The influence of ocean halogen and sulfur emissions in the air quality of a coastal megacity: The case of Los Angeles

Impact and pathway of halogens on atmospheric oxidants in coastal city clusters in the Yangtze River Delta region in China

Halogens (chlorine, bromine, and iodine) are known to profoundly influence atmospheric oxidants (hydroxyl radical (OH), hydroperoxyl radical (HO2), ozone (O3), and nitrate radical (NO3)) in the troposphere and subsequently affecting air quality. However, their impact on atmospheric oxidation and air pollution in coastal areas in China is poorly characterized. In this study, we use the WRF-CMAQ (Weather Research and Forecasting-Community Multiscale Air Quality) model with full halogen chemistry and process analysis to assess the influences and pathways of halogens on atmospheric oxidants in the Yangtze River Delta (YRD) region, a typical coastal city cluster in China. Halogens cause the annual OH radical increase by up to 16.4% and NO3 decrease by up to 45.3%. O3 increases by 2.0% in the YRD but decreases by 3.3% in marine environment. Halogen induced changes in atmospheric oxidants lead to a general increase of atmospheric oxidation capacity by 5.1% (maximum 48.4%). The production rate of OH (POH) in the YRD is enhanced by anthropogenic chlorine through both increased HO2 pathway and hypohalous acid photolysis pathway, while POH over ocean is enhanced by oceanic halogens through converting HO2 into hypohalous acid. Anthropogenic chlorine enhances both O3 and NO3 production (PNO3) rates through influencing their precursors while oceanic halogens reduce PNO3 and directly destroy ozone. Iodine contributed most (on average of 91% in oceanic halogens) in reducing production rates of oxidants. Thus, halogen emissions and potential effects of halogens on air quality need to be considered in air quality policies and regulations in the YRD region.

Potential deterioration of ozone pollution in coastal areas caused by marine-emitted halogens: A case study in the Guangdong-Hong Kong-Macao Greater Bay Area

Ozone (O₃) is one of the most important air pollutants worldwide in terms of its great damage to human health and agriculture. Previous studies show that marine-emitted halogens significantly influence O₃ concentrations, mainly through the consumption of O₃ by bromine and iodine atoms. In this study, we investigate the temporal variation at finer time scales (daily and hourly) than previous studies (annual or monthly) to better characterize the influence of marine-emitted halogens on coastal O₃. In contrast to previous studies that mainly reported a decrease in O₃, our results show significant temporal variations in halogen-induced O₃ changes. More specifically, the halogen-induced decrease in coastal O₃ in southern China is concentrated on clean days, while an unexpected increase in some regions of up to >10 ppbv could occur on polluted days. On polluted days, the activation of particulate chloride (Cl^-) in sea salt aerosol (SSA) is effective due to the high level of dinitrogen pentoxide (N₂O₅) that is formed from the reactions of O₃ and nitrogen dioxide (NO₂). In addition, the wind fields are unfavorable for the transport of marine air masses with large O₃ depletion inland. These two factors together result in the increase in hourly and MDA8 O₃ on polluted days in some regions in the GBA. The locations of O₃ increases are controlled by the distribution of nitryl chloride (ClNO₂) at sunrise, which is influenced by O₃ and NO₂ during the previous night. As a result, the increase in O₃ is a continuation of the O₃ pollution from the previous day, and the whole area is under potential threat of this worsening pollution.

Midlatitude Ozone Depletion and Air Quality Impacts from Industrial Halogen Emissions in the Great Salt Lake Basin

We report aircraft observations of extreme levels of HCl and the dihalogens Cl₂, Br₂, and BrCl in an industrial plume near the Great Salt Lake, Utah. Complete depletion of O₃ was observed concurrently with halogen enhancements as a direct result of photochemically produced halogen radicals. Observed fluxes for Cl₂, HCl, and NO_x agreed with facility-reported emissions inventories. Bromine emissions are not required to be reported in the inventory, but are estimated as 173 Mg year^-1 Br₂ and 949 Mg year^-1 BrCl, representing a major uncounted oxidant source. A zero-dimensional photochemical box model reproduced the observed O₃ depletions and demonstrated that bromine radical cycling was principally responsible for the rapid O₃ depletion. Inclusion of observed halogen emissions in both the box model and a 3D chemical model showed significant increases in oxidants and particulate matter (PM_2.5) in the populated regions of the Great Salt Lake Basin, where winter PM_2.5 is among the most severe air quality issues in the U.S. The model shows regional PM_2.5 increases of 10%-25% attributable to this single industrial halogen source, demonstrating the impact of underreported industrial bromine emissions on oxidation sources and air quality within a major urban area of the western U.S.

Communicating ocean and human health connections: An agenda for research and practice

The emergence of ocean and human health (OHH) science as a distinct scholarly discipline has led to increased research outputs from experts in both the natural and social sciences. Formal research on communication strategies, messaging, and campaigns related to OHH science remains limited despite its importance as part of the social processes that can make knowledge actionable. When utilized to communicate visible, local issues for targeting audiences, OHH themes hold the potential to motivate action in pursuit of solutions to environmental challenges, supplementing efforts to address large-scale, abstract, or politicized issues such as ocean acidification or climate change. Probing peer-reviewed literature from relevant areas of study, this review article outlines and reveals associations between society and the quality of coastal and marine ecosystems, as well as key themes, concepts, and findings in OHH science and environmental communication. Recommendations for future work concerning effective ocean and human health science communication are provided, creating a platform for innovative scholarship, evidence-based practice, and novel collaboration across disciplines.

Halogens Enhance Haze Pollution in China

Severe and persistent haze events in northern China, characterized by high loading of fine aerosol especially of secondary origin, negatively impact human health and the welfare of ecosystems. However, current knowledge cannot fully explain the formation of this haze pollution. Despite field observations of elevated levels of reactive halogen species (e.g., BrCl, ClNO2, Cl2, HBr) at several sites in China, the influence of halogens (particularly bromine) on haze pollution is largely unknown. Here, for the first time, we compile an emission inventory of anthropogenic bromine and quantify the collective impact of halogens on haze pollution in northern China. We utilize a regional model (WRF-Chem), revised to incorporate updated halogen chemistry and anthropogenic chlorine and bromine emissions and validated by measurements of atmospheric pollutants and halogens, to show that halogens enhance the loading of fine aerosol in northern China (on average by 21%) and especially its secondary components (∼130% for secondary organic aerosol and ∼20% for sulfate, nitrate, and ammonium aerosols). Such a significant increase is attributed to the enhancement of atmospheric oxidants (OH, HO2, O3, NO3, Cl, and Br) by halogen chemistry, with a significant contribution from previously unconsidered bromine. These results show that higher recognition of the impact of anthropogenic halogens shall be given in haze pollution research and air quality regulation.

Marine volatile organic compounds and their impacts on marine aerosol-A review

Volatile organic compounds (VOCs) play a vital role in the global carbon budget and in the regional formation of ozone in the troposphere, and are emitted from both natural and anthropogenic activities. They can also serve as a source of secondary organic aerosol (SOA). Field and model studies showed evidences of a strong marine biogenic influence on marine aerosols. Although knowledge of terrestrial VOC emissions and SOA formation mechanisms has been advanced considerably over the last decades, processes constraining marine VOC emissions and marine SOA formation remain poorly understood. Seawater contains an extremely complex, diverse, and largely unidentified mixture of VOCs. Despite the fact that the ocean covers 70% of the Earth's surface, the role of the ocean in the global budget of VOCs is still unclear. The distribution and emission of sea surface VOCs exhibit considerable spatial-temporal variation, with higher concentrations often, but not always, correlated with biological activities. VOCs in surface seawater have been measured in various geographic regions, however, knowledge of the distribution of marine VOCs and the role of the oceans in the global atmospheric chemistry is still insufficient due to the paucity of measurements. This study reviews marine VOCs in terms of current analytical methods, global marine VOCs measurements, their effects on SOA, and future needs for understanding the role of marine VOCs in the chemistry of the atmosphere.

Chemical Interactions Between Ship-Originated Air Pollutants and Ocean-Emitted Halogens

Ocean-going ships supply products from one region to another and contribute to the world's economy. Ship exhaust contains many air pollutants and results in significant changes in marine atmospheric composition. The role of reactive halogen species (RHS) in the troposphere has received increasing recognition and oceans are the largest contributors to their atmospheric burden. However, the impact of shipping emissions on RHS and that of RHS on ship-originated air pollutants have not been studied in detail. Here, an updated Weather Research Forecasting coupled with Chemistry model is utilized to explore the chemical interactions between ship emissions and oceanic RHS over the East Asia seas in summer. The emissions and resulting chemical transformations from shipping activities increase the level of NO and NO2 at the surface, increase O3 in the South China Sea, but decrease O3 in the East China Sea. Such changes in pollutants result in remarkable changes in the levels of RHS (>200% increase of chlorine; ∼30% and ∼5% decrease of bromine and iodine, respectively) as well as in their partitioning. The abundant RHS, in turn, reshape the loadings of air pollutants (∼20% decrease of NO and NO2; ∼15% decrease of O3) and those of the oxidants (>10% reduction of OH and HO2; ∼40% decrease of NO3) with marked patterns along the ship tracks. We, therefore, suggest that these important chemical interactions of ship-originated emissions with RHS should be considered in the environmental policy assessments of the role of shipping emissions in air quality and climate.

Modeling the impact of marine DMS emissions on summertime air quality over the coastal East China Seas

[1] Biogenic emission of dimethyl sulfide (DMS) from seawater is the major natural source of sulfur into the atmosphere. In this study, we use an advanced air quality model (CMAQv5.2) with DMS chemistry to examine the impact of DMS emissions from seawater on summertime air quality over China. A national scale database of DMS concentration in seawater is established based on a five-year observational record in the East China seas including the Bohai Sea, the Yellow Sea and the East China Sea. We employ a commonly used global database and also the newly developed local database of oceanic DMS concentration, calculate DMS emissions using three different parameterization schemes, and perform five different model simulations for July, 2018. Results indicate that in large coastal areas of China, the average DMS emissions flux obtained with the local database is three times higher than that resulting from the global database, with a mean value of 9.1 μmol m^-2 d^-1 in the Bohai Sea, 8.4 μmol m^-2 d^-1 in the Yellow Sea and 13.4 μmol m^-2 d^-1 in the East China Sea. The total DMS emissions flux calculated with the Nightingale scheme is 42% higher than that obtained with the Liss and Merlivat scheme, but is 15% lower than that obtained with the Wanninkhof scheme. Among the three parameterizations, results of the Liss and Merlivat scheme agree better with the ship-based observations over China's coastal waters. DMS emissions with the Liss and Merlivat parametrization increase atmospheric sulfur dioxide (SO₂) and sulfate (SO₄ ^2-) concentration over the East China seas by 6.4% and 3.3%, respectively. Our results indicate that although the anthropogenic source is still the dominant contributor of atmospheric sulfur burden in China, biogenic DMS emissions source is nonnegligible.

Regional and Urban-Scale Environmental Influences of Oceanic DMS Emissions over Coastal China Seas

Marine biogenic dimethyl sulfide (DMS) is an important natural source of sulfur in the atmosphere, which may play an important role in air quality. In this study, the WRF-CMAQ model is employed to assess the impact of DMS on the atmospheric environment at the regional scale of eastern coastal China and urban scale of Shanghai in 2017. A national scale database of DMS concentration in seawater is established based on the historical DMS measurements in the Yellow Sea, the Bohai Sea and the East China Sea in different seasons during 2009~2017. Results indicate that the sea-to-air emission flux of DMS varies greatly in different seasons, with the highest in summer, followed by spring and autumn, and the lowest in winter. The annual DMS emissions from the Yellow Sea, the Bohai Sea and the East China Sea are 0.008, 0.059, and 0.15 Tg S a⁻¹, respectively. At the regional scale, DMS emissions increase atmospheric sulfur dioxide (SO₂) and sulfate (SO42−) concentrations over the East China seas by a maximum of 8% in summer and a minimum of 2% in winter, respectively. At the urban scale, the addition of DMS emissions increase the SO₂ and SO42− levels by 2% and 5%, respectively, and reduce ozone (O₃) in the air of Shanghai by 1.5%~2.5%. DMS emissions increase fine-mode ammonium particle concentration distribution by 4% and 5%, and fine-mode nss-SO42− concentration distributions by 4% and 9% in the urban and marine air, respectively. Our results indicate that although anthropogenic sources are still the dominant contributor of atmospheric sulfur burden in China, biogenic DMS emissions source cannot be ignored.

Potential Effect of Halogens on Atmospheric Oxidation and Air Quality in China

Air pollution has been a hazard in China over recent decades threatening the health of half a billion people. Much effort has been devoted to mitigating air pollution in China leading to a significant reduction in primary pollutants emissions from 2013 to 2017, while a continuously worsening trend of surface ozone (O₃, a secondary pollutant and greenhouse gas) was observed over the same period. Atmospheric oxidation, dominated by daytime reactions involving hydroxyl radicals (OH), is the critical process to convert freshly-emitted compounds into secondary pollutants, and is underestimated in current models of China's air pollution. Halogens (chlorine, bromine, and iodine) are known to profoundly influence oxidation chemistry in the marine environment; however, their impact on atmospheric oxidation and air pollution in China is unknown. In the present study, we report for the first time that halogens substantially enhance the total atmospheric oxidation capacity in polluted areas of China, typically 10% to 20% (up to 87% in winter) and mainly by significantly increasing OH level. The enhanced oxidation along the coast is driven by oceanic emissions of bromine and iodine, and that over the inland areas by anthropogenic emission of chlorine. The extent and seasonality of halogen impact are largely explained by the dynamics of Asian monsoon, location and intensity of halogen emissions, and O₃ formation regime. The omission of halogen emissions and chemistry may lead to significant errors in historical re-assessments and future projections of the evolution of atmospheric oxidation in polluted regions.

Impact of halogen chemistry on summertime air quality in coastal and continental Europe: application of the CMAQ model and implications for regulation

Halogen (Cl, Br, and I) chemistry has been reported to influence the formation of secondary air pollutants. Previous studies mostly focused on the impact of chlorine species on air quality over large spatial scales. Very little attention has been paid to the effect of the combined halogen chemistry on air quality over Europe and its implications for control policy. In the present study, we apply a widely used regional model, the Community Multiscale Air Quality Modeling System (CMAQ), incorporated with the latest halogen sources and chemistry, to simulate the abundance of halogen species over Europe and to examine the role of halogens in the formation of secondary air pollution. The results suggest that the CMAQ model is able to reproduce the level of O₃, NO₂, and halogen species over Europe. Chlorine chemistry slightly increases the levels of OH, HO₂, NO₃, O₃, and NO₂ and substantially enhances the level of the Cl radical. Combined halogen chemistry induces complex effects on OH (ranging from -0.023 to 0.030 pptv) and HO₂ (in the range of -3.7 to 0.73 pptv), significantly reduces the concentrations of NO₃ (as much as 20 pptv) and O₃ (as much as 10 ppbv), and decreases NO₂ in highly polluted regions (as much as 1.7 ppbv); it increases NO₂ (up to 0.20 ppbv) in other areas. The maximum effects of halogen chemistry occur over oceanic and coastal regions, but some noticeable impacts also occur over continental Europe. Halogen chemistry affects the number of days exceeding the European Union target threshold for the protection of human beings and vegetation from ambient O₃. In light of the significant impact of halogen chemistry on air quality, we recommend that halogen chemistry be considered for inclusion in air quality policy assessments, particularly in coastal cities.

Enhanced Chlorine and Bromine Atom Activation by Hydrolysis of Halogen Nitrates from Marine Aerosols at Polluted Coastal Areas

Detailed multiphase chemistry box model studies are carried out, investigating halogen radical activation at polluted coastal areas. Simulations are performed for a nonpermanent cloud and a cloud-free scenario and reveal that ClNO₂ photolysis and ICl photolysis are crucial for gas-phase Cl atom activation. In the cloud scenario, the integrated ClNO₂ and ICl photolysis rates are 3.7 × 10⁷ and 3.1 × 10⁷ molecules cm^-3 s^-1. In the cloud-free scenario, the integrated ClNO₂ and ICl photolysis rates are 8.1 × 10⁷ and 3.6 × 10⁷ molecules cm^-3 s^-1. The simulations show larger contributions of ClNO₂ photolysis in the morning and higher ones of ICl photolysis during afternoon. Throughout the simulation, average contributions to Cl atom activation in the cloud and cloud-free scenarios by ClNO₂ photolysis are 42% and 62% and by ICl photolysis 35% and 28%, respectively. ICl is formed through an aqueous-phase reaction of HOI with chloride. Two thirds of the formed ICl is released into the gas phase. The residual third reacts with bromide, creating IBr. Overall, the simulations emphasize the crucial role of INO₃ hydrolysis for Cl and Br atom activation in polluted coastal areas. Therefore, it needs to be considered in chemical transport models to improve air quality predictions.

Influence of bromine and iodine chemistry on annual, seasonal, diurnal, and background ozone: CMAQ simulations over the Northern Hemisphere

Bromine and iodine chemistry has been updated in the Community Multiscale Air Quality (CMAQ) model to better capture the influence of natural emissions from the oceans on ozone concentrations. Annual simulations were performed using the hemispheric CMAQ model without and with bromine and iodine chemistry. Model results over the Northern Hemisphere show that including bromine and iodine chemistry in CMAQ not only reduces ozone concentrations within the marine boundary layer but also aloft and inland. Bromine and iodine chemistry reduces annual mean surface ozone over seawater by 25%, with lesser ozone reductions over land. The bromine and iodine chemistry decreases ozone concentration without changing the diurnal profile and is active throughout the year. However, it does not have a strong seasonal influence on ozone over the Northern Hemisphere. Model performance of CMAQ is improved by the bromine and iodine chemistry when compared to observations, especially at coastal sites and over seawater. Relative to bromine, iodine chemistry is approximately four times more effective in reducing ozone over seawater over the Northern Hemisphere (on an annual basis). Model results suggest that the chemistry modulates intercontinental transport and lowers the background ozone imported to the United States.