Ozone Episodes Research Articles

An evaluation of the ability of the Ozone Monitoring Instrument (OMI) to observe boundary layer ozone pollution across China: application to 2005–2017 ozone trends

Abstract. Nadir-viewing satellite observations of tropospheric ozone in the UV have been shown to have some sensitivity to boundary layer ozone pollution episodes, but so far they have not yet been compared to surface ozone observations collected by large-scale monitoring networks. Here we use 2013–2017 surface ozone data from China's new Ministry of Ecology and Environment (MEE) network of ∼ 1000 sites, together with vertical profiles from ozonesondes and aircraft, to quantify the ability of tropospheric ozone retrievals from the Ozone Monitoring Instrument (OMI) and to detect boundary layer ozone pollution in China. We focus on summer when ozone pollution in China is most severe and when OMI has the strongest sensitivity. After subtracting the Pacific background, we find that the 2013–2017 mean OMI ozone enhancements over eastern China have strong spatial correlation with the corresponding multiyear means in the surface afternoon observations (R=0.73), and that OMI can estimate these multiyear means in summer afternoon surface ozone with a precision of 8 ppb. The OMI data show significantly higher values on observed surface ozone episode days (>82 ppb) than on non-episode days. Day-to-day correlations with surface ozone are much weaker due to OMI noise and are stronger for sites in southern China (<34∘ N; R=0.3–0.6) than in northern China (R=0.1–0.3) because of weaker retrieval sensitivity and larger upper tropospheric variability in the north. Ozonesonde data show that much of the variability of OMI ozone over southern China in summer is driven by the boundary layer. Comparison of 2005–2009 and 2013–2017 OMI data indicates that mean summer afternoon surface ozone in southern China (including urban and rural regions) has increased by 3.5±3.0 ppb over the 8-year period and that the number of episode days per summer has increased by 2.2±0.4 (as diagnosed by an extreme value model), generally consistent with the few long-term surface records. Ozone increases have been particularly large in the Yangtze River Delta and in the Hubei, Guangxi and Hainan provinces.

Air quality monitoring assessment during the 2016 Olympic Games in Rio de Janeiro, Brazil.

In 2009, the city of Rio de Janeiro was announced as the host city of the 2016 Olympic Games (Rio 2016). For this event, the Brazilian government, in partnership with the International Olympic Committee (IOC), undertook the task of monitoring the air quality in the city. This study discusses the PM10, PM2.5, and O3 profiles at ten sampling sites located near the arenas in 2016, including during the Olympic Games period. At all sampling stations, the annual mean values of PM10 and PM2.5 were below either Brazilian air quality standards or United States Environmental Protection Agency (U.S. EPA) guidelines. In addition, no violations lasting 24h were observed for particulate matter in 2016. Only two ozone episodes occurred in 2016, both in Campos dos Afonsos (163 and 195μgm-3) near the extreme sports arena. However, during the pre-Olympic period (2013-2015), in the same area were registered 16, 81, and 18 violations per year, respectively. The results showed an improvement in air quality in Rio de Janeiro in 2016. The reduction in pollutant levels, especially O3 and PM2.5, is probably due to the conclusion of the structural construction of the Olympic arenas and efforts to improve urban mobility.

Spatial and temporal patterns of ozone at Great Smoky Mountains National Park and implications for plant responses

Abstract Great Smoky Mountains National Park (GRSM) is the most visited National Park in the United States and has the highest levels of biodiversity of any park unit. It is a relatively small park (∼210,433 ha), but topographically complex, with an elevational range of 1757 m. The Park has historically been subject to elevated levels of pollutants, including sulfur dioxide (SO2), ozone (O3), and nitrogen oxides (NOx). Ozone trends are analyzed from 1989 to 2016 for six monitoring sites in and adjacent to GRSM and ranging in elevation from 564 m to 2030 m. Low-elevation sites have minimum O3 concentrations in early morning and maximum concentrations in mid-to late afternoon. High-elevation sites have flatter profiles, smaller diurnal ranges, and maxima that occur in early evening or at night. The W12624-h exposure index increases with elevation up to 823 m, after which it plateaus. W12624- h exposures increased between the years 1989–∼2002, and have substantially decreased afterwards. The highest 1-h concentration ever recorded in the Park was 135 ppb, which occurred on 25 August 1998. At most sites the maximum 3-month W12624-h exposures have shifted from mid-summer to spring (Apr–Jun). Decreases in exposure result primarily from reduction of hourly averaged O3 concentrations ≥ 60 ppb. Ozone episodes (3 or more consecutive hours when O3 ≥ 60 ppb) have decreased in frequency, magnitude, and duration from 1999 to 2016. Decreases in W126 exposures are correlated with lowered NOx emissions from regional TVA power plants and appear to be a direct result of the State Implementation Plan (SIP) call associated with the Clean Air Act Amendments, resulting in the cleanest air in GRSM over the period of record. Lower cumulative W126 O3 exposures and reduction in high O3 concentrations appear to be having beneficial effects on the vegetation within the Park.

Meteorological Conditions During an Ozone Episode in Dallas‐Fort Worth, Texas, and Impact of Their Modeling Uncertainties on Air Quality Prediction

AbstractThe Southern Great Plains experiences an unhealthy level of ozone (O3) at times. The formation mechanisms contributing to these O3 events are not always clear and in some cases are related to particular atmospheric circulation patterns. A severe O3 pollution event on 27 August 2011 in the Dallas‐Fort Worth (DFW) area is investigated with a combination of observations and simulations using the Weather Research and Forecasting model with Chemistry (WRF/Chem). During the O3 episode, a stationary front with a stagnant zone at the leading edge persisted to the west of DFW. At the time, Hurricane Irene was located in western Atlantic, displacing the Bermuda Subtropical High and affecting the circulations over the Southern Great Plains. The stagnant zone confined the pollutant plume originating from DFW, leading to accumulation of primary pollutants and prominent O3 formation. Emission sources from a few urban areas east of DFW as well as power plants near Mount Pleasant and Carthage also contributed to this DFW O3 pollution episode. This scenario is different from the typical summer days over the Southern Great Plains when southerly winds prevail along the west edge of the Bermuda High and the pollutant plumes from DFW are advected downstream, resulting in low O3. Ensemble WRF/Chem predictions driven by the operational Short‐Range Ensemble Forecast outputs are conducted to examine the impact of meteorological uncertainties (particularly transport uncertainties) on air quality forecasting. The ensemble mean gives a better prediction in terms of plume directions than individual members.

The assessment of return probability of maximum ozone concentrations in an urban environment of Delhi: A Generalized Extreme Value analysis approach

High ozone episodes have become a serious issue specially in megacities of developing countries. In this study an attempt has been made to understand the changes in the extreme ozone concentration due to different precursor compounds and meteorological variables in a given time period by applying Generalized Extreme Value (GEV) theory. The return probabilities of the extreme ozone concentrations were estimated for both classical stationary assumption as well as nonstationary assumption given the fact that ozone time series have both trend and multiple periodicities. Under stationary case, the distribution parameters were allowed to remain stationary, but in nonstationary case, distribution parameters were allowed to vary as a function of precursor compounds (Benzene, Toluene, mp-Xylene, NO, NO2) and meteorological variables (temperature and relative humidity). Daily maximum ozone concentrations were found to follow heavy tailed Fréchet distribution in both stationary and nonstationary conditions. The inclusion of covariate into the classical model explains the dynamic nature of ozone depending on its precursor variables. Principal components were also used as covariates in nonstationary GEV distribution model. The estimated return levels of ozone from the stationary model, were found to be 97.89, 144.34, 188.44, 232.10, 310.33, 366.47 and 429.97 μg/m3 for 3, 7, 15, 30, 90, 180 and 365-day return period, respectively. In case of non-stationary GEV model the estimated return levels for 3, 7, 15, 30, 90, 180 and 365-day period were in the range 38.38–140.96, 84.31–184.35, 115.92–225.26, 166.61–337.21, 189.46–388.30 and 214.98–445 μg/m3, respectively. The highest median return level of O3 for 3-day return period (100.35 μg/m3) was observed due to temperature as covariate, for 7-day (146.04 μg/m3) and 15- day (189.85 μg/m3) return period, it was observed due to Benzene as a covariate and for rest of the 4 different return periods, NO was found responsible for the highest median return level of O3 (233.52, 312.40, 369.02 and 433.06 μg/m3 for 30, 90, 180 and 365-day respectively). Seasonal analysis finds O3 extremes to be high in monsoon and premonsoon seasons and low in winter period. The impact of nonstationary condition is exemplified by the fact that the 365 day return level of maximum ozone concentration was found to exceed within just 20 days for a lower concentration of precursor Benzene (i.e. 214.48 μg/m3). Results underline the role of the principal components of the precursor compounds in governing the maximum ozone concentration in any city.

Urban sprawl and ozone episodes in Mexico City

Urban sprawl and ozone episodes in Mexico City

Impact of new fuel specifications on vehicle emissions in Mexico

Volatile Organic Compounds (VOCs) from vehicles are a significant contributor to Mexico's pollution, as a precursor to both ambient ozone and secondary particulate matter (PM). Recent updates to Mexico's gasoline specification NOM-016 allow ethanol blends up to 10 percent (E10) outside of the metropolitan areas of Mexico City, Guadalajara and Monterrey, and leave open the possibility of allowing E10 within these areas as well. The NOM-016 updates also permit E10 to have higher volatility (RVP) than previously allowed. The impact of these changes on vehicle emissions in Mexico was assessed, with a focus on evaporative VOC emissions driven by increased RVP and permeation. Roadside remote sensing (RSD) measurements from Mexico City were analyzed, finding that vehicle fleet hydrocarbon (HC) emissions are much higher than U.S. levels, with indication that high evaporative emissions contribute to this. The impact on vehicle emissions of the NOM-016 revisions and potential introduction of E10 in metropolitan zones was assessed for an ozone episode day using the MOVES-Mexico model, which reflects higher evaporative emissions from Mexico's vehicle fleet. Results show an increase in total motor vehicle VOC emissions of 13–18 percent in metropolitan areas and a 23 percent increase in other areas of the country for NOM-016 compliant E10 fuel, relative to fuel sold in Mexico prior to the NOM-016 update. Results for other pollutants showed an increase in PM2.5 and air toxics from E10 blends, a reduction in CO, and NOx impacts mixed depending on area. Total motor vehicle emissions for toluene, identified in prior work as the compound with the highest ozone forming potential in Mexico City, increased by 29–34 percent with E10 fuel in Mexico City and more in other metropolitan areas. Past studies linking emissions change with health outcomes in Mexico City suggest that the increase in vehicle emissions from NOM-016 updates would have a measurable negative effect on premature mortality, chronic bronchitis, hospital visits, and restricted activity days, with societal costs associated with health care and lost productivity.

Hot Summers: Effect of Extreme Temperatures on Ozone in Sydney, Australia

Poor air quality is often associated with hot weather, but the quantitative attribution of high temperatures on air quality remains unclear. In this study, the effect of elevated temperatures on air quality is investigated in Greater Sydney using January 2013, a period of extreme heat during which temperatures at times exceeded 40 °C, as a case study. Using observations from 17 measurement sites and the Weather Research and Forecasting Chemistry (WRF-Chem) model, we analyse the effect of elevated temperatures on ozone in Sydney by running a number of sensitivity studies in which: (1) the model is run with biogenic emissions generated by MEGAN and separately run with monthly average Model of Emissions of Gases and Aerosols from Nature ( MEGAN) biogenic emissions (for January 2013); (2) the model results from the standard run are compared with those in which average temperatures (for January 2013) are only applied to the chemistry; (3) the model is run using both averaged biogenic emissions and temperatures; and (4 and 5) the model is run with half and zero biogenic emissions. The results show that the impact on simulated ozone through the effect of temperature on reaction rates is similar to the impact via the effect of temperature on biogenic emissions and the relative impacts are largely additive when compared to the run in which both are averaged. When averaged across 17 sites in Greater Sydney, the differences between ozone simulated under standard and averaged model conditions are as high as 16 ppbv. Removing biogenic emissions in the model has the effect of removing all simulated ozone episodes during extreme heat periods, highlighting the important role of biogenic emissions in Australia, where Eucalypts are a key biogenic source.

Impact of Biomass Burning and Vertical Mixing of Residual‐Layer Aged Plumes on Ozone in the Yangtze River Delta, China: A Tethered‐Balloon Measurement and Modeling Study of a Multiday Ozone Episode

AbstractOzone (O3) is one of the most important trace gases in the troposphere because of its impact on human health, crop yield, climate change, and atmospheric oxidizing capacity. In recent years, China has suffered from deleterious O3 pollution in many regions, especially in eastern China during summer. However, understanding the causes of high O3 pollution episodes is still limited because existing measurements were mainly conducted at the surface level. In this study, we conducted an intensive tethered‐balloon measurement at the Station for Observing Regional Processes of the Earth System in the Yangtze River Delta during a multiday O3 episode in mid‐June 2014, with the highest hourly surface O3 record (142 ppbv) at the station in 2014. By integrating available ground‐based, remote sensing and aircraft measurement data together with chemical transport modeling and Lagrangian dispersion modeling, we carried out a comprehensive analysis of the main source and formation mechanism for two typical days during the multiday O3 episode. On 11 June, agricultural straw burning in the north significantly enhanced O3 concentration (40–50 ppbv) above the planetary boundary layer (PBL) over Nanjing and further influenced the downwind surface O3 in the following days. On 14 June, regional photochemically aged air masses from city clusters in the Yangtze River Delta substantially influenced early morning ground O3 concentrations by vertical mixing from the residual layer and contributed to PBL daytime O3 buildup. This study provides a clear picture of the transport and mixing of O3 and relevant pollutants in the PBL from different sources and highlights the significance of regional emission control in the mitigation of photochemical pollution in eastern China.

Photochemistry of ozone over urban area: a case study for Delhi City

Ozone (O3), oxides of nitrogen (NOx), and carbon monoxide (CO) were monitored at CSIR National Physical Laboratory, New Delhi, during May and October, 2014. One-hour O3 concentrations exceeding more than 120 ppb were observed for 9 days in May and 16 days in October months with concurrent high NOx and CO values. The higher ozone episodes in October 2014 as compared to May 2014 may be due to contribution of biomass burning. The nitrogen oxide (NO), nitrogen dioxide (NO2), O3, and the photolysis rate of NO2 (i.e., JNO2) values of May and October, 2014, were used to evaluate the photo-stationary state ratio (or Leighton ratio, ϕ) for clear sky conditions. Leighton ratio ranged from 0.4 to 2.6 (average values 0.89 ± 0.4) for May 2014 and from 0.3 to 3.6 (average value 1.01 ± 0.52) for October 2014. The observed NOy values in October 2014 indicated the production of O3 through hydrocarbon-sensitive O3 chemistry. The sum of reactive nitrogen species’ (i.e., NOy) mixing ratios recorded during October 2014 varied from 259.4 to 10.4 ppb with an average value of 80.5 ± 39 ppb. The averaged diurnal ratio of NOx–NOy of > 0.3 indicated the presence of freshly emitted pollutants.

Co-occurrence of extreme ozone and heat waves in two cities from Morocco

Temperature is the first meteorological factor to be directly involved in leading ozone (O3) extreme events. Generally, upward temperatures increase the probability of having exceedance in ozone adopted thresholds. In the global climate change context more frequent and/or persistent heat waves and extreme ozone (O3) episodes are likely to occur during in coming decades and a key question is about the coincidence and co-occurrence of these extremes. In this paper, using 7 years of surface temperature and air quality observations over two cities from Morocco (Casablanca and Marrakech) and implementing a percentile thresholding approach, we show that the extremes in temperature and ozone (O3) cluster together in many cases and that the outbreak of ozone events generally match the first or second days of heat waves. This co-occurrence of extreme episodes is highly impacted by humidity and may be overlapping large-scale episodes.

Analysis of SAPRC16 chemical mechanism for ambient simulations

SAPRC16 is an interim update to the SAPRC series of chemical mechanisms that includes updated rate constants, a revised representation of radical chemistry, and a new speciation lumping scheme to better develop predictions of SOA precursors. In this study, the ability of the SAPRC16 chemical mechanism to simulate regional ozone episodes is tested in seven (7) major cities across the United States. The UCD-CIT 3-dimensional (3D) airshed model was configured with both SAPRC11 (base mechanism) and SAPRC16 (updated mechanism). Concentrations of ozone, hydroxyl radical (OH) and hydroperoxyl (HO2) radical predicted with both mechanisms were compared to measured values. It was generally observed that SAPRC16 predicts slightly lower ozone concentrations than SAPRC11 in NOx rich urban centers. A box model analysis shows that the SAPRC16 mechanism quenches ozone production earlier than SAPRC11 as NOx concentrations increase (yielding decreasing VOC/NOx ratios). This could be caused by more detailed HO2+RO2 reactions and RO2 isomerization reactions in SAPRC16 that compete with the HO2+NO reaction. Predictions from SAPRC11 are in better agreement with the measurements in the western United States, however SAPRC16 outperforms SAPRC11 in some eastern and southern U.S. cities. These results may also be strongly influenced by the accuracy of emissions inventories in each region. Differences in ozone concentrations predicted by SAPRC16 and SAPRC11 increased as emissions decreased suggesting that the two mechanisms will predict different outcomes from future emissions control programs. In general, the SAPRC11 mechanisms slightly over-predicts OH concentration while under-predicting HO2 radical concentration and the SAPRC16 mechanism slightly under predicts OH and under predicts HO2, sometimes by large amounts. The reasons for this behavior are unknown at this time. These features will influence the secondary organic aerosol (SOA) predictions from each mechanism since the branching between high NOx and low NOx regimes depends on calculated HO2 concentrations. Further analysis of the HO2+RO2 reactions and RO2 isomerization reactions and the reasons for the lower HO2 predictions in the updated SAPRC16 chemical mechanism should be carried out before widespread adoption of the new mechanism.

Summertime high resolution variability of atmospheric formaldehyde and non-methane volatile organic compounds in a rural background area

On rural background areas atmospheric formaldehyde (HCHO) is important for its abundance and chemical reactivity, directly linked to the tropospheric ozone formation processes. HCHO is also toxic and carcinogenic to humans. Atmospheric HCHO was continuously measured in summer 2016 during 81 days (N = 6722, average: 1.42 ppbv) in a rural background area in Northern Spain, Valderejo Natural Park (VNP) using a Hantzsch fluorimetric system. To better characterize the photochemical processes the database was completed with hourly measurements of 63 Non-Methane Hydrocarbons (NMHC) performed by gas chromatography and other common atmospheric pollutants and meteorological parameters.HCHO mixing ratios were highly correlated with ozone and isoprene. Cloudy and rainy days, with low temperature and radiation, led to low HCHO mixing ratios, with maxima (<2 ppbv) registered around 14 UTC. On days with increased radiation and temperature HCHO maxima occurred slightly later (<6 ppbv, ≈16:00 UTC). During clear summer days with high temperature and radiation, two HCHO peaks were registered daily, one synchronized with the radiation maximum (≈3–4 ppbv, ≈13:00 UTC) and an absolute maximum (<10 ppbv, ≈18:00 UTC), associated with the addition of HCHO coming into VNP due to inbound transport of old polluted air masses.In the ozone episode studied, the processes of accumulation and recharge of ozone and of HCHO ran in parallel, leading to similar daily patterns of variation. Finally, HCHO mixing ratios measured in VNP were compared with other measurements at rural, forested, and remote sites all over the world, obtaining similar values.

Impacts of compound extreme weather events on ozone in the present and future

Abstract. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used to study the effect of extreme weather events on ozone in the US for historical (2001–2010) and future (2046–2055) periods under the RCP8.5 scenario. During extreme weather events, including heat waves, atmospheric stagnation, and their compound events, ozone concentration is much higher compared to the non-extreme events period. A striking enhancement of effect during compound events is revealed when heat wave and stagnation occur simultaneously as both high temperature and low wind speed promote the production of high ozone concentrations. In regions with high emissions, compound extreme events can shift the high-end tails of the probability density functions (PDFs) of ozone to even higher values to generate extreme ozone episodes. In regions with low emissions, extreme events can still increase high-ozone frequency but the high-end tails of the PDFs are constrained by the low emissions. Despite the large anthropogenic emission reduction projected for the future, compound events increase ozone more than the single events by 10 to 13 %, comparable to the present, and high-ozone episodes with a maximum daily 8 h average (MDA8) ozone concentration over 70 ppbv are not eliminated. Using the CMIP5 multi-model ensemble, the frequency of compound events is found to increase more dominantly compared to the increased frequency of single events in the future over the US, Europe, and China. High-ozone episodes will likely continue in the future due to increases in both frequency and intensity of extreme events, despite reductions in anthropogenic emissions of its precursors. However, the latter could reduce or eliminate extreme ozone episodes; thus improving projections of compound events and their impacts on extreme ozone may better constrain future projections of extreme ozone episodes that have detrimental effects on human health.

Phenomenology of summer ozone episodes over the Madrid Metropolitan Area, central Spain

Abstract. Various studies have reported that the photochemical nucleation of new ultrafine particles (UFPs) in urban environments within high insolation regions occurs simultaneously with high ground ozone (O3) levels. In this work, we evaluate the atmospheric dynamics leading to summer O3 episodes in the Madrid air basin (central Iberia) by means of measuring a 3-D distribution of concentrations for both pollutants. To this end, we obtained vertical profiles (up to 1200 m above ground level) using tethered balloons and miniaturised instrumentation at a suburban site located to the SW of the Madrid Metropolitan Area (MMA), the Majadahonda site (MJDH), in July 2016. Simultaneously, measurements of an extensive number of air quality and meteorological parameters were carried out at three supersites across the MMA. Furthermore, data from O3 soundings and daily radio soundings were also used to interpret atmospheric dynamics. The results demonstrate the concatenation of venting and accumulation episodes, with relative lows (venting) and peaks (accumulation) in O3 surface levels. Regardless of the episode type, the fumigation of high-altitude O3 (arising from a variety of origins) contributes the major proportion of surface O3 concentrations. Accumulation episodes are characterised by a relatively thinner planetary boundary layer (&lt; 1500 m at midday, lower in altitude than the orographic features), light synoptic winds, and the development of mountain breezes along the slopes of the Guadarrama Mountain Range (located W and NW of the MMA, with a maximum elevation of &gt; 2400 m a.s.l.). This orographic–meteorological setting causes the vertical recirculation of air masses and enrichment of O3 in the lower tropospheric layers. When the highly polluted urban plume from Madrid is affected by these dynamics, the highest Ox (O3+ NO2) concentrations are recorded in the MMA. Vertical O3 profiles during venting episodes, with strong synoptic winds and a deepening of the planetary boundary layer reaching &gt; 2000 m a.s.l., were characterised by an upward gradient in O3 levels, whereas a reverse situation with O3 concentration maxima at lower levels was found during the accumulation episodes due to local and/or regional production. The two contributions to O3 surface levels (fumigation from high-altitude strata, a high O3 background, and/or regional production) require very different approaches for policy actions. In contrast to O3 vertical top-down transfer, UFPs are formed in the planetary boundary layer (PBL) and are transferred upwards progressively with the increase in PBL growth.

Atmospheric Dynamics and Ozone Cycle during Sea Breeze in a Mediterranean Complex Urbanized Coastal Site

AbstractPersistent high pressure conditions over the Mediterranean Basin favor the occurrence of sea breezes that can lead to ozone transport through complex recirculation patterns. These features were investigated during an ozone episode with hourly concentrations exceeding 200 μg m−3 that occurred on July 2015 in Naples (Italy), one of the largest and densest conurbations in the Mediterranean region. Aircraft measurements were taken at heights from 150 to 1500 m AGL and compared and integrated with high-resolution meteorological and air quality model simulations to investigate local circulation and pollutants dynamics. The integration of airborne measurements, surface observations, and modeling established a framework to assess the photochemical phenomena in the area. Sea breezes and local emissions triggered ozone production at inland areas, causing high concentrations between the coast and the Apennine chain. Ozone was then injected into the upper boundary layer and transported toward the sea by the wind rotation occurring above 500 m AGL, causing a complex vertical layering of concentrations, with maxima between 500 and 800 m AGL. Vertical growth of the ozone concentration profile was also caused by the decrease of the boundary layer depth occurring when the breeze front reached the inland area carrying NOx-rich air from the densely populated coast and favoring titration near the surface. Although the whole airshed was a net ozone producer, local surface concentrations were determined by a complex interaction of atmospheric flow and chemistry at different scales, supporting the need for coordinated efforts to control smog precursors over wide areas.

The Controlling Factors of Photochemical Ozone Production in Seoul, South Korea

Author(s): Kim, S; Jeong, D; Sanchez, D; Wang, M; Seco, R; Blake, D; Meinardi, S; Barletta, B; Hughes, S; Jung, J; Kim, D; Lee, G; Lee, M; Ahn, J; Lee, SD; Cho, G; Sung, MY; Lee, YH; Park, R | Abstract: We present the ambient ozone and relevant observed trace gas dataset in Seoul, South Korea, during the Megacity Air Pollution Studies (MAPS)-Seoul field campaign from May to June of 2015 (MAPS-Seoul 2015). We observed two distinctive periods, one with higher and the other with lower daytime ozone levels despite mostly clear conditions for both periods. The importance of peroxy radical contributions to excess ozone production is illustrated by the substantial differences in the Leighton constant (Φ) for the two periods. Moreover, higher levels of hydroxyl radical (OH) reactivity (s–1) were observed during the high ozone episode compared to the low ozone episode by as much as ~5 s–1. The contributions of nitrogen oxides (NOx) to OH reactivity become less important than those of volatile organic compounds (VOCs) during the high ozone episode, which suggests the NOx saturated ozone production regime. It was also notable that the biogenic VOC isoprene consistently contributed the most to OH reactivity from among the observed VOCs during the afternoon throughout the whole field campaign. Finally, we ran multiple box model scenarios to evaluate the ozone production rates of three different air mixtures: a high ozone mixture, a low ozone mixture, and a simulation of the regional air quality. The results indicate that the total OH reactivity levels and the relative contributions of VOCs to NOx play critical roles in ozone production rates. The simulated air quality mixture results in lower OH reactivity, causing lower ozone production rates than those calculated for the high ozone mixture, which clearly indicates the need for further improvements in the regional model to accurately simulate ozone precursors in the region. The results of this study suggest that a comprehensive trace gas dataset combined with observations of the OH reactivity enables us to properly diagnose the photochemistry behind ozone pollution, leading to effective ozone abatement policies.

Air Quality Impact of Biomass Co-Firing with Coal at a Power Plant in the Greater Houston Area

The Houston-Galveston-Brazoria (HGB) area of Texas is a moderate nonattainment region for ozone, and has a history of severe summer ozone episodes. W. A. Parish power plant (WAP) located in the greater Houston area is the largest coal and natural gas based electricity generating unit (EGU) in Texas. Forest residue is an abundant renewable resource, and can be used to offset coal usage at EGUs. This study evaluates the impact of co-firing 5%, 10%, and 15% (energy-basis) of forest residue at WAP on the air quality of the HGB area. Photochemical modeling with Comprehensive Air Quality Model with Extensions (CAMx) was conducted to investigate the air quality at three air quality monitoring sites (C696, C53, C556) in the HGB area, under two source scenarios (all-sources, point + biogenic sources). Significant reduction of SO2 and O3 was observed for 10% and 15% co-firing ratios at monitoring station (C696) close to WAP. The maximum reduction of ozone observed for 15% co-firing is 4.7% and 6.3% for all-sources and point + biogenic sources scenarios respectively. The reduction in other criteria air pollutants is not significant at all locations. The overall results from this study indicate that biomass co-firing at WAP would not lead to a significant reduction in ozone concentrations in the region during periods of peak ozone.

Grain Biodeterioration of Sorghum Converted Lines Inoculated with a Mixture of Fusarium thapsinum and Curvularia lunata

The direct effects of acute on the growth, sporulation and infection of Pyricularia grisea, rece blast pathogen, were investigated to understand the interactions between ozone and the pathogen. Acute exposure of 200 nl ozone for 8 h significantly reduced conidia germination on water atar. Ozone exposure of 200 nl for 8h per day for 5 days had no effect on increase in colony diameter, but severely damaged actively growing aerial mycelia. However, the damage to mycalia was recovered during the following 16 h exposure of unpolluted air. Conidial production was also stimulated by the acute ozone exposure for 5 days. The conidia exposed to the acute ozone for 5 days normally germinates but slightly reduce appressoria formation on rice leaf. However, the conidia produced by artificial stimulation under the same ozone concentration for 10 days showed significant reduction in appressorea for mation on a hydrophobic film. This study suggests that the acute ozone could ingibit appressoria formation as well as vegetative growth of the pathogen, resulting in decrease in rece blast development in the field during summer when high ozone episodes could occur occasionally.

Factors dominating 3-dimensional ozone distribution during high tropospheric ozone period.

Data from an in situ monitoring network and five ozone sondes are analysed during August of 2012, and a high tropospheric ozone episode is observed around the 8th of AUG. The Community Multi-scale Air Quality (CMAQ) model and its process analysis tool were used to study factors and mechanisms for high ozone mixing ratio at different levels of ozone vertical profiles. A sensitive scenario without chemical initial and boundary conditions (ICBCs) from MOZART4-GEOS5 was applied to study the impact of stratosphere-troposphere exchange (STE) on vertical ozone. The simulation results indicated that the first high ozone peak near the tropopause was dominated by STE. Results from process analysis showed that: in the urban area, the second peak at approximately 2km above ground height was mainly caused by local photochemical production. The third peak (near surface) was mainly caused by the upwind transportation from the suburban/rural areas; in the suburban/rural areas, local photochemical production of ozone dominated the high ozone mixing ratio from the surface to approximately 3km height. Furthermore, the capability of indicators to distinguish O3-precursor sensitivity along the vertical O3 profiles was investigated. Two sensitive scenarios, which had cut 30% anthropogenic NOX or VOC emissions, showed that O3-precursor indicators, specifically the ratios of O3/NOy, H2O2/HNO3 or H2O2/NOZ, could partly distinguish the O3-precursor sensitivity between VOCs-sensitive and NOx-sensitive along the vertical profiles. In urban area, the O3-precursor relationship transferred from VOCs-sensitive within the boundary layer to NOx-sensitive at approximately 1-3km above ground height, further confirming the dominant roles of transportation and photochemical production in high O3 peaks at the near-ground layer and 2km above ground height, respectively.