Impact of COVID-19 emission reduction on dust aerosols and marine chlorophyll-a concentration.

The long-range transport of dust aerosols plays a crucial role in biogeochemical cycling, and dust deposition is an important source of nutrients for marine phytoplankton growth. To study the impact of COVID-19 emission reduction on dust aerosols and marine chlorophyll-a (Chl-a) concentration, we selected two similar dust processes from the COVID-19 period (10-15 March 2020) and the non-COVID-19 period (15-20 March 2019) using the Euclidean distance calculation method in combination with the HYSPLIT model and multiple satellite data. During the non-COVID-19 period, the proportion of dust was 6.68 %, approximately half that of the COVID-19 period. Meanwhile, the proportion of polluted dust during the non-COVID-19 period was 4.95 %, which was more than tenfold compared to the COVID-19 period. Furthermore, noticeable discrepancies in Chl-a concentration were observed between the two periods. In the non-COVID-19 period, the maximum daily deposition of dust aerosols can reach 16.23 mg/m2, resulting in a 39-85 % increase in Chl-a concentration. However, during COVID-19 period, the maximum daily dust deposition can reach 33.33 mg/m2, while the increase in Chl-a concentration was <30 %. This conclusion suggests that reductions in anthropogenic emissions during the COVID-19 period have influenced the nutrient content of dust aerosols, resulting in a lesser impact on Chl-a concentrations in the ocean.

Characteristics of aerosol aminiums over a coastal city in North China: Insights from the divergent impacts of marine and terrestrial influences.

Aminium ions, as crucial alkaline components within fine atmospheric particles, have a notable influence on new particle formation and haze occurrence. Their concentrations within coastal atmosphere depict considerable variation due to the interplay of distinctive marine and terrestrial sources, further complicated by dynamic meteorological conditions. This study conducted a comprehensive examination of aminiums ions concentrations, with a particular focus on methylaminium (MMAH+), dimethylaminium (DMAH+), trimethylaminium (TMAH+), and triethylaminium (TEAH+) within PM2.5, over varying seasons (summer, autumn, and winter of 2019 and summer of 2021), at an urban site in the coastal megacity of Qingdao, Northern China. The investigations revealed that the total concentration of particulate aminium ions (∑Aminium) was 21.6 ± 23.6 ng/m3, exhibiting higher values in the autumn and winter compared to the two summer periods. Considering diurnal variations during autumn and winter, concentrations of particulate aminium ions (excluding TEAH+) exhibited a slight increase during the day compared to night, with a notable peak during the morning hours. However, it was not the case for TEAH+, which was argued to be readily oxidized by ambient oxidants in the afternoon. Additionally, the ∑Aminium within the summer demonstrated markedly elevated levels during the day compared to night, potentially attributed to daytime sea fog associated with sea-land breeze interactions. Positive matrix factorization results indicate terrestrial anthropogenic emissions, including vehicle emission mixed with road dust and primary pollution, as the primary sources of MMAH+ and DMAH+. Conversely, TMAH+ was predominantly emitted from agricultural and marine sources. With the dominance of sea breeze in summer, TMAH+ was identified as a primary marine emission correlated with sea salt, while MMAH+, DMAH+, and TEAH+ were postulated to undergo secondary formation. Furthermore, a notable inverse correlation was observed between TMAH+ and methanesulfonate in PM2.5, consistent with dynamic emissions of sulfur-content and nitrogen-content gases reported in the literature.

Unveiling the overlooked direct emissions of particulate organic nitrates from ship.

Particulate organic nitrates (pONs) have drawn growing interests due to their effects on nitrogen cycling, air pollution, and regional climate. While secondary formation is typically considered as the major source of pONs, direct emissions from various sources remain poorly explored. Ship exhausts have been known as an important source of reactive nitrogen species, yet pONs emissions from ship have been rarely characterized. In this study, we conducted atmospheric measurement of pONs during a ship-based cruise measurement campaign in the East China Sea and also emission measurement of pONs from ship exhausts. During the ship-based cruise, total five typical kinds of pONs were determined and the average total concentrations of five pONs were 479 ± 193 and 250 ± 139 ng m-3 when sampling was influenced by ship emissions or not, respectively, indicating the notable impact of ship exhaust plumes on ambient pONs. Further, five typical pONs were successfully identified and quantified from ship exhausts, with the average total concentration of 1123 ± 406 µg m-3. The much higher pONs levels in ship exhausts than in ambient particulate matters demonstrated ship emission as an important source for pONs. Additionally, their emission factors from ship exhausts were determined as at a range of 0.1-12.6 mg kWh-1. The chemical transport model simulations indicate that direct pONs emissions from ship exert a significant contribution to atmospheric pONs, especially in the clean marine atmosphere. These findings provide compelling evidence for direct emission of pONs from ship and its considerable effects. We call for further studies to better characterize the direct pONs emissions from ship and other potential sources, which should be incorporated into global and regional models.

Important Roles and Formation of Atmospheric Organosulfates in Marine Organic Aerosols: Influence of Phytoplankton Emissions and Anthropogenic Pollutants.

Organosulfates (OSs) could be potentially important compounds in marine organic aerosols, while their formation in marine atmospheres is far from clear due to a lack of cruise observations. In this work, shipboard atmospheric observations were conducted over the Yellow Sea and Bohai Sea to investigate the abundance and formation of biogenic isoprene/monoterpene-OSs in marine aerosols. The quantified OSs and NOSs accounted for 0.04-6.9% of marine organic aerosols and were 0.07-2.2% of the non-sea-salt (nss) sulfate in terms of sulfur content. Isoprene-related (nitrooxy-)OSs occupied 27-87% of the total quantified OSs, following the abundance order of summer > autumn > spring or winter. This order was driven by the marine phytoplankton biomass and sea surface temperature (SST), which controlled the seawater and atmospheric isoprene concentration levels. Under the severe impacts of anthropogenic pollutants from the East Asia continent in winter, monoterpene nitrooxy-OSs, generated with NOx involved in, increased to 34.4 ± 35.5 ng/m3 and contributed 68% of the quantified (nitrooxy-)OSs. Our results highlight the notable roles of biogenic OSs in marine organic aerosols over regions with high biological activity and high SST. The formation of biogenic OSs and their roles in altering marine aerosol properties calls for elaboration through cruise observations in different marine environments.

PM2.5 source apportionment identified with total and soluble elements in positive matrix factorization.

Source apportionments of urban aerosols identified with positive matrix factorization (PMF) are sensitive to input variables. So far, total elements were frequently included as effective factors in PMF-based source apportionment. We investigated the advances to involve soluble parts of elements in the source apportionment with four data sets of PM2.5 composition observed at a coastal city (Qingdao) in northern China: water-soluble ions plus organic and elemental carbon (IC set), the IC set plus total elements (ICTE set), the IC set plus soluble elements (ICSE set), and the IC set plus both total elements and soluble elements (ICAE set). The apportionments of six sources, including secondary sulfate, secondary nitrate, secondary oxalate, sea salt, biomass burning and dust, were identified with the IC set. In comparison, pollutants from vehicle + coal combustion, ship emissions, waste incineration and industrial activities were also identified with the ICTE, ICSE, or ICAE sets. We found that the PMF solutions of the ICAE set could distinguish aged and fresh dust, and identify fly ash and aged pollutants from industrial sources. The profiles and corresponding time series of vehicle + coal combustion, secondary aerosols, ship emissions, sea salt, and biomass burning emissions identified with the four data sets were very similar, while discrepancies were encountered for waste incineration, dust, and industrial sources. These results indicate the benefits and potentials with total and soluble elements involved in PMF-based source apportionments.

Fibroblast-epithelial metabolic coupling in laryngeal cancer.

OBJECTIVES: To explore the Fibroblast-epithelial metabolic coupling among laryngeal cancers and its prognostic roles METHODS: We reviewed the clinical information of patients with laryngeal cancer in our department. Paraffin-embedded tissues from included patients were immune-stained with antibodies towards MCT4 and TOMM20 and evaluated for stromal and epithelial expression. Survival analysis and Cox regression analysis were applied to investigate the prognostic factor of laryngeal squamous cell carcinoma. TCGA database was used to validate our result. RESULTS: Stromal MCT4 and TOMM20 were both significantly associated with each other among laryngeal cancer tissues. High expression of both Stromal MCT4 and TOMM20 is related to poor prognosis in laryngeal cancer. Stromal MCT4 expression was an independent prognostic indicator for laryngeal cancer. Furthermore, cancer cell MCT4 expression has no relationship with the clinical characteristics of laryngeal cancer. CONCLUSIONS: Our results support that the phenomenon of metabolic symbiosis was exist in the laryngeal cancer tissue. In addition, TOMM20 and stromal MCT4 could be used as new therapeutic targets for laryngeal cancer.

Investigation of a haze-to-dust and dust swing process at a coastal city in northern China part I: Chemical composition and contributions of anthropogenic and natural sources.

The long retention of dust air masses in polluted areas, especially in winter, may efficiently change the physicochemical properties of aerosols, causing additional health and ecological effects. A large-scale haze-to-dust weather event occurred in the North China Plain (NCP) region during the autumn-to-winter transition period in 2018, affecting the coastal city Qingdao several times between Nov. 27th and Dec. 1st. To study the evolution of the pollution process, we analyzed the chemical characteristics of PM2.5 and PM10-2.5 and source apportionments of PM2.5 and PM10, The dust stagnated around NCP and moved out and back to the site, noted as dust swing process, promoting SO42- formation in PM2.5 and NO3- formation in PM10-2.5. Source apportionments were analyzed using the Positive Matrix Factorization (PMF) receptor model and weighted potential source contribution function (WPSCF). Before the dust invasion, Qingdao was influenced by severe haze; waste incineration and coal burning were the major contributors (~80 %) to PM2.5, and the source region was in the southwest of Shandong Province. During the initial dust event, mineral dust and the mixed factor of dust and sea salt were the major contributors (46.0 % of PM2.5 and 86.5 % of PM10). During the polluted dust period, the contributions of regional transported biomass burning (22.3 %), vehicle emissions (20.8 %), and secondary aerosols (33.8 %) to PM2.5 from the Beijing-Tianjin-Hebei region significantly increased. The secondary aerosols source was more regional than that of vehicle emissions and biomass burning and contributed considerably to PM10 (30.8 %) during the dust swing process. Our findings demonstrate that environmental managers should consider the possible adverse effects of winter dust on regional and local pollution.

The Impact of Meteorology and Emissions on Surface Ozone in Shandong Province, China, during 2014-2019.

China has been experiencing severe ozone pollution problems in recent years. While a number of studies have focused on the ozone-pollution-prone regions such as the North China Plain, Yangtze River Delta, and Pearl River Delta regions, few studies have investigated the mechanisms modulating the interannual variability of ozone concentrations in Shandong Province, where a large population is located and is often subject to ozone pollution. By utilizing both the reanalysis dataset and regional numerical model (WRF-CMAQ), we delve into the potential governing mechanisms of ozone pollution in Shandong Province-especially over the major port city of Qingdao-during summer 2014-2019. During this period, ozone pollution in Qingdao exceeded the tier II standard of the Chinese National Ambient Air Quality (GB 3095-2012) for 75 days. From the perspective of meteorology, the high-pressure ridge over Baikal Lake and to its northeast, which leads to a relatively low humidity and sufficient sunlight, is the most critical weather system inducing high-ozone events in Qingdao. In terms of emissions, biogenic emissions contribute to ozone enhancement close to 10 ppb in the west and north of Shandong Province. Numerical experiments show that the local impact of biogenic emissions on ozone production in Shandong Province is relatively small, whereas biogenic emissions on the southern flank of Shandong Province enhance ozone production and further transport northeastward, resulting in an increase in ozone concentrations over Shandong Province. For the port city of Qingdao, ship emissions increase ozone concentrations when sea breezes (easterlies) prevail over Qingdao, with the 95th percentile reaching 8.7 ppb. The findings in this study have important implications for future ozone pollution in Shandong Province, as well as the northern and coastal areas in China.

Numerical study of the amplification effects of cold-front passage on air pollution over the North China Plain.

Cold-front systems provide scavenging mechanisms for air pollution in the North China Plain (NCP), but the transport of pollutants with cold fronts aggravates air quality downstream. The impact of cold fronts on PM2.5 concentrations over the NCP during 8-14 December 2019 was studied using the WRF-Chem model. Results indicate that cold fronts directly influence PM2.5 concentration through regional transport of pollutants and adjustment of meteorological systems, and they indirectly affect air quality by influencing aerosol-radiation interaction. Pollutants affecting downstream areas may be transported to altitudes of ~3 km along the frontal surface, with near-surface PM2.5 concentrations increasing temporarily at up to 15 µg·m-3·h-1 behind the surface frontal line owing to the inversion layer triggered by the oblique frontal surface. The transport process plays an essential role in affecting air pollution levels, more than vertical mixing and chemical reaction processes. Changes in the meteorological system (eastward shift of the high-pressure center) occurring with the passage of cold fronts facilitate the accumulation and transport of pollutants in the NCP, reducing air quality in the western and northern NCP. Cold fronts may also indirectly exacerbate near-surface pollutant diffusion conditions by affecting solar radiation incidence, with a reduction of the 2-m temperature by as much as 1 °C, increasing near-surface (<1 and 0.5 km agl on the pre- and post-frontal sides, respectively) PM2.5 concentrations by up to 40 µg·m-3, while reducing upper-layer concentrations by up to 30 µg·m-3. This study emphasizes the amplification effect of cold fronts on air pollution, with inter-regional cooperation being essential in improving air quality in the NCP region.

The striking effect of vertical mixing in the planetary boundary layer on new particle formation in the Yangtze River Delta.

New particle formation (NPF) induces a sharp increase in ultrafine particle number concentrations and potentially acts as an important source of cloud condensation nuclei (CCN). As the densely populated area of China, the Yangtze River Delta (YRD) region shows a high frequency of observed NPF events at the ground level, especially in spring. Although recent observational studies suggested a possible connection between NPF at the higher altitudes and ground level, the role played by vertical mixing, particularly in the planetary boundary layer (PBL) is not fully understood. Here we integrate measurements in Nanjing on 15-20 April 2018, and the NPF-explicit Weather Research and Forecast coupled with chemistry (WRF-Chem) model simulations to better understand the governing mechanisms of the NPF and CCN. Our results indicate that newly formed particles at the boundary layer top could be transported downward by vertical mixing as the PBL develops. A numerical sensitivity simulation created by eliminating aerosol vertical mixing suppresses both the downward transport of particles formed at a higher altitude and the dilution of particles at the ground level. The resulting higher Fuchs surface area at the ground level, together with the lack of downward transport, yields a sharp weakening of NPF strength and delayed start of NPF therein. The aerosol vertical mixing, therefore, leads to a more than double increase of surface CN10-40 and a one third decrease of boundary layer top CN10-40. Additionally, the continuous growth of nucleated ultrafine particles at the boundary layer top is strongly steered by the upward transport of condensable gases, with close to half increase of particle number concentrations in Aitken mode and CCN at a supersaturation rate of 0.75%. The findings may bridge the gap in understanding the complex interaction between PBL dynamics and NPF events, reducing the uncertainty in assessing the climate impact of aerosols.

Characterizing the distinct modulation of future emissions on summer ozone concentrations between urban and rural areas over China.

Previous studies on ozone pollution primarily focus on the characterization of ozone on a large regional scale, yet much less attention has been paid towards the contrast between urban and surrounding suburban-rural areas. As anthropogenic emissions are projected to decrease in the coming decades, the evolutions of ozone concentrations over urban and suburban-rural areas are compared using the Community Multiscale Air Quality (CMAQ) model coupled with the Weather Research and Forecasting (WRF). The top 25 city clusters are firstly identified across China based on the amount of NOx emissions and population size. At present, the averages of maximum daily 8-h (MDA8) ozone concentrations over the suburban-rural areas (65.74 ppbv) among these city clusters are higher than the corresponding urban areas (62.00 ppbv). The projections in 2050s depend on the scenarios. Under the Shared Socioeconomic Pathways (SSPs) scenarios such as SSP1-2.6, SSP2-4.5 and SSP5-8.5 indicative of primary decreases in anthropogenic emissions, the mean MDA8 ozone concentrations in suburban-rural areas decrease more than urban areas. In contrast, the gap of higher MDA8 ozone concentrations in suburban-rural than urban areas increases under SSP3-7.0 concomitant with increase in anthropogenic emissions. The strongest response to emission changes is found in SSP1-2.6, in which anthropogenic emission reduction leads to lower MDA8 ozone concentrations in the suburban-rural than urban areas, reversing the present state of higher ozone concentrations in the suburban-rural areas. The feature abovementioned regarding changes of mean MDA8 ozone concentrations is largely reproduced in the response of ozone exceedance. The finding highlights different efficacies of anthropogenic emissions in urban from suburban-rural areas. In a region like China where the population density is much higher in urban than suburban-rural regions, the benefit of ozone reduction due to large emission reductions (e.g., SSP1-2.6) is diminished to some extent because the effect is larger in suburban-rural areas.

Optimal estimation of initial concentrations and emission sources with 4D-Var for air pollution prediction in a 2D transport model.

Attributing sources of air pollution events by deploying an efficient observational network is an important and interesting problem in air quality control and forecast studies, but it is very challenging. In order to estimate the sensitivities of pollution events to emission sources, a comprehensive framework is built based on a horizontal 2-dimensional transport model and its adjoint in solving this problem. In an analysis of an idealized air pollution event of PM2.5 over the region of North China, an objective function is defined to optimally estimate the initial concentrations and emission sources through a series of minimization procedures. Results by means of the 4-dimensional variational approach show that, with the optimal initial conditions and emission sources, the model can successfully forecast the pollution event in a few days. The optimal observing network based on sensitivity analysis takes only one third of the cost but greatly retains predictability skill compared to the full-grid observing system, while nearly no predictability skill is detectable if the same number of observational sites is randomly deployed. We evaluate air pollution predictability in the point of focusing on to what degree the root mean square errors between the modeled concentration and the targeted air pollution are limited by the optimal observational network. Results show that air pollution predictability in association with the optimal observational network is limited in the time scales about 6 days. With the high efficiency and in an economic fashion, such a sensitivity-based optimal observing system holds promise for accurately predicting an air pollution event in the targeted area once the adjoint and variational procedure of a realistic atmosphere model including transport and chemical processes is performed.

The climate impact on atmospheric stagnation and capability of stagnation indices in elucidating the haze events over North China Plain and Northeast China.

In this study, the spatial pattern and temporal evolution of PM2.5 over North China Plain (NCP) and Northeast China (NEC) during 2014-2018 was investigated. The annual mean PM2.5 shows clear decreasing trends over time, but the seasonal mean PM2.5 as well as the seasonal total duration and frequency of haze days shows large inter-annual fluctuation. Based on the atmospheric stagnation index (ASI), this study examined the correlation between ASI and haze events over NCP and NEC. Detailed analysis indicates that location dependency exists of ASI in the capability of capturing the haze events, and the ability is limited in NCP. Therefore, we first propose two alternative methods in defining the ASI to either account for the lag effect or enlarge the threshold value of wind speed at 500 hPa. The new methods can improve the ability of ASI to explain the haze events over NEC, though marginal improvement was achieved in NCP. Furthermore, this study constructed the equation based on the boundary layer height and wind speed at 10-meter, apparently improving the ability in haze capture rate (HCR), a ratio of haze days during the stagnation to the total haze days. Based on a multi-model ensemble analyses under Representative Concentration Pathway (RCP) 8.5, we found that by the end of this century, climate change may lead to increases in both the duration and frequency of wintertime stagnation events over NCP. In contrast, the models predict a decrease in stagnant events and the total duration of stagnation in winter over NEC.

Ship emission of nitrous acid (HONO) and its impacts on the marine atmospheric oxidation chemistry.

Nitrous acid (HONO) is an important reservoir of the hydroxyl radical (OH) and thus plays a central role in tropospheric chemistry. Exhaust from engines has long been known as a major primary source of HONO, yet most previous studies focused on vehicle emissions on land. In comparison, ship emissions of HONO have been rarely characterized, and their impacts on the tropospheric oxidation chemistry have not been quantified. In this study, we conducted cruise measurements of HONO and related species over the East China Sea. Contrasting air masses from pristine marine background air to highly polluted ship plumes were encountered. The emission ratio of ΔHONO/ΔNOx (0.51 ± 0.18%) was derived from a large number of fresh ship plumes. Using the in-situ measured emission ratio, a global ship emission inventory of HONO was developed based on the international shipping emissions of NOx in the Community Emission Data System inventory. The global shipping voyage emits approximately 63.9 ± 22.2 Gg yr-1 of HONO to the atmosphere. GEOS-Chem modelling with the addition of ship-emitted HONO showed that HONO concentrations could increase up to 40-100% over the navigation areas, leading to about 5-15% increases of primary OH production in the early-morning time. This study elucidates the potentially considerable effects of ship HONO emissions on the marine atmospheric chemistry, and calls for further studies to better characterize the ship emissions of HONO and other reactive species, which should be taken into account by global and regional models.

Characteristics and sources of PM2.5 with focus on two severe pollution events in a coastal city of Qingdao, China.

In this study, the seasonal mean PM2.5 concentration in Qingdao, a coastal city, during 2014-2018 was first analyzed and the winter, in particular of 2015, showed the highest concentration. To elucidate the sources and control factors of PM2.5, three dimensional model Weather Research and Forecasting (WRF), Community Multiscale Air Quality model (CMAQ), as well as Flexible Particle model (FLEXPART), were used. During December 2015 and January 2016, modeling results showed that the mean contribution to PM2.5 mass concentrations from local emissions in Qingdao was 25%, and the transport from north and west accounted for almost half. Over the two episodically polluted periods (29-31 December 2015; 15-17 January 2016), the local emissions in Qingdao surprisingly contributed to only 18% and 24% to PM2.5 mass concentrations, respectively, indicating the dominant contributions from other regions, such as areas outside Qingdao in Shandong and Beijing-Tianjin-Hebei (BTH). The results show the sources region and contribution may vary remarkably along with the change in the pathways of the air parcel, inferred by the FLEXPART, while the near-surface PM2.5 enhancement is largely caused by downward vertical advection and enhanced aerosol chemistry reactions, accompanied by simultaneous drop in the boundary layer height. This study also reveals that the transport contribution is sensitive to the air parcel trajectories. We, therefore, recommend the efficient emission control based on transport trajectories in short-term air quality improvement in Qingdao.

[Impact of Marine-atmospheric Process on Aerosol Number Size Distribution in the South China Sea].

The shipboard measurements of aerosol number concentration from August 28th to October 13th, 2012 were conducted to analyze the characteristics of temporal-spatial distribution and number size distribution of marine aerosol in the South China Sea. The impact of meteorological condition on the marine aerosol was also analyzed in this paper. The result showed that the temporal-spatial distribution and number size distribution of marine aerosol were influenced by the backward trajectory of the air masses and the time they spent over the sea, and the meteorological conditions in the local sea area such as wind speed, wind direction, temperature, relative humidity and cloud cover. In terms of temporal-spatial distribution of marine aerosol in the South China Sea, the aerosol in the sea areas under the influence of continental air mass concentration showed much higher concentration than that in the clean maritime air mass (2300 cm-3 vs. 1200 cm-3). During the ship observation, three kinds of number size distribution of marine aerosol were found and classified:polluted type, marine type 1 and marine type 2 (clean). All the distributions had a similar sub-micron mode and a similar coarse mode. The marine type 2 (clean) had the same distribution shape as polluted type except that the value of concentration was much lower. The sub-micron peak of marine type 1 appeared near 0.08 µm and its concentration was significantly higher than that of marine type 2 (clean). Non-precipitating clouds as well as abundant moisture contributed to the growth of ultrafine particles to the size of accumulation mode aerosol (0.05-0.12 µm). Number concentration of particles in this range showed weak positive correlation with wind speed since wind can enhance the efficiency of sea-air exchange and accelerate the release of gas precursors of sulfate aerosol to the atmosphere. The concentration of coarse particles ranging from 0.5-6 µm in diameter strongly depended on the wind speed, and the correlation coefficient reached a value of 0.7.

[Variation of atmospheric particle number concentrations in Qingdao and its impact on visibility].

Atmospheric particle number concentrations were measured from September 2010 to August 2011 with potable light house laser particle counter to study the variation of atmospheric particle concentrations and its impact on visibility in Qingdao. Backward trajectory was calculated by Hysplit model. Statistical analysis was done to discuss the influence of meteorological factors on the atmospheric particle number concentrations and visibility. It was shown that the atmospheric particle number concentrations were the highest in winter and spring, followed by autumn, and the lowest in summer. Air mass from Xinjiang and Gansu regions resulted in higher particle concentrations, while the atmospheric particles from the northeast and the ocean had lower concentrations. The variation of atmospheric particle number concentrations presented a good negative correlation with the variation of wind speed, relative humidity and mixed-layer height. When the air mass came from west or northwest, the surface wind direction was south or southeast and the mixed-layer height was low, the number concentration of fine particles was likely to be higher, which tended to cause low visibility phenomenon.

Seasonal variation, source, and regional representativeness of the background aerosol from two remote sites in western China.

Using observations from two remote sites during July 2004 to March 2005, we show that at Akdala (AKD, 47 degrees 06' N, 87 degrees 58' E, 562 m asl) in northern Xinjiang Province, there were high wintertime loadings of organic carbon (OC), elemental carbon (EC), and water-soluble (WS) SO4(2-), NO3(2-), and NH4+, which is similar to the general pattern in most areas of China and East Asia. However, at Zhuzhang (ZUZ, 28 degrees 00' N, 99 degrees 43' E, 3,583 m asl) in northwestern Yunnan Province, the aerosol concentrations and compositions showed little seasonal variation except for a decreasing trend of OC from August to autumn-winter. Additionally, the OC variations dominated the seasonal variation of PM10 (particles

Aerosol background at two remote CAWNET sites in western China.

The frequency distributions and some statistical features of background aerosol concentrations were investigated at two remote China Atmosphere Watch Network (CAWNET) stations. The estimated elemental carbon (EC) background at Akdala (AKD) in the mid-latitudes of northwestern China (approximately 0.15 microg m(-3)) was only half of that at Zhuzhang (ZUZ) in low-latitude southwestern China (approximately 0.30 microg m(-3)). The contributions of EC to the aerosol mass also differed between sites: EC contributed 3.5% of the PM(10) mass at AKD versus 5.1% at ZUZ. Large percentages of the total organic carbon (OC) apparently were secondary organic carbon (SOC); SOC/OC averaged 81% at ZUZ and 68% at AKD. The OC/EC ratios in PM(10) (ZUZ: 11.9, AKD: 12.2) were comparable with other global background sites, and the OC/EC ratios were used to distinguish polluted periods from background conditions. The SO(4)(2)(-), NH(4)(+) and soil dust loadings at AKD were higher and more variable than at ZUZ, probably due to impacts of pollution from Russia and soil dust from the Gobi and adjacent deserts. In contrast to ZUZ, where the influences from pollution were weaker, the real-time PM(10) mass concentrations at AKD were strongly skew right and the arithmetic mean concentrations of the aerosol populations were higher than their medians. Differences in the aerosol backgrounds between the sites need to be considered when evaluating the aerosol's regional climate effects.