Complex refractive index, single scattering albedo, and mass absorption coefficient of secondary organic aerosols generated from oxidation of biogenic and anthropogenic precursors

Abstract

Refractive index and optical properties of biogenic and anthropogenic secondary organic aerosol (SOA) particles were investigated. Aerosol precursors, namely longifolene, a-pinene, 1-methylnaphthalene, phenol, and toluene were oxidized in a Teflon chamber to produce SOA particles under different initial hydrocarbon concentrations and hydroxyl radical sources, reflecting exposures to different levels of nitrogen oxides (NO x). The real and imaginary components (n and k, respectively) of the refractive index at 375 nm and 632 nm were determined by Mie theory calculations through an iterative process, using the v 2 function to evaluate the fitness of the predicted optical parameters with the measured scattering, absorption, and extinction coefficients from a Photoacoustic Extinctiometer and Cavity Attenuated Phase Shift Spectrometer. Single scattering albedo (SSA) and bulk mass absorption coefficient (MAC) at 375 nm were calculated. SSA values of SOA particles from biogenic precursors (longifolene and a-pinene) were $0.98-0.99 ($6.3% uncertainty), reflecting purely scattering aerosols regardless of the NO x regime. However, SOA particles from aromatic precursors were more absorbing and displayed NO x-dependent SSA values. For 1-methylnaphthalene SOA particles, SSA values of 0.92-0.95 and $0.75-0.90 ($6.1% uncertainty) were observed under intermediate-and high-NO x conditions, respectively, reflecting the absorbing effects of SOA particles and NO x chemistry for this aromatic system. In mixtures of longifolene and phenol or longifolene and toluene SOA under intermediate-and high-NO x conditions, k values of the aromatic-related component of the SOA mixture were higher than that of 1-methylnaphthalene SOA particles. With the increase in OH exposure, k phenol decreased from 0.10 to 0.02 and 0.22 to 0.05 for intermediateand high-NO x conditions, respectively. A simple relative radiative forcing calculation for urban environments at k ¼ 375 nm suggests the influence of absorbing SOA particles on relative radiative forcing at this wavelength is most significant for aerosol sizes greater than 0.4 mm.

Influence of Ammonia and Relative Humidity on the Formation and Composition of Secondary Brown Carbon from Oxidation of 1-Methylnaphthalene and Longifolene

“…The SSA is shown as a function of size parameter at 375 nm. The SSA in the presence and absence of NH 3 remained very high at larger aerosol sizes (with SSA max = 0.95−1), 51 suggesting that aerosols formed from OH oxidation under high-NO x conditions were mostly scattering regardless of NH 3 concentration. Additionally, the MAC org values of longifolene SOA (<0.2 m 2 g −1 ) were significantly lower than the MAC org values from previous studies on absorbing SOA, 4 indicating that the longifolene SOA formed in the current experiments was only weakly absorbing.…”

“…30 Previous work investigated 1-MN SOA formation in a variety of NO x regimes and found that the chemical and physical properties of 1-MN SOA were significantly impacted by NO x conditions. 30,48,51 We hypothesize that, in the presence of NH 51 Essentially, the chamber is a 1.5−2.0 m 3 PFA Teflon bag in a metallic frame enclosure. The initial RH in the chamber was adjusted by injecting dry zero air and water vapor for different experiments resulting in average RH 40−45% for humid experiments and <20% for dry experiments.…”

“…The resulting average concentrations of OH from the above sources in the chamber were calculated as 6.3 × 10 6 and 3.6 × 10 7 molecules cm −3 , respectively. 51 Details of these estimations are presented in the Supporting Information. For high-NO x experiments, we injected additional NO to achieve the initial NO x concentrations of ∼500 ppbv.…”

“…Four of the experiments described by Dingle et al are also presented in the following section for comparison with the high-NH 3 experiments and to better illustrate the influence of NH 3 on the chemical and optical properties of SOA. 51 Once the gas-phase concentration of the parent hydrocarbon was stable in the chamber, irradiation by UV lights initiated photooxidation. A significant amount of SOA particles (with maximum volume concentration ≈30−95 μm 3 cm −3 under different experimental conditions) from VOCs (longifolene or 1-MN) were generated by nucleation and condensation (Figure S1).…”

Climatology of aerosol optical depth over Eastern Europe based on 19 years (2000–2018) MODIS TERRA data

“…Anthropogenic and coarse mode of aerosols, which may impede aerosol properties measurement due to incorrect selection of the aerosol model in the retrieval algorithm (Bilal et al, ), prevail here, where only 68.6% and 55.3% observations are within EE. These results may be associated with estimated surface reflectance and aerosol models at low and medium aerosol loads and with underestimation of surface reflectance and single scattering albedo of aerosols (Dingle et al, ). Meanwhile for Moscow_MSU_MO station there was observed positive bias at low and medium aerosol loads with overestimate of search results at low MAE (0.067) and RMSE (0.130) values and overestimation of retrieval algorithm with RMB = 1.10.…”