Plant-derived Secondary Organic Material in the Air and Ecosystems.

Biogenic secondary organic aerosol (SOA) and deposited secondary organic material (SOM) are formed by oxidation of volatile organic compounds (VOCs) emitted by plants. Many SOA compounds have much longer chemical lifetimes than the original VOC, and may accumulate on plant surfaces and in soil as SOM because of their low volatility. This suggests that they may have important and presently unrecognized roles in plant adaptation. Using reactive plant terpenoids as a model we propose a three-tier (atmosphere-vegetation-soil) framework to better understand the ecological and evolutionary functions of SOM. In this framework, SOA in the atmosphere is known to affect solar radiation, SOM on the plant surfaces influences the interactive organisms, and wet and dry deposition of SOM on soil affects soil organisms.

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

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 k < 0.001((+0.002)(-0.001)). 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 k < 0.001((+0.002)(-0.001)) 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.

Ultrafast photochemistry of methyl hydroperoxide on ice particles.

Simulations show that photodissociation of methyl hydroperoxide, CH(3)OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH(3)OOH is found to prefer the exterior of the cluster, with the CH(3)O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH(3)O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH(3)O to form CH(3)OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH(2)O, H(2)O, H(2), CO, CH(3)OH, and H(2)O(2).