Comparison of black carbon, primary and secondary brown carbon light absorption and direct solar absorption at the foothill and summit of Mt. Hua, China.

Atmospheric black carbon (BC), primary and secondary brown carbon (BrCpri and BrCsec) are the light-absorbing carbonaceous aerosol components. The vertical changes in the BC and BrC distributions are not generally known. Here, we presented a study of the spectral light absorption properties, direct solar absorption, and potential source areas of BC and BrC at the foothill (375 m a.s.l.) and summit (2060 m a.s.l.) of Mt. Hua, China. More than tripled BC and BrC light absorption coefficient were observed at the foothill compared to the summit. The dominant carbonaceous light-absorbing was attributed to BC with the percentages of 77 % (foothill) and 79 % (summit), respectively. The light absorption coefficient and direct solar absorption of BrCpri were much higher than those of BrCsec at foothill, especially in winter. The enhancing contributions of BrCsec light absorption coefficient and direct solar absorption were observed with high RH and visibility at the summit. The light absorption properties of BC, BrCpri, and BrCsec may be attributed to the emissions, meteorological conditions, and photochemical oxidation. The inferred potential source spatial distributions of BC and BrCpri showed different patterns at the foothill and summit. The results underlined the primary emission effects (including BC and BrCpri) at the foothill and the importance of BrCsec at the summit, respectively.

Unraveling the process of aerosols secondary formation and removal based on cosmogenic beryllium-7 and beryllium-10.

The secondary formation and diffusion processes of aerosol are extraordinarily complex and significantly impact the environment and human health. Therefore, exploring the process of aerosol formation and diffusion based on independent new tracer has always been a concern. The 7Be and 10Be, which are generated only by the action of cosmic rays, are chemically stable and adsorbed on aerosol for transmission, so they have the potential characteristics of aerosol tracers. Here, we obtained the daily resolution atmospheric 7Be, 10Be, and 10Be/7Be without dust interference in Xi'an autumn and winter (heavy pollution period in a typical polluted area) by accelerator mass spectrometry. It is found that during the rapid formation of secondary aerosols (SA) under the stable 10Be/7Be ratio, which indicates the stable atmospheric vertical structure, the concentration of 7Be and 10Be is significantly negatively correlated (R2 > 0.9) with the aerosol concentration. Therefore, SA relative content in aerosols can be estimated by the dilution amount of 7Be and 10Be to reveal the secondary-formation process of aerosol (33% average contribution to aerosols during the winter heavy air pollution period). Furthermore, we also revealed the physical removal process of aerosols based on 7Be, 10Be, and 10Be/7Be, including precipitation removal and diffusion of vertical atmospheric movement caused by stratospheric air intrusion. In summary, meteoric cosmogenic 7Be and 10Be will provide a new way to study the secondary chemical formation and physical removal of aerosols.

Spatial distribution of PM2.5-bound elements in eighteen cities over China: policy implication and health risk assessment.

In this study, 30 elements in fine particulate matter (PM2.5) were measured in 18 Chinese cities in 2013. Elemental pollution in northern, southwest, and central China were severe, attributing to excessive coal and biomass combustion in these regions. The concentrations of S, Cl, and K in these areas were 8.21 ± 3.90, 4.03 ± 1.96, and 1.59 ± 0.613 µg/m3, respectively, which were 1.6-2.7 times higher than those in other regions of China. In addition, the industrial emissions in northeast and north China were large, leading to the elevated heavy metal concentration of 1.32 ± 1.17 µg/m3, especially Zn, Pb, Cr, Cd, and Br. Soil dust was the highest in northwest China among the five regions with the concentration of crustal elements of 6.37 ± 4.51 µg/m3. Moreover, although the levels of elemental concentration in east and southeast China were relatively acceptable, regulators must pay attention to elevated level of V (0.009 ± 0.006 µg/m3) in these areas. Compared with 2003, several elements have deteriorated in some cities. For example, As increased by 70%, 18%, and 155% in Changchun, Beijing, and Jinchang, respectively. However, ~ 77% measured elements, e.g., Ti, Fe, and Pb markedly reduced in 2013, with reduction rates of 13-81%. These indicate that the government's policies related to particle-bound elements have shown certain positive environmental effects. For the health risks from the heavy metals in 2013, the non-cancer risks of As and Cd must not be neglected. The cancer risks of As and Pb were much higher than the international safety limit (10-4). More prominent health risks were found in southwest, central, and northwest China. Therefore, the government should accelerate the shift to cleaner energy in underdeveloped areas of China to obtain more environmental and health benefits.

Decreasing concentrations of carbonaceous aerosols in China from 2003 to 2013.

Carbonaceous aerosols were characterized in 19 Chinese cities during winter and summer of 2013. Measurements of organic carbon (OC) and elemental carbon (EC) levels were compared with those from 14 corresponding cities sampled in 2003 to evaluate effects of emission changes over a decade. Average winter and summer OC and EC decreased by 32% and 17%, respectively, from 2003 to 2013, corresponding to nationwide emission control policies implemented since 2006. The extent of carbon reduction varied by season and by location. Larger reductions were found for secondary organic carbon (SOC, 49%) than primary organic carbon (POC, 25%). PM2.5 mass and total carbon concentrations were three to four times higher during winter than summer especially in the northern cities that use coal combustion for heating.

Exploring the impact of chemical composition on aerosol light extinction during winter in a heavily polluted urban area of China.

An intensive measurement campaign was conducted in Xi'an, China from December 2012-January 2013 to investigate the chemical composition, formation, and optical properties of PM1. The PM1 mass concentration (average = 138.8 ±â€¯83.2 µg m-3) accounted for ∼50% of the PM2.5 mass. Organic aerosols (OA) and secondary inorganic aerosols (SIA) were the most abundant PM1 components, contributing 53.0% and 35.0% to the mass, respectively. Both primary emissions and aqueous-phase oxidation of secondary aerosols played roles in the pollution episodes. The average light scattering and absorption coefficients during the campaign were 805 ±â€¯581 Mm-1 and 123 ±â€¯96 Mm-1, respectively. Both the mass scattering and mass absorption efficiencies for PM1 were higher than that for PM2.5-1, indicating stronger ability of light extinction for the smaller particles at visible wavelengths compared with the larger ones. The contributions of aerosol species to light extinction coefficients under two visibility conditions were estimated based on multiple linear regression models, and the OA was found to be the largest contributor to light extinction in both cases. A larger contribution of SIA to light extinction for visibility <5 km demonstrated its greater impacts on visibility during heavy pollution conditions. These findings provide insights into the importance of submicron particles for pollution and visibility degradation in northwestern China.

Characteristics of single atmospheric particles in a heavily polluted urban area of China: size distributions and mixing states.

To investigate the chemical composition, size distribution, and mixing state of aerosol particles on heavy pollution days, single-particle aerosol mass spectrometry was conducted during 9-26 October 2015 in Xi'an, China. The measured particles were classified into six major categories: biomass burning (BB) particles, K-secondary particles, elemental carbon (EC)-related particles, metal-containing particles, dust, and organic carbon (OC) particles. BB and EC-related particles were the dominant types during the study period and mainly originated from biomass burning, vehicle emissions, and coal combustion. According to the ambient air quality index, two typical episodes were defined: clean days (CDs) and polluted days (PDs). Accumulation of BB particles and EC-related particles was the main reason for the pollution in Xi'an. Most types of particle size were larger on PDs than CDs. Each particle type was mixed with secondary species to different degrees on CDs and PDs, indicating that atmospheric aging occurred. The mixing state results demonstrated that the primary tracers were oxidized or vanished and that the amount of secondary species was increased on PDs. This study provides valuable information and a dataset to help control air pollution in the urban areas of Xi'an. Graphical abstract.

Chemical characterization of PM2.5 from a southern coastal city of China: applications of modeling and chemical tracers in demonstration of regional transport.

An intensive sampling campaign of airborne fine particles (PM2.5) was conducted at Sanya, a coastal city in Southern China, from January to February 2012. Chemical analyses and mass reconstruction were used identify potential pollution sources and investigate atmospheric reaction mechanisms. A thermodynamic model indicated that low ammonia and high relative humidity caused the aerosols be acidic and that drove heterogeneous reactions which led to the formation of secondary inorganic aerosol. Relationships among neutralization ratios, free acidity, and air-mass trajectories suggest that the atmosphere at Sanya was impacted by both local and regional emissions. Three major transport pathways were identified, and flow from the northeast (from South China) typically brought the most polluted air to Sanya. A case study confirmed strong impact from South China (e.g., Pearl River Delta region) (contributed 76.8% to EC, and then this result can be extended to primary pollutants) when the northeast winds were dominant. The Weather Research Forecasting Black carbon model and trace organic markers were used to apportion local pollution versus regional contributions. Results of the study offer new insights into the atmospheric conditions and air pollution at this coastal city.

Concentration and sources of atmospheric nitrous acid (HONO) at an urban site in Western China.

Highly time-resolved measurements of nitrous acid (HONO) were carried out with a highly sensitive long path absorption photometer (LOPAP) at an urban site of Xi'an in Western China from 24 July to 6 August 2015 to investigate the atmospheric variations, sources, and formation pathways of HONO. The concentrations of HONO vary from 0.02 to 4.3ppbv with an average of 1.12ppbv for the entire measurement period. The variation trends of HONO and NO2 are very similar and positively correlated which, together with the similar diurnal profiles of HONO/NO2 ratio and HONO, suggest the importance of heterogeneous conversion of HONO from NO2. The nocturnal HONO level is governed by heterogeneous formation from NO2, followed by homogeneous formation of NO with OH and then by direct emissions. Further, it is found that the heterogeneous formation of HONO is largely affected by relative humidity and aerosol surface. Daytime HONO budget analysis indicates that an additional unknown source with HONO production rate of 0.75ppbvh-1 is required to explain the observed HONO concentration, which contributes 60.8% of the observed daytime HONO.

Characteristics and major sources of carbonaceous aerosols in PM2.5 from Sanya, China.

PM2.5 samples were collected in Sanya, China in summer and winter in 2012/2013. Organic carbon (OC), elemental carbon (EC), and non-polar organic compounds including n-alkanes (n-C14-n-C40) and polycyclic aromatic hydrocarbons (PAHs) were quantified. The concentrations of these carbonaceous matters were generally higher in winter than summer. The estimated secondary organic carbon (OCsec) accounted for 38% and 54% of the total organic carbon (TOC) in winter and summer, respectively. The higher value of OCsec in addition to the presences of photochemically-produced PAHs in summer supports that photochemical conversions of organics are much active at the higher air temperatures and with stronger intense solar radiation. Carbon preference index (CPI) and percent contribution of wax n-alkanes suggest that anthropogenic sources were more dominant than derivation from terrestrial plants in Sanya. Diagnostic ratios of atmospheric PAHs further indicate that there was a wide mix of pollution sources in winter while fossil fuel combustion was the most dominant in summer. Positive Matrix Factorization (PMF) analysis with 18 PAHs in the winter samples found that motor vehicle emissions and biomass burning were the two main pollution sources, contributing 37.5% and 24.6% of the total quantified PAHs, respectively.