The EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation and their Effects in the Yangtze River Delta (EXPLORE-YRD) campaign was carried out between May and June 2018 at a regional site in Taizhou, China. The EXPLORE-YRD campaign helped construct a detailed air quality model to understand the formation of O3 and PM2.5 further, identify the key sources of elevated air pollution events, and design efficient emission control strategies to reduce O3 and PM2.5 pollution in YRD. In this study, we predicted the air quality during the EXPLORE-YRD campaign using the Weather Research and Forecasting/Community Multiscale Air Quality modelling system (WRF/CMAQ) and evaluated model performance on O3 and PM2.5 concentrations and compositions. Air quality was predicted using two sets of reanalysis data—NCEP Final (FNL) Operational Global Analysis and ECMWF Reanalysis v5.0 (ERA5)—and three horizontal resolutions of 36, 12, and 4 km. The results showed that PM2.5 concentration was generally under-predicted using both the FNL and ERA5 data. ERA5 yielded slightly higher PM2.5 predictions during the EXPLORE-YRD campaign. Both reanalysis data sets under-predicted the high PM2.5 pollution processes on 29–30 May 2018, indicating that reanalysis data is not essential for under-predicting extreme PM2.5 pollution processes. The performance of O3 was similar in both the reanalysis data sets, because O3 is mostly sensitive to temperature predictions and FNL and ERA5 yielded similar temperature results. Although the average performance of PM2.5 and O3 predictions yielded by FNL and ERA5 was similar, large differences were observed in certain locations on specific days (e.g. in Hangzhou between 29 May and June 6, 2018 and in Hefei on 1–3 June 2018). Therefore, the choice of reanalysis data could be an important factor affecting the predictions of PM2.5 and O3, depending on locations and episodes. Comparable results were obtained using predictions with different horizontal resolutions, indicating that grid resolution was not crucial for determining the model performance of both PM2.5 and O3 during the campaign. © 2020 Elsevier Ltd

The variations of non-refractory submicron aerosol (NR-PM1) were characterized using an high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and other online instruments measurements sampled at an urban site in Shanghai from 2016 to 2017. Spring (from 18 May to 4 June 2017), summer (from 23 August to 10 September 2017) and winter (from 28 November 2016 to 23 January 2017) seasons were chosen for detail investigating the seasonal variations in the aerosol chemical characteristics. The average PM1 (NR-PM1 + BC) mass concentration showed little difference in the three seasons in Shanghai. The average mass concentrations of total PM1 during spring, summer, and winter observations in Shanghai were 23.9 ± 20.7 μg/m3, 28.5 ± 17.6 μg/m3, and 31.9 ± 22.7 μg/m3, respectively. The seasonal difference on chemical compositions was more significant between them. Organic aerosol (OA) and sulfate were dominant contributor of PM1 in summer, whereas OA and nitrate primarily contribution to the increase of PM1 mass loading in spring and winter. As an abundant component in PM1 (accounting for 39%–49%), OA were resolved into two primary organic aerosol (POA) factors and two secondary aerosol (SOA) factors by using positive matrix factorization (PMF), of which OA was overwhelmingly dominated by the SOA (50–60%) across the three seasons in Shanghai. Correlation analysis with relative humidity and odd oxygen indicated that aqueous-phase processing and played an important role in more aged SOA formation in summer and winter. In spring, both aqueous-phase and photochemical processing contributed significantly to fresh SOA formation. Our results suggest the significant role of secondary particles in PM pollution in Shanghai and highlight the importance of control measures for reducing emissions of gaseous precursors, especially need to consider seasonal characteristics. © 2021 Elsevier B.V.

The Sichuan Basin is one of the regions suffering from severe haze pollution in southwest China. However, the secondary aerosol formation in this region is poorly understood. In this study, the chemical compositions of PM2.5 and molecular compositions of water-soluble organics in wintertime Sichuan were measured to investigate the aerosol sources and formation under typical high relative humidity (RH) conditions. Strong correlations between PM2.5, carbonaceous aerosols and K+ suggested the influence of biomass burning. The impacts of biomass burning were also supported by the dominance of primarily emitted reduced/less oxidized nitrogen-containing organics as well as the high peak intensities of secondarily formed nitrocatechols and methyl-nitrocatechols. High humidity (average RH = 80%) and aerosol liquid water (ALW) in Sichuan facilitated the secondary formation of sulfate, nitrate, and secondary organic aerosols (SOA). The average sulfate oxidation ratio and nitrogen oxidation ratio in Sichuan were 2.5 and 3.1 times of those in winter Beijing (average RH = 27%). This suggested higher potentials of SO2 and NOx to form sulfate and nitrate under high-RH conditions. The abundant aqueous-SOA formation in Sichuan was supported by the dominance of organosulfates (OSs) and nitrooxy-OSs in mass spectra of water-soluble organics, while the OSs in winter Beijing were quite limited. The more abundant OS formation in Sichuan was attributed to the much higher RH, ALW, aerosol acidity, and sulfate, which favored the acidic sulfate-catalyzed aqueous-phase reactions for OS formation. Higher concentrations of biogenic volatile organic compounds were additional reasons for the more abundant OSs in Sichuan than in Beijing. © 2021. American Geophysical Union. All Rights Reserved.

Cooking has been proven to be a significant source of primary organic aerosol, especially in megacities. However, the formation of secondary organic aerosol (SOA) derived from cooking emissions is still poorly understood. In this work, four prevalent Chinese domestic cooking types involving complicated cuisines and various cooking methods were chosen to conduct a lab simulation for SOA formation using a Gothenburg potential aerosol mass reactor (Go: PAM). After samples had been aged under OH exposures of 4.3-27.1 × 1010 molecules cm-3 s, the domestic cooking SOA was characterized by mass growth potentialities (1.81-3.16), elemental ratios (O/C = 0.29-0.41), and mass spectra. Compared with other organic aerosol (OA), domestic cooking SOA is a kind of less oxidized oxygenated OA (LO-OOA) with a unique oxidation pathway (alcohol/peroxide pathway) and mass spectra (characteristic peaks at m/z 28, 29, 41, 43, 44, 55, and 57). This study is expected to identify the cooking SOA under actual cooking conditions, which could contribute to the formulation of pollution source control as well as the health risk assessment of exposure to cooking fumes. ©

The variations in physicochemical properties of airborne particles collected during a typical transition from haze to dust were investigated using single particle analysis with transmission and scanning electron microscopes combined with online measurement of chemical compositions of airborne particles in Beijing in February 2013. The transition was divided into three phases based on the weather condition. During haze pollution (Phase 1), gaseous and particle pollutants enhanced gradually. Results from single particle analysis showed that more coatings and more anthropogenic elements (e.g., S) appeared on the surface of fine and coarse particles, which was probably caused by efficient aqueous-phase reactions under high humidity (70%) condition. Phase 2 was dust intrusion episode. PM10 reached over 1000 μg m−3. Larger fractions of mineral particles and bare-like soot particles were observed in fine particles, while the fraction of secondary particles with coatings decreased. The proportion of black carbon in submicron particles also increased. Photochemical oxidation in gas phase likely dominated in secondary formation under high O3 concentration. After the dust episode (Phase 3), secondary formation enhanced obviously. Soot aged quickly and had a larger mode of 0.45 μm than the other phases. The size modes of airborne fine particles during Phases 1 and 3 were 0.35 μm, which were a bit larger than that during Phase 2 (0.24 μm). These results indicate that dust plumes accompanied with strong wind brought mineral particles in both fine and coarse modes and freshly emitted particles with smaller sizes, and swept away pre-presence air pollutants. This study could provide detailed information on the physicochemical properties of airborne particles during typical severe pollution processes in a short time. Such short-term change should be taken into account in order to more accurately assess the environmental, climatic and health-related effects of airborne particles. © 2020 Elsevier B.V.

In the present work, we propose a novel algorithm to determine the scattering coefficient of OA by evaluating the relationships of the MSEs for primary organic aerosol (POA) and secondary organic aerosol (SOA) with their mass concentrations at three distinct sites, i.e. an urban site, a rural site, and a background site in China. Our results showed that the MSEs for POA and SOA increased rapidly as a function of mass concentration in low mass loading. While the increasing rate declined after a threshold of mass loading of 50 μg/m3 for POA, and 15 μg/m3 for SOA, respectively. The dry scattering coefficients of submicron particles (PM1) were reconstructed based on the algorithm for POA and SOA scattering coefficient and further verified by using multi-site data. The calculated dry scattering coefficients using our reconstructing algorithm have good consistency with the measured ones, with the high correlation and small deviation in Shanghai (R2 = 0.98; deviations: 2.9%) and Dezhou (R2 = 0.90; deviations: 4.7%), indicating that our algorithms for OA and PM1 are applicable to predict the scattering coefficient of OA and Submicron particle (PM1) in China. © 2020 Elsevier Ltd

The EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation and their Effects in the Yangtze River Delta (EXPLORE-YRD) campaign was carried out between May and June 2018 at a regional site in Taizhou, China. The EXPLORE-YRD campaign helped construct a detailed air quality model to understand the formation of O3 and PM2.5 further, identify the key sources of elevated air pollution events, and design efficient emission control strategies to reduce O3 and PM2.5 pollution in YRD. In this study, we predicted the air quality during the EXPLORE-YRD campaign using the Weather Research and Forecasting/Community Multiscale Air Quality modelling system (WRF/CMAQ) and evaluated model performance on O3 and PM2.5 concentrations and compositions. Air quality was predicted using two sets of reanalysis data—NCEP Final (FNL) Operational Global Analysis and ECMWF Reanalysis v5.0 (ERA5)—and three horizontal resolutions of 36, 12, and 4 km. The results showed that PM2.5 concentration was generally under-predicted using both the FNL and ERA5 data. ERA5 yielded slightly higher PM2.5 predictions during the EXPLORE-YRD campaign. Both reanalysis data sets under-predicted the high PM2.5 pollution processes on 29–30 May 2018, indicating that reanalysis data is not essential for under-predicting extreme PM2.5 pollution processes. The performance of O3 was similar in both the reanalysis data sets, because O3 is mostly sensitive to temperature predictions and FNL and ERA5 yielded similar temperature results. Although the average performance of PM2.5 and O3 predictions yielded by FNL and ERA5 was similar, large differences were observed in certain locations on specific days (e.g. in Hangzhou between 29 May and June 6, 2018 and in Hefei on 1–3 June 2018). Therefore, the choice of reanalysis data could be an important factor affecting the predictions of PM2.5 and O3, depending on locations and episodes. Comparable results were obtained using predictions with different horizontal resolutions, indicating that grid resolution was not crucial for determining the model performance of both PM2.5 and O3 during the campaign. © 2020 Elsevier Ltd

Severe haze pollution occurs frequently in the winter over the Beijing-Tianjin-Hebei (BTH) region (China), exerting profound impacts on air quality, visibility, and human health. The Chinese Government has taken strict mitigation actions since 2013 and has achieved a significant reduction in the annual mean PM2.5 concentration over this region. However, the level of secondary aerosols during heavy haze episodes showed little decrease during this period. During heavy haze episodes, the concentrations of secondary aerosol components, including sulfate, nitrate and secondary organics, in aerosol particles increase sharply, acting as the main contributors to aerosol pollution. To achieve effective control of particle pollution in the BTH region, the precise and complete secondary aerosol formation mechanisms have been investigated, and advances have been made about the mechanisms of gas phase reaction, nucleation and heterogeneous reactions in forming secondary aerosols. This paper reviews the research progress in aerosol chemistry during haze pollution episodes in the BTH region, lays out the challenges in haze formation studies, and provides implications and directions for future research. [Figure not available: see fulltext.]. © 2020, Higher Education Press.

Formaldehyde (HCHO) is one of the most important intermediate products of atmospheric photochemical reactions and is also a radical source that promotes ozone formation. Given its high solubility, HCHO is likely to exist in particulate form. In this work, gaseous HCHO (HCHOg) and particulate HCHO (HCHOp) were separated and collected by a rotating wet annular denude (RWAD) and an aerosol growth chamber-coil aerosol cooler (AC). The collected HCHO from the RWAD and AC are measured by two online Hantzsch method-based formaldehyde analyzers. The comprehensive campaign was held in the Yangtze River Delta of China from 15 May to 18 June 2018, which is during the harvest season. Several biomass burning events were identified by using acetonitrile as a tracer. During the period influenced by biomass burning, the mixing ratios of HCHOg and HCHOp were respectively 122% and 231% higher than those during other time periods. The enhancement ratio of HCHOg to acetonitrile obtained from this work generally agrees with those from the existing literature. Biomass burning contributed 14.8% to HCHOg, but the abundant freshly discharged precursors it emitted greatly promoted the secondary production of HCHOg. We suggest that the high concentration of HCHOp during the biomass burning period was from uptake of HCHOg by aerosols during their transportation; the liquid state particles are conducive to HCHOg uptake. High relative humidity, a low particle rebound fraction f, as well as low temperatures may result in higher uptake coefficient values.

Particle size distribution, water soluble ions, and black carbon (BC) concentration in a long-term haze-fog episode were measured using a wide-range particle spectrometer (WPS), a monitor for aerosols and gases (MARGA), and an aethalometer (AE33) in Nanjing from 16 to 27 November, 2018. The observation included five processes of clean, mist, mix, haze, and fog. Combined with meteorological elements, the HYSPLIT model, and the IMPROVE model, we analyzed the particle size distribution, chemical composition, and optical properties of aerosols in different processes. The particle number size distribution (PNSD) in five processes differed: It was bimodal in mist and fog and unimodal in clean, mix, and haze. The particle surface area size distribution (PSSD) in different processes showed a bimodal distribution, and the second peak of the mix and fog processes shifted to a larger particle size at 480 nm. The dominant air masses in five processes differed and primarily originated in the northeast direction in the clean process and the southeast direction in the haze process. In the mist, mix, and fog processes local air masses dominated. NO3- was the primary component of water soluble ions, with the lowest proportion of 45.6% in the clean process and the highest proportion of 53.0% in the mix process. The ratio of NH4+ in the different processes was stable at approximately 23%. The ratio of SO42- in the clean process was 26.2%, and the ratio of other processes was approximately 20%. The average concentration of BC in the fog processes was 10,119 ngm(-3), which was 3.55, 1.80, 1.60, and 1.46 times that in the processes of clean, mist, mix, and haze, respectively. In the different processes, BC was primarily based on liquid fuel combustion. NO3-, SO42-, and BC were the main contributors to the atmospheric extinction coefficient and contributed more than 90% in different processes. NO3- contributed 398.43 Mm(-1) in the mix process, and SO42- and BC contributed 167.90 Mm(-1) and 101.19 Mm(-1), respectively, during the fog process.

To clarify the aerosol optical properties under different pollution levels and their impacting factors, hourly organic carbon (OC), elemental carbon (EC), and water-soluble ion (WSI) concentrations in PM2.5 were observed by using monitoring for aerosols and gases (MARGA) and a semicontinuous OC/EC analyzer (Model RT-4) in Wuhan from 9 to 26 January 2018. The aerosol extinction coefficient (b(ext)) was reconstructed using the original Interagency Monitoring of Protected Visual Environment (IMPROVE) formula with a modification to include sea salt aerosols. A good correlation was obtained between the reconstructed b(ext) and measured b(ext) converted from visibility. b(ext) presented a unimodal distribution on polluted days (PM2.5 mass concentrations > 75 mu g.m(-3)), peaking at 19:00. b(ext) on clean days (PM2.5 mass concentrations < 75 mu g.m(-3)) did not change much during the day, while on polluted days, it increased rapidly starting at 12:00 due to the decrease of wind speed and increase of relative humidity (RH). PM2.5 mass concentrations, the aerosol scattering coefficient (b(scat)), and the aerosol extinction coefficient increased with pollution levels. The value of b(ext) was 854.72 Mm(-1) on bad days, which was 4.86, 3.1, 2.29, and 1.28 times of that obtained on excellent, good, acceptable, and poor days, respectively. When RH < 95%, b(ext) exhibited an increasing trend with RH under all pollution levels, and the higher the pollution level, the bigger the growth rate was. However, when RH > 95%, b(ext) on acceptable, poor and bad days decreased, while b(ext) on excellent and good days still increased. The overall b(ext) in Wuhan in January was mainly contributed by NH4NO3 (25.2%) and organic matter (20.1%). The contributions of NH4NO3 and (NH4)(2)SO4 to b(ext) increased significantly with pollution levels. On bad days, NH4NO3 and (NH4)(2)SO4 contributed the most to b(ext), accounting for 38.2% and 27.0%, respectively.

High concentrations of ultrafine particles (UFPs), approaching 1 million/cm3, are frequently produced from new particle formation under urban environments, but the fundamental mechanisms regulating nucleation and growth for UFPs are poorly understood. From simultaneous ambient and environmental chamber measurements, we demonstrate remarkable formation of UFPs from urban traffic emissions. By replicating ambient conditions using an environmental chamber method, we elucidate the roles of existing particles, photochemistry, and synergy of multipollutant photooxidation in nucleation and growth of UFPs. Our results reveal that synergetic oxidation of vehicular exhaust leads to efficient formation of UFPs under urban conditions. Recognition of this large urban source for UFPs is essential to accurately assessing their impacts and to effectively developing mitigation policies.High levels of ultrafine particles (UFPs; diameter of less than 50 nm) are frequently produced from new particle formation under urban conditions, with profound implications on human health, weather, and climate. However, the fundamental mechanisms of new particle formation remain elusive, and few experimental studies have realistically replicated the relevant atmospheric conditions. Previous experimental studies simulated oxidation of one compound or a mixture of a few compounds, and extrapolation of the laboratory results to chemically complex air was uncertain. Here, we show striking formation of UFPs in urban air from combining ambient and chamber measurements. By capturing the ambient conditions (i.e., temperature, relative humidity, sunlight, and the types and abundances of chemical species), we elucidate the roles of existing particles, photochemistry, and synergy of multipollutants in new particle formation. Aerosol nucleation in urban air is limited by existing particles but negligibly by nitrogen oxides. Photooxidation of vehicular exhaust yields abundant precursors, and organics, rather than sulfuric acid or base species, dominate formation of UFPs under urban conditions. Recognition of this source of UFPs is essential to assessing their impacts and developing mitigation policies. Our results imply that reduction of primary particles or removal of existing particles without simultaneously limiting organics from automobile emissions is ineffective and can even exacerbate this problem.

Organosulfates (OSs) are an important group of secondary organic aerosols, but the key influential factors of their formation in polluted atmospheres are not well understood. In this study, we monitored particulate OSs (carboxy OSs, hydroxyacetone sulfate, and isoprene-and monoterpene-derived OSs) at an urban site and a regional site in Beijing and examined their compositions and formation pathways under contrasting atmospheric conditions. The quantified OSs were most abundant in the summer at the regional site due to higher biogenic emissions and favorable formation conditions (higher aerosol acidity and humidity), followed by urban summer and winter conditions. Larger fractions of inorganic sulfate were converted to organosulfur when sulfate was less abundant. This implies that OSs would play more important roles in aerosol properties as the decline of sulfate. Monoterpene-derived nitrooxy-OSs were enhanced via NO3oxidation in the summer under high-NOxconditions at night, while the day-night variations in the winter were not as obvious. Among isoprene-OSs, IEPOX (isoprene epoxydiols)-OS formation was clearly suppressed under high-NOxconditions, while other isoprene-OSs that are favored under high-NOxconditions showed increasing formation with NOx. The results highlight that isoprene-OS formation pathways in polluted atmospheres could be different from the IEPOX-dominated regions reported for the low-NOxenvironments in the literature. © 2020 American Chemical Society. All rights reserved.

To investigate the regional secondary organic aerosol (SOA) formation at Yangtze River Delta (YRD) region, China, the chemical composition of fine particles and their gaseous precursors were simultaneously measured at a regional site, Taizhou, during EXPeriment on the eLucidation of the atmospheric Oxidation capacity, aerosol foRmation and their Effects in Yangtze River Delta (EXPLORE-YRD) intensive field campaign from May to June 2018. Secondary organic carbon (SOC) was estimated by both bottom-up and top-down method, i.e. the yield method from volatile organic compounds (VOCs) oxidation, and the elemental carbon (EC) tracer method. Our result showed that the oxidation of alkanes and aromatics measured by GC-MS/FID based on the yield method could only explain 25.3% of the SOC derived from the EC tracer method, in which aromatics were the dominant contributors (23.9%). This percentage increased to 39.5% while two semi-volatile organic compounds (SVOCs), i.e. naphthalene, and methylnaphthalene, were used in the calculation, suggesting the importance of SVOCs on SOA formation. The SOA formation pathways were further explored. The good correlation of SOC and odd oxygen (Ox) indicated the important role of photochemical reactions on SOA formation in the summer of YRD. Our findings evaluated the contributions of VOCs to SOA formation in Taizhou, revealed the importance of SVOCs to SOA formation and highlighted an urgent need for more exploration of SVOCs in the future. © 2020 The Authors