Atmospheric Environment

Impact of aerosols of sea salt origin in a coastal basin: Sydney, Australia Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Jagoda Crawford, David D. Cohen, Scott D. Chambers, Alastair G. Williams, Armand Atanacio Abstract Sea salt is one of the major aerosols in the atmosphere in both the fine and coarse size ranges. Newly formed sea salt particles have a similar composition to seawater; including elements such as Na, Cl, Mg, S, Ca, Br and K. However, once in the atmosphere sea salt particles can undergo chemical reactions with other airborne pollutants, resulting in a loss of chlorine (whereas Na is conservative). The modified aerosol is commonly referred to as aged sea spray or aged sea salt. Fine aerosols from two sites in the Sydney Basin were analysed for source fingerprints with components that may have originated from the ocean (i.e. fresh and aged sea spray). At Lucas Heights, 18.4 km from the nearest coast, the average source fingerprint concentrations of fresh and aged sea spray were 0.47 ± 0.02 and 1.08 ± 0.03 μg/m3, respectively. At Richmond, 58 km from the coast, the average fingerprint concentrations of fresh and aged sea spray were 0.26 ± 0.01 and 0.87 ± 0.02 μg/m3, respectively. At Lucas heights, fresh and aged sea spray contributed to 11% and 21% of PM2.5, respectively. At Richmond fresh and aged sea spray contributed to 4.8 ± 0.35 and 16 ± 0.5% of the PM2.5, respectively. The Cl/Na ratios of aged sea spray at Lucas Heights and Richmond were 0.72 and 0.87, respectively, in comparison to 1.54 for fresh sea salt. At Richmond the corresponding Ca/Na and K/Na ratios were both 0.037 (close to that of seawater), and at Lucas Heights, the corresponding ratios were 0.038 and 0.026, respectively. Back trajectory and Radon-222 analysis demonstrated that the largest concentrations of aged sea spray occur when the air masses had travelled over regions of anthropogenic sources. This confirms an interaction between anthropogenic precursors and sea spray that liberates chlorine enabling it to contribute to other chemical reactions in the atmosphere, e.g. resulting in an increase in the formation of ozone.

Quantifying the emission potentials of fugitive dust sources in Nanjing, East China Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Mengchun Cui, Huayu Lu, Vic Etyemezian, Quanlong Su Abstract Most cities in China have suffered severe atmospheric particulate matter (PM) pollution, with reduced visibility and worsened air quality. Fugitive dust that primarily originates from soils through the action of wind and anthropogenic activities is recognized to be a major source of urban PM. However, it remains challenging to quantify and parameterize fugitive dust emission from various sources due to a lack of observation data. In this study, a portable wind erosion system, PI-SWERL, was applied to characterize the dynamics and magnitude of PM10 and PM2.5 (particles smaller than 10 μm and 2.5 μm respectively) emissions from several possible sources in Nanjing, a typical large city in the Yangtze River Delta economic regions, East China. Results indicate that aerodynamic suspension and abrasion were two major physical mechanisms responsible for fugitive dust emissions, with varying characteristics by location. Changes of emission characteristics and quantities were primarily associated with differences in the supply of fine particles available for wind erosion, controlled by source-specific surface characteristics and anthropogenic disturbances. Construction sites and unpaved roads were identified as two significant sources, with total emission rates of 95.85 (56.97) and 57.22 (32.62) kg s−1respectively for PM10 (PM2.5) at a friction velocity of 0.82 m s−1. These values were up to five times the amounts of PM emitted from arable lands and woodlands, and 1–2 orders of magnitude greater than those from paved roads, grasslands and bare lands. These data demonstrate the importance of construction sites and unpaved roads as local PM sources which might be significant in policy making for air quality management strategies and prioritization of emission control targets in an urban area.

An assessment of important SPECIATE profiles in the EPA emissions modeling platform and current data gaps Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Casey D. Bray, Madeleine Strum, Heather Simon, Lee Riddick, Mike Kosusko, Marc Menetrez, Michael D. Hays, Venkatesh Rao Abstract The United States (US) Environmental Protection Agency (EPA)'s SPECIATE database contains speciated particulate matter (PM) and volatile organic compound (VOC) emissions profiles. Emissions profiles from anthropogenic combustion, industry, wildfires, and agricultural sources among others are key inputs for creating chemically-resolved emissions inventories for air quality modeling. While the database and its use for air quality modeling are routinely updated and evaluated, this work sets out to systematically prioritize future improvements and communicate speciation data needs to the research community. We first identify the most prominent profiles (PM and VOC) used in the EPA's 2014 emissions modeling platform based on PM mass and VOC mass and reactivity. It is important to note that the on-road profiles were excluded from this analysis since speciation for these profiles is computed internally in the MOVES model. We then investigate these profiles further for quality and to determine whether they were being appropriately matched to source types while also considering regional variability of speciated pollutants. We then applied a quantitative needs assessment ranking system which rates the profile based on age, appropriateness (i.e. is the profile being used appropriately), prevalence in the EPA modeling platform and the quality of the reference. Our analysis shows that the highest ranked profiles (e.g. profile assignments with the highest priority for updates) include PM2.5profiles for fires (prescribed, agricultural and wild) and VOC profiles for crude oil storage tanks and residential wood combustion of pine wood. Top ranked profiles may indicate either that there are problems with the currently available source testing or that current mappings of profiles to source categories within EPA's modeling platform need improvement. Through this process, we have identified 29 emissions source categories that would benefit from updated mapping. Many of these mapping mismatches are due to lack of emissions testing for appropriate source categories. In addition, we conclude that new source emissions testing would be especially beneficial for residential wood combustion, nonroad gasoline exhaust and nonroad diesel equipment.

Can a single water molecule catalyze the OH+CH2CH2 and OH+CH2O reactions? Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Mohamad Akbar Ali, Balaganesh M, Soonmin Jang Abstract The reaction of OH radicals with ethylene (CH2CH2) and formaldehyde (CH2O) molecules with and without water have been investigated using ab initio/DFT potential energy surfaces (PESs) at CCSD(T)/aug-cc-pVTZ//BHandHLYP//aug-cc-pVTZ levels of theory. The rate coefficients for the bimolecular reaction pathways OH + CH2X···H2O (X = CH2, O) and CH2X + H2O⋯HO were calculated using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) correction. The kinetic results show that OH radical adds to C=C bond in CH2CH2, and abstract the hydrogen atoms from CH2O, similar to its isoelectronic analogous OH + CH2NH (+H2O) reaction. The catalytic effect of a single water molecule on OH + CH2X (X = CH2, O) reaction system shows that the initial water complexation step is essential in the rate coefficients calculation. The calculated rate coefficient for the OH + CH2CH2(+H2O) reaction at 300K is 6 × 10−16 cm3 molecule−1 s−1 and for OH + CH2O(+H2O) reaction at 300K is 8.1 × 10−14 cm3 molecule−1 s−1. The rate coefficient for OH + CH2CH2(+H2O) reaction is at least two orders of magnitude smaller than OH + CH2NH(+H2O) reaction (at 300K is 5.1 × 10−14 cm3 molecule−1 s−1) and rate coefficient for OH + CH2O reaction is in good agreement with OH + CH2NH reaction. In general, the rate coefficients for OH + CH2X (X = CH2, O)⋯H2O and for CH2X + H2O⋯HO reactions are ∼3–4 orders of magnitude smaller than reaction without water molecule. Our results predict that catalytic effect of single water molecule on OH + CH2CH2 and OH + CH2O reactions can make a negligible contribution to the gas phase removal of CH2CH2 and CH2O by OH radicals because the dominated water-assisted process depends parametrically on water concentration. As a result, the overall reaction rate coefficients are smaller. The present results provide a better understanding of gas phase catalytic effect of a water molecule on the most important atmospheric and combustion reaction prototypes. Graphical abstract

Kinetics and mechanisms of the gas-phase reactions of OH radicals with three C15 alkanes Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Bo Shi, Weigang Wang, Li Zhou, Junling Li, Jing Wang, Yan Chen, Wenyu Zhang, Maofa Ge Abstract The reaction constants and products for the reactions of OH radicals with different C15 alkanes, were determined by using Proton-Transfer-Reaction Mass Spectrum (PTR-MS) in the smog chamber. Rate coefficients (in units of 10−11 cm3 molecule−1 s−1) k1(pentadecane) = 1.72 ± 0.18,k2(2,6,10-trimethyldodecane) = 1.90 ± 0.20, k3(nonylcyclohexane) = 2.09 ± 0.22 were first obtained by the relative rate method using toluene and m-xylene as reference compounds at (298 ± 1)K and atmospheric pressure. In addition, the products of these reactions were measured by PTR-MS simultaneously. The results show that the major gas phase products observed in the OH radicals reactions were aldehydes and ketones for C15 alkanes. The possible reaction mechanisms were speculated and the atmospheric implications were also discussed.

High resolution, extreme isotopic variability of precipitation nitrate Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Lucy A. Rose, Zhongjie Yu, Daniel J. Bain, Emily M. Elliott Abstract Deposition of atmospheric nitrate (NO3−) in precipitation can be an important source of reactive nitrogen (N) to ecosystems, particularly in regions with high nitrogen oxide (NOx = nitric oxide (NO)+nitrogen dioxide (NO2)) emissions. However, high resolution deposition data are lacking for most systems. We conducted hourly precipitation sampling across six growing season storms in a forested area historically subjected to some of the highest levels of chronic N deposition in the United States. To characterize the influence of electricity generating unit (EGU), vehicle, and biogenic NOx emissions on NO3− deposition, we calculated the total NOx emitted from these sources within a 100 km radius of air mass back trajectories determined for Fernow Experimental Forest (West Virginia, USA). We combined these emissions estimates with established 15N isotope values for NOx sources in a three end-member mixing model to predict source-based δ15N values of deposition reaching the study site on an hourly basis. To evaluate the effect of NOx oxidation pathways on measured δ15N-NO3-values, we compared observed hourly isotope values to a coupled δ15N and Δ17O array representing N isotope exchange between atmospheric oxidized N molecules. Within individual events, δ15N, δ18O, and Δ17O values ranged by as much as 19.5‰, 28.9‰, and 13.8‰, respectively. This extreme short-term isotopic variability suggests a dynamic mix of NOx sources, oxidation pathways, and fractionation processes contributing to HNO3formation. During every storm, precipitation δ15N-NO3- values were lower than those expected to result from predominant HNO3 formation pathways or oxidation of estimated NOx emissions along back trajectories, suggesting a systematic underestimation of NOx contributions to atmospheric HNO3 formation from isotopically depleted soil emissions. Together, these analyses represent the most comprehensive assessment to date relating high temporal resolution δ15N-NO3- observations to NOx emission sources, oxidation chemistry, and isotopic fractionation effects. We present the first observations of extreme intra-storm δ15N, δ18O, and Δ17O variability, emphasizing the need for improved constraints on soil NOx emissions, forest canopy effects, and their role in atmospheric NO3− deposition and isotope dynamics in forests.

Spatio-temporal assessment of nocturnal surface ozone in Malaysia Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Mohd Famey Yusoff, Mohd Talib Latif, Liew Juneng, Md Firoz Khan, Fatimah Ahamad, Jing Xiang Chung, Anis Asma Ahmad Mohtar Abstract This study aims to determine the level and potential sources of nocturnal surface ozone (NSO) in different regions in Malaysia. Eleven-year (2005–2015) ozone data from 37 continuous air quality monitoring stations throughout Malaysia have been analysed to determine spatio-temporal variations in NSO concentrations. NSO daily maximum concentrations from different regions in Malaysia were used for seasonal variation analysis while linear regression and the Mann-Kendall trend test were used for the annual variation analysis. Average ratios of mean NSO to daytime surface ozone (DSO) for the whole country were found to be 60% of DSO (0.58–0.61). The east coast of the Malaysian Peninsula recorded the highest ratio (70% of DSO) while the central region recorded the lowest concentration (50% of DSO). Titration processes, particularly by NO in the urban areas of the central region, and long transboundary movement of ozone to the east coast are expected to influence the concentration of ozone in these two regions, respectively. On certain occasions, the NSO concentrations (with a maximum value of 137 ppb) exceeded the limit of 100 ppb, the value suggested by Malaysian Air Quality Standard for ambient ozone concentration. The monthly diurnal variation analysis revealed the occurrence of secondary/nocturnal peaks at more than 50% of the stations occurring around 0300–0500 h. The NSO was found to be influenced by the monsoonal season with higher concentrations mainly observed during the boreal winter season. The long-term trend analysis presented the country's overall NSO as having an increasing trend at 27% of the stations.

Potential impacts of electric vehicles on air quality and health endpoints in the Greater Houston Area in 2040 Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Shuai Pan, Anirban Roy, Yunsoo Choi, Ebrahim Eslami, Stephanie Thomas, Xiangyu Jiang, H. Oliver Gao Abstract Significant emissions from transportation contribute to the formation of O3 and fine particulate matter (PM2.5), causing poor air quality and health. In this study, four scenarios were developed to understand how future fleet electrification and turnover of both gasoline and diesel vehicles affect air quality and health in the Houston Metropolitan area. These scenarios considered increased vehicle activity and various configurations of emissions controls. Comparing to a base year of 2013, model predictions for 2040 indicated a ∼50% emissions increase in the Business As Usual (BAU) case, and ∼50%, ∼75%, and ∼95% reductions in the three distinct emissions control cases, the Moderate Electrification (ME), Aggressive Electrification (AE), and Complete Turnover (CT) cases, respectively. Each modeling scenario was conducted using a high-resolution (1 km) WRF-SMOKE-CMAQ-BenMAP air quality and health modeling framework, which helped capture urban features in higher detail. The emissions control cases resulted in 1–4 ppb maximum 8 h O3 increase along highways and reductions both in the regions enclosed by the highways and those downwind. Simulated PM2.5 concentrations decreased between 0.5 and 2 μg m−3. Health impact results suggest that increased O3 and PM2.5 concentrations from the BAU case will lead to 122 additional premature deaths with respect to 2013. However, reduced emissions for the control cases (ME, AE, CT) will prevent 114–246 premature deaths. Additionally, about 7,500 asthma exacerbation and 5,500 school loss days will be prevented in the ME case, benefiting younger individuals. The economic benefits generally followed the same trends as health impacts. The analysis framework developed in this study can be applied to other metropolitan areas. The effects of motor vehicle electrification on power plant emissions were estimated using the Argonne National Laboratory's Autonomie data, and indicated the electrification load to be negligible as opposed to projected electricity generation.

Secondary organic aerosol formation from the OH-initiated oxidation of guaiacol under different experimental conditions Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Changgeng Liu, Jun Liu, Yongchun Liu, Tianzeng Chen, Hong He Abstract Guaiacol (2-methoxyphenol) has a high emission rate from wood burning and mainly exists in the gas phase, but the formation potential of secondary organic aerosol (SOA) from the atmospheric oxidation of guaiacol has not been well determined yet. In this work, SOA formation from the gas-phase reaction of guaiacol with OH radicals was investigated using an oxidation flow reactor (OFR) under different experimental conditions. The results showed that SOA yield was dependent on guaiacol concentration, OH exposure, and the presence of SO2 and NO2. SOA yield firstly increased and then decreased as a function of OH exposure. The maximum SOA yield (0.28–0.54) obtained at different guaiacol concentrations could be well-expressed by a one-product model. The SOA oxidation degree was represented by the carbon oxidation state (OSC) and f44/f43 (the ratio of organic mass fractions of m/z 44 to m/z 43), which both increased linearly and significantly with the increase of OH exposure. In addition, SO2 and NO2 promoted SOA formation, for which the maximum yield enhancements were 13.38% and 10.69%, respectively. The N/C ratio (0.034–0.045) indicated that NO2 participated in the OH-initiated reaction of guaiacol, consequently resulting in the formation of organic nitrates. The experimental results would be helpful to further the understanding of SOA formation from the atmospheric oxidation of guaiacol and its subsequent impacts on air quality and climate. Graphical abstract

Sensitivity of simulating a dust storm over Central Asia to different dust schemes using the WRF-Chem model Publication date: 15 June 2019 Source: Atmospheric Environment, Volume 207 Author(s): Tiangang Yuan, Siyu Chen, Jianping Huang, Xiaorui Zhang, Yuan Luo, Xiaojun Ma, Guolong Zhang Abstract Frequent dust storms have harm to human health and agricultural activities in Central Asia. However, there has been a great deal of uncertainty in prediction of dust storms in Central Asia. One of the important reasons is that the adaptability of different dust emission schemes has not been evaluated. Here, the Goddard Chemistry Aerosol Radiation and Transport (GOCART), Air Force Weather Agency (AFWA) and Shao2004 (Shao04) dust schemes coupled to the WRF-Chem model were used to simulate the severe dust storm occurred in Central Asia on 12–15 July 2016. Generally, this dust storm was initialed by a vortex at 500 hPa and surface cold front, and then swept across Turkmenistan, Uzbekistan and Tajikistan. The Shao04 case could represent the spatial-temporal evolution of the dust storm well, especially at the northern Iran and Turkmenistan, due to its better description of the physical process of dust emission. But it overestimated the aerosol optical depth (AOD) to the southeast of the Aral Sea, which might be associated with the uncertainties of the soil particle distribution dataset. The AFWA case simulated AOD as better as Shao04 case with improved soil moisture correction factors, saltation algorithms and particle size distributions although it had smaller particle size, indicating that smaller particles are assignable. The GOCART case showed the largest dust emission areas due to the low threshold velocity. Yet both the AFWA and GOCART scheme underestimated the high AOD over northern Iran owe to its low erodibility factors. The total dust emission of the four-day period in the Shao04 scheme was 11.9 Tg, which was 2–3 times larger than those obtained in the AFWA and GOCART schemes. The significant differences of dust emission between three dust schemes may essentially depend on the sensitivities of threshold friction velocity on surface property.