In-cloud Processes Research Articles

An oxidation flow reactor for simulating and accelerating secondary aerosol formation in aerosol liquid water and cloud droplets

Abstract. Liquid water in cloud droplets and aqueous aerosols serves as an important reaction medium for the formation of secondary aerosol through aqueous-phase reactions (aqSA). Large uncertainties remain in estimates of the production and chemical evolution of aqSA in the dilute solutions found in cloud droplets and the concentrated solutions found in aerosol liquid water, which is partly due to the lack of available measurement tools and techniques. A new oxidation flow reactor (OFR), the Accelerated Production and Processing of Aerosols (APPA) reactor, was developed to measure secondary aerosol formed through gas- and aqueous-phase reactions, both for laboratory gas mixtures containing one or more precursors and for ambient air. For simulating in-cloud processes, ∼ 3.3 µm diameter droplets formed on monodisperse seed particles are introduced into the top of the reactor, and the relative humidity (RH) inside it is controlled to 100 %. Similar measurements made with the RH in the reactor < 100 % provide contrasts for aerosol formation with no liquid water and with varying amounts of aerosol liquid water. The reactor was characterized through a series of experiments and used to form secondary aerosol from known concentrations of an organic precursor and from ambient air. The residence time distributions of both gases and particles are narrow relative to other OFRs and lack the tails at long residence time expected with laminar flow. Initial cloud processing experiments focused on the well-studied oxidation of dissolved SO2 by O3, with the observed growth of seed particles resulting from the added sulfuric acid agreeing well with estimates based on the relevant set of aqueous-phase reactions. The OH exposure (OHexp) for low RH, high RH, and in-cloud conditions was determined experimentally from the loss of SO2 and benzene and simulated from the KinSim chemical kinetics solver with inputs of the measured 254 nm UV intensity profile through the reactor and loss of O3 due to photolysis. The aerosol yield for toluene at high OHexp ranged from 21.4 % at low RH with dry seed particles present in the reactor to 78.1 % with cloud droplets present. Measurement of the composition of the secondary aerosol formed from ambient air using an aerosol mass spectrometer showed that the oxygen-to-carbon ratio (O : C) of the organic component increased with increasing RH (and liquid water content).

How Cloud Droplet Number Concentration Impacts Liquid Water Path and Precipitation in Marine Stratocumulus Clouds—A Satellite-Based Analysis Using Explainable Machine Learning

Aerosol–cloud–precipitation interactions (ACI) are a known major cause of uncertainties in simulations of the future climate. An improved understanding of the in-cloud processes accompanying ACI could help in advancing their implementation in global climate models. This is especially the case for marine stratocumulus clouds, which constitute the most common cloud type globally. In this work, a dataset composed of satellite observations and reanalysis data is used in explainable machine learning models to analyze the relationship between the cloud droplet number concentration (Nd), cloud liquid water path (LWP), and the fraction of precipitating clouds (PF) in five distinct marine stratocumulus regions. This framework makes use of Shapley additive explanation (SHAP) values, allowing to isolate the impact of Nd from other confounding factors, which proved to be very difficult in previous satellite-based studies. All regions display a decrease of PF and an increase in LWP with increasing Nd, despite marked inter-regional differences in the distribution of Nd. Polluted (high Nd) conditions are characterized by an increase of 12 gm−2 in LWP and a decrease of 0.13 in PF on average when compared to pristine (low Nd) conditions. The negative Nd–PF relationship is stronger in high LWP conditions, while the positive Nd–LWP relationship is amplified in precipitating clouds. These findings indicate that precipitation suppression plays an important role in MSC adjusting to aerosol-driven perturbations in Nd.

Variability in morphology of soot particles during non-cloud and in-cloud processes

The morphology of soot particle influences their optical properties and their ability to act as cloud condensation nuclei (CCN). However, the morphology of soot particles has not been thoroughly examined from non-cloud to in-cloud processes. This study analyzed the morphology and mixing structures of cloud interstitial (INT) and cloud residual (RES) soot particles collected at three mountain sites. Soot particles were classified into four categories based on their mixing structures: INT-Bare, INT-Thinly coated, INT-Thickly coated, and RES soot particles. The fractal dimensions (Df) of INT-Thinly coated (1.93) and INT-Thickly coated (1.99) are larger than those of INT-Bare (1.86) soot particles, indicating significant soot compaction due to non-cloud atmospheric aging processes. Interestingly, we found that the Df of RES soot particles (1.95) is close to that of non-cloud internally mixed soot particles in this study (1.97) and previous literature (1.96). Our results indicate that cloud processing may not further compact the soot particles. This discovery has significant implications for the accurate portrayal of soot morphology within cloud droplets in models, ultimately impacting the modeling of radiative and climate effects. Additionally, our study highlights the potential significance of coating density as a factor influencing soot compaction.

Ground-based observations of lightning-related X-ray/gamma-ray emissions in Florida: Occurrence context and new insights

Ground-based observations of energetic radiation (X-rays/gamma-rays) associated with natural lightning discharges are presented and discussed. The emphasis is placed on relating X-ray/gamma-ray emissions to specific lightning processes. X-rays/gamma-rays have been observed in the following three contexts: (1) final stages of the descending leader, (2) collision of opposite-polarity streamers at the onset of lightning attachments process, and (3) in-cloud processes giving rise to energetic radiation bursts characteristic of Terrestrial Gamma-ray Flashes (TGFs). In all three cases, the X-ray/gamma-ray production involves runaway electrons and can be materially influenced (enhanced) by the presence of previously created but decayed lightning channels. Such channels are characterized by elevated temperature (about 3,000 K vs. 300 K for ambient air), which significantly lowers the friction curve (representing the spatial rate of electron energy loss), so that its peak is an order of magnitude lower than that for cold air. As a result, in the electric field of about 4 MV/m (such and even higher fields are briefly produced near the tips of lightning leaders), ambient electrons can be accelerated over the friction-curve peak to the keV range and further to relativistic energies needed for production of X-ray/gamma-ray emissions. Significant avalanching of runaway electrons seems to be possible. For one very intense (55 kA) subsequent stroke, which was a prolific X-ray/gamma-ray producer, we estimated the spatial extent of strong (>4 MV/m) electric field region associated with the descending leader tip to be about 1.5 m, which is sufficient for multiplication of runaway electrons by a factor of 2 × 104 or so.

Below-cloud scavenging of aerosol by rain: a review of numerical modelling approaches and sensitivity simulations with mineral dust in the Met Office's Unified Model

Abstract. Theoretical models of the below-cloud scavenging (BCS) of aerosol by rain yield scavenging rates that are 1–2 orders of magnitude smaller than observations and associated empirical schemes for submicron-sized aerosol. Even when augmented with processes which may explain this disparity, such as phoresis and rear capture in the raindrop wake, the theoretical BCS rates remain an order of magnitude less than observations. Despite this disparity, both theoretical and empirical BCS schemes remain in wide use within numerical aerosol models. BCS is an important sink for atmospheric aerosol, in particular for insoluble aerosol such as mineral dust, which is less likely to be scavenged by in-cloud processes than purely soluble aerosol. In this paper, various widely used theoretical and empirical BCS models are detailed and then applied to mineral dust in climate simulations with the Met Office's Unified Model in order the gauge the sensitivity of aerosol removal to the choice of BCS scheme. We show that the simulated accumulation-mode dust lifetime ranges from 5.4 d in using an empirical BCS scheme based on observations to 43.8 d using a theoretical scheme, while the coarse-mode dust lifetime ranges from 0.9 to 4 d, which highlights the high sensitivity of dust concentrations to BCS scheme. We also show that neglecting the processes of rear capture and phoresis may overestimate submicron-sized dust burdens by 83 %, while accounting for modal widths and mode merging in modal aerosol models alongside BCS is important for accurately reproducing observed aerosol size distributions and burdens. This study provides a new parameterisation for the rear capture of aerosol by rain and is the first to explicitly incorporate the rear-capture mechanism in climate model simulations. Additionally, we answer many outstanding questions pertaining to the numerical modelling of BCS of aerosol by rain and provide a computationally inexpensive BCS algorithm that can be readily incorporated into other aerosol models.

A comprehensive study about the in-cloud processing of nitrate through coupled measurements of individual cloud residuals and cloud water

Abstract. While the formation and evolution of nitrate in airborne particles are extensively investigated, little is known about the processing of nitrate in clouds. Here we present a detailed investigation on the in-cloud formation of nitrate, based on the size-resolved mixing state of nitrate in the individual cloud residual and cloud-free particles obtained by single particle mass spectrometry, and also the mass concentrations of nitrate in the cloud water and PM2.5 at a mountain site (1690 m a.s.l. – above sea level) in southern China. The results show a significant enhancement of nitrate mass fraction and relative intensity of nitrate in the cloud water and the cloud residual particles, respectively, reflecting a critical role of in-cloud processing in the formation of nitrate. We first exclude the gas-phase scavenging of HNO3 and the facilitated activation of nitrate-containing particles as the major contribution for the enhanced nitrate, according to the size distribution of nitrate in individual particles. Based on regression analysis and theoretical calculations, we then highlight the role of N2O5 hydrolysis in the in-cloud formation of nitrate, even during the daytime, attributed to the diminished light in clouds. Nitrate is highly related (R2= ∼ 0.6) to the variations in [NOx][O3], temperature, and droplet surface area in clouds. Accounting for droplet surface area greatly enhances the predictability of the observed nitrate, compared with using [NOx][O3] and temperature. The substantial contribution of N2O5 hydrolysis to nitrate in clouds with diminished light during the daytime can be reproduced by a multiphase chemical box model. Assuming a photolysis rate at 30 % of the default setting, the overall contribution of N2O5 hydrolysis pathway to nitrate formation increases by ∼ 20 % in clouds. Given that N2O5 hydrolysis acts as a major sink of NOx in the atmosphere, further model updates would improve our understanding about the processes contributing to nitrate production in cloud and the cycling of odd nitrogen.

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.

Shipborne Observations of Chemical Characterization and Light Extinction of Aerosol Along the Yangtze River, China

The study of air quality over the Yangtze River is important for the pollution of urban agglomeration along the longest river in China. A comprehensive 15-day shipborne observation was conducted in the Yangtze River of the Jiangsu section in the summer of 2019. Through online observation and offline chemistry analysis of aerosol and gaseous pollutants, the result showed that the air pollution over the Yangtze River was more severe than that in surrounding cities. Sulfate, nitrate, and ammonium (SNA) dominated the water-soluble inorganic species and accounted for 35.0% ± 7.3% of the fine particle concentration (PM2.5) along the Yangtze River. The high concentration of sulfate in the droplet mode (0.56–1.0 μm) was due to the formation of sulfate through in-cloud processes under high sulfur dioxide (SO2) concentration by ship emission and high relative humidity along the river. The strong correlation between the measured mass absorption efficiency value by carbon analyzer and that simulated based on the assumption of core–shell suggested that the core–shell mode was the main composition form of aerosol in the Yangtze River. The scattering effect was the main part of the aerosol light extinction, and the scattering coefficient of 0.4- to 1.1-μm particles accounted for 85.0% of the total extinction coefficient. Positive matrix factorization model was applied for the source apportionment of particle size segment of main extinction contribution (0.4–2.1 μm), and the result showed that secondary nitrate, ship emission, coal combustion, fugitive dust, and biomass burning were the main sources of aerosols in the Yangtze River. After source reanalysis, the result indicated that the contribution of secondary nitrate from nitrogen oxide (NOx) by ship emission and coal combustion should not be ignored.

Catalytic role of formaldehyde in particulate matter formation

Formaldehyde (HCHO), the simplest and most abundant carbonyl in the atmosphere, contributes to particulate matter (PM) formation via two in-cloud processing pathways. First, in a catalytic pathway, HCHO reacts with hydrogen peroxide (H2O2) to form hydroxymethyl hydroperoxide (HMHP), which rapidly oxidizes dissolved sulfur dioxide (SO2,aq) to sulfate, regenerating HCHO. Second, HCHO reacts with dissolved SO2,aq to form hydroxymethanesulfonate (HMS), which upon oxidation with the hydroxyl radical (OH) forms sulfate and also reforms HCHO. Chemical transport model simulations using rate coefficients from laboratory studies of the reaction rate of HMHP with SO2,aq show that the HMHP pathways reduce the SO2 lifetime by up to a factor of 2 and contribute up to ∼18% of global sulfate. This contribution rises to >50% in isoprene-dominated regions such as the Amazon. Combined with recent results on HMS, this work demonstrates that the one-carbon molecules HMHP and HCHO contribute significantly to global PM, with HCHO playing a crucial catalytic role.

Dust and tropical PMx aerosols in Cape Verde: Sources, vertical distributions and stratified transport from North Africa

We investigated the sources and processes affecting the vertical distribution of tropical PMx aerosols (particulate matter -PM- smaller than 10, 2.5 and 1 μm, PM10, PM2.5 and PM1, respectively) in the low troposphere of Santo Antão and São Vicente islands, in Cape Verde archipelago, a region where a better understanding of aerosols is needed due to their involvement in tropical meteorology and their impact on air quality, ocean and climate. We found that local sources had a low-scale impact. From transect measurements at ground level, we found that PMx levels were predominantly low, except near to PMx sources, where distinctive PM1 / PM2.5 ratios were measured, linked to vehicle exhaust (0.96), biomass burning (0.67) and Cape Verdean dust (0.36) emissions. The depth of the marine boundary layer (MBL) and the vertical distributions of PMx showed wide variability prompted by meteorological conditions. The trade winds prevailed in the MBL, whereas other airflows were situated above it: North-Atlantic, African easterly airflow and Saharan Air Layer. Under North-Atlantic airflow conditions, the MBL extended to 1400 m above sea level (m.a.s.l.). Above this altitude, PMx concentrations decreased quickly (< 3 μg/m3) due to the free troposphere conditions. Under Saharan dust conditions, the MBL was confined to just 70 m.a.s.l., whereas a complex dust stratification was observed above, characterized by alternating dry air layers with high dust loads (PM10 ~ 100 μg/m3) and more humid air layers with lower aerosol loads (PM10 ~ 40 μg/m3). Within the dry easterly African airflow occurring above the marine stratocumulus typical of the MBL top (placed at 500 m.a.s.l.), we detected layers enriched in hydrophilic aerosols (PM10: ~ 8 μg/m3). These were imbedded in relatively humid air (RH ~48%), probably linked to secondary aerosol formation by in-cloud processes in the marine stratocumulus situated below. We found that PMx transport from North Africa, both under dust and dust-free conditions, is associated with complex vertical stratifications, even within the dusty Saharan Air Layer.

In-cloud processing as a possible source of isotopically light iron from anthropogenic aerosols: New insights from a laboratory study

Wet atmospheric deposition can account for up to 50% of the total iron input to surface waters, so establishing the extent to which kinetic and equilibrium isotope effects can influence aerosol soluble δ56Fe values is imperative to trace and constrain aerosol sources using Fe isotopes and to understand the differences found between δ56Fe values for bulk and soluble phases of aerosols. In this context, changes in iron solubility and isotopic composition of dissolved Fe during simulated atmospheric processing of industrial ash was investigated. Kinetic and equilibrium experiments were performed under UV/VIS light using ash from a Fe–Mn alloy metallurgical plant and a synthetic solution that mimics cloud water chemistry. The nature of the Fe species of the industrial ash was investigated by Mössbauer Spectroscopy, whereas ash and dissolved δ56Fe values were measured by MC-ICP-MS. Mössbauer Spectroscopy revealed that α-hematite, magnetite, and poorly crystallized manganoferrite nanoparticles are the main Fe species. In the early-stage dissolution (until 60 min) a Fe isotope fractionation (Δ56Fesolution-bulk ash) of −0.284 ± 0.103‰ was found at the minimum contact time evaluated herein (i.e., 5 min) due to kinetic isotopic effects. In the late-stage dissolution (after 60 min) a Δ56Fesolution-ash of 0.227 ± 0.091‰ was found due to equilibrium isotopic effects. The kinetic isotope effect within one ash surface monolayer was modeled with an enrichment factor (ε) of −1‰ in 56Fe/54Fe ratio. Iron fractional dissolution undergone during different atmospheric processing time scales may release Fe with contrasted isotope compositions to solution, changing the original soluble Fe isotope signature (which is linked to its source). This might be especially important when the dissolution process goes from kinetic to near-equilibrium conditions, in which higher amounts of Fe are progressively released from ash surface.

Characterization of dicarboxylic acids, oxoacids, and α-dicarbonyls in PM2.5 within the urban boundary layer in southern China: Sources and formation pathways

Low-molecular-weight dicarboxylic acids, which are important components of secondary organic aerosols, have been extensively studied in recent years. Many studies have focused on ground-level observations and literature reports on the vertical distribution of the organic aerosols within the urban boundary layer are limited. In this study, the vertical profiles of dicarboxylic acids and related organic compounds (DCRCs) in PM2.5 were investigated at altitudinal levels (ground level and 488 m above the ground level) at the Canton Tower in Guangzhou, southern China, to elucidate their primary sources and secondary formation processes. The concentrations of DCRCs at ground level were generally higher than those at 488 m. Oxalic acid (C2) was the most abundant species, followed by succinic acid (C4) and malonic acid (C3) at both heights. The higher ratio of DCRCs-bound carbon to organic carbon (i.e., DCRCs-C/OC) at 488 m (4.8 ± 1.2%) relative to that at ground level (2.7 ± 0.5%) indicated a higher degree of aerosol aging at 488 m. The abundance of C2 was increased and the conversion of C4 to C3 was enhanced due to the photochemical oxidation of its homologues during long-range transport periods. The increase in C2 was associated with in-cloud processes during pollution periods. Principal component analysis showed that DCRCs were mainly derived from atmospheric secondary processing and biomass burning was also an important source of long-chain carboxylic acids during autumn in Guangzhou. Our results illustrate that secondary processing and biomass burning play prominent roles in controlling the abundance of DCRCs. Furthermore, DCRCs are affected by air masses from regional areas, oxidation of their precursors via vertical transport and in-cloud processes.

Aging of Atmospheric Brown Carbon Aerosol

Emitted by numerous primary sources and formed by secondary sources, atmospheric brown carbon (BrC) aerosol is chemically complex. As BrC aerosol ages in the atmosphere via a variety of chemical and physical processes, its chemical composition and optical properties change significantly, altering its impacts on climate. Research in the past decade has considerably expanded our understanding of BrC reactions in both the gas and condensed phases. We review these recent advances in BrC aging chemistry with a focus on gas phase reactions leading to BrC formation, aqueous and in-cloud processes, and aerosol particle reactions. Connections are made between single component BrC proxies and more complex chemical mixtures as well as between laboratory and field measurements of BrC chemistry. General conclusions are that chemical change can darken the BrC aerosol particles over short time scales of hours close to the source and that considerable photobleaching and oxidative whitening will occur when BrC is a day or more removed from its source.

Impact of in-cloud aqueous processes on the chemical compositions and morphology of individual atmospheric aerosols

Abstract. The composition, morphology, and mixing structure of individual cloud residues (RES) and interstitial particles (INT) at a mountaintop site were investigated. Eight types of particles were identified, including sulfate-rich (S-rich), S-organic matter (OM), aged soot, aged mineral dust, aged fly ash, aged metal, refractory, and aged refractory mixture. A shift of dominant particle types from S-rich (29 %) and aged soot (27 %) in the INT to aged refractory mixture (23 %) and S-OM (22 %) in the RES is observed. In particular, particles with organic shells are enriched in the RES (27 %) relative to the INT (12 %). Our results highlight that the formation of more oxidized organic matter in the cloud contributes to the existence of organic shells after cloud processing. The fractal dimension (Df), a morphologic parameter to represent the branching degree of particles, for soot particles in the RES (1.82 ± 0.12) is lower than that in the INT (2.11 ± 0.09), which indicates that in-cloud processes may result in less compact soot. This research emphasizes the role of in-cloud processes in the chemistry and microphysical properties of individual particles. Given that organic coatings may determine the particle hygroscopicity, activation ability, and heterogeneous chemical reactivity, the increase of OM-shelled particles upon in-cloud processes should have considerable implications.

Biogenic Sources of Ice Nucleating Particles at the High Arctic Site Villum Research Station.

The radiative balance in the Arctic region is sensitive to in-cloud processes, which principally depend on atmospheric aerosols, including ice nucleating particles (INPs). High temperature INPs (active at ≥-15 °C) are common in the Arctic. While laboratory and limited in situ studies show that the high-temperature active INPs are associated with bioaerosols and biogenic compounds, there is still little quantitative insight into the Arctic biogenic INPs and bioaerosols. We measured concentrations of bioaerosols, bacteria, and biogenic INPs at the Villum Research Station (VRS, Station Nord) in a large number of snow (15) and air (51) samples. We found that INPs active at high subzero temperatures were present both in spring and summer. Air INP concentrations were higher in summer (18 INP m-3 at ≥-10 °C) than in spring (<4 INP m-3 at ≥-10 °C), when abundant INPs were found in snowfall (1.4 INP mL-1 at ≥-10 °C). Also, in summer, a significantly higher number of microbial and bacterial cells were present compared to the spring. A large proportion (60%-100%) of INPs that were active between -6 °C and -20 °C could be deactivated by heating to 100 °C, which was indicative of their predominantly proteinaceous origin. In addition, there was a significant linear regression between the summer air concentrations of INPs active at ≥-10 °C and air concentrations of bacterial-marker-genes (p < 0.0001, R2 = 0.999, n = 6), pointing at bacterial cells as the source of high-temperature active INPs. In conclusion, the majority of INPs was of proteinaceous, and possibly of bacterial, origin and was found in air during summer and in snowfall during springtime.

Characterization of oxalic acid-containing particles in summer and winter seasons in Chengdu, China

Oxalic acid is the most abundant dicarboxylic acid in the atmosphere, but the processes involved in its formation in ambient aerosols are still uncertainty. We used single particle aerosol mass spectrometer to investigate the sources and formation mechanisms of oxalic acid-containing (C2) particles in ambient aerosols in Chengdu during summer and winter seasons. C2 particles accounted for 2.9 and 0.7% of the total amount of fine particles detected in summer and winter, respectively. The C2 particles in each season were classified as nine different types of particle with significant seasonal differences in their contribution to total C2 particles. The oxalic acid-sulfate (C2-SO4) particles and particles related to biomass burning (i.e., oxalic acid-organic nitrogen (C2-CN) and oxalic acid-elemental carbon (C2-EC)) represented 78% of the total C2 particles in summer, whereas the contributions of oxalic acid-dust (C2-D), oxalic acid-sulfate and nitrate (C2-SN), and oxalic acid-nitrate (C2-NO3) particles clearly increased in winter. Although the peak diameter of the average size distribution of total C2 particles was larger in winter than in summer, there was no significant seasonal difference in the number fraction of the size-resolved particles for each type of particle. The peak concentration of total C2 particle in summer occurred in the afternoon, but there was no obvious diurnal variation in winter. We concluded that local photochemical oxidation in the daytime favored the production of C2 particles in summer, whereas in-cloud processing with subsequent long-distance transport had an important effect on C2 particles in winter. The potential source contribution function (PSCF) analysis indicated that local sources from the eastern, southeastern, and southern regions of Sichuan Province dominated the C2 particles in summer, whereas the contribution from the long-distance transport of aerosols originating outside Sichuan Province was significantly enhanced in winter. These findings will help us to understand the formation of secondary organic aerosols in the atmosphere over the Sichuan Basin.

Mass Concentrations and Size Distributions of Water-soluble Inorganic Ions in Atmospheric Aerosols in Beibei District, Chongqing

In order to study the concentration and distribution characteristics of water-soluble inorganic ions in aerosol particles of the Beibei district of Chongqing, aerosol samples were collected with an Andersen cascade impactor between March 2014 and February 2015. Water-soluble inorganic ions, including Na+, NH4+, K+, Mg2+, Ca2+, F-, Cl-, NO3-, and SO42- were determined for different particle sizes (9.00, 5.80, 4.70, 3.30, 2.10, 1.10, 0.65, and 0.43 μm) using the ion chromatography method. Results showed that SO42-, NH4+, NO3-, Cl-, Na+, and K+ were mainly distributed in fine particles, while Mg2+, Ca2+, and F- were mainly present in coarse particles. SNA (SO42-, NH4+, and NO3-) exhibited clear unimodal distribution, with peaks in the droplet mode of 0.65-1.10 μm, mainly present in the form of (NH4)2SO4 and NH4NO3 in fine particles. The formation of SO42- is mainly attributed to in-cloud processes and partly to oxidation of SO2. Na+, Cl-, and Mg2+ exhibited bimodal distribution in coarse and fine particles; K+ was a single peak distribution in the range of 0.43-1.10 μm, while peaks of F- and Ca2+ concentrations were in coarse particles. Average annual concentrations of total water-soluble ions in PM2.1 and PM9.0 were (32.68±15.28) μg·m-3and (48.01±19.66) μg·m-3 over the observation period. Seasonal variations of PM2.1 and PM9.0concentrations decreased in the order of winter > spring > summer > autumn. This was the same for most ions, but a small number of ions (F-, Mg2+ and Ca2+) had a different pattern in the spring, summer, and winter. The SNA were the major components of water-soluble ions in PM2.1, and Ca2+ was the major component of water-soluble ions in PM9.0 besides SNA. The concentration of cations was significantly higher than that of anions' in PM2.1 and PM9.0, with a certain correlation between different ions. Emissions from motor vehicle exhaust, combustion processes, soil sources, and fugitive dust were the major sources of water-soluble ions in this area. The effect of air temperature on secondary ions is significant (P<0.05), but relative humidity and wind speed have no significant effect (P>0.05).

Radical oxidation of methyl vinyl ketone and methacrolein in aqueous droplets: Characterization of organosulfates and atmospheric implications

In-cloud processing of volatile organic compounds is one of the significant routes leading to secondary organic aerosol (SOA) in the lower troposphere. In this study, we demonstrate that two atmospherically relevant α,β-unsaturated carbonyls, i.e., but-3-en-2-on (methyl vinyl ketone, MVK) and 2-methylopropenal (methacrolein, MACR), undergo sulfate radical-induced transformations in dilute aqueous systems under photochemical conditions to form organosulfates previously identified in ambient aerosols and SOA generated in smog chambers. The photooxidation was performed under sun irradiation in unbuffered aqueous solutions containing carbonyl precursors at a concentration of 0.2 mmol and peroxydisulfate as a source of sulfate radicals (SO4–) at a concentration of 0.95 mmol. UV–vis analysis of solutions showed the fast decay of unsaturated carbonyl precursors in the presence of sulfate radicals. The observation confirms the capacity of sulfate radicals to transform the organic compounds into SOA components in atmospheric waters. Detailed interpretation of high-resolution negative ion electrospray ionization tandem mass spectra allowed to assign molecular structures to multiple aqueous organosulfate products, including an abundant isoprene-derived organosulfate C4H8SO7 detected at m/z 199. The results highlight the solar aqueous-phase reactions as a potentially significant route for biogenic SOA production in clouds at locations where isoprene oxidation occurs. A recent modelling study suggests that such processes could likely contribute to 20–30 Tg year−1 production of SOA, referred to as aqSOA, which is a non-negligible addition to the still underestimated budget of atmospheric aerosol.

A model intercomparison of CCN-limited tenuous clouds in the high Arctic

Abstract. We perform a model intercomparison of summertime high Arctic (&gt; 80∘ N) clouds observed during the 2008 Arctic Summer Cloud Ocean Study (ASCOS) campaign, when observed cloud condensation nuclei (CCN) concentrations fell below 1 cm−3. Previous analyses have suggested that at these low CCN concentrations the liquid water content (LWC) and radiative properties of the clouds are determined primarily by the CCN concentrations, conditions that have previously been referred to as the tenuous cloud regime. The intercomparison includes results from three large eddy simulation models (UCLALES-SALSA, COSMO-LES, and MIMICA) and three numerical weather prediction models (COSMO-NWP, WRF, and UM-CASIM). We test the sensitivities of the model results to different treatments of cloud droplet activation, including prescribed cloud droplet number concentrations (CDNCs) and diagnostic CCN activation based on either fixed aerosol concentrations or prognostic aerosol with in-cloud processing. There remains considerable diversity even in experiments with prescribed CDNCs and prescribed ice crystal number concentrations (ICNC). The sensitivity of mixed-phase Arctic cloud properties to changes in CDNC depends on the representation of the cloud droplet size distribution within each model, which impacts autoconversion rates. Our results therefore suggest that properly estimating aerosol–cloud interactions requires an appropriate treatment of the cloud droplet size distribution within models, as well as in situ observations of hydrometeor size distributions to constrain them. The results strongly support the hypothesis that the liquid water content of these clouds is CCN limited. For the observed meteorological conditions, the cloud generally did not collapse when the CCN concentration was held constant at the relatively high CCN concentrations measured during the cloudy period, but the cloud thins or collapses as the CCN concentration is reduced. The CCN concentration at which collapse occurs varies substantially between models. Only one model predicts complete dissipation of the cloud due to glaciation, and this occurs only for the largest prescribed ICNC tested in this study. Global and regional models with either prescribed CDNCs or prescribed aerosol concentrations would not reproduce these dissipation events. Additionally, future increases in Arctic aerosol concentrations would be expected to decrease the frequency of occurrence of such cloud dissipation events, with implications for the radiative balance at the surface. Our results also show that cooling of the sea-ice surface following cloud dissipation increases atmospheric stability near the surface, further suppressing cloud formation. Therefore, this suggests that linkages between aerosol and clouds, as well as linkages between clouds, surface temperatures, and atmospheric stability need to be considered for weather and climate predictions in this region.

Lightning Characteristics and Electric Charge Structure of a Hail-Producing Thunderstorm on the Eastern Qinghai–Tibetan Plateau

An isolated hail-producing thunderstorm that occurred on 20 August 2012, from the Composite Observing Experiment for Lightning project carried out in the Da Tong Region of Qinghai Province, China, was studied. In most stages, the charge structure in the storm showed a positive tripole structure, which resulted in a low positive ground lightning rate for the whole event. In the thunderstorm process, 202 lightning flashes were detected. The peak lightning rate of lightning was 28 times/5 min. The average number of return strokes of negative ground flash was 2.4, and only one return stroke of positive ground lightning occurred. The duration of the in-cloud process prior to the first return stroke was relatively long. Dynamic transport had a considerable influence on the charge structure of the cloud. Before and after hail landing, the ground electric field of several stations showed a consistent trend of reversal.