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Optical properties of secondary organic aerosols generated by photooxidation of aromatic hydrocarbons.

Li K, Wang W, Ge M, Li J, Wang D - Sci Rep (2014)

Bottom Line: The retrieved RIs at 532 nm for the SOAs range from 1.38-1.59, depending on several factors, such as different precursors and NOx levels.The RIs of the SOAs are altered differently as the NOx concentration increases as follows: the RIs of the SOAs derived from benzene and toluene increase, whereas those of the SOAs derived from ethylbenzene and m-xylene decrease.Finally, by comparing the experimental data with the model values, we demonstrate that the models likely overestimate the RI values of the SOA particles to a certain extent, which in turn overestimates the global direct radiative forcing of the organic particles.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China.

ABSTRACT
The refractive index (RI) is the fundamental characteristic that affects the optical properties of aerosols, which could be some of the most important factors influencing direct radiative forcing. The secondary organic aerosols (SOAs) generated by the photooxidation of benzene, toluene, ethylbenzene and m-xylene (BTEX) under low-NOx and high-NOx conditions are explored in this study. The particles generated in our experiments are considered to be spherical, based on atomic force microscopy (AFM) images, and nonabsorbent at a wavelength of 532 nm, as determined by ultraviolet-visible light (UV-Vis) spectroscopy. The retrieved RIs at 532 nm for the SOAs range from 1.38-1.59, depending on several factors, such as different precursors and NOx levels. The RIs of the SOAs are altered differently as the NOx concentration increases as follows: the RIs of the SOAs derived from benzene and toluene increase, whereas those of the SOAs derived from ethylbenzene and m-xylene decrease. Finally, by comparing the experimental data with the model values, we demonstrate that the models likely overestimate the RI values of the SOA particles to a certain extent, which in turn overestimates the global direct radiative forcing of the organic particles.

No MeSH data available.


Related in: MedlinePlus

Reaction profiles.(a) The low-NOx experiment (200 ppb m-xylene, 5 ppm H2O2); (b) the classical high-NOx experiment (4 ppm ethylbenzene, 3 ppb NO and 480 ppb NO2); (c) the HONO experiment (200 ppb m-xylene, 442 ppb NO and 465 ppb NO2).
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f1: Reaction profiles.(a) The low-NOx experiment (200 ppb m-xylene, 5 ppm H2O2); (b) the classical high-NOx experiment (4 ppm ethylbenzene, 3 ppb NO and 480 ppb NO2); (c) the HONO experiment (200 ppb m-xylene, 442 ppb NO and 465 ppb NO2).

Mentions: As illustrated in Fig. 1, three different NOx conditions were investigated, the details of which are given in the Methods section. Fig. 1a displays a typical reaction profile under the low-NOx condition. In this experiment, the initial m-xylene and H2O2 concentrations were 200 ppb and 5 ppm, respectively. During the entire process, 121.4 ppb m-xylene was consumed. The particles were generated just within a few minutes after the lights were turned on. A small amount of O3 (<20 ppb) was produced during this reaction. After more than 4 hours of illumination, the particle size tended to be stable at approximately 200 nm, and the mass concentration (M) was approximately 350 μg/m3 after a wall-loss correction using the method introduced by McMurry and Grosjean33. The SMPS data (M) in Fig. 1 shows that the mass concentration plateaued at the end of the experiment, indicating that the status in the chamber tend to stabilize. Fig. 1b shows a representative reaction profile under the classical high-NOx condition. In this experiment, the initial ethylbenzene, NO and NO2 concentrations were 4 ppm, 3 ppb and 480 ppb, respectively. During the photooxidation, 402 ppb ethylbenzene was reacted. No particles were generated during the first 2.5 hours. Particles appeared just as the NO concentration approached zero, which is consistent with other classical high-NOx studies161934. At the end of the experiment, the particle surface mean diameter was approximately 300 nm, and the mass concentration was greater than 40 μg/m3. A considerable amount of O3 was generated during the reaction process, especially after particles were generated, leading to a high concentration of greater than 300 ppb when the reaction was complete. Fig. 1c is the reaction profile of a HONO experiment. The initial m-xylene, NO and NO2 concentrations were 200 ppb, 442 ppb and 465 ppb, respectively. During the entire process, 117.2 ppb m-xylene was consumed, yielding approximately 400 μg of secondary particles. Aerosol generation occurred almost immediately, even when the NO concentration was high. The final size mode was approximately 450 nm, which was significantly larger than with the other two conditions. Because the NO concentration was high, the formation of O3 was restrained19.


Optical properties of secondary organic aerosols generated by photooxidation of aromatic hydrocarbons.

Li K, Wang W, Ge M, Li J, Wang D - Sci Rep (2014)

Reaction profiles.(a) The low-NOx experiment (200 ppb m-xylene, 5 ppm H2O2); (b) the classical high-NOx experiment (4 ppm ethylbenzene, 3 ppb NO and 480 ppb NO2); (c) the HONO experiment (200 ppb m-xylene, 442 ppb NO and 465 ppb NO2).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4017213&req=5

f1: Reaction profiles.(a) The low-NOx experiment (200 ppb m-xylene, 5 ppm H2O2); (b) the classical high-NOx experiment (4 ppm ethylbenzene, 3 ppb NO and 480 ppb NO2); (c) the HONO experiment (200 ppb m-xylene, 442 ppb NO and 465 ppb NO2).
Mentions: As illustrated in Fig. 1, three different NOx conditions were investigated, the details of which are given in the Methods section. Fig. 1a displays a typical reaction profile under the low-NOx condition. In this experiment, the initial m-xylene and H2O2 concentrations were 200 ppb and 5 ppm, respectively. During the entire process, 121.4 ppb m-xylene was consumed. The particles were generated just within a few minutes after the lights were turned on. A small amount of O3 (<20 ppb) was produced during this reaction. After more than 4 hours of illumination, the particle size tended to be stable at approximately 200 nm, and the mass concentration (M) was approximately 350 μg/m3 after a wall-loss correction using the method introduced by McMurry and Grosjean33. The SMPS data (M) in Fig. 1 shows that the mass concentration plateaued at the end of the experiment, indicating that the status in the chamber tend to stabilize. Fig. 1b shows a representative reaction profile under the classical high-NOx condition. In this experiment, the initial ethylbenzene, NO and NO2 concentrations were 4 ppm, 3 ppb and 480 ppb, respectively. During the photooxidation, 402 ppb ethylbenzene was reacted. No particles were generated during the first 2.5 hours. Particles appeared just as the NO concentration approached zero, which is consistent with other classical high-NOx studies161934. At the end of the experiment, the particle surface mean diameter was approximately 300 nm, and the mass concentration was greater than 40 μg/m3. A considerable amount of O3 was generated during the reaction process, especially after particles were generated, leading to a high concentration of greater than 300 ppb when the reaction was complete. Fig. 1c is the reaction profile of a HONO experiment. The initial m-xylene, NO and NO2 concentrations were 200 ppb, 442 ppb and 465 ppb, respectively. During the entire process, 117.2 ppb m-xylene was consumed, yielding approximately 400 μg of secondary particles. Aerosol generation occurred almost immediately, even when the NO concentration was high. The final size mode was approximately 450 nm, which was significantly larger than with the other two conditions. Because the NO concentration was high, the formation of O3 was restrained19.

Bottom Line: The retrieved RIs at 532 nm for the SOAs range from 1.38-1.59, depending on several factors, such as different precursors and NOx levels.The RIs of the SOAs are altered differently as the NOx concentration increases as follows: the RIs of the SOAs derived from benzene and toluene increase, whereas those of the SOAs derived from ethylbenzene and m-xylene decrease.Finally, by comparing the experimental data with the model values, we demonstrate that the models likely overestimate the RI values of the SOA particles to a certain extent, which in turn overestimates the global direct radiative forcing of the organic particles.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China.

ABSTRACT
The refractive index (RI) is the fundamental characteristic that affects the optical properties of aerosols, which could be some of the most important factors influencing direct radiative forcing. The secondary organic aerosols (SOAs) generated by the photooxidation of benzene, toluene, ethylbenzene and m-xylene (BTEX) under low-NOx and high-NOx conditions are explored in this study. The particles generated in our experiments are considered to be spherical, based on atomic force microscopy (AFM) images, and nonabsorbent at a wavelength of 532 nm, as determined by ultraviolet-visible light (UV-Vis) spectroscopy. The retrieved RIs at 532 nm for the SOAs range from 1.38-1.59, depending on several factors, such as different precursors and NOx levels. The RIs of the SOAs are altered differently as the NOx concentration increases as follows: the RIs of the SOAs derived from benzene and toluene increase, whereas those of the SOAs derived from ethylbenzene and m-xylene decrease. Finally, by comparing the experimental data with the model values, we demonstrate that the models likely overestimate the RI values of the SOA particles to a certain extent, which in turn overestimates the global direct radiative forcing of the organic particles.

No MeSH data available.


Related in: MedlinePlus