Stratospheric influence on surface ozone pollution in China.

Events of stratospheric intrusions to the surface (SITS) can lead to severe ozone (O3) pollution. Still, to what extent SITS events impact surface O3 on a national scale over years remains a long-lasting question, mainly due to difficulty of resolving three key SITS metrics: frequency, duration and intensity. Here, we identify 27,616 SITS events over China during 2015-2022 based on spatiotemporally dense surface measurements of O3 and carbon monoxide, two effective indicators of SITS. An overview of the three metrics is presented, illustrating large influences of SITS on surface O3 in China. We find that SITS events occur preferentially in high-elevation regions, while those in plain regions are more intense. SITS enhances surface O3 by 20 ppbv on average, contributing to 30-45% of O3 during SITS periods. Nationally, SITS-induced O3 peaks in spring and autumn, while over 70% of SITS events during the warm months exacerbate O3 pollution. Over 2015-2022, SITS-induced O3 shows a declining trend. Our observation-based results can have implications for O3 mitigation policies in short and long terms.

Increasing trend of lightning activity in the South Asia region.

Lightning is an important natural source of wildfires and oxynitride, and hence significantly influences ecological systems and atmospheric chemistry. Here, we choose South Asia, an important region for global water reallocation and global climate changes, to examine lightning variations based on the longest existing lightning dataset from the OTD/LIS observations. We identify a clear increase in lightning density in the research region, increasing at a rate of 0.096 fl km-2 a-1 over the last two decades. Multiple linear regression analysis is adopted to identify the main influencing factors among ten potential thermodynamic or microphysical factors and the crucial areas contributing to the increases in lightning. The surface latent heat flux along the west coast of the Indian subcontinent is the largest contributor, explaining 52% of the lightning variance and contributing to a 0.025 fl km-2 a-1 increase. The sea surface temperature in the Arabian Sea, the convective available potential energy (CAPE) over the northwestern Indian subcontinent, and the wind shear along the northwestern coast also make important contributions to the lightning increase, indicating that the thermodynamic effects overwhelm the microphysical effects on lightning activity over the South Asia region.

The intensification of metallic layered phenomena above thunderstorms through the modulation of atmospheric tides.

We present a multi-instrument experiment to study the effects of tropospheric thunderstorms on the mesopause region and the lower ionosphere. Sodium (Na) lidar and ionospheric observations by two digital ionospheric sounders are used to study the variation in the neutral metal atoms and metallic ions above thunderstorms. An enhanced ionospheric sporadic E layer with a downward tidal phase is observed followed by a subsequent intensification of neutral Na number density with an increase of 600 cm-3 in the mesosphere. In addition, the Na neutral chemistry and ion-molecule chemistry are considered in a Na chemistry model to simulate the dynamical and chemical coupling processes in the mesosphere and ionosphere above thunderstorms. The enhanced Na layer in the simulation obtained by using the ionospheric observation as input is in agreement with the Na lidar observation. We find that the intensification of metallic layered phenomena above thunderstorms is associated with the atmospheric tides, as a result of the troposphere-mesosphere-ionosphere coupling.

Channel branching and zigzagging in negative cloud-to-ground lightning.

A fundamental question in lightning flash concerns why the discharge channel propagates in a zig-zag manner and produces extensive branches. Here we report the optical observation of two negative cloud-to-ground lightning discharges with very high temporal resolution of 180,000 frames per second, which shows in detail the dependence of channel branching and tortuous behavior on the stepping process of the leader development. It is found that the clustered space leaders formed in parallel ahead of the channel tip during an individual step process. The leader branching is due to the multiple connection of the clustered space leaders with the same root channel tip, which occur almost simultaneously, or successively as some space leaders/stems resurrect after interruption. Meanwhile, the irregularity of angles between the clustered space leaders and the advancing direction of leader tip is the origin of channel tortuosity. The statistical analysis on 96 steps shows a geometric-mean value of 4.4 m for the step length, ranging between 1.3 and 8.6 m, while the distance from the center of space leader to the channel is 3.6 m, ranging between 2.1 and 6.9 m. More than 50% steps occurred within an angle range of ±30° from the advancing direction of the leader.

[The spectral properties of an intense lightning return stroke].

The spectrum in the range of 400-600 nm from the first return stroke of an intense cloud-to-ground lightning flash was obtained by a slit-free spectrograph. Applying the atomic structure theory to the research work on lightning spectra, the wavelengths, oscillator strengths and excitation energies of upper levels were calculated for the transitions of related lightning spectrum. Multi-configuration Dirac-Fock method was employed in the calculation. From the results, re-identifications were carried out for the lines of 419.0 and 425.3 nm. It was found by spectral analysis combined with corresponding electrical information finds that the spectrum characteristic is closely related to the intensity of lightning discharge, as during an intense lightning return stroke the lines of O II with high excitation energies are enhanced.

[Temperature study on lightning return stroke in the coastal area of Guangdong].

In the coastal area of Guangdong, slit-less spectra for first return strokes of lightning between the cloud and ground were obtained. After spectrum analysis and line identification were done by using multi-configuration Dirac-Fock (MCDF) method, parameters like wavelengths, oscillator strengths, transition probabilities, and excitation energies were calculated. According to the relative intensities of lines and transition parameters, temperatures for individual lightning strokes and at different heights of the discharge channel were calculated by using multiple-line method. The result shows that the temperatures in return stroke channel varied from stroke to stroke. In general, the more intensive the lightning discharge, the higher the value of channel temperature. On the other hand, for a certain return stroke channel, the temperatures at different positions show signs of falling away with increasing height along the discharge channel. Compared with that of Qinghai plateau, lightning in Guangdong coastal area is more intensive, and the lines from higher excitation potential of N II in these lightning spectra are easier observed.