Photochemical Production Research Articles

Changes in anthropogenic precursor emissions drive shifts in the ozone seasonal cycle throughout the northern midlatitude troposphere

Abstract. Simulations by six Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system models indicate that the seasonal cycle of baseline tropospheric ozone at northern midlatitudes has been shifting since the mid-20th century. Beginning in ∼ 1940, the magnitude of the seasonal cycle increased by ∼10 ppb (measured from seasonal minimum to maximum), and the seasonal maximum shifted to later in the year by about 3 weeks. This shift maximized in the mid-1980s, followed by a reversal – the seasonal cycle decreased in amplitude and the maximum shifted back to earlier in the year. Similar changes are seen in measurements collected from the 1970s to the present. The timing of the seasonal cycle changes is generally concurrent with the rise and fall of anthropogenic emissions that followed industrialization and the subsequent implementation of air quality emission controls. A quantitative comparison of the temporal changes in the ozone seasonal cycle at sites in both Europe and North America with the temporal changes in ozone precursor emissions across the northern midlatitudes found a high degree of similarity between these two temporal patterns. We hypothesize that changing precursor emissions are responsible for the shift in the ozone seasonal cycle; this is supported by the absence of such seasonal shifts in southern midlatitudes where anthropogenic emissions are much smaller. We also suggest a mechanism by which changing emissions drive the changing seasonal cycle: increasing emissions of NOx allow summertime photochemical production of ozone to become more important than ozone transported from the stratosphere, and increasing volatile organic compounds (VOCs) lead to progressively greater photochemical ozone production in the summer months, thereby increasing the amplitude of the seasonal ozone cycle. Decreasing emissions of both precursor classes then reverse these changes. The quantitative parameter values that characterize the seasonal shifts provide useful benchmarks for evaluating model simulations, both against observations and between models.

Annual Variations of Isoprene and Other Non‐Methane Hydrocarbons in the Jiaozhou Bay on the East Coast of North China

AbstractNon‐methane hydrocarbons (NMHCs) play an important role in atmospheric chemistry and the ocean is a source of NMHCs in the atmosphere. Here, we investigated the annual variations of NMHCs concentrations and fluxes (i.e., ethane, ethylene, propane, propylene, and isoprene) in the semi‐enclosed shallow waters of Jiaozhou Bay from September 2016 to August 2017. Ethylene is the most abundant NMHCs, with an annual average concentration of 487.7 ± 239.3 pmol L−1. The concentrations of other NMHCs were lower; the average concentrations for ethane, propane, propylene, and isoprene were 87.0 ± 49.0, 65.7 ± 22.9, 127.4 ± 77.8, and 60.5 ± 47.1 pmol L−1, respectively. NMHCs concentrations varied largely between seasons and higher abundance were observed in summer. The correlations between NMHCs and environmental variables such as chlorophyll a (Chl‐a), dissolved organic carbon (DOC) were ambiguous, and principal component analysis (PCA) suggested nutrients contributed significantly to total variation. These results indicated that the control of NMHCs could be more complex in dynamic and anthropogenically impacted estuarine systems, which could be a result of multiple processes involving phytoplankton release, photochemical production, riverine input, and so forth. The average annual sea‐to‐air fluxes of ethane, ethylene, propane, propylene, and isoprene were 105.9–298.3, 583.5–1,337.0, 36.7–95.2, 85.2–389.6, and 25.2–182.4 nmol m−2 d−1, respectively. Although coastal and estuarine environments only account for a minor fraction of the global ocean (0.26%–0.39%), NMHC emissions from these areas could influence regional atmospheric chemistry and may be relatively important for global budgets (ethane, 1.70%; ethylene, 1.54%; propane, 1.29%; propylene, 0.87%; isoprene, 3.17%).

Atmospheric oxidation capacity and ozone pollution mechanism in a coastal city of southeastern China: analysis of a typical photochemical episode by an observation-based model

Abstract. A typical multi-day ozone (O3) pollution event was chosen to explore the atmospheric oxidation capacity (AOC), OH reactivity, radical chemistry, and O3 pollution mechanism in a coastal city of southeastern China, with an observation-based model coupled to the Master Chemical Mechanism (OBM-MCM). The hydroxyl radical (OH) was the predominant oxidant (90 ± 25 %) for daytime AOC, while the NO3 radical played an important role in AOC during the nighttime (72 ± 9 %). Oxygenated volatile organic compounds (OVOCs; 30 ± 8 %), NO2 (29 ± 8 %), and CO (25 ± 5 %) were the dominant contributors to OH reactivity, accelerating the production of O3 and recycling of ROx radicals (ROx = OH + HO2 + RO2). Photolysis of nitrous acid (HONO, 33 ± 14 %), O3 (25 ± 13 %), formaldehyde (HCHO, 20 ± 5 %), and other OVOCs (17 ± 2 %) was a major ROx source, which played an initiation role in atmospheric oxidation processes. Combined with regional transport analysis, the reasons for this O3 episode were the accumulation of local photochemical production and regional transport. The results of sensitivity analysis showed that volatile organic compounds (VOCs) were the limiting factor of radical recycling and O3 formation, and the 5 % reduction of O3 would be achieved by decreasing 20 % anthropogenic VOCs. Controlling emissions of aromatics, alkenes, and alkanes with ≥4 carbons was beneficial for ozone pollution mitigation. The findings of this study provide significant guidance for emission reduction and regional collaboration for future photochemical pollution control in the relatively clean coastal cities of China and similar countries.

Ozone Formation and Key VOCs of a Continuous Summertime O3 Pollution Event in Ji'nan

In the summer of 2019, field measurements of ozone (O3) and its precursors[volatile organic compounds (VOCs) and nitrogen oxides (NOx)] were carried out at an urban site in Ji'nan. We found that the daily maximum 8-hour averages φ(O3) were (103.0±14.5)×10-9. The average φ(NOx) and φ(VOCs), which are ozone precursors, were (16.7±11.3)×10-9and (22.4±9.4)×10-9, respectively. The ·OH reactivity of VOCs was determined (9.6±3.8) s-1. Ji'nan suffered from serious O3 pollution. An observation-constrained chemical box model was deployed to evaluate in situ photochemical O3 production, which indicated that chemical reactions made positive contributions to O3 production rates between 07:00 and 19:00 LT, with the average hourly O3 production rate of 35.6×10-9 h-1. To evaluate the effectiveness of various ozone precursor control strategies in reducing ozone pollution, we combined the observation-based model (OBM) with the relative incremental reactivity (RIR) method. The key indicators that affect the local ozone production rate were identified. Ji'nan was under VOC-limited conditions and the key VOC precursors were alkenes. The O3 formation mechanism changed from the VOC-limited regime in the morning to the transitional regime in the afternoon. Correspondingly, the simulated local O3 production rate was increased from 18.3×10-9 h-1 to 29.6×10-9 h-1. To further explore the role of anthropogenic emissions in ozone pollution, we used the positive matrix factorization (PMF) model to identify the major sources contributing to VOCs. The major sources in Ji'nan were vehicular exhaust and gasoline evaporation, accounting for more than 50% of the observed VOCs. Therefore, constraints on vehicular emissions is the most effective strategy to control O3 pollution in Ji'nan.

Pathways to Highly Oxidized Products in the Δ3-Carene + OH System.

Oxidation of the monoterpene Δ3-carene (C10H16) is a potentially important and understudied source of atmospheric secondary organic aerosol (SOA). We present chamber-based measurements of speciated gas and particle phases during photochemical oxidation of Δ3-carene. We find evidence of highly oxidized organic molecules (HOMs) in the gas phase and relatively low-volatility SOA dominated by C7-C10 species. We then use computational methods to develop the first stages of a Δ3-carene photochemical oxidation mechanism and explain some of our measured compositions. We find that alkoxy bond scission of the cyclohexyl ring likely leads to efficient HOM formation, in line with previous studies. We also find a surprising role for the abstraction of primary hydrogens from methyl groups, which has been calculated to be rapid in the α-pinene system, and suggest more research is required to determine if this is more general to other systems and a feature of autoxidation. This work develops a more comprehensive view of Δ3-carene photochemical oxidation products via measurements and lays out a suggested mechanism of oxidation via computationally derived rate coefficients.

Hole utilization in solar hydrogen production.

In photochemical production of hydrogen from water, the hole-mediated oxidation reaction is the rate-determining step. A poor solar-to-hydrogen efficiency is usually related to a mismatch between the internal quantum efficiency of photon-induced hole generation and the apparent quantum yield of hydrogen. This waste of photogenerated holes is unwanted yet unavoidable. Although great progress has been made, we are still far away from the required level of dexterity to deal with the associated challenges of wasted holes and its consequential chemical effects that have placed one of the greatest bottlenecks in attaining high solar-to-hydrogen efficiency. A critical assessment of the hole and its related phenomena in solar hydrogen production would, therefore, pave the way moving forward. In this regard, we focus on the contextual and conceptual understanding of the dynamics and kinetics of photogenerated holes and its critical role in driving redox reactions, with the objective of guiding future research. The main reasons behind and consequences of unused holes are examined and different approaches to improve overall efficiency are outlined. We also highlight yet unsolved research questions related to holes in solar fuel production.

Model analysis of vertical exchange of boundary layer ozone and its impact on surface air quality over the North China Plain

In addition to photochemical production and horizontal regional transport, surface O3 concentration can also likely be affected by vertical transport, which is not well known so far. The process analysis was conducted by using the Regional Atmospheric Modeling System Community Multiscale Air Quality (RAMS-CMAQ) model to investigate photochemical production and the vertical transport mechanism of boundary-layer O3 during a typical O3 pollution episode in the North China Plain (NCP), and further quantify the contribution of vertical transport to surface O3. The diurnal variations of vertical budgets of O3 and NO2 in the boundary layer at multiple sites showed that there were substantial differences in the vertical distribution of O3 production and transport between urban and suburban/rural areas. In urban areas, surface O3 is consumed by titration reaction to generate NO2, which is then transported to the upper boundary layer and produces O3 by photochemical reaction. With the development of the boundary layer, the upper-layer O3 stored in the residual layer at nighttime can be transported vertically to the surface as the turbulent diffusion intensifies the next morning. While in suburban and rural areas, the vertical transport is relatively weaker because the photochemical formation of O3 occurs in the whole boundary layer, although it decreases slightly with the altitude. Model simulation showed that 20.6–27.9% of urban surface O3 changes in the morning (09:00–10:00 LST) was attributable to the downward transport from the residual layer, while it is 15.0–22.1% at suburban site. The vertical transport from above the boundary layer contributed 24.0–63.6% to daytime urban surface O3 changes, which was weak in suburban areas. Differences and similarities in O3 formation and transport mechanism in urban and suburban regions revealed here highlight the importance of earlier control and regional collaboration.

Unexpected Fast Photochemistry Emerges in Cool Seasons China

Ozone is a crucial air pollutant that damages human health and vegetation. As it is formed by the sunlit oxidation of the nitrogen oxides and volatile organic compounds, the current ozone mitigation strategies are primarily focused on summer. By analysing ozone pollution episodes in eastern China, observed daily total oxidants accumulation in cool seasons (spring and autumn: 60 ppb and winter 40 ppb) is comparable to that measured in summer (60 ppb) indicating fast photochemical production of secondary pollutants including ozone over the year. Unrecognized fast radical primary productions is found to counteract the increased termination of hydroxyl radical and dis-benefit meteorological conditions to maintain the rapid total oxidant formations in cool seasons. Elucidating and regulate the primary radical sources may be critical for the secondary air pollution control in cool seasons.

Impact of shipping emissions on air pollution and pollutant deposition over the Barents Sea

Arctic warming leading to reduced summertime sea-ice is likely to lead to increased local shipping especially along the Northeast Passage near the northern coasts of Norway and Russia, which are shorter than the traditional southerly routes. Here, the regional chemistry-transport model WRF-Chem is used to examine the effects of shipping emissions on levels of air pollutants and deposition fluxes over the Barents Sea both for present-day and future conditions, based on a high growth scenario. Present-day shipping emissions are found to have already substantial effects on ozone concentrations, but limited effects on sulphate and nitrate aerosols. Predicted future changes in ozone are also important, particularly in regions with low nitrogen oxide concentrations, and results are sensitive to the way in which diversion shipping is distributed due to non-linear effects on photochemical ozone production. Whilst modest future increases in sulphate and nitrate aerosols are predicted, large enhancements in dry deposition of sulphur dioxide and wet deposition of nitrogen compounds to the Barents Sea are predicted. Such levels of future nitrogen deposition would represent a significant atmospheric source of oceanic nitrogen affecting sensitive marine ecosystems.

Evolution and variations of atmospheric VOCs and O3 photochemistry during a summer O3 event in a county-level city, Southern China

Studies on ambient volatile organic compound (VOC) and ozone (O3) pollution in county-level cities in China are scarce. The variations of atmospheric VOCs and O3 photochemistry on O3 pollution and non-O3 pollution period were explored in summer 2019 in a typical county-level city, Southern China. The average O3 concentration was 84.2 ppbv during an O3 episode, which was more than 1.6 times higher than that on non-O3 pollution days. The VOC concentration was 20.0 ± 4.5 ppbv during the sampling period, which was about × 1.5 lower than megacities and suburban areas. The OH, HO2, and RO2 radicals and O3 photochemistry are simulated using a box model. O3 formation was dominated by the reaction of HO2+NO. OH + NO2 was the major contributor to O3 destruction. The net O3 production rates were higher on O3 episode days as compared to non-O3 episode periods. O3 formation regimes were explored using the empirical kinetics modelling approach (EKMA). The shift of wind directions led to the change from transition regimes to NOX-limited regimes. Industrial source was a critical element in the increasing VOC concentrations, compositions, the transition of O3 formation regimes, and formation of O3 episodes. The results revealed that O3 formation in the Yangxin County was governed by interplay of meteorological conditions, O3 precursors, emissions sources, air mass transport, photochemical production conditions, and O3 formation regimes. There were distinct chemical features in the atmosphere of county-level cities and megacities.

Investigation of sources and formation mechanisms of fine particles and organic aerosols in cold season in Fenhe Plain, China

Air quality has been improved significantly from 2017 to 2020 in Fenhe Plain, one of the most polluted regions in China. However, severe haze episodes from days to weeks were still frequently observed in cold seasons, and the sources and formation mechanisms especially organic aerosols are poorly understood due to the limited highly time-resolved measurements. Here we have a comprehensive characterization of chemical composition, variations and sources of fine particles in fall and winter in Fenhe Plain mainly using a PM2.5 Aerosol Chemical Speciation Monitor. Our results showed that organics and nitrate were two major species of PM2.5, on average accounting for 35.4% and 27.0%, respectively, and they played consistently important roles across different pollution levels. Positive matrix factorization revealed three primary sources from traffic, biomass burning and coal combustion emissions, and two secondary factors that are associated with photochemical and aqueous-phase processing. While secondary OA (SOA) dominated OA for the entire study (63%), large increases in primary OA from 19% to 41% were observed during heating period. Particularly, we found a high contribution of photochemical OOA (45%) during severely polluted periods in heating season demonstrating the presence of strong photochemical production in winter. Most aerosol species presented similarly pronounced diurnal cycles with low concentrations during daytime indicating the major influences from planetary boundary layer and primary emissions. The bivariate polar plots further illustrated the different sources of primary and secondary species, i.e., local emissions versus regional transport from the south and southeast. By analyzing four case studies, we found three major formation mechanisms of severe haze episodes that were driven by photochemical processing, aqueous-phase production, and local emissions, respectively. Therefore, different mitigating strategies are suggested in Fenhe Plain depending on the types of severe haze episodes.

Controls on the photochemical production of hydrogen peroxide in Lake Erie.

In Lake Erie, toxin-forming harmful algal blooms (HABs) occur following high concentrations of hydrogen peroxide (H2O2). Correlation between H2O2 concentrations and HABs revealed knowledge gaps on the controls of H2O2 production in Lake Erie. One way H2O2 is produced is upon absorption of sunlight by the chromophoric fraction of dissolved organic matter (CDOM). Rates of this photochemical production of H2O2 may increase in proportion to the apparent quantum yield of H2O2 (ΦH2O2,λ) from CDOM. However, the ΦH2O2,λ for H2O2 production from CDOM remains too poorly constrained to predict the magnitude and range of photochemically produced H2O2, particularly in freshwaters like Lake Erie. To address this knowledge gap, the ΦH2O2,λ was measured approximately biweekly from June-September 2019 in the western basin of Lake Erie along with supporting analyses (e.g., CDOM concentration and composition). The average ΦH2O2,λ in Lake Erie was within previously reported ranges. However, the ΦH2O2,λ varied 5-fold in space and time. The highest ΦH2O2,λ was observed in the Maumee River, a tributary of Lake Erie. In nearshore waters of Lake Erie, the ΦH2O2,λ decreased about five-fold from June through September. Integration of the controls of photochemical production of H2O2 in Lake Erie show that the variability in rates of photochemical H2O2 production was predominantly due to the ΦH2O2,λ. In offshore waters, CDOM concentration also strongly influenced photochemical H2O2 production. Together, the results confirm prior work suggesting that photochemical production of H2O2 contributes but likely cannot account for all the H2O2 associated with HABs in Lake Erie.

Stability of cannabidiol (CBD) in solvents and formulations: A GC–MS approach

The stability of cannabidiol (CBD) under thermal (heating, microwave irradiation, presence of Bronsted and Lewis acids) and photochemical conditions has been tested in organic solvents, including glycerol and propylene glycol that are traditionally employed in the preparation of CBD-containing formulation (e-liquid) for e-cigarettes.The results obtained in the present investigation point out the instability of CBD in the realistic scenario of pharmaceutical preparations and commercial products (e.g., e-liquids). In simulated e-liquid, among the by-products, the psychotropic THCs were found in negligible amounts (<5% added CBD) unless acidic catalysts are used (i.e., 40 mg/L strong acid). Cannabielsoin, Δ8-isoTHC and Δ8-THC were commonly found both as photochemical and thermal degradation products in methanol, acetonitrile and n-hexane. In the experiments performed in methanol, differently from acetonitrile and n-hexane, the solvent tends to be incorporated in the cannabinoids obtained as final products.

Photochemical hydrogen production based on the HCOOH/CO2 cycle promoted by a pentanuclear cobalt complex.

The first catalytic cycle for hydrogen production based on the photochemical two-electron reduction of carbon dioxide (CO2) and the dehydrogenation of formic acid at ambient temperature was demonstrated using a pentanuclear cobalt complex (Co5). A series of mechanistic studies were performed to elucidate the mechanism responsible for the promotion of the photocatalytic cycle by Co5.

Photo-generation of H2 by heterometallic complexes.

Multiple and different metals in a complex can accomplish single and sequential multi-step reactions, providing valuable procedures to obtain chemicals in one-pot synthetic routes. Biology has shown how cooperative catalysis is a powerful method for the synthesis of complicated molecules. One of the most attractive targets for current chemists is the synthesis of H2 by a simple and economical procedure, mainly if it is produced from water using visible light as the energy source. This review aims to show how heterometallic complexes have been recently used to catalyze the photochemical production of H2.

Four- and Five-Carbon Dicarboxylic Acids Present in Secondary Organic Aerosol Produced from Anthropogenic and Biogenic Volatile Organic Compounds

To better understand precursors of dicarboxylic acids in ambient secondary organic aerosol (SOA), we studied C4–C9 dicarboxylic acids present in SOA formed from the oxidation of toluene, naphthalene, α-pinene, and isoprene. C4–C9 dicarboxylic acids present in SOA were analyzed by offline derivatization gas chromatography–mass spectrometry. We revealed that C4 dicarboxylic acids including succinic acid, maleic acid, fumaric acid, malic acid, DL-tartaric acid, and meso-tartaric acid are produced by the photooxidation of toluene. Since meso-tartaric acid barely occurs in nature, it is a potential aerosol tracer of photochemical reaction products. In SOA particles from toluene, we also detected a compound and its isomer with similar mass spectra to methyltartaric acid standard; the compound and the isomer are tentatively identified as 2,3-dihydroxypentanedioic acid isomers. The ratio of detected C4–C5 dicarboxylic acids to total toluene SOA mass had no significant dependence on the initial VOC/NOx condition. Trace levels of maleic acid and fumaric acid were detected during the photooxidation of naphthalene. Malic acid was produced from the oxidation of α-pinene and isoprene. A trace amount of succinic acid was detected in the SOA produced from the oxidation of isoprene.

Measurement report: Photochemical production and loss rates of formaldehyde and ozone across Europe

Abstract. Various atmospheric sources and sinks regulate the abundance of tropospheric formaldehyde (HCHO), which is an important trace gas impacting the HOx (≡ HO2 + OH) budget and the concentration of ozone (O3). In this study, we present the formation and destruction terms of ambient HCHO and O3 calculated from in situ observations of various atmospheric trace gases measured at three different sites across Europe during summertime. These include a coastal site in Cyprus, in the scope of the Cyprus Photochemistry Experiment (CYPHEX) in 2014, a mountain site in southern Germany, as part of the Hohenpeißenberg Photochemistry Experiment (HOPE) in 2012, and a forested site in Finland, where measurements were performed during the Hyytiälä United Measurements of Photochemistry and Particles (HUMPPA) campaign in 2010. We show that, at all three sites, formaldehyde production from the OH oxidation of methane (CH4), acetaldehyde (CH3CHO), isoprene (C5H8) and methanol (CH3OH) can almost completely balance the observed loss via photolysis, OH oxidation and dry deposition. Ozone chemistry is clearly controlled by nitrogen oxides (NOx ≡ NO + NO2) that include O3 production from NO2 photolysis and O3 loss via the reaction with NO. Finally, we use the HCHO budget calculations to determine whether net ozone production is limited by the availability of VOCs (volatile organic compounds; VOC-limited regime) or NOx (NOx-limited regime). At the mountain site in Germany, O3 production is VOC limited, whereas it is NOx limited at the coastal site in Cyprus. The forested site in Finland is in the transition regime.

Measurement report: Fast photochemical production of peroxyacetyl nitrate (PAN) over the rural North China Plain during haze events in autumn

Abstract. Photochemical pollution over the North China Plain (NCP) is attracting much concern. We usually view peroxyacetyl nitrate (PAN) as the second most important photochemical pollutant featuring high mixing ratios during warm seasons. Our observations at a background site in the NCP identified high PAN concentrations, even during haze events in autumn. The substantial increasing ratios of PAN, by 244 % and 178 %, over the morning hours (08:00–12:00 local time) on 20 and 25 October 2020 were 10.6 and 7.7 times larger than those on clean days. Polluted days are characterized by higher temperature, higher humidity, and anomalous southerly winds compared with clean days. Enhanced local photochemistry has been identified as being the dominant factor that controls the PAN increase in the morning at the rural site, as the time when prevailing wind turns to a southerly wind is too late to promote direct transport of PAN from the polluted urban region. By removing the effect of direct transport of PAN, we provide a quantitative assessment of net PAN chemical production rate of 0.45 ppb h−1 for the mornings of polluted days, also demonstrating the strong local photochemistry. Using observations and calculated photolysis rates, we find that acetaldehyde oxidation by hydroxyl radical (OH) is the primary pathway of peroxyacetyl radical formation at the rural site. Acetaldehyde concentrations and production rates of HOx (HOx= OH + HO2) on polluted days are 2.8 and 2 times as large as those on clean days, leading to a remarkable increase in PAN in the morning. Formaldehyde (HCHO) photolysis dominates the daytime HOx production, thus contributing to fast photochemistry of PAN. Our observational results suggest the cause of a rapid increase in PAN during haze events in autumn at a rural site of the NCP and provide evidence of important role of HCHO photolysis in secondary pollutants at lower nitrogen oxide emissions. This highlights the urgency of carrying out strict volatile organic compound controls over the NCP during the cold season and not just in summer.

Observational insights into the compound environmental effect for 2-methyltetrols formation under humid ambient conditions

Laboratory experiments suggest acid-catalyzed aqueous-phase production can promote the formation of isoprene SOA, i.e., 2-methyltetrols. In this study we use ambient observations of the 2-methyltetrols along with other chemical measurements, as well as meteorological factors to investigate the relative importance of environmental influence for isoprene epoxydiols (IEPOX) SOA formation under atmospheric humidity conditions. The 2-Methyltetrols revealed good relationships with temperature and total solar radiation, but were weakly correlated with aerosol acidity and SO42−. EC-scaled 2-methyltetrols were observed to vary in a narrow pH range (1.5–2.0), indicating aerosol acidity was not a limiting factor for 2-methyltetrols formation. High values of 2-methyltetrols were consistently observed at high total solar radiation, the strong dependence of total solar radiation demonstrated that photochemical processes dominated 2-methyltetrols formation in humid environments. Although 2-methyltetrols can be enhanced by acid-catalyzed aqueous-phase reactions, it is not sufficient to compensate the synchronously weakened photochemical activity influence, leading to an obvious net decrease in the formation of 2-methyltetrols in the ambient. Moreover, aerosol droplet acidity was reduced under high liquid water content (LWC) condition, subsequently diminishing the enhancement of SOA formation by acidity. Overall, our results highlight that the environmental impact factors are highly variable and interplay, influencing the production of 2-methyltetrols, and suggest that the formation pathway of 2-methyltetrols is insensitive to aerosol acidity but dominated by photochemical production process in humid environments.

Overview of the Lake Michigan Ozone Study 2017.

The Lake Michigan Ozone Study 2017 (LMOS 2017) was a collaborative multiagency field study targeting ozone chemistry, meteorology, and air quality observations in the southern Lake Michigan area. The primary objective of LMOS 2017 was to provide measurements to improve air quality modeling of the complex meteorological and chemical environment in the region. LMOS 2017 science questions included spatiotemporal assessment of nitrogen oxides (NO x = NO + NO2) and volatile organic compounds (VOC) emission sources and their influence on ozone episodes; the role of lake breezes; contribution of new remote sensing tools such as GeoTASO, Pandora, and TEMPO to air quality management; and evaluation of photochemical grid models. The observing strategy included GeoTASO on board the NASA UC-12 aircraft capturing NO2 and formaldehyde columns, an in situ profiling aircraft, two ground-based coastal enhanced monitoring locations, continuous NO2 columns from coastal Pandora instruments, and an instrumented research vessel. Local photochemical ozone production was observed on 2 June, 9-12 June, and 14-16 June, providing insights on the processes relevant to state and federal air quality management. The LMOS 2017 aircraft mapped significant spatial and temporal variation of NO2 emissions as well as polluted layers with rapid ozone formation occurring in a shallow layer near the Lake Michigan surface. Meteorological characteristics of the lake breeze were observed in detail and measurements of ozone, NOx, nitric acid, hydrogen peroxide, VOC, oxygenated VOC (OVOC), and fine particulate matter (PM2.5) composition were conducted. This article summarizes the study design, directs readers to the campaign data repository, and presents a summary of findings.