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This paper analyzes observation data in the Pearl River Delta (PRD) region from 2012 to 2013, and explores the impact of typhoon periphery on high ozone and high aerosol pollution episodes (double high episodes). Observation analysis show that severe tropical storms to severe typhoons are mainly located in the range of 10°N-30°N, 116°E-135°E when double high episodes occur. Meanwhile, obvious high temperature, low humidity, low wind speed, high actinic flux, high aerosol optical depth (AOD), and high single scattering albedo (SSA) can be observed in double high episodes. The diurnal cycle of the PM2.5 is significant in double high episodes, and the average peak concentration in the afternoon can exceed 90 μg/m3. The diurnal cycle of PM2.5 in non–double high episodes is not significant, and the average value is about 34–39 μg/m3. The ozone peak concentration in double high episodes is 81–103 ppbv, which is about 27–40 ppbv higher than that of non–double high episodes. High correlation can be found between the aerosol and ozone diurnal cycles in double high episodes, and r2 reaches 0.76. In double high episodes, black carbon, nitrate, and sea salt decrease while sulfate, ammonium, secondary organic carbon, and total PM2.5 significantly increase in the afternoon. The growth of PM2.5 in double high episodes is mainly contributed by scattered fine particles from photochemical processes and transmission. The mechanisms that control the double high episodes in the PRD are described below. Ozone and aerosol begin to accumulate under unfavorable meteorological conditions. Via local photochemical processes and external transport, the scattered aerosol increases and leads to an increase in multiple scattering and actinic radiation, which is in turn more favorable for photochemical reaction and further increases the ozone concentration. Meanwhile, high oxidizability promotes the formation of scattered aerosol, creating positive feedback. In addition, the scattered aerosol increases backscattering, which increases the photolysis rate and ozone concentration in the middle and upper boundary layer. Meanwhile, downdraft and turbulence transports high-concentration ozone to the ground.

Weekday/weekend mixing ratios of ozone (O3) and the O3 precursors of non-methane hydrocarbons (NMHCs) and nitrogen oxides (NOx) were recorded at the Guangzhou Panyu Atmospheric Composition Station (GPACS), a suburban site in Guangzhou, during the summer (June, July, and August) and autumn (September, October, and November) of 2011. In both summer and autumn, weekday/weekend O3 differences in the morning and at midday largely depend on how much the O3 precursors are affected by anthropogenic emissions. In the mornings (6:00–9:00 LT), pollutants (i.e. NOx and NMHCs) were more strongly influenced by vehicular emissions in autumn than in summer. In autumn, O3 titration and lower NOx on weekends in NMHCs-limited regimes lead to more rapid O3 production, which resulted in the O3 weekend effect during autumn morning. No O3 weekend effect occurred on summer mornings because O3 formation was in a NOx-limited regime, although O3 titration still existed. At midday (10:00–16:00 LT), the increase of biogenic NMHCs emissions reversed the sensitivity of O3 production from NMHCs-to NOx-sensitive. The weekday/weekend diurnal pattern of vehicular sources was the same at midday, more intense other human industrial activities in autumn not only gave rise to the higher mixing ratios of high-reactive anthropogenic NMHCs (e.g. aromatics) on weekdays, but also could affected the temperature in the city, leading to higher isoprene mixing ratio on weekdays. All these factors are likely to contribute to the O3 weekday effect in autumn. Meanwhile no weekday O3 effect occurred in summer due to the low-intensity industrial anthropogenic activities. Our results show that high-reactive NMHCs and NOx control can be effective for reducing peak O3 mixing ratios in Guangzhou. Further investigation based on numerical models is required to reach more robust conclusions.

In late September 2019, the longest and most extensive ozone (O3) pollution process occurred at Pearl River Delta. Base on the observational data, surface-level O3, vertical distribution characteristics boundary layer O3 as well as its effect on surface-level O3 are thoroughly analyzed. The O3 lidar results showed similar vertical O3 profiles both in pollution episodes and clean periods, from which a high O3 concentration layer between 300 and 500 m and a sub-high O3 concentration layer between 1300 and 1700 m (near the top of the mixing layer) can be found. Besides, the downward O3 transport paths from the high/sub-high O3 concentration layers could be observed along with the boundary layer evolution: At nighttime, large amounts of O3 were effectively stored into the residual layer (RL). Due to the upward development of Mixing layer (ML) in early morning, atmospheric vertical mixing carried the O3 inside the RL down to the surface, which led to a rapid increase in the surface-level O3. The sub-high O3 layer began the downward mixing at noon, and became well-mixed after the boundary layer was fully developed in the afternoon, by which the near surface O3 pollution deteriorated again. Further analysis of the heavy O3 pollution episodes show that, the high O3 concentration inside the RL contributed 54% ± 6% of the surface-level O3 at 9:00 LT and the average contribution of O3 in the sub-high concentration layer to the surface-level O3 at 14:00 LT was 26% ± 9%. Based on the quantitative analysis of the observational data, this paper focus to reveal the importance of the contribution of O3 inside the RL and near the top of the ML to the surface O3.

Submicron aerosols (PM1) have more serious effect on the visibility, air quality, human health and climate change compared to coarse particles (particle size between 2.5 and 10 μm). There is practical significance in understanding the formation mechanisms and sources of these particles. In this study, we conducted real-time measurements on the composition of non-refractory submicron aerosols (NR-PM1) in Guangzhou from 20 October to 1 December, 2016, using a quadripole aerosol chemical speciation monitor (Q-ACSM). Overall, the composition of NR-PM1 was dominated by organics (48.2%), followed by nitrate (NO3−, 20.3%), sulfate (SO42−, 17.4%), ammonium (NH4+, 11.4%), and chloride (Cl−, 2.7%). The high contribution of NO3− and NO3−/SO42− mass ratios were observed in high-PM episodes, suggesting that NOx emission control should be a priority in mitigating PM-pollution in South China. Primary organic aerosols (POA) accounted for 51% of organics as the campaign average, and POA contribution increased with PM concentrations especially during high PM episodes, indicating that local emissions played a very important role in high-PM pollution. The conversion of SO2 to SO42− and aqueous, heterogeneous production may not be the dominant source of SO42− in NR-PM1, while regional transport is likely to be the main source during high-sulfate episodes. The formation of NO3− in this study was related to gas-phase reactions of NO2 (photochemical reaction) during daytime, and heterogeneous reactions (hydrolysis reaction of N2O5) during the nighttime. Gas-phase reactions would be less important for nitrate formation during severe polluted events. Particle acidity analyses showed that particles were generally acidic and the NH4+ was not enough to fully neutralise the NO3− and SO42−, the existing forms of SO42− could be (NH4)2SO4, NH4HSO4 or (NH4)3H(SO4)2. Controlling the emissions of NH3 to reduce the NH4+ concentration can probably be a feasible direction in the future in Guangzhou if the problem of agriculture and aerosol acidity can be solved. Wind rose plots and back-trajectory analyses also demonstrated that local emission was more important to the formation of NO3− and regional transport was more conducive to the increase of SO42− concentration.

Ozone has become the main factor affecting the air quality in the Pearl River Delta (PRD) in recent years, and a clear understanding source of ozone pollution is a top priority of ozone pollution prevention. However, the vertical distribution of tropospheric ozone in the PRD is poorly understood. Based on daily ozone sounding data obtained in Yangjiang (111°58'00" E, 21°50'00" N), Guangdong Province, from 18 November tjo 3 December 2013, the detailed vertical distribution of tropospheric ozone and its influencing factors were determined and compared with weekly autumn ozone profile data obtained at the Hong Kong Observatory (HKO) and corresponding tropospheric column ozone (TCO) datasets from the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) onboard satellite AURA. Overall, the ozone profiles observed in Yangjiang are in accordance with those of the HKO in terms of the vertical structure. The TCO values are 34.2 ± 4.0 and 34.3 ± 4.6 DU, respectively, that are, 4 DU higher than those of satellite data, indicating that the OMI/MLS product underestimates the TCO level in the PRD region. The Yangjiang ozone profiles indicate significant variations in the ozone vertical structure and concentration during the observation period. A significant ozone peak permanently occurs in the lower troposphere at 1.1 ± 0.4 km above the surface. The vertical structural difference is mainly reflected in the presence of low-ozone strata (below 30 ppbv) near the tropopause and ozone peaks and high-ozone stratification in the middle and upper troposphere. The average ozone peak concentrations in the lower, middle, and upper troposphere are 65.2 ± 9.9, 57.8 ± 7.7, 71.8 ± 13.6 ppbv, respectively, which are higher than the average ground-level ozone concentration (48.9 ± 9.4 ppbv). Based on the analysis of the profile characteristics, weather conditions, and backward trajectories, it can be concluded that the low-ozone strata near the tropopause are caused by the regional transport of the tropical air mass from the tropical cyclone storm area. Potential vorticity analysis shows that stratosphere-troposphere exchange (STE) is the main reason for the formation of the ozone peaks and its varying concentrations in the middle and upper troposphere. Photochemical reactions represent the main factor affecting the ozone concentration in the lower troposphere. The STE is also an important factor affecting the ozone in the lower troposphere, leading to an increase in the surface ozone concentration by ~10.9 ppbv.

Absorbing aerosols can significantly modulate short-wave solar radiation in the atmosphere, affecting regional and global climate. The aerosol absorption coefficient (AAC) is an indicator that assesses the impact of absorbing aerosols on radiative forcing. In this study, the near-surface AAC and absorption Ångström exponent (AAE) in the urban area of Nanjing, China, are characterized on the basis of measurements in 2012 and 2013 using the seven-channel Aethalometer (model AE-31, Magee Scientific, USA). The AAC is estimated with direct and indirect corrections, which result in consistent temporal variations and magnitudes of AAC at 532 nm. The mean AAC at 532 nm is about 43.23 ± 28.13 M m−1 in the urban area of Nanjing, which is much lower than that in Pearl River Delta and the same as in rural areas (Lin'an) in Yangtze River Delta. The AAC in the urban area of Nanjing shows strong seasonality (diurnal variations); it is high in cold seasons (at rush hour) and low in summer (in the afternoon). It also shows synoptic and quasi-2-week cycles in response to weather systems. Its frequency distribution follows a typical log-normal pattern. The 532 nm AAC ranging from 15 to 65 M m−1 dominates, accounting for more than 72 % of the total data samples in the entire study period. Frequent high pollution episodes, such as those observed in June 2012 and in winter 2013, greatly enhanced AAC and altered its temporal variations and frequency distributions. These episodes are mostly due to local emissions and regional pollution. Air masses flowing from northern China to Nanjing can sometimes be highly polluted and lead to high AAC at the site. AAE at 660/470 nm from the Schmid correction (Schmid et al., 2006) is about 1.56, which might be more reasonable than from the Weingartner correction (Weingartner et al., 2003). Low AAEs mainly occur in summer, likely due to high relative humidity (RH) in the season. AAC increases with increasing AAE at a fixed aerosol loading. The RH–AAC relationship is more complex. Overall, AAC peaks at RH values of around 40 % (1.3 < AAE < 1.6), 65 % (AAE < 1.3 and AAE > 1.6), and 80 % (1.3 < AAE < 1.6).

An online coupled regional climate-chemistry model called RegCCMS is used to investigate the interactions between anthropogenic aerosols and the East Asian summer monsoon (EASM) over East Asia. The simulation results show that the mean aerosol loading and optical depth over the region are 17.87 mg/m2 and 0.25, respectively. Sulfate and black carbon (BC) account for approximately 61.2% and 7.8% of the total aerosols, respectively. The regional mean radiative forcing (RF) is approximately −3.64, −0.55, and +0.88 W/m2 at the top of the atmosphere for the total aerosol effect, the total aerosol direct effect, and the BC direct effect, respectively. The surface direct RF of BC accounts for approximately 31% of the total RF of all aerosols. Because of the total aerosol effect, both the energy budgets and air temperature are considerably reduced in the region with high aerosol loadings, leading to decreases in the land-ocean air temperature gradient in summer. The total column-absorbed solar radiation and surface air temperature decrease by 8.4 W/m2 and 0.31 K, respectively. This cooling effect weakens horizontal and vertical atmospheric circulations over East Asia. The wind speed at 850 hPa decreases by 0.18 m/s, and the precipitation decreases by 0.29 mm/d. The small responses of solar radiation, air temperature, and atmospheric circulations to the BC warming effect are opposite to those of the total aerosol effect. The BC-induced enhancement of atmospheric circulation can increase local floods in south China, while droughts in north China may worsen in response to the BC semidirect effect. The total aerosol effect is much more significant than the BC direct effect. The East Asian summer monsoon becomes weaker due to the total aerosol effect. However, this weakness could be partially offset by the BC warming effect. Sensitivity analyses further indicate that the influence of aerosols on the EASM might be more substantial in years when the southerlies or southwesterlies at 850 hPa are weak compared with years when the winds are strong. Changes in the EASM can induce variations in the distribution and magnitude of aerosols. Aerosols in the lower troposphere over the region can increase by 3.07 and 1.04 µg/m3 due to the total aerosol effect and the BC warming effect, respectively.

As a short-lived climate forcing agent, black carbon (BC) aerosol plays an important role in climate change, atmospheric environment, and human health. In this study, continuous measurements of BC and trace gases were made at an urban site in Nanjing, Yangtze River Delta (YRD) of China in 2012. The annual mean BC concentration in Nanjing was found to be 4157 ± 2626 ng/m3, with a range of 221–24,686 ng/m3. The value was much lower than that in most megacities of China and also lower than that in LinAn, a rural site in YRD. BC concentrations in Nanjing showed strong seasonality, high in spring and autumn and low in summer. The concentrations also varied diurnally, high at rush hours and low in the afternoon. The diurnal cycles of BC loadings in winter to spring 2012 were similar to those in 2011 in magnitude and shape. BC concentrations followed a typical lognormal pattern, with over 75% of data samples between 1000 and 6000 ng/m3. The maximum frequency occurred in low BC concentrations in summer and high BC concentrations in other seasons. In Nanjing, high levels of BC were mainly caused by local and regional emissions. Low levels of BC were mostly associated with winds from northern and eastern directions of Nanjing. BC and CO varied similarly with season, having a correlation coefficient over 0.7 in most seasons. Annual mean ΔBC/ΔCO was about 7 ng/m3/ppb, implying that the sources of the BC likely came from combustions of bio-fuel, industry-coal, and vehicle-gasoline. Serious pollution episodes with high levels of BC were detected in early June at the site. Analysis suggested that the episodes were due to biomass burning in northwestern region of Nanjing. During this period, ΔBC/ΔCO reached about 9.7 ng/m3/ppb and much higher BC levels were found at mid-night.

The long-term variations of ozone are the combined results of climate change and air quality management. As Guangzhou is under the influence of both subtropical monsoon climate and rapid economic development, the ozone trend in recent years is uncertain. This paper presents the trend analysis of maximum daily average 8 h (MDA8) ozone and daily meteorological observations in Guangzhou from 2008 to 2018, using the Kolmogorov–Zurbenko (KZ) filter method. The observations were conducted at two sites in suburban Guangzhou, thus the datasets were processed in two periods. The first period (P1) is from 2008 to 2013, and the second period (P2) is from 2014 to 2018. Results show that the KZ filter method separates the short-term, seasonal, and long-term components efficiently, leaving a covariance term of 7.3% (5.4%) for P1 (P2). Through linear regression of long-term components, the trends were inferred as −0.06 ± 0.04 ppb year−1 (R2 = 0.00, p

Continuous measurements of atmospheric aerosols were made in Nanjing, a megacity in China, from 18 January to 18 April, 2011 (Phase 1) and from 22 April 2011 to 21 April 2012 (Phase 2). Aerosol characteristics, optical properties, and direct radiative forcing (DRF) were studied through interpretations of these measurements. We found that during Phase 1, mean PM2.5, black carbon (BC), and aerosol scattering coefficient (Bsp) in Nanjing were 76.1 ± 59.3 μg m−3, 4.1 ± 2.2 μg m−3, and 170.9 ± 105.8 M m−1, respectively. High pollution episodes occurred during Spring and Lantern Festivals when hourly PM2.5 concentrations reached 440 μg m−3, possibly due to significant discharge of fireworks. Temporal variations of PM2.5, BC, and Bsp were similar to each other. It is estimated that inorganic scattering aerosols account for about 49 ± 8.6% of total aerosols while BC only accounted for 6.6 ± 2.9%, and nitrate was larger than sulfate. In Phase 2, optical properties of aerosols show great seasonality. High relative humidity (RH) in summer (June, July, August) likely attributed to large optical depth (AOD) and small Angstrom exponent (AE) of aerosols. Due to dust storms, AE of total aerosols was the smallest in spring (March, April, May). Annual mean 550-nm AOD and 675/440-nm AE were 0.6 ± 0.3 and 1.25 ± 0.29 for total aerosols, 0.04 ± 0.02 and 1.44 ± 0.50 for absorbing aerosols, 0.48 ± 0.29 and 1.64 ± 0.29 for fine aerosols, respectively. Annual single scattering albedo of aerosols ranged from 0.90 to 0.92. Real time wavelength-dependent surface albedo from the Moderate Resolution Imaging Spectroradiometer (MODIS) was used to assess aerosol DRFs. Both total and absorbing aerosol DRFs had significant seasonal variations in Nanjing and they were the strongest in summer. Annual mean clear sky TOA DRF (including daytime and nighttime) of total and absorbing aerosols was about −6.9 and +4.5 W m−2, respectively. Aerosol DRFs were found to be sensitive to surface albedo. Over brighter surfaces, solar radiation was more absorbed by absorbing aerosols and less scattered by scattering aerosols.

Long-range transport of dust aerosol has severe impact on the atmospheric environment over vast areas. In this study, the features of a severe dust storm and its transport characteristics were investigated during the period from 28 April to 5 May 2011 in East Asia. The combined impact on Nanjing was studied with the observational PM10, PM2.5, visibility and meteorological data, and the numerical models of HYSPLIT and WRF/Chem. This dust storm was caused by a cyclone on 28 April over arid and semiarid areas in Mongolia and China, and then transported to broad downwind areas including northern, central, and eastern parts of China, the Korean Peninsula, and Japan. Among the available data, the highest hourly PM10 concentration reached 3.916 mg/m3 at Jinchang site near the source area. In East China, the coastal cities had poorer air quality than inland ones for two days. Dust aerosol arrived at Nanjing from the northwest, and then north, owing to the movement of synoptic system, mixed with local anthropogenic emissions, resulting in the highest PM10 concentration of 0.767 mg/m3 with PM2.5 level reaching 0.222 mg/m3. As the dust storm gradually turned eastward, the dust aerosol went over the seas in the east of the continent and then flowed back to Nanjing. Numerical simulations showed that dust aerosol affecting East China was mainly transported below the altitude of 2.5 km. The vertical profiles of PM10 and PM2.5 concentrations showed maxima at the altitude between 0.2 km and 1.3 km.