Based on the ozone（O 3 ） concentration data from national-controlled environmental monitoring sites in Sichuan Basin during August 2019, three regional and persistent O 3 processes with diverse degrees were selected. The spatial and temporal differences in the vertical variation of O 3 concentration and its meteorological causes for the three processes were analyzed using reanalysis data. The results showed that:（1） The distribution of O 3 concentration in the vertical orientation of the three processes showed similar characteristic of increasing and then decreasing, and the extreme value was reached at 900～850 hPa; The tropospheric O 3 was concentrated below 500 hPa or even 700 hPa, and the vertical dispersion became more obvious as the O 3 concentration level increased.（2） The temperature and relative humidity of the three processes were significantly different. The temperature correlated positively with the O 3 concentration and the relative humidity correlated negatively, with high temperature and low relative humidity being more conducive for the production of O 3 .（3） The processes of level Ⅰ～Ⅱ were characterized by northeasterly or northerly wind on the ground of the basin, the cyclonic circulation and wind convergence were likely to cause the accumulation of O 3 pollution in the downstream areas of the wind; The ground dominant wind direction during the process of level Ⅲ was southerly or southeasterly, which made O 3 easily transported to the northwest side of the basin.（4） The intensity of vertical rise was an important reason for the differences in O 3 concentration between the three processes. The relatively intense upward motion of process I was conducive to the upward diffusion of O 3 and transported out of the basin; The wind speed at the lower level of processes II～Ⅲ was relatively weak, and there was a weak vertical circulation circle in the basin, so the vertical diffusion condition was relatively inferior, and O 3 could accumulate and stagnate in the basin and cause pollution.

Method Soils represent a major global source and sink of greenhouse gases (GHGs). Many studies of GHG fluxes between soil, plant and atmosphere rely on chamber measurements. Different chamber techniques have been developed over the last decades, each characterised by different requirements and limitations. In this manuscript, we focus on the non-steady-state technique which is widely used for manual measurements but also in automatic systems. Although the measurement method appears very simple, experience gained over the years shows that there are many details which have to be taken into account to obtain reliable measurement results. Aim This manuscript aims to share lessons learnt and pass on experiences in order to assist the reader with possible questions or unexpected challenges, ranging from the planning of the design of studies and chambers to the practical handling of the chambers and the quality assurance of the gas and data analysis. This concise introduction refers to a more extensive Best Practice Guideline initiated by the Working Group Soil Gases (AG Bodengase) of the German Soil Science Society (Deutsche Bodenkundliche Gesellschaft). The intention was to collect and aggregate the expertise of different working groups in the research field. As a compendium, this Best Practice Guideline is intended to help both beginners and experts to meet the practical and theoretical challenges of measuring soil gas fluxes with non-steady-state chamber systems and to improve the quality of the individual flux measurements and thus entire GHG studies by reducing sources of uncertainty and error.

Abstract. This review provides a community’s perspective on air quality research focussing mainly on developments over the past decade. The article provides perspectives on current and future challenges as well as research needs for selected key topics. While this paper is not an exhaustive review of all research areas in the field of air quality, we have selected key topics that we feel are important from air quality research and policy perspectives. After providing a short historical overview, this review focuses on improvements in characterising sources and emissions of air pollution, new air quality observations and instrumentation, advances in air quality prediction and forecasting, understanding interactions of air quality with meteorology and climate, exposure and health assessment, and air quality management and policy. In conducting the review, specific objectives were (i) to address current developments that push the boundaries of air quality research forward, (ii) to highlight the emerging prominent gaps of knowledge in air quality research and (iii) and to make recommendations to guide the direction for future research within the wider community. This review also identifies areas of particular importance for air quality policy. The original concept of this review was borne at the International Conference on Air Quality 2020 (held online due to the COVID 19 restrictions during 18–26 May 2020), but the article incorporates a wider landscape of research literature within the field of air quality science. On air pollution emissions the review highlights, in particular, the need to reduce uncertainties in emissions from diffuse sources, particulate matter chemical components, shipping emissions and the importance of considering both indoor and outdoor sources. There is a growing need to have integrated air pollution and related observations from both ground based and remote sensing instruments, including especially those on satellites. The research should also capitalize on the growing area of lower cost sensors, while ensuring a quality of the measurements which are regulated by guidelines. Connecting various physical scales in air quality modelling is still a continual issue, with cities being affected by air pollution gradients at local scales and by long range transport. At the same time, one should allow for the impacts from climate change on a longer timescale. Earth system modelling offers considerable potential by providing a consistent framework for treating scales and processes, especially where there are significant feedbacks, such as those related to aerosols, chemistry and meteorology. Assessment of exposure to air pollution should consider both the impacts of indoor and outdoor emissions, as well as apply more sophisticated, dynamic modelling approaches. With particulate matter being one of the most important pollutants for health, research is indicating the urgent need to understand, in particular, the role of particle number and chemical components in terms of health impact, which in turn requires improved emission inventories and models for predicting high resolution distributions of these metrics over cities. The review also examines, how air pollution management needs to adapt to the above-mentioned new challenges and briefly considers the implications from the COVID-19 pandemic for air quality. Finally, we provide recommendations for air quality research and support for policy.

Affected by the COVID-19 epidemic, a series of lockdown control measures adopted by various regions have reduced the emission intensity of air pollutants. Taking Chengdu as an example, this study analyzed the meteorological conditions and pollution concentration characteristics in the first half of 2020 and focused on a detailed analysis of variations in ozone (O3) concentration. The results showed that: ①Compared with 2019, except for O3, all five pollutants in Chengdu showed a decreased trend, and the concentrations of NO2, CO, SO2, PM10 and PM2.5 decreased by 13.60%, 8.96%, 6.30%, 4.56%, 1.80%, respectively. On the contrary, O3 concentration increased abnormally, with the largest increase in February (35.1%) and May (36.1%). ②During the first half of 2020, the high O3 concentration level appeared earlier than in previous years (2015-2019). Meteorological conditions were more supportive to the generation of O3 also. The geopotential heights of 100 hPa and 500 hPa showed positive anomalies, the temperature and sunshine duration was higher than in previous years, and the relative humidity as well as precipitation were lower with a quiet breeze. ③The O3 formation process lasted a long time from April 25th to May 6th, mainly due to the increased emissions of O3 precursors during this period and the relatively stable weather situation, which kept the Chengdu area exposed to unfavorable weather conditions of high temperature (average temperature>30℃), low humidity (40%~60%), and quiet breeze (1.3 m•s-1) for a long time. The backward trajectories and potential sources of air pollution showed that the air quality was affected by the short-distance transportation of high-polluted air masses from the East of Chengdu and southern Sichuan. © 2021, Science Press. All right reserved.

The increasing availability of low-cost air quality sensors has led to novel sensing approaches. Distributed networks of low-cost sensors, together with data fusion and analytics, have enabled unprecedented, spatiotemporal resolution when observing the urban atmosphere. Several projects have demonstrated the potential of different approaches for high-resolution measurement networks ranging from static, low-cost sensor networks over vehicular and airborne sensing to crowdsourced measurements as well as ranging from a research-based operation to citizen science. Yet, sustaining the operation of such low-cost air quality sensor networks remains challenging because of the lack of regulatory support and the lack of an organizational framework linking these measurements to the official air quality network. This paper discusses the logical inclusion of lower-cost air quality sensors into the existing air quality network via a dynamic field calibration process, the resulting sustainable business models, and how this expansion can be self-funded.

Land-sea breeze (LSB) is an atmospheric mesoscale circulation that occurs in the vicinity of the coast and is caused by uneven heating resulting from the difference in specific heat capacity between the sea and land surfaces. The circulation structure of LSB was quantitatively investigated with a Doppler wind lidar Windcube100s on the west coast of the Yellow Sea for the first time. The time of observation was 31 August to 28 September 2018. It was found that the height of LSB development was 700 m to 1300 m. The duration of conversion of LSB was between 6 h and 8 h. The biggest average horizontal sea-breeze wind speed at 425 m was 5.6 m s(-1), and at 375 m it was 4.5 m s(-1). During the conversion process from sea breeze to land breeze, the maximum wind shear exponent was 2.84 at 1300 m altitude. During the conversion process from land breeze to sea breeze, the maximum wind shear exponent was 1.28 at 700 m altitude. The differences in wind shear exponents between sea-breeze and land-breeze systems were between 0.2 and 3.6 at the same altitude. The maximum value of the wind shear exponent can reflect the height of LSB development.

Both PM2.5 and visibility are important physical parameters to measure the air pollution condition. This paper first provided a new normalization approach for the relationship between PM2.5 and visibility in heavy pollutant region, which realized a mathematical calculation for two different physical parameters based on non-dimensional data process (also called Min-Max normalization). The normalization relationship showed a power function (y = axb) with high correlation coefficients. The inflection points of the maximum power function curvature composed the theoretical threshold range of PM2.5 for improving or deteriorating the visibility. In the case city of north China, the threshold range of PM2.5 was 112-121 mu g.m(-3) corresponding to the visibility range 4.5-5.9 km. Within the threshold range, the mean concentration of main pollutant components was 106 mu g.m(-3). The mean extinction coefficient, bext of 743.5 Mm(-1), was mainly contributed by organic matter (34%), ammonium nitrate (19%), ammonium sulfate (18%) and elemental carbon (13%). When PM2.5 was below the range, the visibility increased significantly with the slow decrease of PM2.5. When PM2.5 exceeded the range, the visibility reduced seriously due to the saturated extinction contribution of the main pollutants. The new normalization approach theoretically revealed the significance of threshold and how to improve visibility by controlling PM2.5 at different pollution stages.

使用环境颗粒物分析仪(GRIMM180)对成都市2018年10月-2019年9月0.25～32 μm的大气颗粒物数浓度粒径分布进行观测,结合细颗粒物(PM2.5)质量浓度以及相对湿度(RH)、能见度、降水量等气象要素数据,分析了成都市大气颗粒物粒径分布及其与能见度的关系.结果表明,观测期间成都市大气颗粒物以细粒子为主,0.25～0.5 μm的粒子数浓度占总数浓度的97.75％,数浓度谱呈单峰分布,2018年秋季、2019年夏季表面积谱呈双峰分布,2018年冬季、2019年春季表面积谱呈三峰,体积谱均为三峰分布;在不同RH区间,PM2.5质量浓度及数浓度均与能见度呈负相关,且其中0.25～0.5 μm粒径段的细颗粒物因为对可见光波段的米散射效应而对能见度产生较大影响;在不同RH情况下,不同粒径段颗粒物数浓度对能见度的影响程度有所差别:在低RH(＜70％)下,0.25～0.3 μm粒径段粒子数浓度对能见度影响最大;在中等RH(70％～80％)下,0.3～0.5 μm粒径段粒子数浓度对能见度的影响最大;而在高RH(＞80％)下,0.3～0.5 μm和0.5～1 μm粒径段粒子数浓度对能见度影响相当.

Opening remarks by conference chairs for conference Remote Sensing of Clouds and Atmosphere XXV

In situ measurements are performed to study the size-resolved hygroscopic behaviour of submicron aerosols during pollution and fireworks episodes in winter from late January to February 2019 in Chengdu, a megacity in Sichuan Basin, using a humidity tandem differential mobility analyser (H-TDMA). The H-TDMA is operated at a relative humidity of 90% with dry aerosol diameters between 40 and 200 nm. Three modes of aerosol particles, including nearly hydrophobic mode (NH), less hygroscopic mode (LH), and more hygroscopic mode (MH), are found in the probability distributions of the growth factor (GF-PDF) during the campaign. The GF-PDF shows that aerosol particles are usually externally mixed. The average ensemble mean hygroscopicity parameter values (κMean) over the entire sampling period are 0.16, 0.19, 0.21, 0.23, and 0.26 for aerosols with diameters of 40, 80, 110, 150, and 200 nm, respectively. These averages are lower than those in Shanghai and Nanjing. κMean for aerosols larger than 110 nm, however, are higher than those in Beijing and Guangzhou during winter. Distinct diurnal patterns for all measured sizes are observed for the number fractions of the NH (NFNH) and MH (NFMH) modes as well as κ-PDF and κMean. The NFNH values are lower, but κMean exhibits peak values during daytime. More aerosols are internally mixed because of photochemical ageing during daytime. The number fraction of LH (NFLH) for the 40-nm diameter aerosols in clean periods (CPs) is larger than that in the pollution episode (PEs) because of the increasing amount of SOA formation. More aerosols of diameters larger than 80 nm are internally mixed during CPs and stage of contaminant accumulation, resulting in higher κMean values compared to those in PEs. The aerosol emissions of fireworks that accumulate during the Chinese New Year's Eve contribute to the slow and continuous increasing trend in κMean with average values of 0.19, 0.19, 0.21,0.23, and 0.27 for the 40, 80, 110, 150, and 200-nm diameter aerosols, respectively. These values are higher than those during the pre- and post-fireworks days. The hygroscopic properties of submicron aerosols in Chengdu are essential for understanding the formation and evolution of severe haze events in Sichuan Basin.

Haze pollution by anthropogenic emitted particles is a frequent challenge for Beijing air quality in addition to the well-known dust storm events. Man-made air pollution is able to cause hazy conditions reducing the sight range. Since those air quality conditions became more and more frequent in Beijing recently, this paper focuses on the seasonal variability of the air quality during such haze episodes in relation to clear air situations. In order to find out the characteristics of airborne PM during haze episodes in different seasons as well as of the corresponding PM sources or air flow influences, a continuous one-year daily PM sampling from June 2010 till June 2011 was performed at the campus of the China University of Geoscience (Beijing). The inorganic elements, water-soluble ions, element carbon and organic carbon as well as levoglucosan, hopanes and PAHs were analyzed, respectively. Positive matrix factorization and back trajectory cluster analyses were applied and combined to identify and apportion sources. The results show that the main sources of particles during haze are secondary inorganic ions formations. Beyond that, different sources from season to season are obvious: biomass burning derived particles have high impact on summer and autumn haze, coal combustion is a major source for winter haze whereas mineral dust emissions are most prominent in spring haze. The sources of PM during clear days were dominated by mineral dust emissions and traffic. Nearby southerly industrial regions were found to be the main source areas of particles during haze in Beijing.

A Single Particle Aerosol Mass Spectrometer (SPAMS) was deployed in the urban area of Chengdu to investigate the size-resolved chemical composition and mixing state of single particles from 22 September to 26 October 2016. Ten major single particle types were resolved with the ART-2a algorithm and these types were classified into carbonaceous particles, secondary inorganic particles, and other particles. The mixing state analysis showed that carbonaceous and other particles were commonly internally mixed with sulfate and nitrate by atmospheric processes. Carbonaceous particles were mainly internally mixed with sulfate, while other particle types were mixed with nitrate. The abatement of NOx concentrations is essential for the reduction of PM2.5 (particles with the aerodynamic diameter smaller than 2.5 μm) pollution in Chengdu. The complete process of a pollution event can be summarized as regional (clean), local emission (transition), and accumulation of pollutants (polluted). At high relative humidity levels, the aqueous-phase reaction is a key pathway for the formation of PM2.5 species. The diverse emission characteristics of different types of days (weekdays, weekends, and the National Day holiday) resulted in significantly different diurnal patterns in the PM2.5 mass and total number concentrations. The preliminarily formed particles experienced a variety of atmospheric processes (such as coagulation, condensation, and accumulation), which were responsible for the decrease in the number concentration and subsequent high PM2.5 mass concentration values. All these findings provide new insights into, and improved understanding of, the behavior of single particles and the pollution formation mechanisms in the Sichuan Basin.

Mit dem Forschungsprojekt SmartAQnet wird ein smarter Weg zur raumlichen Bestimmung von Feinstaub untersucht und am Modellstandort Augsburg erprobt. Forschungsansatz ist die Erfassung und Zusammenfuhrung unterschiedlicher Qualitaten von Feinstaubmesswerten mit Fernerkundungsdaten. Feinstaubmesswerte konnen hierbei von Jedermann (z. B. mit Ultra-Low-Cost-Sensoren) bis hin zu offiziellen Messnetzen (mit hochpraziser Messtechnik) in die Datenarchitektur eingespeist werden. Eine neuartige Internet-of-Things-Analyseplattform soll Daten zur Anwendung sowohl fur Planer als auch fur den Burger bieten, welche der nachhaltigen Gesundheitsvorsorge dienen konnen (z. B. App fur eine luftqualitatsbezogene Navigation).

To investigate the spatial representativeness of mixing layer height (MLH) observations in the North China Plain (NCP), we performed several successive MLH measurements with ceilometers from 2013 to 2017, and the MLH observation data were analyzed at different measurement scales. Daytime MLH differences among stations increased with increasing observation scale. The relative coefficient (R) and the coefficient of variation (CV) of the hourly daytime MLH decreased and increased from 0.87 and 18.7% to 0.57 and 27.9%, respectively, by enlarging the observation domain from the urban scale (0–20 km) to the synoptic scale (100–300 km). At inter-station distances of <25 km, the daytime MLH values between two sites are highly consistent. At inter-station distances >25 km but <60 km, the daytime MLH values at any two sites exhibit obvious differences. Additionally, at inter-station distances >60 km, the daytime MLH measurements effectively capture the regional MLH distribution characteristics. Therefore, inter-station distances within the range of 25–60 km are useful for detecting whether obvious differences in daytime MLH exist among stations, and an inter-station distance >60 km is sufficient to obtain daytime MLH observations that represent the regional distribution pattern. Our research results have instructive significance for MLH observation station layouts within the NCP region.

To investigate the spatiotemporal variability of the mixing layer height (MLH) on the North China Plain (NCP), multi-site and long-term observations of the MLH with ceilometers at three inland stations (Beijing, BJ; Shijiazhuang, SJZ; Tianjin, TJ) and one coastal site (Qinhuangdao) were conducted from 16 October 2013 to 15 July 2015. The MLH of the inland stations in the NCP were highest in summer and lowest in winter, while the MLH on the coastal area of Bohai was lowest in summer and highest in spring. As a typical site in southern Hebei, the annual mean of the MLH at SJZ was 464 ± 183 m, which was 15.0 and 21.9 % lower than that at the BJ (594 ± 183 m) and TJ (546 ± 197 m) stations, respectively. Investigation of the shear term and buoyancy term in the NCP revealed that these two parameters in southern Hebei were 2.8 times lower and 1.5 times higher than that in northern NCP within 0–1200 m in winter, respectively, leading to a 1.9-fold higher frequency of the gradient Richardson number > 1 in southern Hebei compared to the northern NCP. Furthermore, combined with aerosol optical depth and PM2.5 observations, we found that the pollutant column concentration contrast (1.2 times) between these two areas was far less than the near-ground PM2.5 concentration contrast (1.5 times). Through analysis of the ventilation coefficient in the NCP, the near-ground heavy pollution in southern Hebei mainly resulted from the lower MLH and wind speed. Therefore, due to the importance of unfavorable weather conditions, heavily polluting enterprises should be relocated and strong emission reduction measures should be introduced to improve the air quality in southern Hebei.

Mit dem Forschungsprojekt SmartAQnet wird ein smarter Weg zur raumlichen Bestimmung von Feinstaub untersucht und am Modellstandort Augsburg erprobt. Forschungsansatz ist die Erfassung und Zusammenfuhrung unterschiedlicher Qualitaten von Feinstaubmesswerten mit Fernerkundungsdaten. Feinstaubmesswerte konnen hierbei von Jedermann (z. B. mit Ultra-Low-Cost-Sensoren) bis hin zu offiziellen Messnetzen (mit hochpraziser Messtechnik) in die Datenarchitektur eingespeist werden. Eine neuartige Internet-of-Things-Analyseplattform soll Daten zur Anwendung sowohl fur Planer als auch fur den Burger bieten, welche der nachhaltigen Gesundheitsvorsorge dienen konnen (z. B. App fur eine luftqualitatsbezogene Navigation).

Passive Fourier Transform Infrared (FTIR) and Differential Optical Absorption Spectroscopy (DOAS) techniques, i.e. those making use of natural radiation sources are used since the 1970s for the remote sensing of trace gases in the atmosphere. Characteristic for these methods is that they can be put to stationary or mobile use (e.g. cars, multicopters, balloons, aircraft, satellites), are capable of very quickly generating indicative data over a large measurement range and permit quantitative determinations of defined substances. A number of distinct method variations has been developed for specific purposes that cover a broad range of applications in emission and ambient air measurements. These reasons prompted the Commission on Air Pollution Prevention of VDI and DIN (a standards committee of the German Standards Institute) to establish the German Standards VDI 4211 and 4212 on these techniques. Both Standards are also intended to support and encourage manufacturers and start-ups to develop and produce marketable instruments in consideration of future air monitoring concepts (small, mobile, versatile, multi-component analysers). In order to pave the way for these methods to be recognized by the authorities as equivalent methods to established air quality monitoring methods experience-based minimum requirements for selected performance characteristics are recommended which may be the basis for a future type testing procedure.