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Issue 22, 2014
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Complex refractive indices in the near-ultraviolet spectral region of biogenic secondary organic aerosol aged with ammonia

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Abstract

Atmospheric absorption by brown carbon aerosol may play an important role in global radiative forcing. Brown carbon arises from both primary and secondary sources, but the mechanisms and reactions of the latter are highly uncertain. One proposed mechanism is the reaction of ammonia or amino acids with carbonyl products in secondary organic aerosol (SOA). We generated SOA in situ by reacting biogenic alkenes (α-pinene, limonene, and α-humulene) with excess ozone, humidifying the resulting aerosol, and reacting the humidified aerosol with gaseous ammonia. We determined the complex refractive indices (RI) in the 360–420 nm range for these aerosols using broadband cavity enhanced spectroscopy (BBCES). The average real part (n) of the measured spectral range of the NH3-aged α-pinene SOA increased from n = 1.50 (±0.01) for the unreacted SOA to n = 1.57 (±0.01) after 1.5 h of exposure to 1.9 ppm NH3, whereas the imaginary component (k) remained below Image ID:c4cp01009d-t1.gif. For the limonene and α-humulene SOA the real part did not change significantly, and we observed a small change in the imaginary component of the RI. The imaginary component increased from k = 0.000 to an average k = 0.029 (±0.021) for α-humulene SOA, and from Image ID:c4cp01009d-t2.gif to an average k = 0.032 (±0.019) for limonene SOA after 1.5 h of exposure to 1.3 and 1.9 ppm of NH3, respectively. Collected filter samples of the aged and unreacted α-pinene SOA and limonene SOA were analyzed off-line by nanospray desorption electrospray ionization high resolution mass spectrometry (nano-DESI/HR-MS), and in situ using a Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS), confirming that the SOA reacted and that various nitrogen-containing reaction products formed. If we assume that NH3 aging reactions scale linearly with time and concentration, which will not necessarily be the case in the atmosphere, then a 1.5 h reaction with 1 ppm NH3 in the laboratory is equivalent to 24 h reaction with 63 ppbv NH3, indicating that the observed aerosol absorption will be limited to atmospheric regions with high NH3 concentrations.

Graphical abstract: Complex refractive indices in the near-ultraviolet spectral region of biogenic secondary organic aerosol aged with ammonia

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Publication details

The article was received on 09 Mar 2014, accepted on 10 Apr 2014 and first published on 10 Apr 2014


Article type: Paper
DOI: 10.1039/C4CP01009D
Citation: Phys. Chem. Chem. Phys., 2014,16, 10629-10642
  • Open access: Creative Commons BY license
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    Complex refractive indices in the near-ultraviolet spectral region of biogenic secondary organic aerosol aged with ammonia

    J. M. Flores, R. A. Washenfelder, G. Adler, H. J. Lee, L. Segev, J. Laskin, A. Laskin, S. A. Nizkorodov, S. S. Brown and Y. Rudich, Phys. Chem. Chem. Phys., 2014, 16, 10629
    DOI: 10.1039/C4CP01009D

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