Evolution of the light-absorption properties of combustion brown carbon aerosols following reaction with nitrate radicals

Figures & data

Figure 1. Schematic of the experimental setup for generating unoxidized and oxidized (with NO₃) BrC.

Figure 2. (a) Evolution of the imaginary part of refractive index (k) at λ = 473, 532, and 671 nm of light, medium, and dark BrC, quantified as the ratio of k of the oxidized BrC (k_oxidized) to k of the unoxidized BrC (k_unoxidized). (b) The corresponding evolution of the wavelength dependence (w). Smaller w is indicative of darker BrC. Error bars represent standard deviations over time for one experiment. Numerical values of k_unoxidized, k_oxidized, and w are given in Table S1.

Figure 3. AMS and SP-AMS UMR spectra of organic aerosol components in unoxidized light BrC and dark BrC aerosols.

Figure 4. (a) AMS and (b) SP-AMS HR spectra of organic and nitrate signals in the unoxidized light BrC, and (c) AMS and (d) SP-AMS HR spectra of organic and nitrate signals in the oxidized light BrC (exposed to NO₃ radicals at 2.4 nights of equivalent atmospheric NO₃ exposure).

Figure 5. Van Krevelen diagram showing the H/C versus O/C for unoxidized (open symbols) and oxidized (solid symbols) BrC. Symbols are colored by N/C. Error bars represent ±12% uncertainty in O/C and ±4% uncertainty in H/C (Canagaratna et al. 2015).

Figure 6. Calculated relative contribution of heterogeneous oxidation to adding nitrogen (ΔN_het/ΔN_tot) and oxygen (ΔO_het/ΔO_tot) to the condensed phase as a function of assumed average carbon number of the condensing SOA molecules for (a, b) light, (c, d) medium, and (e, f) dark BrC.

Figure 7. Effect of heterogeneous oxidation on light-absorption properties of the light, medium, and dark BrC. The bars represent ranges of k_het/k_unoxidized calculated based on Equation (3)(3) $$k_{\text{oxidized}}={\mathrm{ }\mathrm{y}}_{\text{SOA}}{k}_{\text{SOA}}+{\mathrm{ }\mathrm{y}}_{\text{het}}{k}_{\text{het}}$$(3) . They reflect the assumed range of k_SOA as well as the uncertainty in k_oxidized and k_unoxidized (Table S1). The diamonds represent k_oxidized/k_unoxidized (the same as in Figure 2), added here for reference to compare the effect of heterogeneous oxidation to the overall observed oxidation. Error bars represent standard deviations over time for one.

Canagaratna, M. R., J. L. Jimenez, J. H. Kroll, Q. Chen, S. H. Kessler, P. Massoli, L. Hildebrandt Ruiz, E. Fortner, L. R. Williams, K. R. Wilson, et al, 2015. Elemental ratio measurements of organic compounds using aerosol mass spectrometry: Characterization, improved calibration, and implications. Atmos. Chem. Phys. 15 (1):253–72. doi:10.5194/acp-15-253-2015.

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