The mechanisms leading to the formation of secondary organic aerosol (SOA) are an important subject of ongoing research for both air quality and climate. Recent laboratory experiments suggest that ...reactions taking place in the atmospheric liquid phase represent a potentially significant source of SOA mass. Here, we report direct ambient observations of SOA mass formation from processing of biomass-burning emissions in the aqueous phase. Aqueous SOA (aqSOA) formation is observed both in fog water and in wet aerosol. The aqSOA from biomass burning contributes to the “brown” carbon (BrC) budget and exhibits light absorption wavelength dependence close to the upper bound of the values observed in laboratory experiments for fresh and processed biomass-burning emissions. We estimate that the aqSOA from residential wood combustion can account for up to 0.1–0.5 Tg of organic aerosol (OA) per y in Europe, equivalent to 4–20% of the total OA emissions. Our findings highlight the importance of aqSOA from anthropogenic emissions on air quality and climate.
Surface ocean bubble-bursting generates aerosols composed of microscopic salt-water droplets, enriched in marine organic matter. The organic fraction profoundly influences aerosols' properties, by ...scattering solar radiations and nucleating water particles. Still little is known on the biochemical and microbiological composition of these organic particles. In the present study, we experimentally simulated the bursting of bubbles at the seawater surface of the North-Eastern Atlantic Ocean, analysing the organic materials and the diversity of the bacteria in the source-seawaters and in the produced aerosols. We show that, compared with seawater, the sub-micron aerosol particles were highly enriched in organic matter (up to 140,000x for lipids, 120,000x for proteins and 100,000x for carbohydrates). Also DNA, viruses and prokaryotes were significantly enriched (up to 30,000, 250 and 45x, respectively). The relative importance of the organic components in the aerosol did not reflect those in the seawater, suggesting their selective transfer. Molecular analyses indicate the presence of selective transfers also for bacterial genotypes, highlighting higher contribution of less abundant seawater bacterial taxa to the marine aerosol. Overall, our results open new perspectives in the study of microbial dispersal through marine aerosol and provide new insights for a better understanding of climate-regulating processes of global relevance.
The light‐absorbing properties of water‐soluble brown carbon (WS‐BrC) and methanol‐soluble brown carbon (MeS‐BrC) were studied in PM10 aerosols collected at the “Nepal Climate Observatory‐Pyramid” ...(NCO‐P) station (5079 m above sea level) during the period 2013–2014. The light absorption coefficients of WS‐BrC and MeS‐BrC were the highest during the premonsoon season and the lowest during monsoon. MeS‐BrC absorbs about 2 times higher at 365 nm and about 3 times more at 550 nm compared to WS‐BrC. The mass absorption cross section (MAC) of WS‐BrC measured at 365 nm is similar to that observed previously at South Asian low‐altitude sites. Fractional solar radiation absorption by BrC compared to BC considering the full solar spectrum showed that WS‐BrC absorbs 4 ± 1% and MeS‐BrC absorbs 9 ± 2% compared to BC at NCO‐P. Such ratios become 8 ± 1% (for WS‐BrC respect to BC) and 17 ± 5% (for MeS‐BrC respect to BC) when accounting for correction factors proposed by previous studies to convert absorption coefficients in bulk solutions into light absorption by accumulation mode aerosol particles. These results confirm the importance of BrC in contributing to light‐absorbing aerosols in this region of the world. However, the BrC absorption at 550 nm appears small compared to that of BC (1–5%, or 3–9% with conversion factors), and it is lower compared to global model estimates constrained by Aerosol Robotic Network observations. Finally, our study provides no clear evidence of a change in the fractional contribution of BrC with respect to BC to light absorption in the middle troposphere respect to the Indo‐Gangetic plain boundary layer.
Key Points
Light absorption coefficient of BrC in the high Himalayas has a strong seasonal variability
BrC absorbs on average 4 +/‐ 1% (WS‐BrC) and 9 +/‐ 2% (MeS‐BrC) of solar radiation compared to BC
BrC light absorption properties in the high Himalayas are similar to those at low altitudes
The chemical properties of sea‐spray aerosol particles produced by artificially generated bubbles using oceanic waters were investigated during a phytoplankton bloom in the North Atlantic. Spray ...particles exhibited a progressive increase in the organic matter (OM) content from 3 ± 0.4% up to 77 ± 5% with decreasing particle diameter from 8 to 0.125 μm. Submicron OM was almost entirely water insoluble (WIOM) and consisted of colloids and aggregates exuded by phytoplankton. Our observations indicate that size dependent transfer of sea water organic material to primary marine particles is mainly controlled by the solubility and surface tension properties of marine OM. The pattern of WIOM and sea‐salt content in the different size intervals observed in bubble bursting experiments is similar to that measured in atmospheric marine aerosol samples collected during periods of high biological activity. The results point to a WIOM/sea‐salt fingerprint associated with submicron primary marine aerosol production in biologically rich waters.
Bursting bubbles at the ocean-surface produce airborne salt-water spray-droplets, in turn, forming climate-cooling marine haze and cloud layers. The reflectance and ultimate cooling effect of these ...layers is determined by the spray's water-uptake properties that are modified through entrainment of ocean-surface organic matter (OM) into the airborne droplets. We present new results illustrating a clear dependence of OM mass-fraction enrichment in sea spray (OMss) on both phytoplankton-biomass, determined from Chlorophyll-a (Chl-a) and Net Primary Productivity (NPP). The correlation coefficient for OMss as a function of Chl-a increased form 0.67 on a daily timescale to 0.85 on a monthly timescale. An even stronger correlation was found as a function of NPP, increasing to 0.93 on a monthly timescale. We suggest the observed dependence is through the demise of the bloom, driven by nanoscale biological processes (such as viral infections), releasing large quantities of transferable OM comprising cell debris, exudates and other colloidal materials. This OM, through aggregation processes, leads to enrichment in sea-spray, thus demonstrating an important coupling between biologically-driven plankton bloom termination, marine productivity and sea-spray modification with potentially significant climate impacts.
We investigate optical–microphysical–chemical properties of brown carbon (BrC) in the urban ambient atmosphere of the Po Valley. In situ ground measurements of aerosol spectral optical properties, ...PM1 chemical composition (HR-ToF-AMS), and particle size distributions were carried out in Bologna. BrC was identified through its wavelength dependence of light absorption at visible wavelengths, as indicated by the absorption Ångström exponent (AAE). We found that BrC occurs in particles with a narrow monomodal size distribution peaking in the droplet mode, enriched in ammonium nitrate and poor in black carbon (BC), with a strong dependance on OA-to-BC ratios, and SSA530 of 0.98 ± 0.01. We demonstrate that specific complex refractive index values (k530 = 0.017 ± 0.001) are necessary in addition to a proper particle size range to match the large AAEs measured for this BrC (AAE467 − 660 = 3.2 ± 0.9 with values up to 5.3). In terms of consistency of these findings with literature, this study i. provides experimental evidence of the size distribution of BrC associated with the formation of secondary aerosol;ii. shows that in the lower troposphere AAE increases with increasing OA-to-BC ratios rather than with increasing OA – contributing to sky radiometer retrieval techniques (e.g., AERONET);iii. extends the dependence of AAE on BC-to-OA ratios previously observed in chamber experiments to ambient aerosol dominated by wood-burning emissions. These findings are expected to bear important implications for atmospheric modeling studies and remote sensing observations as regards the parametrization and identification of BrC in the atmosphere.
To study the influence of regional biomass burning emissions and secondary processes, ambient samples of fog and aerosol were collected in the Po Valley (Italy) during the 2013 Supersito field ...campaign. After the extent of fresh vs. aged biomass burning influence was estimated from proton nuclear magnetic resonance (1H NMR) and high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS), two samples of fog water and two samples of PM1 aerosol were selected for ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. Molecular compositions indicated that the water-soluble organic matter was largely non-polymeric without clearly repeating units. The selected samples had an atypically large frequency of molecular formulas containing nitrogen and sulfur (not evident in the NMR composition) attributed to multifunctional organonitrates and organosulfates. Higher numbers of organonitrates were observed in aerosol, and higher numbers of organosulfates were observed in fog water. Consistent with the observation of an enhanced aromatic proton signature in the 1H-NMR analysis, the average molecular formula double-bond equivalents and carbon numbers were higher in the fresh biomass-burning-influenced samples. The average O : C and H : C values from FT-ICR MS were higher in the samples with an aged influence (O : C = 0.50–0.58, and H : C = 1.31–1.37) compared to those with fresh influence (O : C = 0.43–0.48, and H : C = 1.13–1.30). The aged fog had a large set of unique highly oxygenated CHO fragments in the HR-ToF-AMS, which reflects an enrichment of carboxylic acids and other compounds carrying acyl groups, highlighted by the NMR analysis. Fog compositions were more oxidized and SOA (secondary organic aerosol)-like than aerosols as indicated by their NMR measured acyl-to-alkoxyl ratios and the observed molecular formula similarity between the aged aerosol and fresh fog, implying that fog nuclei must be somewhat aged. Overall, functionalization with nitrate and sulfate moieties, in addition to aqueous oxidation, triggers an increase in the molecular complexity in this environment, which is apparent in the FT-ICR MS results. This study demonstrates the significance of the aqueous phase in transforming the molecular chemistry of atmospheric organic matter and contributing to secondary organic aerosol.
Climate warming affects the development and distribution of sea ice, but at present the evidence of polar ecosystem feedbacks on climate through changes in the atmosphere is sparse. By means of ...synergistic atmospheric and oceanic measurements in the Southern Ocean near Antarctica, we present evidence that the microbiota of sea ice and sea ice-influenced ocean are a previously unknown significant source of atmospheric organic nitrogen, including low molecular weight alkyl-amines. Given the keystone role of nitrogen compounds in aerosol formation, growth and neutralization, our findings call for greater chemical and source diversity in the modelling efforts linking the marine ecosystem to aerosol-mediated climate effects in the Southern Ocean.
Relevant concentrations of dimethyl- and diethylammonium salts (DMA+ and DEA+) were measured in submicrometer marine aerosol collected over the North Atlantic during periods of high biological ...activity (HBA) in clean air masses (median concentration (minimum−maximum) = 26 (6−56) ng m−3). Much lower concentrations were measured during periods of low biological activity (LBA): 1 (<0.4−20) ng m−3 and when polluted air masses were advected to the sampling site: 2 (<0.2−24) ng m−3. DMA+ and DEA+ are the most abundant organic species, second only to MSA, detected in fine marine particles representing on average 11% of the secondary organic aerosol (SOA) fraction and a dominant part (35% on average) of the water-soluble organic nitrogen (WSON). Several observations support the hypothesis that DMA+ and DEA+ have a biogenic oceanic source and are produced through the reaction of gaseous amines with sulfuric acid or acidic sulfates. Moreover, the water-soluble fraction of nascent marine aerosol particles produced by bubble-bursting experiments carried out in parallel to ambient aerosol sampling over the open ocean showed WSON, DMA+, and DEA+ concentrations always below the detection limit, thus excluding an important primary sea spray source.
Rising air pollution levels in South Asia will have worldwide environmental consequences. Transport of pollutants from the densely populated regions of India, Pakistan, China, and Nepal to the ...Himalayas may lead to substantial radiative forcing in South Asia with potential effects on the monsoon circulation and, hence, on regional climate and hydrological cycles, as well as to dramatic impacts on glacier retreat. An improved description of particulate sources is needed to constrain the simulation of future regional climate changes. Here, the first evidence of very frequent new particle formation events occurring up to high altitudes is presented. A 16-month record of aerosol size distribution from the Nepal Climate Observatory at Pyramid (Nepal, 5,079 m above sea level), the highest atmospheric research station, is shown. Aerosol concentrations are driven by intense ultrafine particle events occurring on >35% of the days at the interface between clean tropospheric air and the more polluted air rising from the valleys. During a pilot study, we observed a significant increase of ion cluster concentrations with the onset of new particle formation events. The ion clusters rapidly grew to a 10-nm size within a few hours, confirming, thus, that in situ nucleation takes place up to high altitudes. The initiation of the new particle events coincides with the shift from free tropospheric downslope winds to thermal upslope winds from the valley in the morning hours. The new particle formation events represent a very significant additional source of particles possibly injected into the free troposphere by thermal winds.
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