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1. Examining the competing effects of contemporary land management vs. land cover changes on global air quality

   https://doi.org/10.5194/acp-21-16479-2021  

   Wong, Anthony Y.; Geddes, Jeffrey A. (January 2021, Atmospheric Chemistry and Physics)

   Abstract. Our work explores the impact of two important dimensions of landsystem changes, land use and land cover change (LULCC) as well as directagricultural reactive nitrogen (Nr) emissions from soils, on ozone(O3) and fine particulate matter (PM2.5) in terms of air quality overcontemporary (1992 to 2014) timescales. We account for LULCC andagricultural Nr emissions changes with consistent remote sensingproducts and new global emission inventories respectively estimating theirimpacts on global surface O3 and PM2.5 concentrations as well as Nrdeposition using the GEOS-Chem global chemical transport model. Over thistime period, our model results show that agricultural Nr emissionchanges cause a reduction of annual mean PM2.5 levels over Europe andnorthern Asia (up to −2.1 µg m−3) while increasing PM2.5 levels in India, China and the eastern US (up to +3.5 µg m−3). Land cover changes induce small reductions in PM2.5 (up to −0.7 µg m−3) over Amazonia, China and India due to reduced biogenic volatile organic compound (BVOC) emissions and enhanced deposition of aerosol precursor gases (e.g., NO2, SO2). Agricultural Nr emissionchanges only lead to minor changes (up to ±0.6 ppbv) in annual meansurface O3 levels, mainly over China, India and Myanmar. Meanwhile, ourmodel result suggests a stronger impact of LULCC on surface O3 over the time period across South America; the combination of changes in drydeposition and isoprene emissions results in −0.8 to +1.2 ppbv surfaceozone changes. The enhancement of dry deposition reduces the surface ozone level (up to −1 ppbv) over southern China, the eastern US and central Africa. The enhancement of soil NO emission due to crop expansion also contributes to surface ozone changes (up to +0.6 ppbv) over sub-Saharan Africa. Incertain regions, the combined effects of LULCC and agricultural Nr emission changes on O3 and PM2.5 air quality can be comparable (>20 %) to anthropogenic emission changes over the same time period. Finally, we calculate that the increase in global agricultural Nr emissions leads to a net increase in global land area (+3.67×106km2) that potentially faces exceedance of the critical Nr load (>5 kg N ha−1 yr−1). Our result demonstrates the impacts of contemporary LULCC and agricultural Nr emission changes on PM2.5 and O3 in terms of air quality, as well as the importanceof land system changes for air quality over multidecadal timescales. 

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2. Substantial ozone enhancement over the North China Plain from increased biogenic emissions due to heat waves and land cover in summer 2017

   https://doi.org/10.5194/acp-19-12195-2019  

   Ma, Mingchen; Gao, Yang; Wang, Yuhang; Zhang, Shaoqing; Leung, L. Ruby; Liu, Cheng; Wang, Shuxiao; Zhao, Bin; Chang, Xing; Su, Hang; et al (January 2019, Atmospheric Chemistry and Physics)

   Abstract. In the summer of 2017, heavy ozone pollution swamped most of the North ChinaPlain (NCP), with the maximum regional average of daily maximum 8 h ozoneconcentration (MDA8) reaching almost 120 ppbv. In light of the continuingreduction of anthropogenic emissions in China, the underlying mechanisms forthe occurrences of these regional extreme ozone episodes are elucidated fromtwo perspectives: meteorology and biogenic emissions. The significantpositive correlation between MDA8 ozone and temperature, which is amplifiedduring heat waves concomitant with stagnant air and no precipitation,supports the crucial role of meteorology in driving high ozoneconcentrations. We also find that biogenic emissions are enhanced due tofactors previously not considered. During the heavy ozone pollution episodesin June 2017, biogenic emissions driven by high vapor pressure deficit(VPD), land cover change and urban landscape yield an extra mean MDA8 ozoneof 3.08, 2.79 and 4.74 ppbv, respectively, over the NCP, which togethercontribute as much to MDA8 ozone as biogenic emissions simulated using theland cover of 2003 and ignoring VPD and urban landscape. In Beijing, thebiogenic emission increase due to urban landscape has a comparable effect onMDA8 ozone to the combined effect of high VPD and land cover change between2003 and 2016. In light of the large effect of urban landscape on biogenicemission and the subsequent ozone formation, the types of trees may becautiously selected to take into account of the biogenic volatile organic compound (BVOC) emission during the afforestation of cities. This study highlights the vital contributions ofheat waves, land cover change and urbanization to the occurrence of extremeozone episodes, with significant implications for ozone pollution control ina future when heat wave frequency and intensity are projected to increaseunder global warming. 

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3. The long-term trend and production sensitivity change in the US ozone pollution from observations and model simulations

   https://doi.org/10.5194/acp-20-3191-2020  

   He, Hao; Liang, Xin-Zhong; Sun, Chao; Tao, Zhining; Tong, Daniel Q. (January 2020, Atmospheric Chemistry and Physics)

   Abstract. We investigated the ozone pollution trend and its sensitivity to keyprecursors from 1990 to 2015 in the United States using long-term EPA Air Quality System (AQS)observations and mesoscale simulations. The modeling system, a coupledregional climate–air quality model (CWRF-CMAQ; Climate-Weather Research Forecast andthe Community Multiscale Air Quality), captured well the summersurface ozone pollution during the past decades, having a mean slope oflinear regression with AQS observations of ∼0.75. While theAQS network has limited spatial coverage and measures only a few keychemical species, CWRF-CMAQ provides comprehensive simulations to enablea more rigorous study of the change in ozone pollution and chemicalsensitivity. Analysis of seasonal variations and diurnal cycle of ozoneobservations showed that peak ozone concentrations in the summer afternoondecreased ubiquitously across the United States, up to 0.5 ppbv yr−1 in majornon-attainment areas such as Los Angeles, while concentrations at certainhours such as the early morning and late afternoon increased slightly.Consistent with the AQS observations, CMAQ simulated a similar decreasingtrend of peak ozone concentrations in the afternoon, up to 0.4 ppbv yr−1, andincreasing ozone trends in the early morning and late afternoon. A monotonicallydecreasing trend (up to 0.5 ppbv yr−1) in the odd oxygen (Ox=O3+NO2) concentrations are simulated by CMAQ at all daytime hours.This result suggests that the increased ozone in the early morning and lateafternoon was likely caused by reduced NO–O3 titration, driven bycontinuous anthropogenic NOx emission reductions in the past decades.Furthermore, the CMAQ simulations revealed a shift in chemical regimes ofozone photochemical production. From 1990 to 2015, surface ozone productionin some metropolitan areas, such as Baltimore, has transited from aVOC-sensitive environment (>50 % probability) to aNOx-sensitive regime. Our results demonstrated that the long-termCWRF-CMAQ simulations can provide detailed information of the ozonechemistry evolution under a changing climate and may partially explain theUS ozone pollution responses to regional and national regulations. 

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4. Direct measurements of ozone response to emissions perturbations in California

   https://doi.org/10.5194/acp-22-4929-2022  

   Wu, Shenglun; Lee, Hyung Joo; Anderson, Andrea; Liu, Shang; Kuwayama, Toshihiro; Seinfeld, John H.; Kleeman, Michael J. (January 2022, Atmospheric Chemistry and Physics)

   Abstract. A new technique was used to directly measure O3 response to changes inprecursor NOx and volatile organic compound (VOC) concentrations in the atmosphere using threeidentical Teflon smog chambers equipped with UV lights. One chamberserved as the baseline measurement for O3 formation, one chamber addedNOx, and one chamber added surrogate VOCs (ethylene, m-xylene,n-hexane). Comparing the O3 formation between chambers over a3-hour UV cycle provides a direct measurement of O3 sensitivity toprecursor concentrations. Measurements made with this system at Sacramento,California, between April–December 2020 revealed that theatmospheric chemical regime followed a seasonal cycle. O3 formation wasVOC-limited (NOx-rich) during the early spring, transitioned toNOx-limited during the summer due to increased concentrations ofambient VOCs with high O3 formation potential, and then returned toVOC-limited (NOx-rich) during the fall season as the concentrations ofambient VOCs decreased and NOx increased. This seasonal pattern ofO3 sensitivity is consistent with the cycle of biogenic emissions inCalifornia. The direct chamber O3 sensitivity measurements matchedsemi-direct measurements of HCHO/NO2 ratios from the TROPOsphericMonitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor (Sentinel-5P) satellite. Furthermore, the satellite observations showed thatthe same seasonal cycle in O3 sensitivity occurred over most of theentire state of California, with only the urban cores of the very largecities remaining VOC-limited across all seasons. The O3-nonattainmentdays (MDA8 O3>70 ppb) have O3 sensitivity in theNOx-limited regime, suggesting that a NOx emissions controlstrategy would be most effective at reducing these peak O3concentrations. In contrast, a large portion of the days with MDA8 O3concentrations below 55 ppb were in the VOC-limited regime, suggesting thatan emissions control strategy focusing on NOx reduction would increaseO3 concentrations. This challenging situation suggests that emissionscontrol programs that focus on NOx reductions will immediately lowerpeak O3 concentrations but slightly increase intermediate O3concentrations until NOx levels fall far enough to re-enter theNOx-limited regime. The spatial pattern of increasing and decreasingO3 concentrations in response to a NOx emissions control strategyshould be carefully mapped in order to fully understand the public healthimplications. 

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5. Impact of Fire Emissions on U.S. Air Quality from 1997 to 2016–A Modeling Study in the Satellite Era

   https://doi.org/10.3390/rs12060913  

   Tao, Zhining; He, Hao; Sun, Chao; Tong, Daniel; Liang, Xin-Zhong (March 2020, Remote Sensing)

   A regional modeling system that integrates the state-of-the-art emissions processing (SMOKE), climate (CWRF), and air quality (CMAQ) models has been combined with satellite measurements of fire activities to assess the impact of fire emissions on the contiguous United States (CONUS) air quality during 1997–2016. The system realistically reproduced the spatiotemporal distributions of the observed meteorology and surface air quality, with a slight overestimate of surface ozone (O3) by ~4% and underestimate of surface PM2.5 by ~10%. The system simulation showed that the fire impacts on primary pollutants such as CO were generally confined to the fire source areas but its effects on secondary pollutants like O3 spread more broadly. The fire contribution to air quality varied greatly during 1997-2016 and occasionally accounted for more than 100 ppbv of monthly mean surface CO and over 20 µg m−3 of monthly mean PM2.5 in the Northwest U.S. and Northern California, two regions susceptible to frequent fires. Fire emissions also had implications on air quality compliance. From 1997 to 2016, fire emissions increased surface 8-hour O3 standard exceedances by 10% and 24-hour PM2.5 exceedances by 33% over CONUS. 

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