Recent Progress in Atmospheric Chemistry Research in China: Establishing a Theoretical Framework for the "Air Pollution Complex".

Atmospheric chemistry research has been growing rapidly in China in the last 25 years since the concept of the "air pollution complex" was first proposed by Professor Xiaoyan TANG in 1997. For papers published in 2021 on air pollution (only papers included in the Web of Science Core Collection database were considered), more than 24 000 papers were authored or co-authored by scientists working in China. In this paper, we review a limited number of representative and significant studies on atmospheric chemistry in China in the last few years, including studies on (1) sources and emission inventories, (2) atmospheric chemical processes, (3) interactions of air pollution with meteorology, weather and climate, (4) interactions between the biosphere and atmosphere, and (5) data assimilation. The intention was not to provide a complete review of all progress made in the last few years, but rather to serve as a starting point for learning more about atmospheric chemistry research in China. The advances reviewed in this paper have enabled a theoretical framework for the air pollution complex to be established, provided robust scientific support to highly successful air pollution control policies in China, and created great opportunities in education, training, and career development for many graduate students and young scientists. This paper further highlights that developing and low-income countries that are heavily affected by air pollution can benefit from these research advances, whilst at the same time acknowledging that many challenges and opportunities still remain in atmospheric chemistry research in China, to hopefully be addressed over the next few decades.

Sources of elevated organic acids in the mountainous background atmosphere of southern China.

Formic acid (FA) and acetic acid (AA) are pivotal organic acids in the troposphere, significantly influencing atmospheric chemistry. However, their abundance and sources in the mountainous background atmosphere remain underexplored. We undertook continuous measurements of FA and AA in Nanling mountains, southern China, during autumn 2020 using a high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS). Both acids registered higher concentrations than in other global high-altitude or forested locations, averaging at 0.89 (max: 3.91) and 0.95 (max: 3.52) ppbv for FA and AA, respectively. High concentrations of FA and AA in this forested background area arose from secondary formation and biomass burning, collectively contributing 71 % to 89 %. During episodes, FA and AA concentrations surged 2-3 times, owing to the enhanced atmospheric oxidation capacity. The secondary FA production was predominantly due to isoprene oxidation among the VOC precursors studied. However, observed inconsistencies between calculated and actual FA concentrations suggest overlooked precursors or mechanisms warranting further investigation. Our findings can enhance the understanding of organic acid characteristics and the interplay of biogenic and anthropogenic sources in the background atmosphere.

Evidence of reactive aromatics as a major source of peroxy acetyl nitrate over China.

We analyze the observations of near-surface peroxy acetyl nitrate (PAN) and its precursors in Beijing, China in August of 2007. The levels of PAN are remarkably high (up to 14 ppbv), surpassing those measured over other urban regions in recent years. Analyses employing a 1-D version of a chemical transport model (Regional chEmical and trAnsport Model, REAM) indicate that aromatic non-methane hydrocarbons (NMHCs) are the dominant (55-75%) PAN source. The major oxidation product of aromatics that produces acetyl peroxy radicals is methylglyoxal (MGLY). PAN and O(3) in the observations are correlated at daytime; aromatic NMHCs appear to play an important role in O(3) photochemistry. Previous NMHC measurements indicate the presence of reactive aromatics at high levels over broad polluted regions of China. Aromatics are often ignored in global and (to a lesser degree) regional 3D photochemical transport models; their emissions over China as well as photochemistry are quite uncertain. Our findings suggest that critical assessments of aromatics emissions and chemistry (such as the yields of MGLY) are necessary to understand and assess ozone photochemistry and regional pollution export in China.

How does El Niño-Southern Oscillation modulate the interannual variability of winter haze days over eastern China?

The haze pollution over eastern China has evident interannual variability. Based on the observed daily visibility at the meteorological stations from 1980 to 2018, the interannual variability of winter haze days (WHD) and its relation to El Niño-Southern Oscillation (ENSO) are investigated. ENSO has a significant negative modulation on the WHD over southern China (south of 30°N) and an insignificant positive modulation on the WHD over North China (north of 35°N). Based on the analyses on the haze-prone weather condition and climate variability conducive to excessive WHD anomaly at interannual time scale, anomalous subsidence with suppressed precipitation plays a dominant role for haze over southern China where wet deposition is the most important, while southerly wind anomaly plays a dominant role for haze over North China. An anomalous anticyclone (cyclone) over western North Pacific is stimulated by El Niño (La Niña) during its peak phase in winter, and the anomalous ascending (descending) motion on the northwestern flank of this anomalous anticyclone (cyclone) induces a significant excessive (deficient) precipitation over southern China, conducive to less (more) WHD over southern China during El Niño (La Niña) winters via wet deposition. The insignificant southerly (northerly) wind anomaly associated with weakened (enhanced) winter monsoon during El Niño (La Niña) is responsible for the slightly more (less) haze days over North China during El Niño (La Niña) winters.

An observation-based perspective of winter haze days in four major polluted regions of China.

An observation-based approach is used to examine key characteristics of winter haze days in four major polluted regions in China. Major findings in this study are: first, there was no significant trend in the number of winter haze days in most provinces and districts in eastern China from 1973 to 2012, contrary to the 2.5-fold increase in the emissions of particulate matter and its precursors (PM emissions) in the same period of time. Second, meteorological and climate conditions rather than PM emissions are in control of the interannual variabilities and trends of winter haze days. These interannual variabilities (ranging from 24 to 125%) pose a substantial masking effect that must be overcome by any control of PM emissions before its impact becomes statistically detectable. Finally, we find that global warming may have contributed significantly to the trend of winter haze days in eastern China.

Variations of Cd/Pb and Zn/Pb ratios in Taipei aerosols reflecting long-range transport or local pollution emissions.

Along with windblown dust, large quantities of pollutants are annually brought out of continental China by the westerlies in winter and spring; thereafter, they are partly subjected to transport by northeastern monsoon winds to Taiwan. To characterize the heavy metal composition differences between long-range transported and local aerosols and to evaluate metal contributions from long-range transported aerosols during the northeastern monsoon season, both PM(10) and PM(2.5) aerosols collected from Taipei, Taiwan from February 2002 to March 2003 were analyzed for three selected heavy metals, namely Pb, Cd and Zn using ICP-MS. Monthly patterns show that Pb concentrations in winter (62 ng/m(3)) were over two times higher than those in the other seasons, which is attributed to long-range transport from areas under development in China. Low Cd/Pb (0.017) and Zn/Pb (1.82) ratios were measured in aerosols collected during the Asian dust period, in which the ambient aerosols consisted predominantly of long-range transported pollutants. By contrast, high Cd/Pb (0.030) and Zn/Pb (3.44) ratios were observed during the summer monsoon season, in which aerosols were dominated by local pollutant emissions. Cd/Pb and Zn/Pb ratios appear to be successfully applied to identify the pollutants originating principally from the long-range transport or from local emissions. In addition, by assuming that a significant fraction of heavy metals associated with coarse airborne dust have settled to the sea prior to reaching Taiwan in spring, a mechanism is suggested to explain why higher anthropogenic metal concentrations occurred in winter than those in dust-rich spring.

Risk of asthma associated with energy-dense but nutrient-poor dietary pattern in Taiwanese children.

Childhood asthma has rapidly increased over the past few decades, possibly due to changes in lifestyle and dietary patterns. We aimed to determine associations between dietary patterns and asthma in schoolchildren in Taiwan. The Nutrition and Health survey in Taiwan Elementary School Children was carried out by using a multi-staged complex sampling design. A total of 2,082 elementary school children with complete data on dietary, lifestyle, demographics and asthma were included in the analysis. We used a Chinese version of the International Study of Asthma and Allergies in Childhood questionnaire to generate an asthma symptom score and to define asthma outcomes. Dietary intake was assessed by a food frequency questionnaire. Reduced rank regression (RRR) was used to identify the dietary pattern associated with the asthma symptoms score. Asthma outcomes included; current asthma, current severe asthma, nocturnal cough, exercise-induced wheeze and asthma ever. The RRR-derived dietary pattern was characterized by high consumption of fast foods, high-fat snacks, candy, and cheese; and low consumption of fruit, vegetables and rice. The RRR-derived dietary pattern was associated with an increased risk of current asthma (OR [95% CI]) (2.42 [1.19-4.93] for Q4/Q1, p-for-trend=0.01), current severe asthma (3.21 [1.11-9.25] for Q3/Q1, 4.45 [1.59-12.5] for Q4/Q1; p-for-trend=0.003), and nocturnal cough (1.79 [1.06-3.05] for Q2/Q1, 1.74 [1.02-2.97] for Q3/Q1, 1.82 [1.07-3.11] for Q4/Q1; p-for-trend=0.049). Our results suggest that a diet with a high intake of fat and simple sugars and low intake of fruit, vegetables and rice is associated with an increased risk of asthma in Taiwanese children.