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Stratosphere-to-troposphere transport results in the stratospheric intrusion (SI) of O3 into the free troposphere through the tropopause folding. However, the mechanism of SI influencing the atmospheric environment with the cross-layer transport of O3 from the stratosphere, free troposphere to the atmospheric boundary layer has not been elucidated thoroughly. In this study, a SI event over the North China Plain (NCP) was taken to investigate the mechanism of the cross-layer transport of stratospheric O3 with the impact on the near-surface O3 based on the multi-source reanalysis and observation data and air quality modeling. The results revealed a significant mechanism of stratospheric O3 intrusion to the atmospheric environment induced by an extratropical cyclone system. The SI with downward transport of stratospheric O3 to near-surface layer was driven by the extratropical cyclone system with vertical coupling of "upper westerly trough-middle the Northeast Cold Vortex (NECV)-lower extratropical cyclone" in the troposphere. The deep trough in the westerly jet aroused the tropopause folding, and the lower stratospheric O3 penetrated the folded tropopause into the upper and middle troposphere; the westerly trough was cut off to form a typical cold vortex in the upper and middle troposphere. The compensating downdrafts of the NECV pushed the further downward transport of stratospheric O3 in the free troposphere; The NECV activated an extratropical cyclone in the lower troposphere, and the vertical cyclonic circulation governed the stratospheric O3 from the free troposphere across the boundary layer top invading the near-surface atmosphere. In this SI event, the averaged contribution of stratospheric O3 to near-surface O3 was accounted for 26.77%. The proposed meteorological mechanism of vertical transport of stratospheric O3 into the near-surface atmosphere driven by an extratropical cyclone system could improve the understanding of the influence of stratospheric O3 on atmospheric environment with implications for the coordinated control of atmospheric pollution. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) affect secondary pollutant formation via active chemistry. Proton-transfer-reaction mass spectrometry (PTR-MS) is one of the most important techniques to study the highly variable spatial and temporal characteristics of VOCs. The response of protonated, adduct, and fragmented ions in PTR-MS in changing instrument settings and varying relative humidity (RH) requires rigorous characterization. Herein, dedicatedly designed laboratory experiments were conducted to investigate the response of these ions for 21 VOCs, including 12 oxygenated VOCs and two nitriles, using the recently developed Vocus PTR-MS. Our results show that the focusing ion-molecule reactor (FIMR) axial voltage increases sensitivity by three to four orders of magnitude but does not significantly change the fractions of protonated ions. Reducing the FIMR pressure, however, substantially increases fragmentation. Applying a high radio frequency (RF) amplitude radially on FIMR can enhance sensitivity by one to two orders of magnitude without affecting the protonated ion fractions. The change in big segmented quadrupole (BSQ) amplitude mainly affects sensitivity and protonated ion fraction by modifying ion transmission. The relationship between sensitivity and proton-transfer reaction rate constant is complicated by the influences from both ion transmission and protonated ion fraction. The protonated ions of most VOCs studied (19 out of 21) show less than 15% variations in sensitivity as RH increases from ~5% to ~85%, except for some long-chain aldehydes which show a positive RH variation of up to 30%. Our results suggest that the Vocus PTR-MS can reliably quantify the majority of VOCs under ambient conditions with varying RH. However, caution is advised for small oxygenates such as formaldehyde and methanol due to their low sensitivity, as well as for long-chain aldehydes for their slight RH dependence and fragmentation. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) affect secondary pollutant formation via active chemistry. Proton-transfer-reaction mass spectrometry (PTR-MS) is one of the most important techniques to study the highly variable spatial and temporal characteristics of VOCs. The response of protonated, adduct, and fragmented ions in PTR-MS in changing instrument settings and varying relative humidity (RH) requires rigorous characterization. Herein, dedicatedly designed laboratory experiments were conducted to investigate the response of these ions for 21 VOCs, including 12 oxygenated VOCs and two nitriles, using the recently developed Vocus PTR-MS. Our results show that the focusing ion-molecule reactor (FIMR) axial voltage increases sensitivity by three to four orders of magnitude but does not significantly change the fractions of protonated ions. Reducing the FIMR pressure, however, substantially increases fragmentation. Applying a high radio frequency (RF) amplitude radially on FIMR can enhance sensitivity by one to two orders of magnitude without affecting the protonated ion fractions. The change in big segmented quadrupole (BSQ) amplitude mainly affects sensitivity and protonated ion fraction by modifying ion transmission. The relationship between sensitivity and proton-transfer reaction rate constant is complicated by the influences from both ion transmission and protonated ion fraction. The protonated ions of most VOCs studied (19 out of 21) show less than 15 % variations in sensitivity as RH increases from ~5 % to ~85 %, except for some long-chain aldehydes which show a positive RH variation of up to 30 %. Our results suggest that the Vocus PTR-MS can reliably quantify the majority of VOCs under ambient conditions with varying RH. However, caution is advised for small oxygenates such as formaldehyde and methanol due to their low sensitivity, as well as for long-chain aldehydes for their slight RH dependence and fragmentation. [ABSTRACT FROM AUTHOR]