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. 2022 Apr 19;56(8):4816-4827.
doi: 10.1021/acs.est.1c07328. Epub 2022 Apr 6.

Optical Properties of Secondary Organic Aerosol Produced by Photooxidation of Naphthalene under NOx Condition

Quanfu He  1 , Chunlin Li  1 , Kyla Siemens, Ana C Morales, Anusha Priyadarshani Silva Hettiyadura, Alexander Laskin, Yinon Rudich  1
Affiliations

Optical Properties of Secondary Organic Aerosol Produced by Photooxidation of Naphthalene under NOx Condition

Quanfu He et al. Environ Sci Technol. .

Abstract

Secondary organic aerosols (SOAs) affect incoming solar radiation by interacting with light at ultraviolet and visible wavelength ranges. However, the relationship between the chemical composition and optical properties of SOA is still not well understood. In this study, the complex refractive index (RI) of SOA produced from OH oxidation of naphthalene in the presence of nitrogen oxides (NOx) was retrieved online in the wavelength range of 315-650 nm and the bulk chemical composition of the SOA was characterized by an online high-resolution time-of-flight mass spectrometer. In addition, the molecular-level composition of brown carbon chromophores was determined using high-performance liquid chromatography coupled to a photodiode array detector and a high-resolution mass spectrometer. The real part of the RI of the SOA increases with both the NOx/naphthalene ratio and aging time, likely due to the increased mean polarizability and decreased molecular weight due to fragmentation. Highly absorbing nitroaromatics (e.g., C6H5NO4, C7H7NO4, C7H5NO5, C8H5NO5) produced under higher NOx conditions contribute significantly to the light absorption of the SOA. The imaginary part of the RI linearly increases with the NOx/VOCs ratio due to the formation of nitroaromatic compounds. As a function of aging, the imaginary RI increases with the O/C ratio (slope = 0.024), mainly attributed to the achieved higher NOx/VOCs ratio, which favors the formation of light-absorbing nitroaromatics. The light-absorbing enhancement is not as significant with extensive aging as it is under a lower aging time due to the opening of aromatic rings by reactions.

Keywords: NOx effect during photooxidation; atmospheric aging; chemical composition; chromophore characterization; optical properties; secondary organic aerosol.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Elemental ratios (H/C vs O/C) of naph-SOA produced from OH oxidation. Circles show the influence of NOx/naphthalene (N00–N40), and hexagon markers exhibit the effect of aging (A08–A49) on the bulk chemical composition of naph-SOA. Marker colors indicate the NOx/naphthalene ratio (0–3.2), while the marker size represents the aging time (0.8–4.9 days).
Figure 2
Figure 2
Complex refractive index (RI) of naph-SOA generated by OH oxidation. Panels (a, b) show BBCES-determined real and imaginary parts of RI for naph-SOA produced under various aging times (A08–A49). These two panels share the same bottom axis (wavelength) and legend. Panels (c, d) display the RI retrieved from PAS-CRD measurements at 404 nm for naph-SOA generated with different initial NOx/naphthalene ratios (c, N00–N40) and aging times (d, A08–A49), respectively. Pink squares and blue circles represent the real and imaginary parts of the RI, respectively. For BBCES, the uncertainties arise from the particle number concentration (0.3%), temperature (0.1%), pressure (0.3%), light intensity measurement (≪0.2%), and the Rayleigh scattering cross section of N2 (1%). The first three parameters also contribute to the uncertainties in PAS-CRD results. Moreover, uncertainty in the calibration (1.5%) was included in PAS results.
Figure 3
Figure 3
HPLC-PDA (a, b) and extracted HPLC-ESI/HRMS (c) chromatograms of the naph-SOA samples. Panels (a, b) are obtained for SOA generated under NOx/naphthalene ratios of 0 (N00, 3.2 days aging) and 2.7 (N40, 2.4 days aging), respectively. Possible molecular structures are proposed for these dominant absorbing species. Panel (c) shows a compilation of the selected extracted ion chromatograms (EICs) of the most abundant peaks. The color map in panels (a, b) represents the UV–vis absorbance. The left y-axis of panel (c) represents the intensities of C8H6O6, C11H12O7, C11H12O5, C10H9NO4, C8H5NO6, C7H7NO4, and C8H5NO5, while the right y-axis in panel (c) shows the signal intensities of C9H9NO6, C6H5NO4, C7H5NO5, and C7H6O3.
Figure 4
Figure 4
Evolution of optical properties of naph-SOA due to aging, as indicated by increasing H/C and O/C ratios, in the presence of NOx. The upper and lower panels show the change of the imaginary part and the real part as a function of nitrate fraction (fNO3), H/C ratio, and O/C ratio measured with HR-ToF-AMS. The dashed lines show the RI values at 330 nm (purple), 404 nm (cyan), and 532 nm (green).
Figure 5
Figure 5
RI (404 nm) changes as a function of the NOx/VOC ratio (a, b) and aging time, which is represented by O/C (c, d). Blue markers indicate the results from experiments N00–N40, which are conducted with a fixed aging time but with different NOx/VOC ratios. Red markers indicate results from experiments A08–A49, which were performed with different aging times and NOx/VOC ratios. Brown markers represent literature results at 405 nm for naph-SOA produced without NOx under different aging times.k obtained from experiment N40 was not included in the fitting (panel b) due to the shorter aging time than N00–N20. To constrain similar NOx/VOCs and O/C ratios for comparison, only part of the results is included in the fitting, as shown in panels (b)–(d).
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