Aiming at improving the efficiency of cavitation treatment, this work investigates characteristics of acoustic streaming and cavitation generated in water by dumbbell-shaped sonotrodes with plane, truncated and conical tips. The main emphasis was placed on elucidating the effects of tip shape and vibration amplitude ranged from 40 to 60 μm. The PIV technique and Weissler reaction were used to measure flow pattern and velocity of acoustic streaming, and cavitation efficiency, respectively. To provide a theoretical explanation to the experimental results, a self-developed mathematical model was used to simulate the acoustic streaming and predict the size of cavitation zone numerically. Both the experimental and numerical results revealed that the sonotrode tip shape affects the acoustic streaming significantly, altering the flow magnitude and direction from fast and downward under the plane and truncated tips to relatively slow and upward near the conical tip. Besides, the conical tip provides a more efficient cavitation treatment in comparison with the plane and truncated tips. The simulation results showed that widening of cavitation zone and altering of acoustic streaming velocity and direction near the sonotrode tip are responsible for the enhancement of cavitation treatment efficiency.
Nitric acid (HNO<sub>3</sub>) or nitrate (NO<sub>3</sub><sup>−</sup>) is the dominant sink for reactive nitrogen oxides (NO<sub>x</sub> = NO + NO<sub>2</sub>) in the atmosphere. In many Chinese cities, HNO<sub>3</sub> is becoming a significant contributor to acid deposition. In the present study, we used the denitrifier method to measure nitrogen (N) and oxygen (O) isotopic composition of NO<sub>3</sub><sup>−</sup> in 113 precipitation samples collected from Guangzhou City in southern China over a two-year period (2008 and 2009). We attempted to better understand the spatial and seasonal variability of atmospheric NO<sub>x</sub> sources and the NO<sub>3</sub><sup>−</sup> formation pathways in this N-polluted city in the Pearl River Delta region. The δ<sup>15</sup>N values of NO<sub>3</sub><sup>−</sup> (versus air N<sub>2</sub>) ranged from −4.9 to +10.1‰, and averaged +3.9‰ in 2008 and +3.3‰ in 2009. Positive δ<sup>15</sup>N values were observed throughout the year, indicating the anthropogenic contribution of NO<sub>x</sub> emissions, particularly from coal combustion. Different seasonal patterns of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> were observed between 2008 and 2009, which might reflect different human activities associated with the global financial crisis and the intensive preparations for the 16th Asian Games. Nitrate δ<sup>18</sup>O values (versus Vienna Standard Mean Ocean Water) varied from +33.4 to +86.5‰ (average +65.0‰ and +67.0‰ in 2008 and 2009, respectively), a range being lower than those reported for high altitude and polar areas. Several δ<sup>18</sup>O values were observed lower than the expected minimum of 50‰ at our study site. This was likely caused by the reaction of NO with peroxy radicals; peroxy radicals can compete with O<sub>3</sub> to convert NO to NO<sub>2</sub>, thereby donate O atoms with much lower δ<sup>18</sup>O value than that of O<sub>3</sub> to atmospheric NO<sub>3</sub><sup>−</sup>. Our results highlight that the influence of human activities on atmospheric chemistry can be recorded by the N and O isotopic composition of atmospheric NO<sub>3</sub><sup>−</sup> in a N-polluted city
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