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Large aldehydes are widespread in the atmosphere and their oxidation leads to secondary organic aerosols. The current understanding of their chemical transformation processes is limited to hydroxyl radical (OH) oxidation during daytime and nitrate radical (NO3) oxidation during nighttime. Here, we report quantitative kinetics calculations of the reactions of hexanal (C5H11CHO), pentanal (C4H9CHO), and butanal (C3H7CHO) with hydroperoxyl radical (HO2) at atmospheric temperatures and pressures. We find that neither tunneling nor multistructural torsion anharmonicity should be neglected in computing these rate constants; strong anharmonicity at the transition states is also important. We find rate constants for the three reactions in the range 3.2–7.7 × 10−14 cm3 molecule−1 s−1 at 298 K and 1 atm, showing that the HO2 reactions can be competitive with OH and NO3 oxidation under some conditions relevant to the atmosphere. Our findings reveal that HO2-initiated oxidation of large aldehydes may be responsible for the formation of highly oxygenated molecules via autoxidation. We augment the theoretic studies with laboratory flow-tube experiments using an iodide-adduct time-of-flight chemical ionization mass spectrometer to confirm the theoretical predictions of peroxy radicals and the autoxidation pathway. We find that the adduct from HO2 + C5H11CHO undergoes a fast unimolecular 1,7-hydrogen shift with a rate constant of 0.45 s−1. We suggest that the HO2 reactions make significant contributions to the sink of aldehydes. [ABSTRACT FROM AUTHOR]

Isoprene has the largest global non-methane hydrocarbon emission, and the oxidation of isoprene plays a crucial role in the formation of secondary organic aerosol (SOA). Two primary processes are known to contribute to SOA formation from isoprene oxidation: (1) the reactive uptake of isoprene-derived epoxides on acidic or aqueous particle surfaces and (2) the absorptive gas–particle partitioning of low-volatility oxidation products. In this study, we developed a new multiphase condensed isoprene oxidation mechanism that includes these processes with key molecular intermediates and products. The new mechanism was applied to simulate isoprene gas-phase oxidation products and SOA formation from previously published chamber experiments under a variety of conditions and atmospheric observations during the Southern Oxidant and Aerosol Studies (SOAS) field campaign. Our results show that SOA formation from most of the chamber experiments is reasonably reproduced using our mechanism, except when the concentration ratios of initial nitric oxide to isoprene exceed ∼ 2, the formed SOA is significantly underpredicted. The SOAS simulations also reasonably agree with the measurements regarding the diurnal pattern and concentrations of different product categories, while the total isoprene SOA remains underestimated. The molecular compositions of the modeled SOA indicate that multifunctional low-volatility products contribute to isoprene SOA more significantly than previously thought, with a median mass contribution of ∼ 57 % to the total modeled isoprene SOA. However, this contribution is intricately intertwined with IEPOX-derived SOA (IEPOX: isoprene-derived epoxydiols), posing challenges for their differentiation using bulk aerosol composition analysis (e.g., the aerosol mass spectrometer with positive matrix factorization). Furthermore, the SOA from these pathways may vary greatly, mainly dependent on the volatility estimation and treatment of particle-phase processes (i.e., photolysis and hydrolysis). Our findings emphasize that the various pathways to produce these low-volatility species should be considered in models to more accurately predict isoprene SOA formation. The new condensed isoprene chemical mechanism can be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess isoprene SOA formation on a larger scale. [ABSTRACT FROM AUTHOR]

Atmospheric black carbon (BC) is the strongest visible solar radiative absorber in the atmosphere, exerting significant influences on the earth's radiation budget. The mass absorption cross-section (MAC) is a crucial parameter for converting light absorption coefficient (bab) to mass equivalent BC concentration (mBC). Traditional filter-based instrument, such as AE33, uses a constant MAC of 7.77 m2/g to derive mBC, which may lead to uncertainty in mBC. In this paper, a new method of converting light absorption coefficient to BC mass concentration is proposed by incorporating the variations of MAC attributed to the influences of aerosol coating state. Mie simulation showed that MAC varied dramatically with different core-shell structures. We compared our new method with traditional method during a field measurement at a site of North China Plain. The results showed that the MAC was smaller (larger) than 7.77 m2/g for particle smaller (larger) than 280 nm, resulting in BC mass size distribution derived from new method was higher (lower) than traditional method for particle smaller (larger) than 280 nm. Size-integrated BC mass concentration derived from new method was 16 % higher than traditional method. Sensitivity analysis indicated that the uncertainty in mBC caused by refractive index (RI) was with in 35 % and the imaginary part of RI had dominant influence on the derived mBC. This study emphasizes the necessity to take variations of MAC into account when deriving mBC from bab and can help constrain the uncertainty in mBC measurements. [ABSTRACT FROM AUTHOR]

The Humidified Tandem Differential Mobility Analyzer (HTDMA) is widely used to obtain the hygroscopic properties of submicron particles. Aerosol size-resolved hygroscopicity parameter κ measured by HTDMA will be influenced by the contribution of multiply charged aerosols, and this effect has seldom been discussed in previous field measurements. Our calculation demonstrates that the number ratio of multiply charged particles is quite considerable for some specific sizes between 100 nm and 300 nm, especially during the polluted episode. The multi charges will further lead to the shrinking effect of aerosol hygroscopicity in HTDMA measurements. Therefore, we propose a new algorithm to do the multi-charge correction for the size-resolved hygroscopicity κ considering both the shrinking effect and multi-charge number contribution. The application in field measurements shows that the relatively high hygroscopicity in the accumulation size range will lead to the overestimation of the particle's hygroscopicity smaller than 200 nm. The low hygroscopicity in coarse mode particles will lead to the underestimation of accumulation particles between 200 nm and 500 nm. The difference between corrected and measured κ can reach as large as 0.05, highlighting that special attention needs to be paid to the multi-charge effect when the HTDMA is used for the aerosol hygroscopicity measurement. [ABSTRACT FROM AUTHOR]

There is evidence that black carbon (BC) particles may affect cirrus formation and hence global climate by acting as potential ice nucleating particles (INPs) in the troposphere. Nevertheless, the ice nucleation (IN) ability of bare BC and BC coated with secondary organic aerosol (SOA) material remains uncertain. We have systematically examined the IN ability of 100-400 nm size-selected BC particles with different morphologies and different SOA coatings representative of anthropogenic (toluene and n-dodecane) and biogenic (ß-caryophyllene) sources in the cirrus regime (-46 to -38 °C). Several aerosolized BC proxies were selected to represent different particle morphologies and oxidation levels. Atmospheric aging was further replicated with exposure of SOA-coated BC to OH. The results demonstrate that the 400 nm hydrophobic BC types nucleate ice only at or near the homogeneous freezing threshold (-42 to -46 °C). Deposition IN, as opposed to purely homogeneous freezing, was observed to occur for some BC types between 100-200 nm within the investigated temperature range. More fractal BC particles did not consistently act as superior deposition INPs over more spherical ones. SOA coating generated by oxidizing ß-caryophyllene with O3 did not seem to affect BC IN ability. However, SOA coatings generated from OH oxidation of various organic species did exhibit higher IN onset supersaturation ratio with respect to ice (SS) compared with bare BC particles, with toluene SOA coating showing an increase of SSi by 0.1-0.15 while still below the homogeneous threshold. n- dodecane and ß-caryophyllene-derived SOA only froze in the homogeneous regime. We attribute the inhibition of IN ability to the filling of the pores on the BC surface by the SOA material coating. OH exposure levels of all SOA coating experiments, from an equivalent atmospheric 10 days to 90 days, did not render significant differences in IN potential. Our study suggests that BC particles with large sizes and/or oxidized surfaces generally exhibit better IN ability, and that the organic coating materials can inhibit ice formation. [ABSTRACT FROM AUTHOR]

Submicron particles larger than 300nm dominate the aerosol light extinction and mass concentration in the atmosphere. The water uptake ability of this size range greatly influences the particle mass, visibility degradation and particle chemistry. However, most of previous field measurements on aerosol hygroscopicity are limited within 350nm. In this study, thesize-resolved aerosol hygroscopic properties over extended size range (50-600nm) at 85% relative humidity were investigated in Beijing winter from 27 November 2019 to 14 January 2020 using a Humidity Tandem Differential Mobility Analyser (HTDMA) instrument. The corresponding aerosol optical properties were also analyzed using the Mie scattering theory. Results show that the averaged probability distribution of GF (GF-PDF) is generally a constitute of a more-hygroscopic (MH) group and a less-hygroscopic (LH) group (including hydrophobic). For the particles larger than 300nm, there exist a large fraction of LH group particles, resulting in an unexpected low hygroscopicity. During the development of pollution when particles are gradually aged and accumulated, the bulk hygroscopicity above 300nm is enhanced significantly by the growth and expansion of MH group. This result is supported by previous chemical composition analysis and we give a more direct and detailed evidence from growth factor and mixing state aspects. Our calculations indicate that the optical contribution of particles larger than 300nm constitutes about 2/3 of the total aerosol extinction. The large hygroscopic variation of aerosols above 300nm will influence the light degradation comparably with the increase of aerosol loading in the low visibility haze events. Our studies highlight that the hygroscopic properties above 300nm are complex and vary greatly with different pollution stages, therefore more field measurements and investigations need to be done in the future. [ABSTRACT FROM AUTHOR]

Emissions of ice nucleating particles from sea spray can impact climate and precipitation by changing cloud formation, precipitation, and albedo. However, the relationship between seawater biogeochemistry and the ice nucleation activity of sea spray aerosols remains unclarified. Here, we demonstrate a link between the biological productivity in seawater and the ice nucleation activity of sea spray aerosol under conditions relevant to cirrus and mixed-phase cloud formation. We show for the first time that aerosol generated from both subsurface and microlayer seawater from the highly productive Eastern Tropical North Pacific Ocean are effective ice nucleating particles in the deposition and immersion freezing modes. Jet droplets aerosolized from the subsurface waters of highly productive regions may therefore be an unrealized source of effective INPs. In contrast, the subsurface water from the less productive Florida Straits produced less effective immersion mode INPs and ineffective depositional mode INPs. These results indicate that the regional biogeochemistry of seawater can strongly affect the ice nucleation activity of sea spray aerosol. [ABSTRACT FROM AUTHOR]

Large aldehydes are widespread in the atmosphere and their oxidation leads to secondary organic aerosols. The current understanding of their chemical transformation processes is limited to hydroxyl radical (OH) oxidation during daytime and nitrate radical (NO 3 ) oxidation during nighttime. Here, we report quantitative kinetics calculations of the reactions of hexanal (C 5 H 11 CHO), pentanal (C 4 H 9 CHO), and butanal (C 3 H 7 CHO) with hydroperoxyl radical (HO 2 ) at atmospheric temperatures and pressures. We find that neither tunneling nor multistructural torsion anharmonicity should be neglected in computing these rate constants; strong anharmonicity at the transition states is also important. We find rate constants for the three reactions in the range 3.2-7.7 × 10 -14 cm 3 molecule -1 s -1 at 298 K and 1 atm, showing that the HO 2 reactions can be competitive with OH and NO 3 oxidation under some conditions relevant to the atmosphere. Our findings reveal that HO 2 -initiated oxidation of large aldehydes may be responsible for the formation of highly oxygenated molecules via autoxidation. We augment the theoretic studies with laboratory flow-tube experiments using an iodide-adduct time-of-flight chemical ionization mass spectrometer to confirm the theoretical predictions of peroxy radicals and the autoxidation pathway. We find that the adduct from HO 2 + C 5 H 11 CHO undergoes a fast unimolecular 1,7-hydrogen shift with a rate constant of 0.45 s -1 . We suggest that the HO 2 reactions make significant contributions to the sink of aldehydes.