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We present here a fully coupled global aerosol and chemistry model for the troposphere. The model is used to assess the interactions between aerosols and chemical oxidants in the troposphere, including (1) the conversion from gas-...
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We present here a fully coupled global aerosol and chemistry model for the troposphere. The model is used to assess the interactions between aerosols and chemical oxidants in the troposphere, including (1) the conversion from gas-phase oxidants into the condensed phase during the formation of aerosols, (2) the heterogeneous reactions occurring on the surface of aerosols, and (3) the effect of aerosols on ultraviolet radiation and photolysis rates. The present study uses the global three-dimensional chemical/transport model, Model for Ozone and Related Chemical Tracers, version 2 (MOZART-2), in which aerosols are coupled with the model. The model accounts for the presence of sulfate, soot, primary organic carbon, ammonium nitrate, secondary organic carbon, sea salt, and mineral dust particles. The simulated global distributions of the aerosols are analyzed and evaluated using satellite measurements (Moderate-Resolution Imaging Spectroradiometer (MODIS)) and surface measurements. The results suggest that in northern continental regions the tropospheric aerosol loading is highest in Europe, North America, and east Asia. Sulfate, organic carbon, black carbon, and ammonium nitrate are major contributions for the high aerosol loading in these regions. Aerosol loading is also high in the Amazon and in Africa. In these areas the aerosols consist primarily of organic carbon and black carbon. Over the southern high-latitude ocean (around 60°S), high concentrations of sea-salt aerosol are predicted. The concentration of mineral dust is highest over the Sahara and, as a result of transport, spread out into adjacent regions. The model and MODIS show similar geographical distributions of aerosol particles. However, the model overestimates the sulfate and carbonaceous aerosol in the eastern United States, Europe, and east Asia. In the region where aerosol loading is high, aerosols have important impacts on tropospheric ozone and other oxidants. The model suggests that heterogeneous reactions of HO2 and CH2O on sulfate have an important impact on HOx (OH + HO2) concentrations, while the heterogeneous reaction of O3 on soot has a minor effect on O3 concentrations in the lower troposphere. The heterogeneous reactions on dust have very important impacts on HOx and O3 in the region of dust mobilization, where the reduction of HOx and O3 concentrations can reach a maximum of 30% and 20%, respectively, over the Sahara desert. Dust, organic carbon, black carbon, and sulfate aerosols have important impacts on photolysis rates. For example, the photodissociation frequencies of ozone and nitrogen dioxide are reduced by 20% at the surface in the Sahara, in the Amazon, and in eastern Asia, leading to 5–20% reduction in the concentration of HOx and to a few percent change in the O3 abundance in these regions.
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Changes in land use and land cover (LULC) influence meteorological fields and biogenic emissions, further affecting the atmospheric chemistry and air quality. Combining the satellite measurements and WRF-Chem model simulations, we...
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Changes in land use and land cover (LULC) influence meteorological fields and biogenic emissions, further affecting the atmospheric chemistry and air quality. Combining the satellite measurements and WRF-Chem model simulations, we evaluate the impacts of the LULC change between 2001 and 2018 on the summertime ozone (O_3) formation in North China Plain and surrounding areas (NCPs). Satellite measurements have revealed that from Taihang to Yanshan Mountain, the fraction of broadleaf and needle forest coverage has increased by 5%-20% and the urban area has increased by up to 25% in the NCP. Additionally, the vegetation density has increased significantly in the NCPs except for urban areas. The LULC change generally enhances biogenic volatile compounds emissions in the NCPs, particularly over Taihang and Yanshan mountain, but the O_3 variation is divergent. The maximum daily 8-ihr average (MDA8) O_3 concentrations are reduced by 1%-7% over Taihang and Yanshan Mountain because the raised vegetation density increases O_3 dry deposition velocity to accelerate the O_3 loss. The raised vegetation density enhances the evapotranspiration to decrease the near-surface temperature by 0.1°C-1.5°C, which also generates a divergence in the low-level atmosphere in the NCPs, causing secondary northerly or easterly winds in the NCP. The O_3 enhancement along the coastal areas of the NCP is attributed to the perturbation of wind fields and photolysis induced by the LULC change. The divergent variation of the MDA8 O_3 concentrations in the NCP is generally caused by the variations of biogenic emissions and photolysis.
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This study evaluates the strengths and weaknesses of aerosol distributions and optical depths that are used to force the GFDL coupled climate model CM2.1. The concentrations of sulfate, organic carbon, black carbon, and dust are s...
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This study evaluates the strengths and weaknesses of aerosol distributions and optical depths that are used to force the GFDL coupled climate model CM2.1. The concentrations of sulfate, organic carbon, black carbon, and dust are simulated using the MOZART model (Horowitz, 2006), while sea-salt concentrations are obtained from a previous study by Haywood et al. (1999). These aerosol distributions and precalculated relative-humidity-dependent specific extinction are utilized in the CM2.1 radiative scheme to calculate the aerosol optical depth. Our evaluation of the mean values (1996–2000) of simulated aerosols is based on comparisons with long-term mean climatological data from ground-based and remote sensing observations as well as previous modeling studies. Overall, the predicted concentrations of aerosol are within a factor 2 of the observed values and have a tendency to be overestimated. Comparison with satellite data shows an agreement within 10% of global mean optical depth. This agreement masks regional differences of opposite signs in the optical depth. Essentially, the excessive optical depth from sulfate aerosols compensates for the underestimated contribution from organic and sea-salt aerosols. The largest discrepancies are over the northeastern United States (predicted optical depths are too high) and over biomass burning regions and southern oceans (predicted optical depths are too low). This analysis indicates that the aerosol properties are very sensitive to humidity, and major improvements could be achieved by properly taking into account their hygroscopic growth together with corresponding modifications of their optical properties.
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The formation of chemical oxidants, particularly ozone, in Mexico City were studied using a newly developed regional chemical/dynamical model (WRF-Chem). The magnitude and timing of simulated diurnal cycles of ozone (O_3), carbon ...
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The formation of chemical oxidants, particularly ozone, in Mexico City were studied using a newly developed regional chemical/dynamical model (WRF-Chem). The magnitude and timing of simulated diurnal cycles of ozone (O_3), carbon monoxide (CO) and nitrogen oxides (NO_x) and the maximum and minimum O_3 concentrations are generally consistent with surface measurements. Our analysis shows that the strong diurnal cycle in O_3 is mainly attributable to photochemical variations, while diurnal cycles of CO and NO_x mainly result from variations of emissions and boundary layer height. In a sensitivity study, oxidation reactions of aromatic hydrocarbons (HCs) and alkenes yield highest peak O_3 production rates (20 and 18 ppbv h~(-1), respectively). Alkene oxidations, which are generally faster, dominate in early morning. By late morning, alkene concentrations drop, and oxidations of aromatics dominate, with lesser contributions from alkanes and CO. The sensitivity of O_3 concentrations to NO_x and HC emissions was assessed. Our results show that daytime O_3 production is HC-limited in the Mexico City metropolitan area, so that increases in HC emissions increase O_3 chemical production, while increases in NO_x emissions decrease O_3 concentrations. However, increases in both NO_x and HC emissions yield even greater O_3 increases than increases in HCs alone. Uncertainties in HC emissions estimates give large uncertainties in calculated daytime O_3, while NO_x emissions uncertainties are less influential. However, NO_x emissions are important in controlling O_3 at night.
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Although stringent emission mitigation strategies have been carried out since 2013 in Beijing–Tianjin–Hebei (BTH), China, heavy haze with high levels of fine particulate matter (PM2.5) still frequently engulfs the region during ...
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Although stringent emission mitigation strategies have been carried out since 2013 in Beijing–Tianjin–Hebei (BTH), China, heavy haze with high levels of fine particulate matter (PM2.5) still frequently engulfs the region during wintertime and the nitrate contribution to PM2.5 mass has progressively increased. N2O5 heterogeneous hydrolysis is the most important pathway of nitrate formation at nighttime. In the present study, the WRF-Chem model is applied to simulate a heavy haze episode from 10 to 27?February 2014 in BTH to evaluate contributions of N2O5 heterogeneous hydrolysis to nitrate formation and effects of organic coating. The model generally performs reasonably well in simulating meteorological parameters, air pollutants, and aerosol species against observations in BTH. N2O5 heterogeneous hydrolysis with all the secondary organic aerosol assumed to be involved in coating considerably improves the nitrate simulations compared to the measurements in Beijing. On average, organic coating decreases nitrate concentrations by 8.4 % in BTH during an episode, and N2O5 heterogeneous hydrolysis with organic coating contributes about 30.1 % of nitrate concentrations. Additionally, the reaction also plays a considerable role in the heavy haze formation, with a PM2.5 contribution of about 11.6 % in BTH. Sensitivity studies also reveal that future studies need to be conducted to predict the organic aerosol hygroscopicity for accurately representing the organic coating effect on N2O5 heterogeneous hydrolysis.
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Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol–radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibit an i...
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Water vapor has been proposed to amplify the severe haze pollution in China by enhancing the aerosol–radiation feedback (ARF). Observations have revealed that the near-surface PM2.5 concentrations ([PM2.5]) generally exhibit an increasing trend with relative humidity (RH) in the North China Plain (NCP) during 2015 wintertime, indicating that the aerosol liquid water (ALW) caused by hygroscopic growth could play an important role in the PM2.5 formation and accumulation. Simulations during a persistent and heavy haze pollution episode from 5?December?2015 to 4?January?2016 in the NCP were conducted using the WRF-Chem Model to comprehensively quantify contributions of the ALW effect to near-surface [PM2.5]. The WRF-Chem Model generally performs reasonably well in simulating the temporal variations in RH against measurements in the NCP. The factor separation approach (FSA) was used to evaluate the contribution of the ALW effect on the ARF, photochemistry, and heterogeneous reactions to [PM2.5]. The ALW not only augments particle sizes to enhance aerosol backward scattering but also increases the effective radius to favor aerosol forward scattering. The contribution of the ALW effect on the ARF and photochemistry to near-surface [PM2.5] is not significant, being generally within 1.0 μg m?3 on average in the NCP during the episode. Serving as an excellent substrate for heterogeneous reactions, the ALW substantially enhances the secondary aerosol (SA) formation, with an average contribution of 71 %, 10 %, 26 %, and 48 % to near-surface sulfate, nitrate, ammonium, and secondary organic aerosol concentrations. Nevertheless, the SA enhancement due to the ALW decreases the aerosol optical depth and increases the effective radius to weaken the ARF, reducing near-surface primary aerosols. The contribution of the ALW total effect to near-surface [PM2.5] is 17.5 % on average, which is overwhelmingly dominated by enhanced SA. Model sensitivities also show that when the RH is less than 80 %, the ALW progressively increases near-surface [PM2.5] but commences to decrease when the RH exceeds 80 % due to the high occurrence frequencies of precipitation.
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The fine particles (PM2.5) in China have decreased significantly in recent years as a result of the implementation of Chinese Clean Air Action Plan since 2013, while the O3 pollution is getting worse, especially in megacities such...
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The fine particles (PM2.5) in China have decreased significantly in recent years as a result of the implementation of Chinese Clean Air Action Plan since 2013, while the O3 pollution is getting worse, especially in megacities such as Beijing and Shanghai. Better understanding of the elevated O3 pollution in Chinese megacities and its response to emission change is important for developing an effective emission control strategy in the future. In this study, we analyze the significant increasing trend of daily maximum O3 concentration from 2006 to 2015 in the megacity Shanghai with the variability of 0.8–1.3 ppbv yr?1. It could likely be attributed to the notable reduction in NOx concentrations with the decreasing rate of 1.86–2.15 ppbv yr?1 accompanied by the small change in VOCs during the same period by excluding the weak trends of meteorological impacts on local dispersion (wind speed), regional transport (wind direction), and O3 photolysis (solar radiation). It is further illustrated by using a state-of-the-art regional chemical and dynamical model (WRF-Chem) to explore the O3 variation response to the reduction in NOx emissions in Shanghai. The control experiment conducted for September of 2009 shows excellent performance for O3 and NOx simulations, including both the spatial distribution pattern and the day-by-day variation through comparison with six in situ measurements from the MIRAGE-Shanghai field campaign. Sensitivity experiments with 30 % reduction in NOx emissions from 2009 to 2015 in Shanghai estimated by Shanghai Environmental Monitoring Center shows that the calculated O3 concentrations exhibit obvious enhancement by 4–7 ppbv in urban zones with increasing variability of 0.96–1.06 ppbv yr?1, which is consistent with the observed O3 trend as a result of the strong VOC-limited condition for O3 production. The large reduction in NOx combined with less change in VOCs in the past 10 years promotes the O3 production in Shanghai to move towards an NOx-limited regime. Further analysis of the WRF-Chem experiments and O3 isopleth diagram suggests that the O3 production downtown is still under a VOC-limited regime after 2015 despite the remarkable NOx reduction, while it moves to the transition regime between NOx-limited and VOC-limited in sub-urban zones. Supposing the insignificant VOC variation persists, the O3 concentration downtown would keep increasing until 2020 with the further 20 % reduction in NOx emission after 2015 estimated by Shanghai Clean Air Action Plan. The O3 production in Shanghai will switch from a VOC-limited to an NOx-limited regime after 2020 except for downtown area, which is likely close to the transition regime. As a result the O3 concentration will decrease by 2–3 ppbv in sub-urban zones and by more than 4 ppbv in rural areas as a response to a 20 % reduction in NOx emission after 2020, whereas it is not sensitive to both NOx and VOC changes downtown. This result reveals that the control strategy of O3 pollution is a very complex process and needs to be carefully studied.
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Due to rapid economic development in recent years, China has become a major global source of refractory black carbon (rBC) particles. However, surface rBC measurements have been limited, and the lower troposphere suffers from a co...
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Due to rapid economic development in recent years, China has become a major global source of refractory black carbon (rBC) particles. However, surface rBC measurements have been limited, and the lower troposphere suffers from a complete lack of measurements, especially in heavily rBC-polluted regions such as China’s capital, Beijing (BJ). In this study, we present the first concentration measurements using an airborne Single Particle Soot Photometer (SP2) instrument, including vertical distributions, size distributions, and the mixing state of rBC particles in the lower troposphere in BJ and its surrounding areas. The measurements were conducted from April to June 2012 during 11 flights. The results show that the vertical rBC distributions had noticeable differences between different air masses. When an air mass originated from the south of BJ (polluted region), the rBC particles were strongly compressed in the planetary boundary layer (PBL), and showed a large vertical gradient at the top of the PBL. In contrast, when an air mass originated from the north of BJ (clean region), there was a small vertical gradient. This analysis suggests that there was significant regional transport of rBC particles that enhanced the air pollution in BJ, and the transport not only occurred near the surface but also in the middle levels of the PBL (around 0.5 to 1 km). The measured size distributions show that about 80% of the rBC particles were between the diameters of 70 and 400 nm, and the mean diameter of the peak rBC concentrations was about 180–210 nm. This suggests that the rBC particles were relatively small particles. The mixing state of the rBC particles was analyzed to study the coating processes that occurred on the surface of these particles. The results indicate that the air mass strongly affected the number fraction (NF) of the coated particles. As for a southern air mass, the local air pollution was high, which was coupled with a lower PBL height and higher humidity. Consequently, hygroscopic growth occurred rapidly, producing a high NF value (~65%) of coated rBC particles. The correlation coefficient between the NF and the local relative humidity (RH) was 0.88, suggesting that the rBC particles were quickly converted from hydrophobic to hydrophilic particles. This rapid conversion is very important because it suggests a shorter lifetime of rBC particles under heavily polluted conditions. In contrast, under a northern air mass, there was no clear correlation between the NF and the local humidity. This suggests that the coating process occurred during the regional transport in the upwind region. In this case, the lifetime was longer than the southern air mass condition.
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Qualification of the sources of volatile organic compounds (VOCs) and their effects on city air pollution are crucial issues to develop an effective air pollution control strategy in many polluted large cities of China. In this st...
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Qualification of the sources of volatile organic compounds (VOCs) and their effects on city air pollution are crucial issues to develop an effective air pollution control strategy in many polluted large cities of China. In this study, the VOC concentrations measured in Shanghai, China from 2006 to 2008 are analyzed. A receptor model (PCA/ APCS; Principal Component Analysis/Absolute Principal Component Scores) is applied for identifying the contributions of individual VOC sources to VOC concentrations. Using the PCA/APCS technique, five and four surrogated VOC sources are classified in the center of Shanghai city in summer and in winter. In summer, the five VOC sources include PCs1 (liquefied petroleum gas/natural gas leakage and gasoline evaporation), PCs2 (vehicle related emissions), PCs3 (solvent usages), PCs4 (industrial productions), and PCs5 (biomass/biofuel/coal burning and other natural sources). In winter, the four VOC sources include PCwl (liquefied petroleum gas/natural gas leakage and gasoline evaporation), PCw2 (solvent usages and industrial productions), PCw3 (vehicle related emissions), and PCw4 (biomass/biofuel/coal burning). The result suggests that during summer, 24, 28, 17, 18, and 13% of the measured VOC concentrations were estimated due to the PCsl, PCs2, PCs3, PCs4, and PCs5 VOC sources, respectively. During winter, 17, 48, 23, and 12% of the measured VOC concentrations were attributed to the PCwl, PCw2, PCw3, and PCw4 VOC sources, respectively. For aromatic concentrations, 35% of the concentrations were resulted from solvent usage (PCs3), following by industrial productions (PCs4) of 27%, and vehicle emissions (PCs2) of 19%. For alkene concentrations, the two largest contributors
were due to gasoline industrial and vehicle emissions in both summer and winter. For alkane concentrations, the largest sources were due to gasoline industrial emissions (PCs1) and vehicle emissions (PCs2) in summer. In winter, vehicle emissions (PCw3), solvent usages/industrial productions (PCw2), and gasoline industrial emissions (PCw1) were the major sources. For halo-hydrocarbon concentrations, biomass/biofuel/coal burning and other natural sources were the major sources in both summer and winter.
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Satellite data (MODIS, GOME, and MOPITT) together with a chemical transport global model of the atmosphere (MOZART-2) are used to characterize air pollution in Eastern China and the Eastern US to assess the differences between the...
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Satellite data (MODIS, GOME, and MOPITT) together with a chemical transport global model of the atmosphere (MOZART-2) are used to characterize air pollution in Eastern China and the Eastern US to assess the differences between the photochemical conditions in these two regions. Observations show that aerosol concentrations (both fine (radius < 0.5 μm) and coarse modes (radius > 0.5 μm)) are higher in Eastern China than in the Eastern US. The NO_x concentrations in both regions are substantially higher than in remote regions such as over the oceans (150 compared to 5 (10~(14)#cm~(-2)) over the Pacific Ocean). The CO concentrations are high in both urbanized areas (30 compared to 10 (10~(17)#cm~(-2)) over the Pacific Ocean). However, the concentrations of non-methane hydrocarbons from both anthropogenic and biogenic sources are considerably lower in Eastern China than in the Eastern US. As a result, the rate of photochemical ozone production and ozone concentrations during summer is significantly lower in Eastern China (daily averaged concentrations of 40-50 ppbv in summer) than in the Eastern US (daily averaged values of 60-70 ppbv). The analysis also shows that in Eastern China, the O_3 production is mainly due to the oxidation of carbon monoxide (54% of total O_3 production), while, in the Eastern US, the O_3 production is attributed primarily to the oxidation of reactive hydrocarbons (68% of total O_3 production). The results also indicate that biogenic emissions of hydrocarbons contribute substantially to the production of O_3 in the Eastern US. The O_3 production due to the oxidation of biogenic hydrocarbons represents approximately one third of total O_3 photochemical production in this region. Measurements of surface ozone in the Eastern US and Eastern China seem to support that the summer ozone production is lower in Eastern China than in the Eastern US. However, additional surface measurements, especially of reactive hydrocarbons and ozone are needed in Eastern China in order to improve the present analysis and to confirm our current conclusions. A sensitivity study shows that with increase in anthropogenic emissions of HCs, the surface ozone concentrations significantly increase in Eastern China, indicating that the increase in the emissions of HCs plays an important role for the enhancement in surface ozone in this region.
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