Abstract Intense and frequent new particle formation (NPF) events have been observed in polluted urban environments, yet the dominant mechanisms are still under debate. To understand the key species and governing processes of NPF in polluted urban environments, we conducted comprehensive measurements in downtown Beijing during January – March 2018. We performed detailed analyses on sulfuric acid cluster composition and budget, as well as the chemical and physical properties of oxidized organic molecules. Our results demonstrate that the fast clustering of sulfuric acid (H2SO4) and base molecules triggered the NPF events, and oxidized organic molecules (OOMs) further helped grow the newly formed particles towards climate- and health-relevant sizes. This synergistic role of H2SO4, base species, and OOMs in NPF is likely representative of polluted urban environments where abundant H2SO4 and base species usually co-exist, and OOMs are with moderately low volatility when produced under high NOx concentrations.

Abstract Despite the recent decrease in pollution events in Chinese urban areas, the World Health Organization air quality guideline values are still exceeded. Observations from monitoring networks show a stronger decrease of organic aerosol directly emitted to the atmosphere relative to secondary organic aerosol (SOA) generated from oxidation processes. Here, the uptake of water-soluble gas-phase oxidation products is reported as a major SOA contribution to particulate pollution in Beijing, triggered by the increase of aerosol liquid water. In pollution episodes, this pathway is enough to explain the increase in SOA mass, with formaldehyde, acetaldehyde, glycolaldehyde, formic, and acetic acid alone explaining 15 to 25% of the SOA increase. Future mitigation strategies to reduce non-methane volatile organic compound emissions should be considered to reduce organic particulate pollution in China.

Organic aerosols, a major constituent of fine particulate mass in megacities, can be directly emitted or formed from secondary processing of biogenic and anthropogenic volatile organic compound emissions. The complexity of volatile organic compound emission sources, speciation and oxidation pathways leads to uncertainties in the key sources and chemistry leading to formation of organic aerosol in urban areas. Historically, online measurements of organic aerosol composition have been unable to resolve specific markers of volatile organic compound oxidation, while offline analysis of markers focus on a small proportion of organic aerosol and lack the time resolution to carry out detailed statistical analysis required to study the dynamic changes in aerosol sources and chemistry. Here we use data collected as part of the joint UK–China Air Pollution and Human Health (APHH-Beijing) collaboration during a field campaign in urban Beijing in the summer of 2017 alongside laboratory measurements of secondary organic aerosol from oxidation of key aromatic precursors (1,3,5-trimethyl benzene, 1,2,4-trimethyl benzene, propyl benzene, isopropyl benzene and 1-methyl naphthalene) to study the anthropogenic and biogenic contributions to organic aerosol. For the first time in Beijing, this study applies positive matrix factorisation to online measurements of organic aerosol composition from a time-of-flight iodide chemical ionisation mass spectrometer fitted with a filter inlet for gases and aerosols (FIGAERO-ToF-I-CIMS). This approach identifies the real-time variations in sources and oxidation processes influencing aerosol composition at a near-molecular level. We identify eight factors with distinct temporal variability, highlighting episodic differences in OA composition attributed to regional influences and in situ formation. These have average carbon numbers ranging from C5–C9 and can be associated with oxidation of anthropogenic aromatic hydrocarbons alongside biogenic emissions of isoprene, α-pinene and sesquiterpenes.

China submitted the Greenhouse gas emission reduction target in the form of Nationally Determined Contributions (NDC) to the Paris Agreement. To reduce the negative impact of global warming, a tighter target is needed, such as the 2-degree target. This study investigated how China could reach its emissions peak and decarbonize its economy through different key countermeasures in various sectors in line with the NDC and 2 degrees C targets by 2030. A dynamic CGE model is used to develop ten scenarios that contain two dimensions consisting of two stringency levels of carbon emission limitation and the availability of different low-carbon options. We found that in the baseline scenario, China's total CO2 emissions in 2030 would reach 14.7 Gt. To meet China's NDC target, it is essential to develop non-fossil fuel energy, restrict the over-expansion of energy-intensive industries and improve end-use efficiency. Meanwhile, the global 2 degrees C target poses higher requirements for China to develop various non-fossil technologies both in electricity production and demand sectors, and vigorously promote low-carbon consumption pattern. Furthermore, we estimated the economic impacts and found that if low-carbon measures are adopted properly, the mitigation cost in 2030 could decline by 92 and 226 USD/ton-CO2 under the NDC target and 2 degrees C target, respectively. Accordingly, GDP loss could fall from 3.8% to barely 0.004% under the NDC target, and from 11.6% to 1.6% under the 2 degrees C target. The welfare will almost not be affected significantly under all scenarios. Moreover, carbon reduction will also bring co-benefits on the air pollution improvement in China. (c) 2020 Elsevier Ltd. All rights reserved.

The Guanzhong basin is a part of the three top priority regions in China's blue sky action as of 2019. Understanding the chemical composition, sources, and atmospheric process of aerosol in this region is therefore imperative for improving air quality. In this study, we present, for the first time, the seasonal variations of organic aerosol (OA) in Xi'an, the largest city in the Guanzhong basin. Biomass burning OA (BBOA) and oxidized OA (OOA) contributed N50% of OA in both autumn and winter. The average concentrations of BBOA in autumn (14.8 +/- 5.1 mu g m(-3)) and winter (11.6 +/- 6.8 mu g m(-3)) were similar. The fractional contribution of BBOA to total OA, however, decreased from 31.9% in autumn to 15.3% in winter, because of enhanced contributions from other sources in winter. The OOA fraction in OA increased largely from 20.9% in autumn to 34.9% in winter, likely due to enhanced emissions of precursors and stagnant meteorological conditions which facilitate the accumulation and secondary formation. A large increase in OOA concentration was observed during polluted days, by a factor of similar to 4 in autumn and similar to 6 in winter compared to clean days. In both seasons, OOA formation was most likely dominated by photochemical oxidation when aerosol liquid water content was b30 mu g m(-3) or by aqueous-phase processes when Ox was b35 ppb. A higher concentration of BBOA was observed for air masses circulated within the Guanzhong basin (16.5-18.1 mu g m(-3)), compared to air masses from Northwest and West (10.9-14.5 mu g m(-3)). Furthermore, compared with OA fraction in non-refractory PM1 in other regions of China, BBOA (17-19%) and coal combustion OA (10-20%) were major emission sources in the Guanzhong Basin and the BTH region, respec-tively, whereas OOA (10-34%) was an important source in all studied regions. (C) 2020 Elsevier B.V. All rights reserved.

An unmanned aerial vehicle (UAV) equipped with miniature monitors was used to study the vertical profiles of PM2.5 (particulate matter with a <= 2.5-mu m diameter) and black carbon (BC) in Macau, China, from the surface to 500 m above ground level (AGL). Twelve- and 11-day measurements were conducted during February and March 2018, respectively. In total, 46 flights were conducted between 05:00 and 06:00 AM Local Time (LT). The average concentrations of PM2.5 and BC were significantly lower in March (40.1 +/- 17.9 and 2.3 +/- 2.0 mu g m(-3), respectively) when easterly winds prevailed, compared with those in February (69.8 +/- 35.7 and 3.6 +/- 2.0 mu g m(-3), respectively) when northerly winds dominated. In general, PM2.5 concentrations decreased with height, with a vertical decrement of 0.2 mu g m(-3) per 10 m. BC concentrations exhibited diverse vertical profiles with an overall vertical decrement of 0.1 mu g m(-3) per 10 m. Meteorological analyses including back-trajectory analysis and atmospheric stability categorization revealed that both advection and convection transports may have notable influences on the vertical profiles of PM pollutants. The concentration of PM pollutants above the boundary layer was lower than that within the layer, thus exhibiting a sigmoid profile in some cases. In addition, the lighting of firecrackers and fireworks on February 16 (first day of the Chinese New Year) resulted in the elevated concentrations of PM2.5 and BC within 150 m AGL. The takeoff of a civil flight on February 10 may have resulted in a substantial increase in the PM2.5 concentrations from 80.8 (+/- 2.1) mu g m(-3) at the ground level to 119.2 (+/- 9.3) mu g m(-3) at a height of 330 m. Although the results are confined to a height of 500 mAGL, the current study provides a useful dataset for PM vertical distributions, complementing the spatiotemporal variations by ground-based measurements. (C) 2019 Elsevier B.V. All rights reserved.

To get a comprehensive source apportionment of the non-refractory submicron aerosol (NR-PM,), a merged dataset of the organic fragments and the inorganic species, measured by an aerosol chemical speciation monitor (ACSM) during winter 2014 in Shijiazhuang, was used as input for positive matrix factorization (PMF) analysis using the multilinear engine (ME-2) algorithm. Four primary factors were resolved by constraining the profiles of the previously separated organic factors, while three unconstrained secondary factors were resolved. Secondary factors (sum of organic and inorganic components) accounted for over half of NR-PM, during normal days (NDs, 58% or 105.7 mu g m(-3)) and Chinese New Year (CNY, 79% or 72.6 mu g m(-3)). Among the organic components of the total secondary aerosol, 38-48% (8.0-14.4 mu g m(-3)) of the oxygenated organic aerosol (OOA) was attributed to the nitrate-rich OOA (i.e., OOA-NO3) factor, indicating that a part of the OOA was freshly formed and/or had similar volatility as nitrate. In comparison, a portion of 25-26% (5.5-7.7 mu g m(-3)) of the OOA was attributed to the regionally transported sulfate-rich OOA (i.e., OOA-SO4) while 26-37% (7.3-7.4 mu g m(-3)) of the OOA to aged primary aerosol. The positive relationship between OOA-SO4 and aerosol liquid water content (ALWC) in the same air mass suggested an aqueous-phase reaction pathway, which produced nearly half as much OOA as sulfate (12.0-17.0 mu g m(-3)), while photochemical reactions could produce similar amounts of OOA as nitrate (8.6-15.4 mu g m(-3)), as indicated by the positive relationship between OOA-NO3 and O-x (O-3 + NO2). During CNY, the NR-PM, concentrations (91.9 mu g m(-3)) were reduced by similar to 50% when compared to the nonholiday periods (182.7 mu g m(-3)). This reduction was primarily due to the reduced anthropogenic activities, resulting in a 65-89% reduction in the primary emissions from traffic, cooking, biomass burning, and coal combustion, as well as a 1-44% reduction in secondary factors. The results in our study have significant implications for controlling primary emissions, while joint measures over a regional scale are needed to reduce the secondary aerosols in Shijiazhuang.

Although there are many studies of particulate matter (PM) pollution in Beijing, the sources and processes of secondary PM species during haze periods remain unclear. Limited studies have investigated the PM formation in highly polluted environments under low- and high-relative-humidity (RH) conditions. Herein, we present a systematic comparison of species in submicron particles (PM1) in wintertime Beijing (29 December 2014 to 28 February 2015) for clean periods and pollution periods under low- and high-RH conditions. PM1 species were measured with an aerosol chemical species monitor (ACSM) and an Aethalometer. Sources and processes for organic aerosol (OA) were resolved by positive matrix factorization (PMF) with a multilinear engine 2 (ME-2). The comparisons for clean, low-RH pollution and high-RH pollution periods are made from three different aspects, namely (a) mass concentration, (b) mass fraction and (c) growth rate in diurnal profiles. OA is the dominant component of PM1, with an average mass concentration of 56.7 mu g m(-3) (46 %) during high-RH pollution and 67.7 mu g m(-3) (54 %) during low-RH pollution periods. Sulfate had higher concentration and mass fraction during high-RH pollution periods, while nitrate had higher concentration and mass fraction during low-RH pollution periods. The diurnal variations of nitrate and oxygenated organic aerosol (OOA) showed a daytime increase in their concentrations during all three types of periods. Nitrate had similar growth rates during low-RH (0.40 mu g m(-3) h(-1)) and high-RH (0.55 mu g m(-3) h(-1)) pollution periods. OOA had a higher growth rate during low- RH pollution periods (1.0 mu g m(-3) h(-1)) than during high-RH pollution periods (0.40 mu g m(-3) h(-1)). In contrast, sulfate had a decreasing trend during low-RH pollution periods, while it increased significantly with a growth rate of 0.81 mu g m(-3) h(-1) during high-RH pollution periods. These distinctions in mass concentrations, mass fractions and daytime growth rates may be explained by the difference in the formation processes affected by meteorological conditions. In particular, photochemical oxidation and aqueous-phase processes may both produce sulfate and nitrate. The relative importance of the two pathways, however, differs under different meteorological conditions. Additional OOA formation under high-RH (> 70 %) conditions suggests aqueous-related formation pathways. This study provides a general picture of the haze formation in Beijing under different meteorological conditions.

The chromophores responsible for light absorption in atmospheric brown carbon (BrC) are not well characterized, which hinders our understanding of BrC chemistry, the links with optical properties, and accurate model representations of BrC to global climate and atmospheric oxidative capacity. In this study, the light absorption properties and chromophore composition of three BrC fractions of different polarities were characterized for urban aerosol collected in Xi'an and Beijing in winter 2013-2014. These three BrC fractions show large differences in light absorption and chromophore composition, but the chromophores responsible for light absorption are similar in Xi'an and Beijing. Water-insoluble BrC (WI-BrC) fraction dominates the total BrC absorption at 365 nm in both Xi'an (51 +/- 5%) and Beijing (62 +/- 13%), followed by a humic-like fraction (HULIS-BrC) and high-polarity water-soluble BrC. The major chromophores identified in HULIS-BrC are nitrophenols and carbonyl oxygenated polycyclic aromatic hydrocarbons (OPAHs) with 2-3 aromatic rings (in total 18 species), accounting for 10% and 14% of the light absorption of HULIS-BrC at 365 nm in Xi'an and Beijing, respectively. In comparison, the major chromophores identified in WI-BrC are PAHs and OPAHs with 4-6 aromatic rings (in total 16 species), contributing 6% and 8% of the light absorption of WI-BrC at 365 nm in Xi'an and Beijing, respectively.