Effects of air masses and synoptic weather on aerosol formation in the continental boundary layer

Nucleation of near nm sized aerosol particles and subsequent growth to ∼100 nm in 1–2 days has in recent years been frequently observed in the continental boundary layer at several European locations. In 1998–99, this was the focus of the BIOFOR experiment in Hyytiälä in the boreal Finnish forest. Nucleation occurred in arctic and to some extent in polar air masses, with a preference for maritime air in transition to continental air masses, and never in sub‐tropical air. The air masses originated north of the BIOFOR experiment by paths from the southwest to northeast sector. The nucleation was also associated with cold air advection behind cold fronts, never warm air advection. This may relate to low pre‐existing aerosol concentration, low cloudiness and large diurnal amplitudes in the continental boundary layer associated with cold air advection and clear skies. Arctic and polar air together with cold air advection did not always lead to nucleation. The most important limiting meteorological factors were cold front passages and high cloudiness, probably through reduced photochemistry and wet scavenging of precursor gases and new aerosol particles. The preference for nucleation to occur in arctic air masses, which seldom form in the summer, suggests a meteorological explanation for the annual cycle of nucleation, which has a minimum in summer. The connection to cold‐air outbreaks suggests that the maximum in nucleation events during spring and autumn may be explained by the larger latitudinal temperature gradients and higher cyclone activity at that time of the year. Nucleation was observed on the same days over large parts (1000‐km distance) of the same air mass. This suggests that the aerosol nucleation spans from the microphysical scale to the synoptic scale, perhaps connected through boundary layer and mesoscale processes.     

Variability of Near-Ground Ozone Concentrations During Cold Front Passages – a Possible Effect of Tropopause Folding Events

The analysis of surface ozone variability requires besides chemicalstudies the consideration of meteorological conditions and dynamicprocesses. Our research focuses on the mechanisms in connection with coldfront passages. A statistical study and case studies of cold front passageswere carried out at six German ground-based sites during the year 1990.After the passage of cold fronts three typical developments of thenear-ground ozone concentrations could be identified. Usually the ozoneconcentrations decrease due to advection of clean air masses or due toenhanced cloudiness preventing the photochemical production of ozone,chemical destruction by nitrogenoxides, and heterogeneous chemistry. In somecases the concentrations increased by reason of downward mixing of ozoneenriched air intruded from the stratosphere into the troposphere bytropopause foldings. For a few cases no modification set in. The decreasewas mostly twice as strong as the increase. The latter was between 4 and 8ppb on the average. Special emphasis is given to the transport ofstratospheric ozone down to the ground. There is no direct evidence forstratospheric ozone at ground level, because it cant be distinguished fromthe tropospheric one, but from case studies circumstantial evidence is foundin favour of it. As an example of increasing ozone behind the passage ofcold fronts one case study typical of all other case studies is presented.It shows the characteristic properties of the corresponding fronts, whichare fast movement, a vertical split structure and strong convection.     

Nowcasting precipitation, clouds, and surface state in winter

Freezing of rainwater on a surface is often caused by nocturnal clearing after precipitation, which goes along with radiative cooling. Forecasting cloudiness, and especially the risk of clearing, on a scale of 1 to 3 h is of interest in the domains of road weather, agriculture, or water management.Cloud amount and the difference between air and surface temperature are important parameters of the radiation balance. In this contribution, we show the relationship between them, proved at several stations all over Switzerland. We found a correlation coefficient of 0.88 and improved it considering other meteorological parameters like wind speed. We conclude that temperature difference is a signature for nocturnal cloudiness.We investigated nocturnal cloudiness for a case from winter 2001/02 in northern Switzerland. An ultra-dense combination of two networks with 70 stations in total is operated, measuring air and surface temperature, wind and other parameters. With the aid of our equations, these measurements where converted into cloud maps, including also precipitation seen by radar. We identified a frontal precipitation area, postfrontal clearing, freezing, and the first clouds of a following frontal passage.All these findings will contribute to a better observation and predictability of surface state and other risks connected with radiative cooling.     

Climate variability of cold surge and its impact on the air quality of Taiwan

Climate change has been receiving wide attention in the last few decades. In order to quantify the climate variability of extreme weather events and their possible impacts on weather parameters and air quality, cold surge events in the past 45 years and the difference in characteristics of air pollutants before and after frontal passage has been examined after December 1993 in Taiwan. The potential impact of climate change on air pollutant concentration and its health implication were presented and discussed. In the past 45 years, the cold surge days (about 18.7 days, or 0.42 day/year) decreased significantly and the average lowest daily temperature for winter in northern Taiwan increased nearly 3°C (0.067°C/year). Based on the definition of cold surge in Taiwan and excluding the stagnation frontal passage, 21 cold surge frontal passage (CSFP) cases and 89 common frontal passage (CFP) events in winter (December–February) were identified in the past 12 years (1993–2005). We take the frontal passage day as the baseline and the differences in air pollutant concentrations and weather-related parameters between the two days before and after the frontal passage days were examined for each case. The averages of the above-mentioned differences during CSFP were compared to the corresponding differences during CFP. During CSFP, the air temperatures after the frontal passage were nearly 4–6°C lower than before the passage at both the background windward stations and urban stations. The average wind speed was about 4–5 m/s higher at the windward stations and less than 2 m/s higher in the major urban areas in Taiwan. During CFP, there was a 2°C increase in temperature but 1 m/s decrease in wind speeds on the day after frontal passage. Because of these meteorological differences, the concentration change of air pollutants during CSFP is significantly greater than that during CFP, especially for PM₁₀ concentration. The difference of PM₁₀ concentration during CSFP can be as large as 20–40 μg/m³ while that during CFP is only about 10 μg/m³. The differences in the other air pollutants such as CO, SO₂, and O₃ during CSFP are greater than those during CFP, but the difference is insignificant. Under the warming trend, less frequent CSFP’s are expected; the impacts on deterioration of air quality and human health are noteworthy.     

Synoptic Typing and Its Application for Evaluating Climatic Impact on Air Quality in Urumqi

An automated procedure employing principal-component analysis and a two-stage cluster analysis was developed to classify the synoptic meteorological conditions prevailing over Urumqi, one of the most heavily polluted cities in the world. Six clusters representing different circulation patterns and air-mass characteristics were classified using surface- and upper-meteorological variables during the heating period from 2001 to 2008, and the relationships between synoptic clusters and air quality were evaluated. The heaviest air-pollution episodes occurred when Urumqi was in either an extremely cold, strong anticyclone or at the front of a migrating cyclone, both with light winds, wet surface air, and relatively dry upper air. Moderate pollution was seen when Urumqi was in the pre-cold/cold frontal passages with lower temperatures and light winds or moderate anticyclone with relatively warmer, drier air. When Urumqi was at the front of a migrating anticyclone or in a weak anticyclone with moderate winds and most warm, dry air, or in the cold/post-cold frontal passages with relatively strongly northerly airflows and precipitation, relatively good air quality could be seen. These results suggest that air pollution in Urumqi is very closely related to the synoptic meteorological conditions, which provides an important basis for not only the prediction and control of urban air-quality problems here but also for the analysis of the differential impacts of weather and pollution on human morbidity.     

The Summer Arctic Boundary Layer during the Arctic Ocean Experiment 2001 (AOE-2001)

Boundary-layer measurements made from the Swedish icebreaker Oden during the Arctic Ocean Experiment 2001 (AOE-2001) are analysed. They refer mainly to ice drift in the central Arctic during the period 2–21 August 2001. On board Oden a remote sensing array with a wind profiler, cloud radar and a scanning microwave radiometer, and a regular weather station operated continuously; soundings were also released during research stations. Turbulence and profile measurements on an 18-m mast were deployed on the ice, along with two sodar systems, a microbarograph array and a tethered sounding system. Surface flux and meteorological stations were also deployed on nearby ice floes. There is a clear diurnal cycle in radiation and also in wind speed, cloud base and visibility. It is absent in temperature and humidity, probably due to the very strong control by melting/ freezing ice and snow. In the advection of warm air, latent heat of melting maintains the surface temperature at 0 °C, while with a negative energy balance the latent heat of freezing of the salty ocean water acts to maintain the surface temperature > −2 °C. The constant presence of water at the surface maintains a relative humidity close to 100%, and this is also often facilitated by an increasing specific humidity through the capping inversion, making entrainment a moisture source. This ensures cloudy conditions, with low cloud and fog prevailing most of the time. Intrusions of warm and moist air from beyond the ice edge are frequent, but the local Arctic boundary layer remains at a relatively constant temperature, and is shallow and well mixed with strong capping inversions. Power spectra of surface-layer wind speed sometimes show large variance at low frequency. A scanning radiometer provides a monitoring of the vertical thermal structure with a spatial and temporal resolution not seen before in the Arctic. There are often two inversions, an elevated main inversion and a weak surface inversion, and occasionally additional inversions occur. Enhanced entrainment across the main inversion appears to occur during frontal passages. Variance of the scanning radiometer temperatures occurs in large pulses rather than varying smoothly, and the height to the maximum variance appears to be a reasonable proxy for the boundary-layer depth.     

气象条件对梅州城区臭氧浓度的影响研究

利用2014—2017年梅州城区的O3监测数据和相关气象观测资料,对本地区的O3的变化特征及影响O3的气象条件进行了相关分析。分析表明:梅州地区2014—2017年O3超标日数维持稳定水平,O3二级超标日数全部集中在春季和夏季; O3二级超标天数占AQI超标日数的比例快速上升,O3二级超标主要发生在3—8月,最多的是4月,O3是影响梅州市区空气质量重要因素。O3的月平均浓度与同期气温、日照显著正相关,与同期的气压呈显著负相关; O3污染过程中,O3的逐日浓度与同期气温、日照显著正相关,与同期的总云量、低云量、湿度呈显著负相关; O3污染过程中,O3浓度与地面及高空天气形势配置关系密切,O3浓度变化是多种气象要素共同作用的结果。     

Predictability of the Meteorological Conditions Favourable to Radiative Fog Formation During the 2011 ParisFog Campaign

Radiative fog formation is a complex phenomenon involving local physical and microphysical processes that take place when particular meteorological conditions occur. This study aims at quantifying the ability of a regional numerical weather model to analyze and forecast the conditions favourable to radiative fog formation at an instrumental site in the Paris area. Data from the ParisFog campaign have been used in order to quantify the meteorological conditions favorable to radiative fog formation (pre-fog conditions) by setting threshold values on the key meteorological variables driving this process: 2-m temperature tendency, 10-m wind speed, 2-m relative humidity and net infrared flux. Data from the ParisFog observation periods of November 2011 indicate that use of these thresholds leads to the detection of 87 % of cases in which radiative fog formation was observed. In order to evaluate the ability of a regional weather model to reproduce adequately these conditions, the same thresholds are applied to meteorological model fields in both analysis and forecast mode. It is shown that, with this simple methodology, the model detects 74 % of the meteorological conditions finally leading to observed radiative fog, and 48 % 2 days in advance. Finally, sensitivity tests are conducted in order to evaluate the impact of using larger time or space windows on the forecasting skills.     

云南辐射雾的气候分布特征及天气成因

应用1976～2005年云南126个气象站观测资料及2001～2005年的高空探测资料,统计分析了云南辐射雾的时空分布气候特征、形成雾的天气环流背景及要素成因.结果发现:云南雾H数逐年呈现波状变化,总体趋势上呈下降趋势,但雾的日变化规律较为稳定;形成雾的天气形势有偏西气流型和偏北气流型,其中偏北气流型成雾重、持续时间长.成雾物理机制分析表明,近地层良好的水汽条件、静风或弱的风力条件、晴空少云、地面变性冷高压后部的均压场环境、低层大气弱的上升运动、中高层大气较强的下沉运动、低层暖平流、中高层冷平流以及边界层存在逆温层等特征是形成雾的重要条件.     

河源市空气质量首要污染物特征及其与气象条件的关系

基于2017-2019年河源市空气质量数据,分析了河源市首要污染物的年际变化特征,同时利用2019年东埔国控站点的首要污染物与气象要素进行了相关性分析,并以典型污染日为案例,分析了气象条件对污染过程的影响。结果表明：2017-2019年细颗粒物（PM_2.5）污染日比重大幅度降低,以臭氧（O₃）为首要污染物的污染日逐年增加,污染形式逐渐从颗粒物污染向臭氧污染发生转变。O₃浓度与温度和湿度分别呈正负相关关系,高浓度O₃主要出现在（20-30℃,25%-55%）阈值之间,在吹西北偏北风时O₃浓度也较高。PM_2.5和PM₁₀与湿度也呈负相关关系,温度与湿度组合在（8-13℃,40%-55%）范围内时两者容易同时出现高值;在夏季PM_2.5和PM₁₀还与温度具有较强的正相关关系,这意味着高温情况下河源有出现颗粒物与O₃复合污染的可能。河源市典型污染日具有风速较小局部扩散不利的特征,低温低湿条件下容易出现PM_2.5污染,且主要受到区域的传输影响;而高温低湿条件下容易发生O₃污染,且较高的前体物浓度容易加剧O₃的本地污染。     

癌症死亡的气象影响因素探讨

采用广西南宁1991～2003年13年癌症死亡数据及同期气象资料,分析癌症死亡与气象条件的关系。分析表明气象条件对癌症死亡确实有一定的影响：紫外线辐射强度偏强时癌症死亡人数明显偏多;肺癌死亡率随气温升高而增大,而肝癌死亡率随气温升高而降低,肺癌受气温的影响比肝癌大;各种癌症死亡人数均与气压呈反相关。部分分析结论与国外在本领域研究的结果一致。     

城市空气质量与气象条件的关系及空气质量预报系统

利用2002年6月1日至2004年5月31日沧州市环境监测站的逐日空气质量报告数据,对沧州市空气质量及其污染物进行了统计分析,发现沧州市空气质量优良的天数居多,两年中有236天污染天气。影响沧州市空气质量的首要污染物为PM10,其次为SO2,空气质量受季节影响较为严重,冬季取暖期污染最为严重,属于“煤烟型”污染,并与天气现象、气压场、风场、总云量等气象因子密切相关。对比沧州市每日运行的空气质量预报系统,CAPPS 1.0模式比统计预报模式预报更为准确。     

Modeling the evolution of the family of Mediterranean cyclones using the regional model of the atmosphere

Due to the complex orography and the presence of the moisture-saturated air, the Mediterranean region is characterized by the increased baroclinic and convective instability, that leads to the sudden cyclogenesis and the formation of dangerous weather phenomena. The results are given of the investigation of formation mechanisms of Mediterranean cyclones, peculiarities of stages of their evolution and dynamical processes, which occur throughout the atmosphere, using the regional numerical ETA model of the atmosphere by the example of individual cases of the cyclogenesis over the Mediterranean Sea. It is revealed that the cold Arctic air outbreak (the intrusion of the cold Arctic air) to the south of the Western Europe, leading to the formation of the areas of the baroclinic instability and the increased moisture content of the air in the area of the vortex origin, favors the cyclogenesis. The use of the vertical coordinate η in the model enabled to compute more precisely the vertical wind speed, therefore, the influence of the orography on the moisture content and precipitation increase became pronounced. The transformation of the structure of meteorological fields in the course of the development of vortexes is considered. The computation of the helicity is made, and it is shown that this characteristic can be one of the earliest predictors of cyclogenesis.     

Small-particle concentration fluctuations at a coastal site

Aerosol size spectra (d=10 nm–10 μm) were measured with an electrical aerosol spectrometer (EAS) at Mace Head on the west coast of Ireland. Several small aerosol particle (diameter 10–32 nm) concentration bursts were observed during the measurement period. Relationships between the events, air mass trajectories, tide height, and meteorological parameters are examined. Series of bursts were observed when a spectral transformation due to subsequent particle growth from 10 to 56–100 nm can be identified in an Eulerian experiment. Particle growth rates of between 1 and 3 nm/h were determined. These bursts appear in cold and comparatively clean arctic or polar air masses with temperature and relative humidity fluctuations, and do not correlate with low tide in some cases. These episodes, similar to those frequently found in the continental boundary layer, are thought to occur over a wide area and, for clear detection, require stable airflow for a few days. Elevated small-particle concentration events are more common during low tide or shortly after, and are typically associated with low wind speeds. Here, the increased shore exposure during low tide is thought to influence the nucleation and the subsequent growth of these aerosol particles. The occurrences of the bursts are found to depend on local wind direction. The highest d=10–32 nm particle concentrations appeared for wind sectors furthest from the tidal regions when the wind direction was 150–160°(south-easterly). Most of the events occurred during daytime when solar irradiation is most intense.     

黑龙江省沙尘天气发生的气候背景和预报方法

对2000～2003年黑龙江省较典型的7次沙尘天气过程进行气候条件和地面天气形势分析,并对沙尘天气影响下哈尔滨地区的空气污染状况特别是颗粒物污染进行了分析。结果表明：持续少雨、高温、春季大风是发生沙尘天气的气候条件;沙尘天气发生时,中等以上污染的地面天气形势有高低压同时发展、强低压发展、春季其它类型鞍形场均值区和弱低压内3种类型;在沙尘天气影响下,哈尔滨地区的颗粒污染物浓度迅速上升,形成中、重度污染。建立了未来24h发生沙尘天气的高空及地面预报模型。     

Climatology of winter transition days for the contiguous USA, 1951�C2007

In middle and high latitudes, climate change could impact the frequency and characteristics of frontal passages. Although transitions between air masses are significant features of the general circulation that influence human activities and other surface processes, they are much more difficult to objectively identify than single variables like temperature or even extreme events like fires, droughts, and floods. The recently developed Spatial Synoptic Classification (SSC) provides a fairly objective means of identifying frontal passages. In this research, we determine the specific meteorological patterns represented by the SSC??s Transition category, a ??catch-all?? group that attempts to identify those days that cannot be characterized as a single, homogeneous air mass type. The result is a detailed transition climatology for the continental USA. We identify four subtypes of the Transition category based on intra-day sea level pressure change and dew point temperature change. Across the contiguous USA, most transition days are identified as cold fronts and warm fronts during the winter season. Among the two less common subtypes, transition days in which the dew point temperature and pressure both rise are more frequently observed across the western states, and days in which both variables fall are more frequently observed in coastal regions. The relative frequencies of wintertime warm and cold fronts have changed over the period 1951?C2007. Relative cold front frequency has significantly increased in the Northeast and Midwest regions, and warm front frequencies have declined in the Midwest, Rocky Mountain, and Pacific Northwest regions. The overall shift toward cold fronts and away from warm fronts across the northern USA arises from a combination of an enhanced ridge over western North America and a northward shift of storm tracks throughout the mid-latitudes. These results are consistent with projections of climate change associated with elevated greenhouse gas concentrations.     

1953—2008年厦门地区的灰霾天气特征

利用厦门气象局气象观测站1953—2008年地面气象观测资料和厦门市环保局2001—2005年PM10、SO2、NOX环境监测资料,分析了厦门地区霾天气的气候特征、气象要素特征,及其影响因素,探讨了大气污染物与霾天气的关系。结果表明,在过去的50多a中,年日照时数及能见度有明显下降的趋势;厦门市灰霾日数总体呈上升趋势,尤其近十年来更加明显;厦门夏秋季节霾日少于冬春季节;2000年以来,随着霾日的增加,霾的持续日数也迅速增加;风速增大不利于霾的形成,高相对湿度有利于霾的形成;近年来大气污染日益严重,污染物（SO2、NOX、PM10）浓度增大导致霾日数增加,能见度下降。     

Observation of regional new particle formation in the urban atmosphere

Long-term measurements of fine particle number-size distributions were carried out over 9.5 yr (May 1997–December 2006), in the urban background atmosphere of Helsinki. The total number of days was 3528 with about 91.9% valid data. A new particle formation event (NPF) is defined if a distinct nucleation mode of aerosol particles is observed below 25 nm for several hours, and it shows a growth pattern. We observed 185 NPF events, 111 d were clear non-events and most of the days (around 83.5%) were undefined. The observed events were regional because they were observed at Hyytiälä (250 km north of Helsinki). The events occurred most frequently during spring and autumn. The observed formation rate was maximum during the spring and summer (monthly median 2.87 cm⁻³ s⁻¹) and the modal growth rate was maximum during late summer and Autumn (monthly median 6.55 nm h⁻¹). The events were observed around noon, and the growth pattern often continued on the following day. The observation of weak NPF events was hindered due to pre-existing particles from both local sources. It is clear that regional NPF events have a clear influence on the dynamic behaviour of aerosol particles in the urban atmosphere.     

Measurements of Carbon Monoxide and Nonmethane Hydrocarbons During POPCORN

During the field campaign POPCORN (Photo oxidant formation by plant emitted compounds and OH radicals in North-eastern Germany) in Pennewitt (Mecklenburg-Vorpommern, Germany) in August 1994, carbon monoxide and nonmethane hydrocarbons were measured over a large maize field by in-situ gas chromatography. Throughout the campaign CO and NMHC showed, even for a remote rural area, unexpectedly low mixing ratios. Except a few episodes, CO mixing ratios were around 120 ppb. Ethane was the only hydrocarbon showing mixing ratios exceeding 1 ppb. The mixing ratios of all other NMHC ranged between several hundred ppt and the lower limit of detection which was between 20 and 5 ppt depending on the compound. During three frontal passages CO and NMHC mixing ratios increased significantly, while between August 13 and 16, 1994, polar air masses were encountered with CO and NMHC mixing ratios dropping to values which are typical for North Atlantic background air. During this period average CO mixing ratios were 85 ppb and ethane as the most abundant hydrocarbon decreased to 650 ppt. The large-scale meteorological situation is reflected in an unusual frequency distribution of CO. The distribution shows three maxima which can be assigned to the periods of the frontal passages, to the observation of polar air masses and the rest of the campaign. Two-day backward trajectories were calculated in order to obtain information about the origin of the air masses transported to the site. The observed NMHC and CO data can be attributed to the origin of the air masses and the air mass trajectories. NMHC and CO mixing ratios were well correlated indicating that these compounds originated from similar mostly anthropogenic sources. An exception was isoprene which showed no correlation with CO. With values below 100 ppt the mixing ratio of isoprene, which is emitted by terrestrial vegetation, was also unexpectedly low during the first half of the campaign although the maximum temperatures were around 35°C.     

江西省霾天气气候特征及其与气象条件的关系

利用1980-2011年江西83个常规站和南昌、赣州2个探空站的观测资料,对江西霾的气候特征及其与气象条件的关系进行了分析。结果表明：近32 a来江西霾日总体呈上升趋势;霾日的季节、年际和年代际变化明显,秋冬季多（12月最多）春夏季少（7月最少）,4 a左右和8 a左右的年际变化周期显著,年代际变化主要为15 a左右的变化周期;江西霾日空间分布不均,呈现中北部多,南部及山区少的分布形势。霾与地面风速、大气逆温、海平面气压、降雨量和相对湿度密切相关,低风速、大气逆温、高气压、高湿度和少降雨有利于霾天气的发生,反之,高风速、低气压、低湿度和多降雨不利于霾天气的发生。