Distributed fiber sparse-wideband vibration sensing by sub-Nyquist additive random sampling.

The round-trip time of the light pulse limits the maximum detectable vibration frequency response range of phase-sensitive optical time domain reflectometry (ϕ-OTDR). Unlike the uniform laser pulse interval in conventional ϕ-OTDR, we randomly modulate the pulse interval so that an equivalent sub-Nyquist additive random sampling (sNARS) is realized for every sensing point of the long interrogation fiber. For a ϕ-OTDR system with 10 km sensing length, the sNARS method is optimized by theoretical analysis and Monte Carlo simulation, and the experimental results verify that a wideband sparse signal can be identified and reconstructed. Such a method can broaden the vibration frequency response range of ϕ-OTDR, which is of great significance in sparse-wideband-frequency vibration signal detection.

Significance of carbonyl compounds to photochemical ozone formation in a coastal city (Shantou) in eastern China.

Carbonyl compounds are ubiquitous in the troposphere, yet their contributions to ambient ozone (O3) formation have rarely been quantified in China. To better understand their roles in O3 pollution, a field campaign was conducted at an urban site of Shantou, a coastal city in eastern China, during 7th-29th October 2019. Seven carbonyls were quantified (average ± standard deviation: 14.42 ± 3.05 ppbv), among which formaldehyde (4.12 ± 1.02 ppbv), acetaldehyde (1.57 ± 0.30 ppbv), acetone (7.55 ± 2.10 ppbv), and methyl ethyl ketone (0.94 ± 0.28 ppbv) were the most abundant species. Relative incremental reactivity (RIR) analysis indicated that O3 formation in Shantou was VOC-limited, specifically most sensitive to carbonyls, and formaldehyde showed the largest RIR values in terms of individual species. Budgets of O3 and ROx (OH, HO2, and RO2) radicals were elucidated with a chemical box model. Carbonyls played a vital role in both the primary formation and recycling of the ROx; more than 80% of the primary source of HO2 and RO2 came from photolysis of formaldehyde and other oxygenated VOCs. Zero-out sensitivity studies showed that the seven measured carbonyls accounted for 37% of the peak net O3 production rate, mainly by affecting the concentrations of HO2 and RO2. These results highlight the significance of carbonyls, especially formaldehyde, to photochemical O3 formation, and carbonyls should be paid more attention to mitigate the worsening O3 pollution in China.