Search

Understanding the near-ground vertical and temporal photochemical O3 formation mechanism is important to mitigate the O3 pollution. Here, we measured the vertical profiles of O3 and its precursors at six different heights from 5-335 m using a newly built vertical observation system in Pearl River Delta (PRD) region, China. The net photochemical ozone production rate (P (O3)net) and O3 formation sensitivities at various heights were diagnosed using an observation-based model coupled with the Master Chemical Mechanism (MCM v3.3.1). Moreover, for the assessment of model performance and the causative factors behind O3 pollution episodes, the net photochemical ozone production rate (P (O3)net) was measured at 5 m ground level utilizing a custom-built detection system. In total three O3 pollution episodes and two non-episodes were captured. The identified O3 pollution episodes were found to be jointly influenced by both photochemical production and physical transport, with local photochemical reactions play a dominate role. The high index of agreement (IOA) calculated from comparing the modelled and measured P (O3)net values indicated the rationality to investigate the vertical and temporal variability of O3 formation mechanism using modelling results. However, the measured P (O3)net values were generally higher than the modelled P (O3)net values, particularly under high NOx conditions, which may indicate a potential underestimation of total RO2 by the model. Throughout the measurement period, the contribution of different reaction pathways to O3 production remained consistent across various heights, with HO2+NO as the major O3 production pathway, followed by RO2+NO. We saw P (O3)net decreased with the increase of the measurement height, primarily attributed to the decreased O3 precursors anthropogenic organic compounds (AVOC) and oxygenated volatile organic compounds (OVOC). O3 formation regimes were similar at different heights during both episodes and non-episodes, which was located either in volatile organic compounds (VOCs) sensitive regime or in transition regime and more sensitive to VOCs. Diurnally, photochemical O3 formation typically remained in the VOCs sensitive regime during the morning and noon time, but in the transitional regime and more sensitive to VOCs in the afternoon at around 16:00 LT. The vertical and temporal O3 formation are most sensitive to AVOC and OVOC, which suggests that targeting VOCs, especially AVOC and OVOC, for control measures is more practical and feasible at the observation site. The vertical temporal analysis of O3 formation mechanisms near the ground surface in this study provides critical foundational knowledge for formulating effective short-term emergency and long-term strategies to combat O3 pollution in the PRD region of China. [ABSTRACT FROM AUTHOR]

Luminescent carbon dots (CDs) have received increasing attention from many fields during the past decade. Unfortunately, the luminescent mechanisms of CDs remain unclear due to insufficient experimental and theoretical knowledge, which significantly hinders the development of CDs with desired optical properties. Currently, surface states of CDs, which are based on synergistic hybridization between the carbon backbones and the connected functional groups, have been considered as the dominant luminescence origins. This tutorial paper, thus, aims to offer an overview of the key features on the surface of CDs, such as particle size, surface functional groups, defects and heteroatom doping, and their influences on the photoluminescence of CDs. In addition, optical characteristics of surface state-derived luminescence emissions of CDs are also summarized. Finally, the potential approaches of characterizing surface states of CDs are introduced, followed by an outlook of synthesizing high-quality CDs through modulation of the surface states. [ABSTRACT FROM AUTHOR]

Current process-based research mainly uses box models to evaluate photochemical ozone production and destruction rates, and it is unclear to what extent the photochemical reaction mechanisms are elucidated. Here, we modified and improved a net photochemical ozone production rate (NPOPR, P (O 3) net) detection system based on the current dual-channel reaction chamber technique, which makes the instrument applicable to different ambient environments, and its various operating indicators were characterized, i.e., "airtightness", light transmittance, wall losses of the reaction and reference chambers, conversion rate of O 3 to NO 2 , air residence time, and performance of the reaction and reference chambers. The limits of detection of the NPOPR detection system were determined to be 0.07, 1.4, and 2.3 ppbv h -1 at sampling flow rates of 1.3, 3, and 5 L min -1 , respectively. We further applied the NPOPR detection system to field observations at an urban site in the Pearl River Delta (China). During the observation period, the maximum value of P (O 3) net was 34.1 ppbv h -1 , which was ∼ 0 ppbv h -1 at night within the system detection error and peaked at approximately noon local time. The daytime (from 06:00–18:00 LT) average value of P (O 3) net was 12.8 (± 5.5) ppbv h -1. We investigated the detailed photochemical O 3 formation mechanism in the reaction and reference chambers of the NPOPR detection system using a zero-dimensional box model. We found that the photochemical reactions in the reaction chamber were very close to those in ambient air, but there was not zero chemistry in the reference chamber because the reaction related to the production and destruction of RO 2 (= HO 2 + RO 2) continued in the reference chamber, which led to a small amount of P (O 3) net. Therefore, the P (O 3) net measured here can be regarded as the lower limit of the real P (O 3) net in the atmosphere; however, the measured P (O 3) net was still ∼ 7.5 to 9.3 ppbv h -1 higher than the modeled P (O 3) net value depending on different modeling methods, which may be due to the inaccurate estimation of HO 2 / RO 2 radicals in the modeling study. Short-lived intermediate measurements coupled with direct P (O 3) net measurements are needed in the future to better understand O 3 photochemistry. Our results show that the NPOPR detection system can achieve high temporal resolution and continuous field observations, which helps us to better understand photochemical O 3 formation and provides a key scientific basis for continuous improvement of air quality in China. [ABSTRACT FROM AUTHOR]