tract regions decreased in the order: tracheobronchial (TB) > pulmonary (PUL) > extrathoracic (ET) on NPF days, while it was PUL > TB > ET on non-NPF days. The number of deposited nucleation mode particles, which were deposited mainly in the TB region (45%), was 2 times higher on NPF days than that on non-NPF days. Our results demonstrated that the deposition potential due to UFPs in terms of particle number concentrations is high in Beijing regardless of the aerosol mass concentration. More toxicological studies related to UFPs on NPF days, especially those targeting tracheobronchial and pulmonary impairment, are required in the future.

Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition.

Detailed knowledge of the physical and chemical properties and sources of particles that form clouds is especially important in pristine areas like the Arctic, where particle concentrations are often low and observations are sparse. Here, we present in situ cloud and aerosol measurements from the central Arctic Ocean in August-September 2018 combined with air parcel source analysis. We provide direct experimental evidence that Aitken mode particles (particles with diameters ≲70 nm) significantly contribute to cloud condensation nuclei (CCN) or cloud droplet residuals, especially after the freeze-up of the sea ice in the transition toward fall. These Aitken mode particles were associated with air that spent more time over the pack ice, while size distributions dominated by accumulation mode particles (particles with diameters ≳70 nm) showed a stronger contribution of oceanic air and slightly different source regions. This was accompanied by changes in the average chemical composition of the accumulation mode aerosol with an increased relative contribution of organic material toward fall. Addition of aerosol mass due to aqueous-phase chemistry during in-cloud processing was probably small over the pack ice given the fact that we observed very similar particle size distributions in both the whole-air and cloud droplet residual data. These aerosol-cloud interaction observations provide valuable insight into the origin and physical and chemical properties of CCN over the pristine central Arctic Ocean.

Survival of newly formed particles in haze conditions.

Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s-1. We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO3) and ammonia (NH3), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO3 and NH3 can be expected to contribute to particle survival during haze.

What controls the observed size-dependency of the growth rates of sub-10 nm atmospheric particles?

The formation and growth of atmospheric particles involving sulfuric acid and organic vapors is estimated to have significant climate effects. To accurately represent this process in large-scale models, the correct interpretation of the observations on particle growth, especially below 10 nm, is essential. Here, we disentangle the factors governing the growth of sub-10 nm particles in the presence of sulfuric acid and organic vapors, using molecular-resolution cluster population simulations and chamber experiments. We find that observed particle growth rates are determined by the combined effects of (1) the concentrations and evaporation rates of the condensing vapors, (2) particle population dynamics, and (3) stochastic fluctuations, characteristic to initial nucleation. This leads to a different size-dependency of growth rate in the presence of sulfuric acid and/or organic vapors at different concentrations. Specifically, the activation type behavior, resulting in growth rate increasing with the particle size, is observed only at certain vapor concentrations. In our model simulations, cluster-cluster collisions enhance growth rate at high vapor concentrations and their importance is dictated by the cluster evaporation rates, which demonstrates the need for accurate evaporation rate data. Finally, we show that at sizes below ∼2.5-3.5 nm, stochastic effects can importantly contribute to particle population growth. Overall, our results suggest that interpreting particle growth observations with approaches neglecting population dynamics and stochastics, such as with single particle growth models, can lead to the wrong conclusions on the properties of condensing vapors and particle growth mechanisms.

The contribution of new particle formation and subsequent growth to haze formation.

We investigated the contribution of atmospheric new particle formation (NPF) and subsequent growth of the newly formed particles, characterized by high concentrations of fine particulate matter (PM2.5). In addition to having adverse effects on visibility and human health, these haze particles may act as cloud condensation nuclei, having potentially large influences on clouds and precipitation. Using atmospheric observations performed in 2019 in Beijing, a polluted megacity in China, we showed that the variability of growth rates (GR) of particles originating from NPF depend only weakly on low-volatile vapor - highly oxidated organic molecules (HOMs) and sulphuric acid - concentrations and have no apparent connection with the strength of NPF or the level of background pollution. We then constrained aerosol dynamic model simulations with these observations. We showed that under conditions typical for the Beijing atmosphere, NPF is capable of contributing with more than 100 µg m-3 to the PM2.5 mass concentration and simultaneously >103 cm-3 to the haze particle (diameter > 100 nm) number concentration. Our simulations reveal that the PM2.5 mass concentration originating from NPF, strength of NPF, particle growth rate and pre-existing background particle population are all connected with each other. Concerning the PM pollution control, our results indicate that reducing primary particle emissions might not result in an effective enough decrease in total PM2.5 mass concentrations until a reduction in emissions of precursor compounds for NPF and subsequent particle growth is imposed.

Influence of Aerosol Chemical Composition on Condensation Sink Efficiency and New Particle Formation in 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.

Synergistic HNO3-H2SO4-NH3 upper tropospheric particle formation.

New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN)1-4. However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region5,6. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles-comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO3-H2SO4-NH3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere.