Hygroscopic and chemical characterisation of Po Valley aerosol

Continental summer-time aerosol in the Italian Po Valley was characterised in terms of hygroscopic properties and the influence of chemical composition therein. Additionally, the ethanol affinity of particles was analysed. The campaign-average minima in hygroscopic growth factors (HGFs, at 90% relative humidity) occurred just before and during sunrise from 03:00 to 06:00 LT (all data are reported in the local time), but, more generally, the hygroscopicity during the whole night is very low, particularly in the smaller particle sizes. The average HGFs recorded during the low HGF period were in a range from 1.18 (for the smallest, 35nm particles) to 1.38 (for the largest, 165 nm particles). During the day, the HGF gradually increased to achieve maximum values in the early afternoon hours 12:00–15:00, reaching 1.32 for 35 nm particles and 1.46 for 165 nm particles. Two contrasting case scenarios were encountered during the measurement period: Case 1 was associated with westerly air flow moving at a moderate pace and Case 2 was associated with more stagnant, slower moving air from the north-easterly sector. Case 1 exhibited weak diurnal temporal patterns, with no distinct maximum or minimum in HGF or chemical composition, and was associated with moderate non-refractory aerosol mass concentrations (for 50% size cut at 1 μ) of the order of 4.5 μg m⁻³. For Case 1, organics contributed typically 50% of the mass. Case 2 was characterised by >9.5 μg m⁻³ total non-refractory mass (<1 μ) in the early morning hours (04:00), decreasing to ~3 μg m⁻³ by late morning (10:00) and exhibited strong diurnal changes in chemical composition, particularly in nitrate mass but also in total organic mass concentrations. Specifically, the concentrations of nitrate peaked at night-time, along with the concentrations of hydrocarbon-like organic aerosol (HOA) and of semi-volatile oxygenated organic aerosol (SV-OOA). In general, organic growth factors (OGFs) followed a trend which was opposed to HGF and also to the total organic mass as measured by the aerosol mass spectrometer. The analysis of the HGF probability distribution function (PDF) reveals an existence of a predominant "more hygroscopic" (MH) mode with HGF of 1.5 around noon, and two additional modes: one with a "less hygroscopic" (LH) HGF of 1.26, and another with a "barely hygroscopic" (BH) mode of 1.05. Particles sized 165 nm exhibited moderate diurnal variability in HGF, ranging from 80% at night to 95% of "more hygroscopic" growth factors (i.e. HGFs 1.35–1.9) around noon. The diurnal changes in HGF progressively became enhanced with decreasing particle size, decreasing from 95% "more hygroscopic" growth factor fraction at noon to 10% fraction at midnight, while the "less hygroscopic" growth factor fraction (1.13–1.34) increased from 5% at noon to > 60% and the "barely hygroscopic" growth factor fraction (1.1–1.2) increased from less than 2% at noon to 30% at midnight. Surprisingly, the lowest HGFs occurred for the period when nitrate mass reached peak concentrations (Case 2). We hypothesised that the low HGFs of nitrate-containing particles can be explained by a) an organic coating suppressing the water-uptake, and/or by b) the existence of nitrates in a less hygroscopic state, e.g. as organic nitrates. The latter hypothesis allows us to explain also the reduced OGFs observed during the early morning hours (before dawn) when nitrate concentrations peaked, based on the evidence that organic nitrates have significant lower ethanol affinity than other SV-OOA compounds

Melt pond biogeochemistry in central Arctic: first insights from MOSAiC campaign

We undertook a melt pond survey during the international drift campaign MOSAiC Leg 5 (from 22 August to 18 September 2020) to understand variations in climate gases (CO2, CH4, N2O and DMS) and nutrients in melt ponds during the open water and freezing periods, and to study the interactions with atmospheric and ecological parameters (Figure 1a). Inside those melt ponds with a darker color, we found significant quantities of floating organic material within the pond water, along with significant further organic material settled at the bottom of the pond and frozen into the ice (Figure 1b). These floating and sedimented materials were both white and green/brown; the green/brown material was mainly composed of phytoplankton “Melosira arctica”, while the white material was composed of re-mineralized organic matter during degradation (including the remains of krill and other zooplankton). There were strong vertical gradients in physical parameters from the surface to the bottom of the melt pond (within 1 m depth): from +0.2°C to –1.5°C for temperature, from 0 to 29 psu for salinity, and 9.2 to 13.5 mg L–1 for dissolved oxygen (DO). The DO minimum layer (below 9 mg L–1) corresponded with a salinity of 25 psu, which generally occurred at approximately 0.6 m depth, and it increased to over 13 mg L–1 at the atmospheric interface. At the end of Leg 5 (mid-September 2020), these strong gradients disappeared, likely due to the mixing events during the cooling and freezing periods. Prior to and during the freezing period, CO2 flux was measured periodically within the melt pond with a floating chamber system. Because measured in situ CO2 concentration at the melt pond surface (top 10 cm) was low (321 ppm) compared to the atmosphere (approximately 400 ppm), air–to-melt pond CO2 flux was negative (melt pond was acting as a sink for atmospheric CO2) around –3.9 mmol m–2 day–1. Therefore, the melt pond water absorbs significant amounts of CO2 from the atmosphere. We also found extremely low CO2 concentrations (170 ppm) at the freshwater/seawater interface (0.6 m depth) corresponding to the same depth as the DO minimum. Therefore, we expected that if melt pond water is mixed vertically by the wind, cooling, crack formation, and ice movement, the melt pond could become an even stronger sink for atmospheric CO2. Ice cores collected from the bottom of the melt pond were porous at the top 0.50 m, and contain large quantities of organic material similar to that identified floating in the water column This accumulation of material and ongoing degradation processes over the pond bottom ice would contribute significantly to the turnover of carbon, sulphur and nitrogen containing gases cycles within melt pond water and thereby gas exchange process with the atmosphere.MOSAi

Abiotic and biotic sources influencing spring new particle formation in North East Greenland

9 pages, 4 figuresIn order to improve our ability to predict cloud properties, radiative balance and climate, it is crucial to understand the mechanisms that trigger the formation of new particles and their growth to activation sizes. Using an array of real time aerosol measurements, we report a categorization of the aerosol population taken at Villum Research Station, Station Nord (VRS) in North Greenland during a period of 88 days (February–May 2015). A number of New Particle Formation (NPF) events were detected and are herein discussed. Air mass back trajectories analysis plotted over snow-sea ice satellite maps allowed us to correlate early spring (April) NPF events with air masses travelling mainly over snow on land and sea ice, whereas late spring (May) NPF events were associated with air masses that have passed mainly over sea ice regions. Concomitant aerosol mass spectrometry analysis suggests methanesulfonic acid (MSA) and molecular iodine (I) may be involved in the NPF mechanisms. The source of MSA was attributed to open leads within the sea ice. By contrast, iodine was associated with air masses over snow on land and over sea ice, suggesting both abiotic and biotic sources. Measurements of nucleating particle composition as well as gas-phase species are needed to improve our understanding of the links between emissions, aerosols, cloud and climate in the Arctic; therefore our ability to model such processesThe study was supported by the Spanish Ministry of Economy through project BIONUC (CGL 2013-49020-R), PIICE (CTM 2017-89117-R) and the Ramon y Cajal fellowship (RYC-2012-11922). The National Centre for Atmospheric ScienceNCAS Birmingham group is funded by the UK Natural Environment Research Council. [...]. This work was financially supported by the Danish Environmental Protection Agency with means from the MIKA/DANCEA funds for Environmental Support to the Arctic Region, which is part of the Danish contribution to “Arctic Monitoring and Assessment Program” (AMAP) and to the Danish research project “Short lived Climate Forcers” (SLCF), and the Danish Council for Independent Research (project NUMEN, DFF-FTP-4005-00485B). [...] This work was also supported by the Nordic Centre of Excellence (NCoE) Cryosphere-Atmosphere Interactions in a Changing Arctic Climate (CRAICC). The Villum Foundation is acknowledged for funding the construction of Villum Research Station, Station NordPeer Reviewe

Frequent new particle formation over the high Arctic pack ice by enhanced iodine emissions

In the central Arctic Ocean the formation of clouds and their properties are sensitive to the availability of cloud condensation nuclei (CCN). The vapors responsible for new particle formation (NPF), potentially leading to CCN, have remained unidentified since the first aerosol measurements in 1991. Here, we report that all the observed NPF events from the Arctic Ocean 2018 expedition are driven by iodic acid with little contribution from sulfuric acid. Iodic acid largely explains the growth of ultrafine particles (UFP) in most events. The iodic acid concentration increases significantly from summer towards autumn, possibly linked to the ocean freeze-up and a seasonal rise in ozone. This leads to a one order of magnitude higher UFP concentration in autumn. Measurements of cloud residuals suggest that particles smaller than 30 nm in diameter can activate as CCN. Therefore, iodine NPF has the potential to influence cloud properties over the Arctic Ocean

The hemispheric contrast in cloud microphysical properties constrains aerosol forcing

The change in planetary albedo due to aerosol−cloud interactions during the industrial era is the leading source of uncertainty in inferring Earth’s climate sensitivity to increased greenhouse gases from the historical record. The variable that controls aerosol−cloud interactions in warm clouds is droplet number concentration. Global climate models demonstrate that the present-day hemispheric contrast in cloud droplet number concentration between the pristine Southern Hemisphere and the polluted Northern Hemisphere oceans can be used as a proxy for anthropogenically driven change in cloud droplet number concentration. Remotely sensed estimates constrain this change in droplet number concentration to be between 8 cm−3 and 24 cm−3. By extension, the radiative forcing since 1850 from aerosol−cloud interactions is constrained to be −1.2 W⋅m−2 to −0.6 W⋅m−2. The robustness of this constraint depends upon the assumption that pristine Southern Ocean droplet number concentration is a suitable proxy for preindustrial concentrations. Droplet number concentrations calculated from satellite data over the Southern Ocean are high in austral summer. Near Antarctica, they reach values typical of Northern Hemisphere polluted outflows. These concentrations are found to agree with several in situ datasets. In contrast, climate models show systematic underpredictions of cloud droplet number concentration across the Southern Ocean. Near Antarctica, where precipitation sinks of aerosol are small, the underestimation by climate models is particularly large. This motivates the need for detailed process studies of aerosol production and aerosol−cloud interactions in pristine environments. The hemispheric difference in satellite estimated cloud droplet number concentration implies preindustrial aerosol concentrations were higher than estimated by most models

On the origin of water-soluble organic tracer compounds in fine aerosols in two cities: the case of Los Angeles and Barcelona

Water soluble organic compounds (WSOCs), represented by anhydro-saccharides, dicarboxylic acids and polyols, were analyzed by gas chromatography interfaced to mass spectrometry (GC/MS) in extracts from 103 PM1 and 22 PM2.5 filter samples collected in an urban background and road site in Barcelona (Spain) and an urban background site in Los Angeles (USA), respectively, during one-month intensive sampling campaigns in 2010. Both locations have similar Mediterranean climates, with relatively high solar radiation and frequent anti-cyclonic conditions, and are influenced by a complex mixture of emission sources. Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) analyses were applied on the database in order to resolve differences and similarities in WSOC compositions in the studied sites. Five consistent clusters for the analyzed compounds were obtained, representing primary regional biomass burning organic carbon (regional BBOC), three secondary organic components (aged SOC, isoprene SOC and -pinene SOC), and a less clear component, called urban oxygenated organic carbon (urban OOC). This last component is probably influenced by in-situ urban activities, such as food cooking and traffic emissions and oxidation processes.Peer reviewe

Hygroscopic and chemical characterisation of po valley aerosol

Continental summer-time aerosol in the Italian Po Valley was characterised in terms of hygroscopic properties and the influence of chemical composition therein. Additionally, the ethanol affinity of particles was analysed. The campaign-average minima in hygroscopic growth factors (HGFs, at 90% relative humidity) occurred just before and during sunrise from 03: 00 to 06: 00 LT (all data are reported in the local time), but, more generally, the hygroscopicity during the whole night is very low, particularly in the smaller particle sizes. The average HGFs recorded during the low HGF period were in a range from 1.18 (for the smallest, 35nm particles) to 1.38 (for the largest, 165 nm particles). During the day, the HGF gradually increased to achieve maximum values in the early afternoon hours 12: 0015: 00, reaching 1.32 for 35 nm particles and 1.46 for 165 nm particles. Two contrasting case scenarios were encountered during the measurement period: Case 1 was associated with westerly air flow moving at a moderate pace and Case 2 was associated with more stagnant, slower moving air from the north-easterly sector. Case 1 exhibited weak diurnal temporal patterns, with no distinct maximum or minimum in HGF or chemical composition, and was associated with moderate non-refractory aerosol mass concentrations (for 50% size cut at 1 mu) of the order of 4.5 mu g m(-3). For Case 1, organics contributed typically 50% of the mass. Case 2 was characterised by &amp;gt;9.5 mu g m(-3) total non-refractory mass (&amp;lt;1 mu) in the early morning hours (04:00), decreasing to similar to 3 mu g m(-3) by late morning (10: 00) and exhibited strong diurnal changes in chemical composition, particularly in nitrate mass but also in total organic mass concentrations. Specifically, the concentrations of nitrate peaked at night-time, along with the concentrations of hydrocarbon-like organic aerosol (HOA) and of semi-volatile oxygenated organic aerosol (SV-OOA). In general, organic growth factors (OGFs) followed a trend which was opposed to HGF and also to the total organic mass as measured by the aerosol mass spectrometer. The analysis of the HGF probability distribution function (PDF) reveals an existence of a predominant &amp;quot;more hygroscopic&amp;quot; (MH) mode with HGF of 1.5 around noon, and two additional modes: one with a &amp;quot;less hygroscopic&amp;quot; (LH) HGF of 1.26, and another with a &amp;quot;barely hygroscopic&amp;quot; (BH) mode of 1.05. Particles sized 165 nm exhibited moderate diurnal variability in HGF, ranging from 80% at night to 95% of &amp;quot;more hygroscopic&amp;quot; growth factors (i.e. HGFs 1.35-1.9) around noon. The diurnal changes in HGF progressively became enhanced with decreasing particle size, decreasing from 95% &amp;quot;more hygroscopic&amp;quot; growth factor fraction at noon to 10% fraction at midnight, while the &amp;quot;less hygroscopic&amp;quot; growth factor fraction (1.13-1.34) increased from 5% at noon to &amp;gt;60% and the &amp;quot;barely hygroscopic&amp;quot; growth factor fraction (1.1-1.2) increased from less than 2% at noon to 30% at midnight. Surprisingly, the lowest HGFs occurred for the period when nitrate mass reached peak concentrations (Case 2). We hypothesised that the low HGFs of nitrate-containing particles can be explained by a) an organic coating suppressing the wateruptake, and/or by b) the existence of nitrates in a less hygroscopic state, e. g. as organic nitrates. The latter hypothesis allows us to explain also the reduced OGFs observed during the early morning hours (before dawn) when nitrate concentrations peaked, based on the evidence that organic nitrates have significant lower ethanol affinity than other SV-OOA compounds

Ubiquity of organic nitrates from nighttime chemistry in the European submicron aerosol

In the atmosphere nighttime removal of volatile organic compounds is initiated to a large extent by reaction with the nitrate radical (NO3) forming organic nitrates which partition between gas and particulate phase. Here we show based on particle phase measurements performed at a suburban site in the Netherlands that organic nitrates contribute substantially to particulate nitrate and organic mass. Comparisons with a chemistry transport model indicate that most of the measured particulate organic nitrates are formed by NO3 oxidation. Using aerosol composition data from three intensive observation periods at numerous measurement sites across Europe, we conclude that organic nitrates are a considerable fraction of fine particulate matter (PM1) at the continental scale. Organic nitrates represent 34% to 44% of measured submicron aerosol nitrate and are found at all urban and rural sites, implying a substantial potential of PM reduction by NOx emission control