Nonlinear Photochemistry Research Articles

Development of a recurrent spatiotemporal deep-learning method coupled with data fusion for correction of hourly ozone forecasts

Ambient ozone (O3) predictions can be very challenging mainly due to the highly nonlinear photochemistry among its precursors, and meteorological conditions and regional transport can further complicate the O3 formation processes. The emission-based chemical transport models (CTM) are broadly used to predict O3 formation, but they may deviate from observations due to input uncertainties such as emissions and meteorological data, in addition to the treatment of O3 nonlinear chemistry. In this study, an innovative recurrent spatiotemporal deep-learning (RSDL) method with model-monitor coupled convolutional recurrent neural networks (ConvRNN) has been developed to improve O3 predictions of CTM. The RSDL method was first used to build the ConvRNN within a 24-h scale to characterize the spatiotemporal relationships between the monitored O3 data and CTM simulations, and then incorporated the recurrent pattern to achieve 72-h multi-site forecasts based on a pilot study over the Pearl River Delta (PRD) region of China. The results showed that the RSDL method predicted O3 with high accuracy over this case study, with an increase of 27.54% in the correlation coefficient (R) average for all sites as well as an increase in R of 0.14–0.21 for all cities compared to CTM. Moreover, the regional distribution of CTM was further improved by the RSDL predictions with the data fusion technique, which greatly reduced the underpredictions of O3 concentrations, particularly in high O3-level areas (concentrations >160 μg/m3), with a 33.55% reduction in the mean absolute error (MAE).

Control of fine particulate nitrate during severe winter haze in “2+26” cities

The “2+26” cities, suffering the most severe winter haze pollution, have been the key region for air quality improvement in China. Increasing prominent nitrate pollution is one of the most challenging environmental issues in this region, necessitating development of an effective control strategy. Herein, we use observations, and state-of-the-art model simulations with scenario analysis and process analysis to quantify the effectiveness of the future SO2-NOX-VOC-NH3 emission control on nitrate pollution mitigation in “2+26” cities. Focusing on a serious winter haze episode, we find that limited NOX emission reduction alone in the short-term period is a less effective choice than VOC or NH3 emission reduction alone to decrease nitrate concentrations, due to the accelerated NOX-HNO3 conversion by atmospheric oxidants and the enhanced HNO3 to NO3− partition by ammonia, although deep NOX emission reduction is essential in the long-term period. The synergistic NH3 and VOC emission control is strongly recommended, which can counteract the adverse effects of nonlinear photochemistry and aerosol chemical feedback to decrease nitrate more. Such extra benefits will be reduced if the synergistic NH3 and VOC reduction is delayed, and thus reducing emission of multiple precursors is urgently required for the effective control of increasingly severe winter nitrate pollution in “2+26” cities.

Probing neuronal functions with precise and targeted laser ablation in the living cortex: comment

The paper [Optica 8, 1559 (2021)OPTIC82334-253610.1364/OPTICA.433562] is devoted to probing neuronal functions by targeted cell ablation in living cortex. It demonstrates the advantage of cell ablation by single high-energy pulses from an amplified femtosecond (fs) laser system, which leaves adjacent structures intact. The single-pulse effects rely on mechanical disruption by laser-induced microcavitation, which goes along with a lack of diffusive collateral damage. In comparison, cell damage by low-energy pulses from fs laser oscillators is shown to be associated with a damage zone with tens of micrometer radius around the targeted cell. The collateral damage is attributed to heat accumulation and diffusion. We demonstrate through numerical simulations of plasma formation and temperature evolution for the experimentally employed irradiation conditions that a large number of free electrons is produced but the temperature rise at focus is only 0.3 K. Therefore, the damage is caused by nonlinear photochemistry and plasma-mediated chemical effects rather than by thermal effects.

Persistent Heavy Winter Nitrate Pollution Driven by Increased Photochemical Oxidants in Northern China.

Nitrate is an increasingly important component of fine particulate matter (PM2.5) during winter in northern China. Past emission control has been ineffective in reducing winter nitrate. Here, we use extensive observations and a model with state-of-the-art nitrogen chemistry to identify the key factors that control the nitrate formation in the heavily polluted North China Plain (NCP). In contrast to the previous view of weak winter photochemistry, we show that the O3 and OH productions are sufficiently high in winter to facilitate fast gas-phase and heterogeneous conversion of NOX to nitrate over the NCP. Increasing O3 and OH productions from higher precursor levels and fast ROX cycling accelerate the nitrate generation during heavy pollution. We find that the 31.8% reduction of NOX emissions from 2010 to 2017 in the NCP lowers surface nitrate by only 0.2% and even increases nitrate in some polluted areas. This is mainly due to the increase of O3 and OH (by ∼30%), which has subsequently increased the conversion efficiency of NOX to HNO3 (by 38.7%). Future control strategies for the winter haze should also aim to lower photochemical oxidants, via larger and synchronized NOX and VOCs emissions reduction, to overcome the effects of nonlinear photochemistry and aerosol chemical feedback.

NOx-VOC-O3 Sensitivity in Urban Environments of Sri Lanka

Physical phenomenon of the relation among the ground level O3, NOx and VOC governed by complex nonlinear photochemistry in urban environments is explained in detail using the ambient pollutant concentration data of eleven cities in Sri Lanka. The time-series analysis was conducted using the 24-hour average ambient concentrations of PM10, NO2, CO, O3 and SO2 air pollutants obtained from fixed air pollution monitoring station located in Colombo since 2008. Further analysis was carried out from the mobile air pollution monitoring station for eleven cities. The hourly averaged ambient real time air quality data i.e. VOC, NO2, NO, O3 pollutants and the corresponding meteorological parameters were analyzed and presented in weekly results for the base year 2013, 2014 and 2015. It was identified that there exist two regimes of NOx-VOC-O3 sensitivity among these cities. Colombo, Kurunegala, Jaffna, Matara, Badulla, Pollonnaruwa, and Gampaha are the NOx-sensitive regime. While Rathnapura, Anuradhapura, Kandy and Nuwaraelliya are the VOC-sensitive regime. In the NOx-sensitive regime (with relatively low NOx and high VOC), O3 increases with the increasing NOx and slightly changes in response to the increasing VOC levels. In the NOx-saturated or VOC-sensitive regime, O3 decreases with increasing NOx level and increases with increasing VOC levels. In the immediate vicinity of very large emissions of NO, O3 concentrations are depressed through the process of NOx titration. Mathematical relationships were developed to calculate the steady state ozone concentration (O3ss) that gives the values for both NOx-sensitive regime and the VOC-sensitive regime. Establishment of these relationships are essential for Sri Lanka to develop the appropriate interventions for controlling O3 pollution in each city.

Experimental Scanning Laser Lithography with Exposure Optimization

Laser scanning lithography is a maskless method for exposing films of photoresist during semiconductor manufacturing. In this method a focused beam is scanned over a surface with varying intensity to create features in the photoresist. Given the shape of a desired feature, an exposure pattern must be found that approximates this shape in the developed photoresist. This can be cast as an optimization problem, which is complicated by the non-negative nature of the exposure function and the non-linear photochemistry of the film. In this article, a nonlinear programming approach is described that results in a tractable optimization problem which accounts for all of the practical constraints encountered in laser scanning lithography. This method is demonstrated to create a sub-wavelength feature which is verified by optical finite element simulation with a resolution of 20 nm.

Two-photon fabrication of three-dimensional silver microstructures in microfluidic channels for volumetric surface-enhanced Raman scattering detection

We report on the nonlinear photochemistry fabrication of three-dimensional silver (Ag) microstructures in microfluidic channels for volumetric surface-enhanced Raman scattering (3D SERS). The fabrication of high resolution 3D Ag microstructures is obtained by a two-photon induced reduction process of silver cations, which is restricted at the focal point of a Q-switched Nd:YAG microlaser (sub-nanosecond pulses at 1064 nm). Firstly, 3D Ag micro-pillars made on cover glass showed a 3D SERS detection limit of Oxazine 720 as low as 10−8 M. Secondly, we directly fabricated 3D microstructures within microfluidic channels, and demonstrated their 3D SERS capability. The micro-cube geometry gave a significantly larger 3D SERS signal than the micro-pillar geometry. This result demonstration is paving the way for further optimization routes by varying the geometry, the size, and the density of complex 3D structures which can be obtained by direct laser writing based on two-photon induced chemistry.

Photoactive high explosives: linear and nonlinear photochemistry of petrin tetrazine chloride.

Pentaerythritol tetranitrate (PETN), a high explosive, initiates with traditional shock and thermal mechanisms. In this study, the tetrazine-substituted derivative of PETN, pentaerythritol trinitrate chlorotetrazine (PetrinTzCl), is being investigated for a photochemical initiation mechanism that could allow control over the chemistry contributing to decomposition leading to initiation. PetrinTzCl exhibits a photochemical quantum yield (QYPC) at 532 nm not evident with PETN. Using static spectroscopic methods, we observe energy absorption on the tetrazine (Tz) ring that results in photodissociation yielding N2, Cl-CN, and Petrin-CN as the major photoproducts. The QYPC was enhanced with increasing irradiation intensity. Experiment and theoretical calculations imply this excitation mechanism follows sequential photon absorption. Dynamic simulations demonstrate that the relaxation mechanism leading to the observed photochemistry in PetrinTzCl is due to vibrational excitation during internal conversion. PetrinTzCl's single photon stability and intensity dependence suggest this material could be stable in ambient lighting, yet possible to initiate with short-pulsed lasers.

Nonlinear Photochemistry Squared: Quartic Light Power Dependence Realized in Photon Upconversion

Simultaneous two-photon excitation of a solution mixture of [Ru(dmb)3]2+ and 9,10-diphenylanthracene (DPA) using 860 nm light pulses from a Ti:Sapphire laser resulted in triplet energy transfer followed by triplet-triplet annihilation (TTA), ultimately leading to upconverted DPA fluorescence from the sensitized DPA triplets. The photochemistry sequence was confirmed by the unprecedented quartic (x4) incident light power dependence exhibited by this process, which incidentally generated a record anti-Stokes shift of 1.38 eV for sensitized TTA.

Development of a reactive puff model for simultaneous consideration of plume dispersion and plume chemistry

To comprehensively assess and understand the environmental impacts of air pollutants emitted from large-scale point sources, the complex, non-linear photochemistry inside the plumes should be considered together with atmospheric turbulent dispersion of the air pollutants. For this purpose, a reactive puff model that can consider both atmospheric turbulent dispersion and photochemical reactions was developed in this study. The model’s performance was evaluated by comparing the model-predicted concentrations with the ship-plume chemical concentrations measured from the Intercontinental Transport and Chemical Transformation (ITCT) 2K2 field campaign. This comparison study confirmed the ability of the developed reactive puff model to capture the major characteristics of both plume dispersion and photochemical reactions. Based on these findings, the reactive puff model was applied to a Korean power plant (Hadong) as a case study.

Two-photon treatment

The combination of nonlinear optics and photochemistry may soon be able to provide cataract patients with optimum long-term vision following surgery.

The effects of gravity waves on distributions of chemically active constituents in the mesopause region

The influence of a monochromatic atmospheric gravity wave of fixed amplitude on the mean vertical distribution of mesospheric minor species is studied using a dynamical‐photochemical gravity wave model. The fluctuations of winds, temperature, and species concentrations produced by the wave are calculated by a coupled linear gravity wave model. The effects of the wave on the mean mixing ratio of 19 key chemically active species is derived from a coupled system of continuity equations which accounts for full nonlinear photochemistry in a vertical column. This study focuses on the impact that gravity wave induced nonlinearities in the chemical reaction rates has on the chemical species distribution. The effects of gravity wave induced chemical transport and background diffusion are also calculated. Calculations indicate that the impacts of gravity waves on the vertical distribution (mean concentration) of atmospheric minor constituents are largest in the mesopause region, especially during nighttime. A comparison of the three effects indicates that for short lifetime chemical species such as O3 and OH, the tendency due to chemical reaction perturbations induced by gravity waves is much greater than that due to transport and diffusion. The altitude range over which these nonlinearities is important is narrowly confined to the chemically active region near the mesopause.

Multiphoton dissociation of some aromatic molecules. Formation of atomic carbon in the 2p21S0 excited state

Carbon atoms in the high energy metastable state 2p2 1S0 have been detected for the first time in multiphoton dissociation of some aromatic molecules, using tunable laser light in the region of 380 to 393 nm. The detection is based on the two-photon resonances2p 1S0 → → 3p1D2 and 2p1S0 → → 3p1S0 three-photon ionization of atomic carbon, following the dissociation of the molecule, during the same laser pulse. The results are of particular interest in the field of nonlinear photochemistry.

Non-linear photochemistry of polymer films: laser ablation of poly (n-vinylcarbazole)

Irradiation of some poly(N-vinylcarbazole) films with a 248 nm excimer laser resulted in their ablation, giving an etched hole with a submicrometer depth. A characteristic periodic pattern and small particles with diameter of the order of submicrometers were formed in the etched area. This ablation behavior was affected by doping excitation energy and electron acceptors, discussed from a photophysical viewpoint.

Nonlinear photochemistry of polymer laser media in high-intensity light fluxes

An investigation was made of photobleaching of organic dyes in polyurethane matrices irradiated with high-intensity monochromatic light fluxes corresponding to the main absorption band. The probability of dissociation of dye molecules k, reduced to a unit dose, depended nonlinearly on the intensity of the exciting light I. In the range I > 100–150 MW/cm2 the dependence k (I) reached its maximum, which depended on the structure of the dye and differed by several orders of magnitude for dyes of different classes. The observed dependences were described using a three-level model of electronic–vibrational states. Some practical recommendations were made for increasing the service life of dye-activated polymer laser elements.

Laser multiphoton processes in thin films: Non-linear photochemistry of organosilane polymers

Irradiation of organosilane polymers in the ultraviolet leads to photodegradation of the chain via a bond scission process. Here photodegradation by visible laser multi-quantum excitation is reported. The process is characterized by a sharp threshold at power densities of approximately 2.5 GW/cm 2. The photochemistry of the system beyond this power density proceeds non-linearly leading to vaporization of the absorbing film and 100% bleaching of the ultraviolet absorption bands. Utilization of laser wavelengths whose two-photon energies are insufficient to access the ultraviolet system, produced no measurable photodecomposition.

Nonlinear laser photophysics, photochemistry and photobiology of nucleic acids

The present paper is concerned with photophysics, photochemistry and photobiology of nucleic-acid molecules and their components under intense laser UV irradiation. Attempts have been made to follow the effect of nonlinear two-quantum excitation on the course of subsequent physical-chemical and biological processes. It has been shown that high-intensity laser UV irradiation of nucleic-acid molecules and their components with nanosecond and picosecond pulses enables us to realize two-step excitation of chromophorebases above the ionization limit. At the same time in some cases two-photon excitation of the solvent, water, forming reactive products occurs. Nonlinear two-quantum excitation of biomolecules and water provides realization of chemical reactions with the dissolved molecules participating, and the products of such reactions differ qualitatively from those of classical linear photochemistry. Besides, as the irradiation intensity increases, the quantitative relation between one-step and two-step nucleic-acid lesions changes, since the quantum yield of one-step lesions drops with increasing intensity and the quantum yield of two-step lesions increases. The products formed in nonlinear photochemical reactions may be important in biology. As a result, the biological processes after nonlinear excitation also begin to depend on intensity, which is illustrated in some instances in the last section of the paper. Thus the nonlinearity of the process of excitation of molecules under intense laser UV radiation (nonlinear photophysics) is responsible for the nonlinear character of subsequent chemical and biological processes (nonlinear photochemistry and photobiology).

Nonlinear laser UV photochemistry of maleic acid in aqueous solution

This work deals with photochemical transformations of maleic acid in an aqueous solution under powerful picosecond fourth-harmonic pulses of a YAG-Nd3+ laser with λ=266 nm and nanosecond pulses of an excimer KrF laser with λ-248 nm. It has been found that by changing the irradiation intensity only it is possible to produce selectively different products with a high quantum yield resulting from the following reactions of the −C=C− bond of the maleic acid molecule: 1) cis-trans isomerization at radiation intensities up to 108 W/cm2 as a result of linear photochemistry; 2) radical dimerization of the maleic acid withJ=108÷109 W/cm2 and water addition to the −C=C− bond of the maleic acid withJ≃109 W/cm2 as a result of nonlinear photochemistry.

Nonlinear photochemistry in organic, inorganic, and organometallic systems

Nonlinear photochemistry in organic, inorganic, and organometallic systems