Photolysis Rates Research Articles

In situ investigation of N deposit effects on polycyclic aromatic hydrocarbons (PAHs) photolysis in snow

As snow is an ideal scavenger of polycyclic aromatic hydrocarbons (PAHs) for both particulate- and gas-phase PAHs, a large amount of PAHs are accumulated in snowpacks. These PAHs could be released from seasonal snowpacks into environment over a short time during the snow melting period, causing a flush of contaminants. Understanding the fate of PAHs in snow therefore is important to evaluate the contaminant flush during snow melting. However, as an important natural process, PAH photolysis in snow is still unclear, especially in the context of global climate change. Therefore, photolysis of the 16 PAHs, which are listed as priority controlled pollutants by United States Environmental Protection Agency (US EPA), was investigated with N deposit in field in snow in this study. A mixture of the 16 US EPA priority PAHs and nitrate/ammonia were spiked into sealed quartzose bottles containing snow samples in field. The bottles were then exposed to solar radiation in field for 6 h. Experimental results showed that >50% of PAHs were lost during 6 h photolysis, mostly occurred in the first hour. Generally, HMW PAHs were photolyzed faster than LMW PAHs, though the photolysis rate did not strictly increase with molecular weight. Low concentrations of nitrate can enhance photolysis of Ant and Pyr, while photolysis of investigated PAHs was inhibited with nitrate concentration increasing. In contrast, both low and high concentration of ammonium can inhibit PAH photolysis. The inhibiting effect of both nitrate and ammonia on photolysis of HMW PAH is stronger than that of LMW PAHs, indicating direct photolysis of PAHs is the main pathway of PAH degradation in snow in situ. This study firstly showed the effect of N deposition on PAH photolysis in snow. Under current increasing N deposition in northeast China, this study implied that more PAHs in snow could be preserved during winter and released shortly during snowmelt, resulting in high ecological risk in spring.

Evaluation of the nitrogen oxide emission inventory with TROPOMI observations

To compensate for the emissions missed or underestimated in the national bottom-up emission inventories, we apply the high spatial resolution satellite data from TROPOspheric Monitoring Instrument (TROPOMI) to estimate the top-down nitrogen oxide (NOx) emissions in regional scales. The NOx chemical lifetime is derived based on ground-based measurements of ozone photolysis rate, ultra-violet (UV) index, and temperature. For five designated regions of western Taiwan, the derived lifetime is about 1–2 h in summer and 2–4 h in winter. The retrieved 2021 annual emissions for regions near two major pollution sources, Taichung thermal power plant and Mailiao Industrial Zone, are comparable with the emission from the Continuous Emission Monitoring System (CEMS), with a difference of 6% and −12%, respectively. After validating the data and methods, the NOx emissions for five regions of western Taiwan are derived and applied to evaluate the bottom-up inventories. For northern and southern Taiwan, the top-down emissions agree well with emission inventories. The top-down emissions are 12%, 23%, and 16% higher than emission inventories for north-central, central, and south-central Taiwan, respectively. This indicates that the bottom-up inventories are underestimated from north-central to south-central Taiwan, which may be associated with the uncertainties from traffic sources. Given the various complex pollution sources, deriving NOx emissions from space allows us to acquire a better understanding of emissions on urban scales and improve the bottom-up emission inventories.

Photolysis of 3-Nitro-1,2,4-triazol-5-one: Mechanisms and Products.

Insensitive munitions formulations that include 3-nitro-1,2,4-triazol-5-one (NTO) are replacing traditional explosive compounds. While these new formulations have superior safety characteristics, the compounds have greater environmental mobility, raising concern over potential contamination and cleanup of training and manufacturing facilities. Here, we examine the mechanisms and products of NTO photolysis in simulated sunlight to further inform NTO degradation in sunlit surface waters. We demonstrate that NTO produces singlet oxygen and that dissolved oxygen increases the NTO photolysis rate in deionized water. The rate of NTO photolysis is independent of concentration and decreases slightly in the presence of Suwannee River Natural Organic Matter. The apparent quantum yield of NTO generally decreases as pH increases, ranging from 2.0 × 10-5 at pH 12 to 1.3 × 10-3 at pH 2. Bimolecular reaction rate constants for NTO with singlet oxygen and hydroxyl radical were measured to be (1.95 ± 0.15) × 106 and (3.28 ± 0.23) × 1010 M-1 s-1, respectively. Major photolysis reaction products were ammonium, nitrite, and nitrate, with nitrite produced in nearly stoichiometric yield upon the reaction of NTO with singlet oxygen. Environmental half-lives are predicted to span from 1.1 to 5.7 days. Taken together, these data enhance our understanding of NTO photolysis under environmentally relevant conditions.

Photolysis frequency of nitrophenols derived from ambient measurements

Nitrophenols, a class of important intermediate products from the oxidation of aromatics, can participate in photochemistry and influence the atmospheric oxidative capacity. However, the reported photolysis frequencies of nitrophenols show considerable discrepancies. Here, measurements of nitrophenol, and methyl nitrophenol using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) at both urban and regional sites in southern China are used to constrain photolysis frequencies of nitrophenols. Considerable concentrations with a campaign average of 58 ± 32 ppt for nitrophenol and 97 ± 59 ppt for methyl nitrophenol were observed at the regional site. Based on the in-situ measurement dataset, a steady-state calculation was performed along with a zero-dimensional box model to analyze the budgets of nitrophenols. The result indicates that both primary emission and photolysis have significant impacts on nitrophenols. Primary emission contributes up to 88 % of the total nitrophenols production while photolysis accounts for up to 98 % of the total removal rate. The dominant sink of nitrophenols is photolysis with a rate of about 3.5 % ± 1.3 % of jNO2 for nitrophenol and 2.4 % ± 1.0 % of jNO2 for methyl nitrophenol. The results of this study suggest that using advanced mass spectrometry to accurately measure ambient nitrophenols, supplemented by an observation-based box model for budget analysis, provides an important indication for determining photolysis rate constants of nitrophenols.

Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols.

Particulate nitrate ([Formula: see text]) has long been considered a permanent sink for NOx (NO and NO2), removing a gaseous pollutant that is central to air quality and that influences the global self-cleansing capacity of the atmosphere. Evidence is emerging that photolysis of [Formula: see text] can recycle HONO and NOx back to the gas phase with potentially important implications for tropospheric ozone and OH budgets; however, there are substantial discrepancies in "renoxification" photolysis rate constants. Using aircraft and ground-based HONO observations in the remote Atlantic troposphere, we show evidence for renoxification occurring on mixed marine aerosols with an efficiency that increases with relative humidity and decreases with the concentration of [Formula: see text], thus largely reconciling the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies. Active release of HONO from aerosol has important implications for atmospheric oxidants such as OH and O3 in both polluted and clean environments.

Photolysis of Gas-Phase Atmospherically Relevant Monoterpene-Derived Organic Nitrates.

Organic nitrates (ONs) can impact spatial distribution of reactive nitrogen species and ozone formation in the atmosphere. While photolysis of ONs is known to result in the release of NO2 back to the atmosphere, the photolysis rate constants and mechanisms of monoterpene-derived ONs (MT-ONs) have not been well constrained. We investigated the gas-phase photolysis of three synthetic ONs derived from α-pinene, β-pinene, and d-limonene through chamber experiments. The measured photolysis rate constants ranged from (0.55 ± 0.10) × 10-5 to (2.3 ± 0.80) × 10-5 s-1 under chamber black lights. When extrapolated to solar spectral photon flux at a solar zenith angle of 28.14° in summer, the photolysis rate constants were in the range of (4.1 ± 1.4) × 10-5 to (14 ± 6.7) × 10-5 s-1 (corresponding to lifetimes of 2.0 ± 0.96 to 6.8 ± 2.4 h) and (1.7 ± 0.60) × 10-5 to (8.3 ± 4.0) ×10-5 s-1 (3.3 ± 1.6 to 17 ± 6.0 h lifetimes) by using wavelength-dependent and average quantum yields, respectively. Photolysis mechanisms were proposed based on major products detected during photolysis. A zero-dimensional box model was further employed to simulate the photolysis of α-pinene-derived ON under ambient conditions. We found that more than 99% of α-pinene-derived ON can be converted to inorganic nitrogen within 12 h of irradiation and ozone was formed correspondingly. Together, these findings show that photolysis is an important atmospheric sink for MT-ONs and highlight their role in NOx recycling and ozone chemistry.

Visible-light activatable coumarin-based phototriggers for fluorescence imaging with ultra-high photolysis efficiency.

Photoactivated fluorophores (PAFs) are powerful imaging tools for observing subcellular structures and tracking dynamic biological processes. However, photoremovable protecting groups (PPGs) widely used to construct PAFs suffer from the drawbacks of short-wavelength excitation and/or low photolysis efficiency. Herein, a class of coumarin-based PPGs with electron-rich thiophene derived substitutions at the C3-position of a coumarin scaffold were prepared. The modification not only leads to the redshift of the absorption band to the blue light region (400-500 nm), but also the increases of uncaging quantum yield (Φu) as well as molar extinction coefficient (εmax), thus enhancing the photolysis efficiency (Φu × εmax) up to 34.2 × 103 M-1 cm-1. The exceptionally high photolysis efficiency enables efficient photolysis in blue light as weak as 2 mW cm-2 or in blue light from a Luminol chemiluminescence system. Based on the excellent photolysis properties, the PAF constructed by the new PPG exhibits fast photoactivation and a low background signal, and the resulting fluorescence images display a signal-to-noise ratio greater than 780. It is anticipated that the superior photolysis performance makes the PPGs a novel platform for the construction of photo responsive systems in a variety of applications.

Characterization of a smog chamber for studying formation of gas-phase products and secondary organic aerosol

Smog chambers provide a potent approach to explore the secondary organic aerosol formation under varied conditions. This study describes the construction and characterization of a new smog chamber facility for studying the formation mechanisms of gas-phase products and secondary organic aerosol from the photooxidation of volatile organic compounds. The chamber is a 5.4 m3 Fluorinated Ethylene Propylene (FEP) Teflon reactor with the potential to perform photooxidation experiments at controlled temperature and relative humidity. Detailed characterizations were conducted for evaluation of stability of environmental parameters, mixing time, background contamination, light intensity, and wall losses of gases and particles. The photolysis rate of NO2 (JNO2) ranged from (1.02−3.32) ×10−3 sec−1, comparable to the average JNO2 in ambient environment. The wall loss rates for NO, NO2, and O3 were 0.47 × 10−4, 0.37 × 10−4, and 1.17 × 10−4 min−1, while wall loss of toluene was obsoletely found in a 6 hr test. The particle number wall loss rates are (0.01−2.46) ×10−3 min−1 for 40−350 nm with an average lifetime of more than one day. A series of toluene photooxidation experiments were carried out in absence of NOx under dry conditions. The results of the simulation experiments demonstrated that the chamber is well designed to simulate photolysis progress in the atmosphere.

Solvent effect on the photolysis of 5‐fluorouracil: A kinetic study

AbstractA photodegradation study of 5‐fluorouracil (5‐FU) has been carried out in the water (pH 7.0) and organic solvents (chloroform, ethyl acetate, acetone, 1‐butanol, 1‐propanol, ethanol, methanol, and acetonitrile) using developed and validated high‐performance liquid chromatography (HPLC) method. The test method was found to be simple, accurate (% recovery, 100.25%–101.08%), precise (%RSD < 2.00), and sensitive. The apparent first‐order rate constants are in the range of 0.85 (chloroform) to 4.25 × 10−4 min−1 (water). The values obtained for kobs were found to be linear as a function of the dielectric constant indicating the involvement of the dipolar intermediate in the reaction. The increase in the rate of degradation of this intermediate is enhanced by the polarity of the medium. Rates of photolysis for 5‐FU show a linear relationship with the solvent acceptor number (AN) which indicates the degree of solute–solvent interaction. The values of kobs were found to be linear to the inverse of the viscosity of the medium due to the diffusion‐controlled process.

Multiple Impacts of Aerosols on O3 Production Are Largely Compensated: A Case Study Shenzhen, China.

Tropospheric ozone (O3) is a harmful gas compound to humans and vegetation, and it also serves as a climate change forcer. O3 is formed in the reactions of nitrogen oxides and volatile organic compounds (VOCs) with light. In this study, an O3 pollution episode encountered in Shenzhen, South China in 2018 was investigated to illustrate the influence of aerosols on local O3 production. We used a box model with comprehensive heterogeneous mechanisms and empirical prediction of photolysis rates to reproduce the O3 episode. Results demonstrate that the aerosol light extinction and NO2 heterogeneous reactions showed comparable influence but opposite signs on the O3 production. Hence, the influence of aerosols from different processes is largely counteracted. Sensitivity tests suggest that O3 production increases with further reduction in aerosols in this study, while the continued NOx reduction finally shifts O3 production to an NOx-limited regime with respect to traditional O3-NOx-VOC sensitivity. Our results shed light on the role of NOx reduction on O3 production and highlight further mitigation in NOx not only limiting the production of O3 but also helping to ease particulate nitrate, as a path for cocontrol of O3 and fine particle pollution.

Fractal analysis of impact of PM2.5 on surface O3 sensitivity regime based on field observations

Properties of PM2.5 that can change aerosol chemistry and photolysis rates have great impacts on O3 sensitivity regime, further affecting the production rate of surface O3. However, responses of O3 sensitivity regime to changes in PM2.5 levels are difficult to be accurately determined, due to the complexity and nonlinearity of atmospheric chemistry. Here, based on long-term time series (2016–2020) of air quality variables in north and south Taiwan, fractal analysis along with Pearson correlation analysis are used to directly reveal the impacts of PM2.5 on O3 sensitivity regime in real atmosphere, by capturing the nonlinear dynamic relations among air pollutants. Great regional and seasonal difference in impacts of PM2.5 on O3 sensitivity regime may be ascribed to meteorological factors, PM2.5 components and levels of SO2, NO, NO2, etc. For north Taiwan, increased PM2.5 level can enhance the sensitivity of O3 formation to VOC in spring and summer, whereas the opposite effect can be observed in winter. But for south Taiwan, the influence of PM2.5 on O3 sensitivity regime is not statistically significant, excluding spring. Furthermore, feasibility and availability of fractal analysis is tested by simulations with Empirical Kinetics Modeling Approach (EKMA). The results demonstrate the capability of fractal analysis to identify the impacts of PM2.5 on O3 sensitivity regime in real atmosphere, which can provide suggestions for PM2.5-O3 coordinated control strategies in regions suffering combined air pollution.

Reactive Chlorine Emissions from Cleaning and Reactive Nitrogen Chemistry in an Indoor Athletic Facility.

Indoor gas-phase radical sources are poorly understood but expected to be much different from outdoors. Several potential radical sources were measured in a windowless, light-emitting diode (LED)-lit room in a college athletic facility over a 2 week period. Alternating measurements between the room air and the supply air of the heating, ventilation, and air-conditioning system allowed an assessment of sources. Use of a chlorine-based cleaner was a source of several photolabile reactive chlorine compounds, including ClNO2 and Cl2. During cleaning events, photolysis rates for these two compounds were up to 0.0023 pptv min-1, acting as a source of chlorine atoms even in this low-light indoor environment. Unrelated to cleaning events, elevated ClNO2 was often observed during daytime and lost to ventilation. The nitrate radical (NO3), which is rapidly photolyzed outdoors during daytime, may persist in low-light indoor environments. With negligible photolysis, loss rates of NO3 indoors were dominated by bimolecular reactions. At times with high NO2 and O3 ventilated from outdoors, N2O5 was observed. Elevated ClNO2 measured concurrently suggests the formation through heterogeneous reactions, acting as an additional source of reactive chlorine within the athletic facility and outdoors.

A review of gas-phase chemical mechanisms commonly used in atmospheric chemistry modelling

The atmospheric chemical mechanism is an essential component of airshed models used for investigating the chemical behaviors and impacts of species. Since the first tropospheric chemical mechanism was proposed in the 1960s, various mechanisms including Master Chemical Mechanism (MCM), Carbon Bond Mechanism (CBM), Statewide Air Pollution Research Center (SAPRC) and Regional Atmospheric Chemistry Mechanism (RACM) have been developed for different research purposes. This work summarizes the development and applications of these mechanisms, introduces their compositions and lumping methods, and compares the ways the mechanisms treat radicals with box model simulations. CBM can reproduce urban pollution events with relatively low cost compared to SAPRC and RACM, whereas the chemical behaviors of radicals and the photochemical production of ozone are described in detail in RACM. The photolysis rates of some oxygenated compounds are low in SAPRC07, which may result in underestimation of radical levels. As an explicit chemical mechanism, MCM describes the chemical processes of primary pollutants and their oxidation products in detail. MCM can be used to investigate certain chemical processes; however, due to its large size, it is rarely used in regional model simulations. A box model case study showed that the chemical behavior of OH and HO2 radicals and the production of ozone were well described by all mechanisms. CBM and SAPRC underestimated the radical levels for different chemical treatments, leading to low ozone production values in both cases. MCM and RACM are widely used in box model studies, while CBM and SAPRC are often selected in regional simulations.

Release Timing and Duration Control the Fate of Photolytic Compounds in Stream‐Hyporheic Systems

AbstractPredicting environmental fate requires an understanding of the underlying, spatiotemporally variable interaction of transport and transformation processes. Photolytic compounds, for example, interact with both time‐variable photolysis and the perennially dark hyporheic zone, generating potentially unexpected dynamics that arise from time‐variable reactivity. This interaction has been found to significantly impact environmental fate but is commonly oversimplified in predictive models. Our primary objective was to explore how time‐variable photolysis and hyporheic storage interact across a range of photolysis rates to control the fate and transport of photolytic solutes in stream‐hyporheic systems. In this study, we released a photolytic compound in a natural system at different times of day and simulated variable release timing and durations of photolytic compounds spanning half‐lives ranging from about 2 min to 900 hr. To contextualize these results, we simulated five photolytic compounds with varied half‐lives to systematically study the interaction between release timing, duration, and reactivity. We found that the environmental fate and transport of photolytic compounds are highly variable as a function of release timing, which controls when, where, and for how long solute is stored in the hyporheic zone or exposed to in‐channel photolysis. This knowledge can be used to improve predictions for photolytic compounds or assess potential impacts for an anticipated discharge or treatment.

Vertical distributions of atmospheric HONO and the corresponding OH radical production by photolysis at the suburb area of Shanghai, China

Nitrous acid (HONO) is considered as one of the main sources of the hydroxyl radical (OH), the most relevant oxidant in the atmosphere. Multi-AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were conducted to obtain the vertical profiles of aerosol and HONO from November 1, 2020 to January 31, 2021 at a suburb site of Shanghai, China. HONO was mainly distributed near the surface, but high values HONO occasionally occurred around 0.7 km, indicating an unaccounted source of daytime HONO at high altitudes. The positive correlation between HONO and aerosols suggested that the photo-enhanced heterogeneous reactions on the aerosol surface were an important source of daytime HONO at high altitudes. To obtain the vertical distribution of OH production by HONO photolysis (P(OH)HONO), the vertical profiles of photolysis rate of HONO (JHONO) were calculated by establishing a method of combining observations with empirical relationship based on heterogeneous atmospheric and radiative transfer models. The JHONO increased approximately linearly with increasing altitudes and the noontime averages value of JHONO near the ground were 6.68 × 10−4 s−1, which was strongly negatively affected by aerosols in the morning and afternoon. The P(OH)HONO profile varied in different months (November, December, January) that the changes were mainly affected by HONO and JHONO. P(OH)HONO was more positively affected by JHONO at high altitude and noon but greatly influenced by HONO concentrations in the morning and afternoon.

Changes in tropospheric ozone concentration over Indo-Gangetic Plains: the role of meteorological parameters

This study seeks to understand and quantify the changes in tropospheric ozone (O3) in lower troposphere (LT), middle troposphere (MT) and upper middle troposphere (UMT) over the Indo-Gangetic Plains (IGPs), India during the COVID-19 lockdown 2020 with that of pre-lockdown 2019. The gridded datasets of ozone from the European Centre for Medium-range Weather Forecasts (ECMWF) reanalysis product, ERA5 in combination with statistical interpolated (IDWs) surface NO2 observations, present a consistent picture and indicate a significant tropospheric ozone enhancement over IGP during COVID-19 lockdown restrictions in May 2020. The Paper also examines the influencing role of meteorological parameters on increasing ozone concentration. Over LT, an increase in O3 concentration (23%) is observed and in MT to UMT an enhancement of about 9–18% in O3 concentration have been seen during May 2020 with respect to May 2019. An investigation on causes of increasing ozone concentration (35–85 ppbv) from MT to UMT during May 2020 reveals that there was significant rise (by 1–6%) in low cloud cover (LCC). Notably, higher LCC increases the backscattering of upward solar radiation from the top of the atmosphere. A positive difference of 5–25 W/m2 in upward solar radiation (USR) is observed across the entire study region. The result suggests that higher LCC significantly contributed to the enhanced USR. Thereby, resulting in higher photolysis rate that lead to an increase in mid tropospheric ozone concentration during May 2020. The results highlight the importance of LCC as an important pathway in ozone formation and aid in scientific understanding of it.

Low Pressure UV Photolysis of the Pharmaceutical Compounds Acetaminophen, Atenolol, Bezafibrate, Diclofenac and Ibuprofen

Pharmaceutically active compounds (PhACs) are continuously introduced into the environment by human and livestock excretion, hospital sewage and pharmaceutical effluents. While the performance of UV photolysis regarding PhACs degradation may be limited by low quantum yields, it may be efficient when the contaminants significantly absorb UV radiation. In this work, the direct photolysis under 254 nm UVC radiation of acetaminophen (ACT), atenolol (ATL), bezafibrate (BZF), diclofenac (DIC) and ibuprofen (IBU), isolated and in mixture, was investigated. The results showed that PhAC photolysis followed apparent first-order kinetics, with removals ranging from 32% to 99% after 60 min, while all the compounds exhibited lower photolysis rates when mixed in solution. Less than 13% mineralization was achieved. The toxicity of irradiated solutions of Vibrio fischeri remained the same or slightly decreased for ATL, BZF and IBU, increased for ACT, and notably decreased for DIC; nevertheless, the solution of mixed PhACs became very toxic following irradiation, showing the need for oxidant addition for removing residual toxicity.

Correcting ozone biases in a global chemistry–climate model: implications for future ozone

Abstract. Weaknesses in process representation in chemistry–climate models lead to biases in simulating surface ozone and to uncertainty in projections of future ozone change. We here develop a deep learning model to demonstrate the feasibility of ozone bias correction in a global chemistry–climate model. We apply this approach to identify the key factors causing ozone biases and to correct projections of future surface ozone. Temperature and the related geographic variables latitude and month show the strongest relationship with ozone biases. This indicates that ozone biases are sensitive to temperature and suggests weaknesses in representation of temperature-sensitive physical or chemical processes. Photolysis rates are also an important factor, highlighting the sensitivity of biases to simulated cloud cover and insolation. Atmospheric chemical species such as the hydroxyl radical, nitric acid and peroxyacyl nitrate show strong positive relationships with ozone biases on a regional scale. These relationships reveal the conditions under which ozone biases occur, although they reflect association rather than direct causation. We correct model projections of future ozone under different climate and emission scenarios following the shared socio-economic pathways. We find that changes in seasonal ozone mixing ratios from the present day to the future are generally smaller than those simulated without bias correction, especially in high-emission regions. This suggests that the ozone sensitivity to changing emissions and climate may be overestimated with chemistry–climate models. Given the uncertainty in simulating future ozone, we show that deep learning approaches can provide improved assessment of the impacts of climate and emission changes on future air quality, along with valuable information to guide future model development.

Hydrocarbon chemistry in the atmosphere of a Warmer Exo-Titan

Hosting a ∼1.5 bar N2 atmosphere and reducing atmospheric composition, Titan has the energy sources needed to drive disequilibrium chemistry and hosts an aerosol layer which shields the surface from incident UV radiation. This world draws parallels to an early Earth-like world (although ∼200 K cooler), and the atmospheric chemistry may be capable of forming relevant prebiotic species. Exo-Titan worlds at close-in orbits host photochemistry relevant to habitability with rich hydrocarbon chemistry. We investigate the effect of stellar type of the host star, equilibrium temperature, incident radiation, and vertical transport efficiency on the production of higher-order hydrocarbons. We find a greater incident radiation (a closer orbit) increases the rate of methane photolysis as well as photolysis of hydrocarbons. A larger H2 abundance and warmer temperature increases the rate of the back reaction H2 + CH3 → CH4 + H, and the temperature dependence is so great that CH3 recycles back into CH4 instead of forming C2H6. A larger H2 abundance and warmer temperature also encourages interesting cycling between C2H2, C2H3, and C2H4via reactions with atomic H.

Which aerosol type dominate the impact of aerosols on ozone via changing photolysis rates?

The impact of aerosols on ozone via influencing photolysis rates is a combined effect of absorbing aerosols (AA) and scattering aerosols (SA). However, AA and SA show different optical properties and influence photolysis rates differently, which then cause different impacts on ozone. Till now, the dominate factor is disconfirmed, which is largely due to the impact of SA on ozone not reaching to a consistent conclusion. In this study, the WRF-Chem model was implemented to simulate the air pollutants over the North China Plain (NCP). The impacts of AA and SA on ozone via influencing photolysis rates were quantitatively isolated and analyzed. Our results also demonstrated the decreasing effect of AA on ozone within planet boundary layer (PBL) which is consistent with the conclusions of previous studies. But for SA, it decreased the ozone chemical contribution (CHEM) near surface but increased which in the upper layers of PBL, that enlarge the ozone vertical gradients. In this case, more vertical exchanges of ozone would occur with the effect of vertical mixing motion of atmosphere, then the opposite CHEM variations were counteracted with each other and finally led to very slight changes in ozone within PBL. Thus, it can be summarized that AA dominate this impact of aerosols on ozone. Reducing AA could cause a general increase in ozone (ΔO3) over the NCP. Based on the aerosol levels of this case, ΔO3 would be seen over 86 % of the areas in the NCP when reducing AA by 3/4 and ΔO3 was more significant in the megacities. Our study highlights the different relationships between ozone and aerosol types, which suggests that more attentions should be paid on aerosol types, especially AA, when making the synergetic control strategy of aerosols and ozone in China.