Optical properties of secondary organic aerosols and their changes by chemical processes.

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.

https://www.pnas.org/content/pnas/113/36/10013.full.pdf

Direct observation of aqueous secondary organic aerosol from biomass-burning emissions.

Modeling the processing of aerosol and trace gases in clouds and fogs.

Atmospheric submicron particles were collected on Teflon filters downstream of a three-stage concentrator aboard the National Center for Atmospheric Research C-130 aircraft near Japan during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). Particle-phase organic carbon (OC) was quantified using Fourier transform infrared (FTIR) transmission spectroscopy. Silicate, carbonate, alkane, alkene, aromatic, alcohol, carbonyl, amine, and organosulfate functional groups were identified and separated with a four-solvent rinsing procedure. X-ray fluorescence identified elemental composition. Total OC constructed from FTIR measurements agreed with simultaneous thermal-optical OC measurements with a slope of 0.91 and an R value of 0.93. OC varied from 0.4 to 14.2 μg m -3 , and organic mass varied from 0.6 to 19.6 μg m -3 , representing on average 36% of the identified submicron aerosol mass. Measured carbon monoxide (CO) to OC slopes illustrate 10 groups of air from regions described by an Asian emissions inventory. The CO/OC slope is used to compare sources and their influence on organic composition. Fifty-two percent of ACE-Asia samples have CO/OC slopes indicative of biomass combustion. Unitless CO/OC slopes above 15 are associated with increased fractions of alcohol groups, unsaturated C-H groups, and inorganic nitrate. Increased carbonyl carbon fractions in air originating over northern Asia are consistent with secondary OC formation. Case studies in the boundary layer demonstrate that aerosol compositions downwind of large Asian aerosol sources show clear regional composition signatures.

Source signatures of carbon monoxide and organic functional groups in Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) submicron aerosol types : Characterization of Asian aerosols and their radiative impacts on climate

. We analyse the development of a radiation fog event and its gradual
transition from optically thin fog in a stable boundary layer to
well-mixed optically thick fog. A comparison of observations and a
detailed large-eddy simulation demonstrate that aerosol growth and
activation is the key process in determining the onset of adiabatic
fog. Weak turbulence and low supersaturations lead to the growth of
aerosol particles which can significantly affect the visibility but
do not significantly interact with the long-wave radiation, allowing
the atmosphere to remain stable. Only when a substantial fraction of
the aerosol activates into cloud droplets can the fog
interact with the radiation, becoming optically thick and
well mixed. Modifications to the parameterisation of cloud droplet
numbers in fog, resulting in lower and more realistic
concentrations, are shown to give significant improvements to an NWP
model, which initially struggled to accurately simulate the
transition. Finally, the consequences of this work for common aerosol
activation parameterisations used in climate models are discussed,
demonstrating that many schemes are reliant on an artificial minimum
value when activating aerosol in fog, and adjustment of this minimum
can significantly affect the sensitivity of the climate system to
aerosol radiative forcing.

https://acp.copernicus.org/articles/18/7827/2018/acp-18-7827-2018.pdf

Aerosol–fog interaction and the transition to well-mixed radiation fog

. The interaction of aerosol with atmospheric water affects the processing and wet removal of atmospheric particles. Understanding such interaction is mandatory to improve model description of aerosol lifetime and ageing. We analyzed the aerosol–water interaction at high relative humidity during fog events in the Po Valley within the framework of the Agenzia Regionale per la Prevenzione e l'Ambiente (ARPA) – Emilia Romagna supersite project. For the first time in this area, the changes in particle chemical composition caused by fog are discussed along with changes in particle microphysics. During the experiment, 14 fog events were observed. The average mass scavenging efficiency was 70% for nitrate, 68% for ammonium, 61% for sulfate, 50% for organics, and 39% for black carbon. After fog formation, the interstitial aerosol was dominated by particles smaller than 200 nm Dva (vacuum aerodynamic diameter) and enriched in carbonaceous aerosol, mainly black carbon and water-insoluble organic aerosol. For each fog event, the size-segregated scavenging efficiency of nitrate and organic aerosol (OA) was calculated by comparing chemical species size distribution before and after fog formation. For both nitrate and OA, the size-segregated scavenging efficiency followed a sigmoidal curve, with values close to zero below 100 nm Dva and close to 1 above 700 nm Dva. OA was able to affect scavenging efficiency of nitrate in particles smaller than 300 nm Dva. A linear correlation between nitrate scavenging and particle hygroscopicity (κ) was observed, indicating that 44–51% of the variability of nitrate scavenging in smaller particles (below 300 nm Dva) was explained by changes in particle chemical composition. The size-segregated scavenging curves of OA followed those of nitrate, suggesting that organic scavenging was controlled by mixing with water-soluble species. In particular, functional group composition and OA elemental analysis indicated that more oxidized OA was scavenged more efficiently than less oxidized OA. Nevertheless, the small variability of organic functional group composition during the experiment did not allow us to discriminate the effect of different organic functionalities on OA scavenging.

/pdf/fog-scavenging-of-organic-and-inorganic-aerosol-in-the-po-48g3vakcmn.pdf

Fog scavenging of organic and inorganic aerosol in the Po Valley

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Fog scavenging of organic and inorganic aerosol in the Po Valley