Coating enhancement of black carbon (BC) light absorption (E_abs) is a large uncertainty in modelling direct radiative forcing (DRF) by BC. Reported E_abs values after atmospheric aging vary widely and the mechanisms responsible for enhancing BC absorption remain elusive. Here, we report on the direct field measurement of size-resolved mixing state, E_abs, and aerosol single-scattering albedo (SSA) at λ = 532 nm at a rural site in east China from June to July 2016. Strong diurnal variability of E_abs, SSA, and O_x (O_x = NO₂ + O₃, a proxy for atmospheric photochemical aging) was observed. A method that combined E_abs and SSA was developed to retrieve the fraction contribution of BC absorption (f_BC), lensing-driven enhancement (f_Lens), as well as the fractional contribution of coating absorption (fraction absorption contribution (f_Shell), the coated shell diameter (D_Shell) and the imaginary part of the complex refractive index (CRI) of the shell (k_Shell)). Parameterization of E_abs and SSA captures much of the influence of BC coating and the particle absorption. In our measurements at this site, the results showed that the absorption amplification depended on the coating thickness and the absorption of coating materials, and photochemistry plays a role in modifying the absorption of BC-containing particles. The lensing-driven enhancement was reduced by light absorption of the shell. One implication of these findings is that the contribution of light-absorbing organic compounds (brown carbon, BrC) at a longer aging time should be included in climate models.

This article describes the development and field application of a portable broadband cavity enhanced spectrometer (BBCES) operating in the spectral range of 440-480 nm for sensitive, real-time, in situ measurement of ambient glyoxal (CHOCHO) and nitrogen dioxide (NO₂). The instrument utilized a custom cage system in which the same SMA collimators were used in the transmitter and receiver units for coupling the LED light into the cavity and collecting the light transmitted through the cavity. This configuration realised a compact and stable optical system that could be easily aligned. The dimensions and mass of the optical layer were 676 × 74 × 86 mm³ and 4.5 kg, respectively. The cavity base length was about 42 cm. The mirror reflectivity at λ = 460 nm was determined to be 0.9998, giving an effective absorption pathlength of 2.26 km. The demonstrated measurement precisions (1σ) over 60 s were 28 and 50 pptv for CHOCHO and NO₂ and the respective accuracies were 5% and 4%. By applying a Kalman adaptive filter to the retrieved concentrations, the measurement precisions of CHOCHO and NO₂ were improved to 8 pptv and 40 pptv in 21 s.

We report on the development of a cavity-enhanced aerosol single-scattering albedometer based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) combined with an integrating sphere (IS) for simultaneous in situ measurements of aerosol scattering and extinction coefficients in an exact same sample volume. The cavity-enhanced albedometer employed a blue light-emitting-diode (LED)-based IBBCEAS approach for the measurement of wavelength-resolved aerosol optical extinction over the spectral range of 445–480 nm and an integrating sphere nephelometer coupled to the IBBCEAS setup for the measurement of aerosol scattering. The scattering signal was measured with a single-channel photomultiplier tube (PMT), providing an averaged value over a narrow bandwidth (full-width at half-maximum, FWHM, ~ 9 nm) in the spectral region of 465–474 nm. A scattering coefficient at a wavelength of 470 nm was deduced as an averaged scattering value over the spectral region of 465–474 nm and used for data analysis and instrumental performance comparison. Performance evaluation of the albedometer was carried out using laboratory-generated particles and ambient aerosol. The scattering and extinction measurements of monodisperse polystyrene latex (PSL) spheres generated in the laboratory proved excellent correlation between two channels of the albedometer. The retrieved refractive index (RI) of the PSL particles from the measured scattering and extinction efficiencies agreed well with the values reported in previously published papers. Aerosol light scattering and extinction coefficients, single-scattering albedo (SSA) and NO₂ concentrations in an ambient sample were directly and simultaneously measured using the albedometer developed. The instrument developed was validated via an intercomparison of the measured aerosol scattering coefficients and NO₂ trace gas concentrations to a TSI 3563 integrating nephelometer and a chemiluminescence detector, respectively.

The optical properties and chemical composition of PM_1.0 particles in a suburban environment (Huairou) near the megacity of Beijing were measured during the HOPE-J³A (Haze Observation Project Especially for Jing–Jin–Ji Area) field campaign. The campaign covered the period November 2014 to January 2015 during the winter coal heating season. The average values and standard deviations of the extinction, scattering, absorption coefficients, and the aerosol single scattering albedo (SSA) at λ = 470 nm during the measurement period were 201 ± 240, 164 ± 202, 37 ± 43 Mm⁻¹, and 0.80 ± 0.08, respectively. The average values for the real and imaginary components of the effective complex refractive index (CRI) over the campaign were 1.40 ± 0.06 and 0.03 ± 0.02, while the average mass scattering and absorption efficiencies (MSEs and MAEs) of PM_1.0 were 3.6 and 0.7 m² g⁻¹, respectively. Highly time-resolved air pollution episodes clearly show the dramatic evolution of the PM_1.0 size distribution, extensive optical properties (extinction, scattering, and absorption coefficients), and intensive optical properties (SSA and CRI) during haze formation, development, and decline. Time periods were classified into three different pollution levels (clear, slightly polluted, and polluted) for further analysis. It was found that (1) the relative contributions of organic and inorganic species to observed aerosol composition changed significantly from clear to polluted days: the organic mass fraction decreased from 50 to 43 % while the proportion of sulfates, nitrates, and ammonium increased strongly from 34 to 44 %. (2) Chemical apportionment of extinction, calculated using the IMPROVE algorithm, tended to underestimate the extinction compared to measurements. Agreement with measurements was improved by modifying the parameters to account for enhanced absorption by elemental carbon (EC). Organic mass was the largest contributor (52 %) to the total extinction of PM_1.0, while EC, despite its low mass concentration of ∼ 4 %, contributed about 17 % to extinction. When the air quality deteriorated, the contribution of nitrate aerosol increased significantly (from 15 % on clear days to 22 % on polluted days). (3) Under polluted conditions, the average MAEs of EC were up to 4 times as large as the reference MAE value for freshly generated black carbon (BC). The temporal pattern of MAE values was similar to that of the OC / EC ratio, suggesting that non-BC absorption from secondary organic aerosol also contributes to particle absorption.

The influences of ammonia (NH ₃ ) on secondary organic aerosol (SOA) formation from ozonolysis of styrene have been investigated using chamber experiments and quantum chemical calculations. With the value of [O ₃ ] ₀ /[styrene] ₀ ratios between 2 and 4, chamber experiments were carried out without NH ₃ or under different [NH ₃ ]/[styrene] ₀ ratios. The chamber experiments reveal that the addition of NH ₃ led to significant decrease of SOA yield. The overall SOA yield decreased with the [NH ₃ ] ₀ /[styrene] ₀ increasing. In addition, the addition of NH ₃ at the beginning of the reaction or several hours after the reaction occurs had obviously different influence on the yield of SOA. Gas phase reactions of Criegee intermediates (CIs) with aldehydes and NH ₃ were studied in detail by theoretical methods to probe into the mechanisms behind these phenomena. The calculated results showed that 3,5-diphenyl-1,2,4-trioxolane, a secondary ozonide formed through the reactions of C ₆ H ₅ ĊHOO· with C ₆ H ₅ CHO, could make important contribution to the aerosol composition. The addition of excess NH ₃ may compete with aldehydes, decreasing the secondary ozonide yield to some extent and thus affect the SOA formation.