A. Bougiatioti

Effects of anthropogenic emissions on aerosol formation from isoprene and monoterpenes in the southeastern United States

Significance Atmospheric secondary organic aerosol has substantial impacts on climate, air quality, and human health. However, the formation mechanisms of secondary organic aerosol remain uncertain, especially on how anthropogenic pollutants (from human activities) control aerosol formation from biogenic volatile organic compounds (emitted by vegetation) and the magnitude of anthropogenic influences. Although possible mechanisms have been proposed based on laboratories studies, a coherent understanding of anthropogenic−biogenic interactions in ambient environments has not emerged. Here, we provide direct observational evidence that secondary organic aerosol formed from biogenic isoprene and monoterpenes is greatly mediated by anthropogenic SO2 and NOx emissions based on integrated ambient measurements and laboratory studies. Secondary organic aerosol (SOA) constitutes a substantial fraction of fine particulate matter and has important impacts on climate and human health. The extent to which human activities alter SOA formation from biogenic emissions in the atmosphere is largely undetermined. Here, we present direct observational evidence on the magnitude of anthropogenic influence on biogenic SOA formation based on comprehensive ambient measurements in the southeastern United States (US). Multiple high-time-resolution mass spectrometry organic aerosol measurements were made during different seasons at various locations, including urban and rural sites in the greater Atlanta area and Centreville in rural Alabama. Our results provide a quantitative understanding of the roles of anthropogenic SO2 and NOx in ambient SOA formation. We show that isoprene-derived SOA is directly mediated by the abundance of sulfate, instead of the particle water content and/or particle acidity as suggested by prior laboratory studies. Anthropogenic NOx is shown to enhance nighttime SOA formation via nitrate radical oxidation of monoterpenes, resulting in the formation of condensable organic nitrates. Together, anthropogenic sulfate and NOx can mediate 43–70% of total measured organic aerosol (29–49% of submicron particulate matter, PM1) in the southeastern US during summer. These measurements imply that future reduction in SO2 and NOx emissions can considerably reduce the SOA burden in the southeastern US. Updating current modeling frameworks with these observational constraints will also lead to more accurate treatment of aerosol formation for regions with substantial anthropogenic−biogenic interactions and consequently improve air quality and climate simulations.

Instrumentation and Measurement Strategy for the NOAA SENEX Aircraft Campaign as Part of the Southeast Atmosphere Study 2013

Natural emissions of ozone-and-aerosol-precursor gases such as isoprene and monoterpenes are high in the southeast of the US. In addition, anthropogenic emissions are significant in the Southeast US and summertime photochemistry is rapid. The NOAA-led SENEX (Southeast Nexus) aircraft campaign was one of the major components of the Southeast Atmosphere Study (SAS) and was focused on studying the interactions between biogenic and anthropogenic emissions to form secondary pollutants. During SENEX, the NOAA WP-3D aircraft conducted 20 research flights between 27 May and 10 July 2013 based out of Smyrna, TN. Here we describe the experimental approach, the science goals and early results of the NOAA SENEX campaign. The aircraft, its capabilities and standard measurements are described. The instrument payload is summarized including detection limits, accuracy, precision and time resolutions for all gas-and-aerosol phase instruments. The inter-comparisons of compounds measured with multiple instruments on the NOAA WP-3D are presented and were all within the stated uncertainties, except two of the three NO2 measurements. The SENEX flights included day- and nighttime flights in the Southeast as well as flights over areas with intense shale gas extraction (Marcellus, Fayetteville and Haynesville shale). We present one example flight on 16 June 2013, which was a daytime flight over the Atlanta region, where several crosswind transects of plumes from the city and nearby point sources, such as power plants, paper mills and landfills, were flown. The area around Atlanta has large biogenic isoprene emissions, which provided an excellent case for studying the interactions between biogenic and anthropogenic emissions. In this example flight, chemistry in and outside the Atlanta plumes was observed for several hours after emission. The analysis of this flight showcases the strategies implemented to answer some of the main SENEX science questions.

Worldwide data sets constrain the water vapor uptake coefficient in cloud formation

Cloud droplet formation depends on the condensation of water vapor on ambient aerosols, the rate of which is strongly affected by the kinetics of water uptake as expressed by the condensation (or mass accommodation) coefficient, αc. Estimates of αc for droplet growth from activation of ambient particles vary considerably and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. We present an analysis of 10 globally relevant data sets of cloud condensation nuclei to constrain the value of αc for ambient aerosol. We find that rapid activation kinetics (αc > 0.1) is uniformly prevalent. This finding resolves a long-standing issue in cloud physics, as the uncertainty in water vapor accommodation on droplets is considerably less than previously thought.

Microphysical explanation of the RH‐dependent water affinity of biogenic organic aerosol and its importance for climate

Abstract A large fraction of atmospheric organic aerosol (OA) originates from natural emissions that are oxidized in the atmosphere to form secondary organic aerosol (SOA). Isoprene (IP) and monoterpenes (MT) are the most important precursors of SOA originating from forests. The climate impacts from OA are currently estimated through parameterizations of water uptake that drastically simplify the complexity of OA. We combine laboratory experiments, thermodynamic modeling, field observations, and climate modeling to (1) explain the molecular mechanisms behind RH‐dependent SOA water‐uptake with solubility and phase separation; (2) show that laboratory data on IP‐ and MT‐SOA hygroscopicity are representative of ambient data with corresponding OA source profiles; and (3) demonstrate the sensitivity of the modeled aerosol climate effect to assumed OA water affinity. We conclude that the commonly used single‐parameter hygroscopicity framework can introduce significant error when quantifying the climate effects of organic aerosol. The results highlight the need for better constraints on the overall global OA mass loadings and its molecular composition, including currently underexplored anthropogenic and marine OA sources.

Mineral Dust and its Microphysical Interactions with Clouds

Our understanding of the interactions of aerosols and clouds has a strong heritage in laboratory experiments, field measurements, and process modeling. We present a review on the state of knowledge for mineral dust emitted from major global dust source regions. Laboratory studies and field measurements have given insights on processes and mechanisms taking place when mineral dust is released into the atmosphere and reacts with the atmospheric constituents. Furthermore, theoretical approaches and parameterizations have been established to interpret the observations and quantitatively express the mechanisms by which dust can act as cloud condensation nuclei (CCN) and ice nuclei (IN). Finally, model simulations have been used in order to study the effects of dust particles to different aerosol-cloud-climate interactions. Dust can act as efficient CCN in clouds solely based on their relatively large size combined with the hydrophilicity from the adsorption of water vapor on their insoluble core. When mixed with even small fractions of hygroscopic material from emission or atmospheric processing, their hygroscopicity and CCN activity are significantly enhanced. The theoretical frameworks of adsorption activation and Kohler theory are presented to explain dust CCN activity, together with a summary on the potential contributions of dust to cloud droplet number concentration (CDNC), and its role in regulating supersaturation. Mineral dust aerosol is an effective IN and, combined with their concentration, can dominate ice production in cirrus and mixed-phase clouds even at great distances from source regions. The pathways to nucleation of ice are different for different cloud types and have distinct effects in those clouds. Our fundamental understanding of ice nucleation lags behind that for CCN activation, and a key challenge is that we cannot predict a priori which aerosol materials will make effective IN. Nevertheless, numerous field and laboratory studies have shown that mineral dust from deserts is one of the most important ice-nucleating aerosol types around the globe.

Collocated observations of cloud condensation nuclei, particle size distributions, and chemical composition

Cloud condensation nuclei (CCN) number concentrations alongside with submicrometer particle number size distributions and particle chemical composition have been measured at atmospheric observatories of the Aerosols, Clouds, and Trace gases Research InfraStructure (ACTRIS) as well as other international sites over multiple years. Here, harmonized data records from 11 observatories are summarized, spanning 98,677 instrument hours for CCN data, 157,880 for particle number size distributions, and 70,817 for chemical composition data. The observatories represent nine different environments, e.g., Arctic, Atlantic, Pacific and Mediterranean maritime, boreal forest, or high alpine atmospheric conditions. This is a unique collection of aerosol particle properties most relevant for studying aerosol-cloud interactions which constitute the largest uncertainty in anthropogenic radiative forcing of the climate. The dataset is appropriate for comprehensive aerosol characterization (e.g., closure studies of CCN), model-measurement intercomparison and satellite retrieval method evaluation, among others. Data have been acquired and processed following international recommendations for quality assurance and have undergone multiple stages of quality assessment.

Non-Methane Hydrocarbons (NMHC s ) Variability in the Eastern Mediterranean

Non-methane hydrocarbons (NMHCS) are important constituents of the atmosphere that contribute both to the oxidation capacity of the atmosphere and to the formation of the secondary organic aerosol. In Europe, over the last decades, many efforts have been devoted to the assignment of NMHCs sources and although NMHCS role is well established large uncertainties in their emissions still exist. Measurements of NMHCS in the atmosphere of the Eastern Mediterranean are very scarce in the literature and, therefore, the present work is willing to provide an assessment of individual NMHCS sources on both spatial and temporal basis in the Eastern Mediterranean. Intensive campaigns of several days took place each month by in situ continuous hourly measurements of NMHCS from C2 to C8 in different locations on the island of Crete, Greece (marine, rural and urban areas). All samples were analysed for 45 HMHCS with a gas chromatographic (GC) system equipped with a flame ionisation detector (FID). A simple statistical analysis of the relationship between various hydrocarbon pairs indicates influences from common sources. The average measured hydrocarbon concentrations show seasonal variations in agreement with previous published measurements. Evidence for both chemical processing and source dominating the variability of NMHCS mixing ratios were obtained. Chlorine atom concentrations were indirectly derived from changes in the patterns of the measured NMHCS. The result of the present study suggests that the Cl-atom induced reaction may be as well of considerable importance in the troposphere of the region. A. Mellouki and A.R. Ravishankara (eds.), Regional Climate Variability and its Impacts in the 197 Mediterranean Area, 197–206.

Formation and growth of atmospheric nanoparticles in the eastern Mediterranean: Results from long-term measurements and process simulations

Atmospheric new particle formation (NPF) is a common phenomenon all over the world. In this study we present the longest time series of NPF records in the eastern Mediterranean region by analyzing 10 years of aerosol number size distribution data obtained with a mobility particle sizer. The measurements were performed at the Finokalia environmental research station on Crete, Greece, during the period June 2008–June 2018. We found that NPF took place on 27 % of the available days, undefined days were 23 % and non-event days 50 %. NPF is more frequent in April and May probably due to the terrestrial biogenic activity and is less frequent in August. Throughout the period under study, nucleation was observed also during the night. Nucleation mode particles had the highest concentration in winter and early spring, mainly because of the minimum sinks, and their average contribution to the total particle number concentration was 8 %. Nucleation mode particle concentrations were low outside periods of active NPF and growth, so there are hardly any other local sources of sub-25 nm particles. Additional atmospheric ion size distribution data simultaneously collected for more than 2 years were also analyzed. Classification of NPF events based on ion spectrometer measurements differed from the corresponding classification based on a mobility spectrometer, possibly indicating a different representation of local and regional NPF events between these two measurement data sets. We used the MALTE-Box model for simulating a case study of NPF in the eastern Mediterranean region. Monoterpenes contributing to NPF can explain a large fraction of the observed NPF events according to our model simulations. However the adjusted parameterization resulting from our sensitivity tests was significantly different from the initial one that had been determined for the boreal environment. Published by Copernicus Publications on behalf of the European Geosciences Union. 2672 N. Kalivitis et al.: Formation and growth of atmospheric nanoparticles in the eastern Mediterranean

Fine Particle Water and PH in an Urban and Remote Location and the Role of Biomass Burning

Particle water (LWC) and pH are calculated for the fine fraction of aerosols sampled at two locations in Greece; an urban (Athens) and a remote background (Finokalia). Using concurrent measurements of aerosol chemical composition, tandem light scattering coefficients and the thermodynamic model ISORROPIA-II, LWC mass concentrations associated with the aerosol inorganic and organic components are determined for both locations. The predicted pHs are interpreted based on different sources that influence air quality in these locations. For Finokalia closure between predicted aerosol water and that determined by comparison of ambient with dry light scattering coefficients was achieved within 8 % (slope = 0.92, R2 = 0.8). For both locations a significant diurnal variability is found for both organic and inorganic water constituents. The average value for total aerosol water was 2.19 ± 1.75 μg m−3 and 13.86 ± 17.26 μg m−3 for Finokalia and Athens, respectively. The large difference in LWC is associated with intense biomass burning activities during wintertime in Athens. The aerosol at Finokalia was found to be highly acidic with pH varying from 0.5 to 2.8 throughout a 5-month study period. Biomass burning exhibited the highest pH (2.77 ± 0.88) with values being comparable to the ones derived from Athens from days with high biomass burning influence (2.83 ± 0.47).

High Aerosol Acidity Despite Declining Atmospheric Sulfate Concentrations: Lessons from Observations and Implications for Models

Particle acidity affects aerosol concentrations, chemical composition, toxicity and nutrient bioavailability. We present a summary of thermodynamic analysis of comprehensive observations of ambient aerosol collected over the US and E.Mediterranean to understand the levels and drivers of aerosol pH. We find that acidic aerosol in the fine mode is ubiquitous, with levels that range between 0 and 2 throughout most of the data examined. The strong acidity is largely from the large difference in volatility between sulfate (the main acidic compound, which resides completely in the aerosol phase) and ammonia (the main neutralizing agent, which partitions between aerosol and gas-phase). This counterintuitive, but thermodynamically consistent finding explains why aerosol acidity in the southeastern United States has not decreased over the last decades, despite a 70% reduction in sulfates and a constant ammonia background. We then demonstrate that evaluation of model-predicted pH is critical for model predictions of semi-volatile species, e.g., nitrate.