|Title||Optical property measurements and single particle analysis of secondary organic aerosol produced from the aqueous-phase reaction of ammonium sulfate with methylglyoxal|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Kwon D., Or V.W, Sovers M.J, Tang M.J, Kleiber P.D, Grassian VH, Young M.A|
|Type of Article||Article|
|Keywords||atmospheric aerosols; atmospheric chemistry; atomic force microscopy; brown carbon; brown carbon formation; cavity ring; cavity ring-down; chemistry; constants; extinction; Geochemistry & Geophysics; glyoxal; hygroscopic growth; mixed particles; optical; refractive index; refractive-index; region; ring-down spectroscopy; secondary organic aerosol; spectroscopy; ultraviolet spectral|
Reactions involving the dicarbonyl species methylglyoxal (MG) have been suggested as an important pathway for the production of secondary organic aerosol (SOA) in the atmosphere. Reaction in an aqueous inorganic salt solution, such as ammonium sulfate (AS), leads to the formation of light-absorbing brown carbon (BrC) product. We report on an investigation of the optical properties of BrC aerosol generated from the aqueous-phase reaction between MG and AS as a function of aging time using calibrated cavity ring-down spectroscopy (CRDS) at a wavelength of 403 nm. The retrieved real index of refraction at 403 nm is n = 1.558 +/- 0.021, with an imaginary index value of k = 0.002 +/- 0.004; these values do not appear to change significantly with aging time over the course of 22 days and are similar to the AS aerosol values. The small complex index suggests that BrC aerosol formed from this pathway may not significantly impact radiative forcing. Measurements of the aerosol optical properties show significant deviation from Mie theory simulations for particles with diameters of greater than or similar to 500 nm, probably as a result of nonspherical particle shape effects. In addition to the CRDS study, we used UV-vis spectroscopy to measure the mass absorption coefficient of the solution-phase reaction products as a function of aging. We also employed atomic force microscopy (AFM)-based IR spectroscopy to investigate the morphology and chemical composition of single SOA particles. AFM analysis of the particle morphology shows that a significant fraction of BrC particles with diameters of greater than or similar to 500 nm are nonspherical in shape, consistent with our observed breakdown in the applicability of Mie theory for larger particles.