Air Mass Types Research Articles

Characterization of aerosol particles over the southern and South-Eastern Baltic Sea

Abstract Studies of aerosol particle number concentration (PNC) level over open sea in the southern and south-eastern Baltic Sea regions were performed during five cruises (2005–2006, 2008–2010) by RV Oceania and at the coastal Preila station (Lithuania). A comparative analysis was performed of aerosol PNC variation at open sea versus measurements taken at the coastal site under various conditions (breaking waves, fog events, NPF), considering transport of forest fire products and advection of different air mass types to the investigation sites. It was found that the average PNC over the open sea (3000–4000 cm− 3) was generally higher than at the coastal site (2000–3000 cm− 3). Breaking waves caused by the latitudinal or longitudinal long-lasting (3–4 days) constant-direction wind under stormy conditions had the strongest effect on the aerosol PNC over the Baltic Sea. The aerosol PNC in the marine boundary layer (MBL) of the south-eastern Baltic Sea during fog did not depend on the air mass type over open sea, in contrast to the coastal site, that is, the average PNC level (3000–4000 cm− 3) in the MBL, which could be constantly affected by specific environmental factors as a source of condensation nuclei for the fog formation. Short-term increases of aerosol PNC in the MBL (5000–10,000 cm− 3) were higher than in the same periods of the time scale at the coastal site (3000–6000 cm− 3) during NPF events, and therefore the open sea environment may facilitate a more favourable condition for the NPF, that is, exhaust and biogenic emissions from ships and the marine environment.

Concentrations and stable carbon isotope compositions of oxalic acid and related SOA in Beijing before, during, and after the 2014 APEC

Abstract. To ensure good air quality for the 2014 Asia-Pacific Economic Cooperation (APEC) summit, stringent emission controls were implemented in Beijing and its surrounding regions, leading to a significant reduction in PM2.5 loadings. To investigate the impact of the emission controls on aerosol chemistry, high-volume PM2.5 samples were collected in Beijing from 8 October to 24 November 2014 and determined for secondary inorganic aerosols (SIA, i.e., SO42−, NO3−, and NH4+), dicarboxylic acids, keto-carboxylic acid, and α-dicarbonyls, as well as stable carbon isotope composition of oxalic acid (C2). Our results showed that SIA, C2, and related secondary organic aerosols in PM2.5 during APEC were 2–4 times lower than those before APEC, which is firstly ascribed to the strict emission control measures and secondly attributed to the relatively colder and drier conditions during the event that are unfavorable for secondary aerosol production.C2 in the polluted air masses, which mostly occurred before APEC, are abundant and enriched in 13C. On the contrary, C2 in the clean air masses, which mostly occurred during APEC, is much less abundant but still enriched in 13C. In the mixed type of clean and polluted air masses, which mostly occurred after APEC, C2 is lower than that before APEC but higher than that during APEC and enriched in lighter 12C. A comparison on chemical composition of fine particles and δ13C values of C2 in two events that are characterized by high loadings of PM2.5 further showed that after APEC SIA and the total detected organic compounds (TDOC) are much less abundant and fine aerosols are enriched with primary organics and relatively fresh, compared with those before APEC.

Aerosol arriving on the Caribbean island of Barbados: physical properties and origin

Abstract. The marine aerosol arriving at Barbados (Ragged Point) was characterized during two 3-week long measurement periods in November 2010 and April 2011, in the context of the measurement campaign CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArbados). Through a comparison between ground-based and airborne measurements it was shown that the former are representative of the marine boundary layer at least up to cloud base. In general, total particle number concentrations (Ntotal) ranged from as low as 100 up to 800 cm−3, while number concentrations for cloud condensation nuclei (NCCN) at a supersaturation of 0.26 % ranged from some 10 to 600 cm−3. Ntotal and NCCN depended on the air mass origin. Three distinct types of air masses were found. One type showed elevated values for both Ntotal and NCCN and could be attributed to long-range transport from Africa, by which biomass burning particles from the Sahel region and/or mineral dust particles from the Sahara were advected. The second and third type both had values for NCCN below 200 cm−3 and a clear minimum in the particle number size distribution (NSD) around 70 to 80 nm (Hoppel minimum). While for one of these two types the accumulation mode was dominating (albeit less so than for air masses advected from Africa), the Aitken mode dominated the other and contributed more than 50 % of all particles. These Aitken mode particles likely were formed by new particle formation no more than 3 days prior to the measurements. Hygroscopicity of particles in the CCN size range was determined from CCN measurements to be κ = 0.66 on average, which suggests that these particles contain mainly sulfate and do not show a strong influence from organic material, which might generally be the case for the months during which measurements were made. The average κ could be used to derive NCCN from measured number size distributions, showing that this is a valid approach to obtain NCCN. Although the total particulate mass sampled on filters was found to be dominated by Na+ and Cl−, this was found to be contributed by a small number of large particles ( > 500 nm, mostly even in the super-micron size range). Based on a three-modal fit, a sea spray mode observed in the NSDs was found to contribute 90 % to the total particulate mass but only 4 to 10 % to Ntotal and up to 15 % to NCCN. This is in accordance with finding no correlation between Ntotal and wind speed.

Aerosol optical extinction during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) 2014 summertime field campaign, Colorado, USA

Abstract. Summertime aerosol optical extinction (βext) was measured in the Colorado Front Range and Denver metropolitan area as part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) campaign during July–August 2014. An Aerodyne cavity attenuated phase shift particle light extinction monitor (CAPS-PMex) was deployed to measure βext (at average relative humidity of 20 ± 7 %) of submicron aerosols at λ = 632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret βext behavior in various categories of air masses and sources. Extinction enhancement ratios relative to CO (Δβext ∕ ΔCO) were higher in aged urban air masses compared to fresh air masses by ∼ 50 %. The resulting increase in Δβext ∕ ΔCO for highly aged air masses was accompanied by formation of secondary organic aerosols (SOAs). In addition, the impacts of aerosol composition on βext in air masses under the influence of urban, natural oil and gas operations (O&G), and agriculture and livestock operations were evaluated. Estimated non-refractory mass extinction efficiency (MEE) values for different air mass types ranged from 1.51 to 2.27 m2 g−1, with the minimum and maximum values observed in urban and agriculture-influenced air masses, respectively. The mass distribution for organic, nitrate, and sulfate aerosols presented distinct profiles in different air mass types. During 11–12 August, regional influence of a biomass burning event was observed, increasing the background βext and estimated MEE values in the Front Range.

Short-Term Forecasting of Nitrogen Dioxide (NO2) Levels Using a Hybrid Statistical and Air Mass History Modelling Approach

A novel hybrid model has been developed to support the provision of real-time air quality forecasts. Statistical techniques have been applied in parallel with air mass history modelling to provide an efficient and accurate forecasting system with the ability to identify high NO2 events, which tend to be the episodes of most significance in Ireland. Air mass history modelling and k-means clustering are used to identify air mass types that lead to high NO2 levels in Ireland. Trajectory matching techniques allow data associated with these air masses to be partitioned during model development. Non-parametric regression (NPR) has been applied to describe nonlinear variations in concentration levels with wind speed, direction and season and produce a set of linearized factors which, together with other meteorological variables, are employed as inputs to a multiple linear regression. The model uses an innovative integrated approach to combine the NPR with the air mass history modelling results. On validation, a correlation coefficient of 0.75 was obtained, and 91 % of daily maximum (hourly averaged) NO2 predictions were within a factor of two of the measured value. High pollution events were well captured, as indicated by strong agreement between measured and modelled high percentile values. The model requires only simple input data, does not require an emission inventory and utilises very low computational resources. It represents an accurate and efficient means of producing real-time air quality forecasts and, when used in combination with forecaster experience, is a useful tool for identifying periods of poor air quality 24 h in advance. The hybrid approach outlined in this paper can easily be applied to produce high-quality forecasts of both NO2 and additional pollutants at new locations/countries where historical monitoring data are available.

Characteristics of carbonaceous aerosols in large-scale Asian wintertime outflows at Cape Hedo, Okinawa, Japan

We have performed long-term measurements of organic carbon (OC) and elemental carbon (EC) in atmospheric aerosols at Cape Hedo Atmosphere and Aerosol Monitoring Station (CHAAMS). Large-scale East Asian continental outflows of aerosols have frequently been observed at CHAAMS in both winter and spring, but large-scale air pollution (LSP) events in winter have not yet been described in the literature. Examination of PM2.5 in LSP events showed that it consisted mainly of carbonaceous aerosols and sulfate. We compared OC and EC concentrations measured at CHAAMS during LSP events with those measured at the same time in East Asian cities, and with concentrations measured at CHAAMS during an Asian dust event in spring 2006. The maximum OC and EC concentrations at Cape Hedo during LSP events were comparable to those at China Atmosphere Watch Network sites in China. The average OC concentration was also similar to concentrations measured at urban sites in Taipei (Taiwan), Seoul (Korea), and Tokyo (Japan); EC, however, was several times lower at Cape Hedo than EC concentrations at these urban sites. We identified three types of air masses associated with LSP events: Type 1, a polluted air mass composed of freshly emitted pollutants; Type 2, an air mass that underwent photochemical aging during transport from China to Okinawa; and Type 3, an air mass that was almost fully aged in China before being transported to Japan. Simulations of LSP events with a 3-D chemical transport model (Chemical Weather Forecasting System) indicated that Type 1 air masses were associated with “cone type” transport in a low-pressure system, and Type 2 air masses were associated with “giant puff type” transport by a migratory high-pressure system. Generally speaking, air pollutants are transported from China to CHAAMS in winter in association with the north or northwest monsoon. High levels of air pollutants in China in winter are due to higher emissions and poorer dispersion because of the low boundary layer height and low wind speeds. Our results suggest that LSP events occur when a cold front forms under accumulated air pollutants. The mass contribution of carbonaceous materials to PM2.5 during the LSP event in winter 2008 was 36%, much larger than the contribution of 19% during the spring 2006 dust event.

Air temperature in the lower troposphere over moscow during heat wave in the summer of 2010

The dynamics of the air temperature in the lower 4-km layer have been studied during the heat wave in the summer of 2010 by the radio sounding data from Dolgoprudny and ground-based meteorological measurements at Moscow State University. The results have been compared with the aeroclimatic data for the previous 19 years. According to these data, the long-term average estimates of vertical thermal gradients and thickness and intensity of nocturnal surface inversions (200–300 m and 2.0°С, respectively) are presented for summer months in the Moscow region. A record high air temperature for the period from at least 1991 has been shown in the air layer up to 2 km above Moscow in the summer of 2010. Among others, the 30°С value has been detected for the first time in the air layer from 400 to 800 m. During the heat wave in 2010, as a result of clearly pronounced anticyclone conditions, the thickness and intensity of nocturnal surface inversions were higher than usual (up to 700 m and 12°С, respectively). The mean temperature profiles have been studied for different types of air masses. It is shown that the tropical air mass predominated over the Moscow region in the summer of 2010 during more than half of the period.

Study of the Tropospheric Aerosol Structure Under Changing of the Air Mass Type from Lidar Observations in Tomsk

The aerosol optical characteristics in the main tropospheric layers are investigated based on joint interpretation of data of multi-frequency lidar sensing (110 sessions) and results of modeling of back air mass trajectories. Methodical problems for separating layers with different scattering properties and estimating their vertical boundaries are considered. Three optical criteria are simultaneously used to distinguish aerosol layers from cloud formations, including the gradient of the backscattering coefficient, optical depth, and the depolarization ratio. High values of the lidar ratio (66 sr) and of the Angstrom exponent (1.62) in the shortwavelength spectral range are observed in the boundary layer for Arctic transport. At the same time, low values of these optical parameters are characteristic for Asian transport: the lidar ratio is 54 sr and the Angstrom exponent is 1.1, which is explained by different relative contributions of the coarse and fine aerosol fractions to the air mass.

Trends and sources vs air mass origins in a major city in South-western Europe: Implications for air quality management

This study presents a 17-years air quality database comprised of different parameters corresponding to the largest city in the south of Spain (Seville) where atmospheric pollution is frequently attributed to traffic emissions and is directly affected by Saharan dust outbreaks. We identify the PM10 contributions from both natural and anthropogenic sources in this area associated to different air mass origins. Hourly, daily and seasonal variation of PM10 and gaseous pollutant concentrations (CO, NO2 and SO2), all of them showing negative trends during the study period, point to the traffic as one of the main sources of air pollution in Seville. Mineral dust, secondary inorganic compounds (SIC) and trace elements showed higher concentrations under North African (NAF) air mass origins than under Atlantic. We observe a decreasing trend in all chemical components of PM10 under both types of air masses, NAF and Atlantic. Principal component analysis using more frequent air masses in the area allows the identification of five PM10 sources: crustal, regional, marine, traffic and industrial. Natural sources play a more relevant role during NAF events (20.6μg·m−3) than in Atlantic episodes (13.8μg·m−3). The contribution of the anthropogenic sources under NAF doubles the one under Atlantic conditions (33.6μg·m−3 and 15.8μg·m−3, respectively). During Saharan dust outbreaks the frequent accumulation of local anthropogenic pollutants in the lower atmosphere results in poor air quality and an increased risk of mortality. The results are relevant when analysing the impact of anthropogenic emissions on the exposed population in large cities. The increase in potentially toxic elements during Saharan dust outbreaks should also be taken into account when discounting the number of exceedances attributable to non-anthropogenic or natural origins.

Variations of surface ozone at Ieodo Ocean Research Station in the East China Sea and the influence of Asian outflows

Abstract. Ieodo Ocean Research Station (IORS), a research tower (~ 40 m a.s.l.) for atmospheric and oceanographic observations, is located in the East China Sea (32.07° N, 125.10° E). The IORS is almost equidistant from South Korea, China, and Japan and, therefore, it is an ideal place to observe Asian outflows without local emission effects. The seasonal variation of ozone was distinct, with a minimum in August (37 ppbv) and two peaks in April and October (62 ppbv), and was largely affected by the seasonal wind pattern over east Asia. At IORS, six types of air masses were distinguished with different levels of O3 concentrations by the cluster analysis of backward trajectories. Marine air masses from the Pacific Ocean represent a relatively clean background air with a lowest ozone level of 32 ppbv, which was most frequently observed in summer (July–August). In spring (March–April) and winter (December–February), the influence of Chinese outflows was dominant with higher ozone concentrations of 62 and 49 ppbv, respectively. This study confirms that the influence of Chinese outflows was the main factor determining O3 levels at IORS and its extent was dependent on meteorological state, particularly at a long-term scale.

Extreme weather and air pollution effects on cardiovascular and respiratory hospital admissions in Cyprus

In many regions of the world, climatic change is associated with increased extreme temperatures, which can have severe effects on mortality and morbidity. In this study, we examine the effect of extreme weather on hospital admissions in Cyprus, for inland and coastal areas, through the use of synoptic weather classifications (air mass types). In addition, the effect of particulate air pollution (PM10) on morbidity is examined. Our results show that two air mass types, namely (a) warm, rainy days with increased levels of water vapour in the atmosphere and (b) cold, cloudy days with increased levels of precipitation, were associated with increased morbidity in the form of hospital admissions. This was true both for cardiovascular and respiratory conditions, for all age groups, but particularly for the elderly, aged over 65. Particulate air pollution was also associated with increased morbidity in Cyprus, where the effect was more pronounced for cardiovascular diseases.

Airborne measurements of new particle formation in the free troposphere above the Mediterranean Sea during the HYMEX campaign

Abstract. While atmospheric new particle formation (NPF) has been observed in various environments and was found to contribute significantly to the total aerosol particle concentration, the production of new particles over open seas is poorly documented in the literature. Nucleation events were detected and analysed over the Mediterranean Sea using two condensation particle counters and a scanning mobility particle sizer on board the ATR-42 research aircraft during flights conducted between 11 September and 4 November 2012 in the framework of the HYMEX (HYdrological cycle in Mediterranean EXperiment) project. The main purpose of the present work was to characterize the spatial extent of the NPF process, both horizontally and vertically. Our findings show that nucleation is occurring over large areas above the Mediterranean Sea in all air mass types. Maximum concentrations of particles in the size range 5–10 nm (N5–10) do not systematically coincide with lower fetches (time spent by the air mass over the sea before sampling), and significant N5–10 values are found for fetches between 0 and 60 h depending on the air mass type. These observations suggest that nucleation events could be more influenced by local precursors originating from emission processes occurring above the sea, rather than linked to synoptic history. Vertical soundings were performed, giving the opportunity to examine profiles of the N5–10 concentration and to analyse the vertical extent of NPF. Our observations demonstrate that the process could be favoured above 1000 m, i.e. frequently in the free troposphere, and more especially between 2000 and 3000 m, where the NPF frequency is close to 50 %. This vertical distribution of NPF might be favoured by the gradients of several atmospheric parameters, together with the mixing of two air parcels, which could also explain the occurrence of the process at preferential altitudes. In addition, increased condensation sinks collocated with high concentrations of small particles suggest the occurrence of NPF events promoted by inputs from the boundary layer, most probably associated with convective clouds and their outflow. After their formation, particles slowly grow at higher altitudes to diameters of at least 30 nm while not being greatly depleted or affected by coagulation. Our analysis of the particle size distributions suggests that particle growth could decrease with increasing altitude.

A Study of Convective Band with Heavy Rainfall Occurred in Honam Region

On the study of the characteristics and life cycle of mesoscale convective band in type of airmass that occurred in the Honam area from June to September for only 4 years in the period of 2009~2012, 10 examples based on the amount of rainfall with AWS 24 hours/60 minutes rainfalls, Mt. Osung radar 1.5 km CAPPI/X-SECT images and KLAPS data for convective band with heavy rainfall event were selected. There were analyzed and classified by using the convective band with heavy rainfall occurred along the convergence line of sea wind in the form of individual multi-cellular cell and moving direction of convective band appeared in a variety of patterns; toward southwestern (2 cases), northeastern (4 cases), congesting (2 cases), and changing its moving direction (2 cases). The case study dated of the 17th Aug. 2012 was chosen and implemented by sequentially different evolution of its shape along the convergence line of sea wind cell and moving direction of convective band as equivalent potential temperatures at the lower layer have increased to the upper layer 500 hPa, that the individual cells were developed vertically and horizontally through their merger, but owing to divergence caused by weakened rainfall and descending air current, the growth of new cell was inhibited resulting in dissipation of convective cells.

The GAW‐PFR aerosol optical depth network: The 2008–2013 time series at Cape Point Station, South Africa

AbstractA ground‐based aerosol optical depth (AOD) climatology is presented for Cape Point (CPT) station, South Africa, for the 2008–2013 period. CPT is part of the Global Atmosphere Watch–Precision Filter Radiometer network which conducts long‐term AOD measurements at remote background sites. AOD (λ = 500 nm) and Ångström exponent (368 to 862 nm; α368–862) averages for the entire period were 0.059 and 0.68, displaying only a weak seasonality. Based on an established method for air mass classification using the in situ wind direction and 222Rn concentration, the following four air mass types were used to further investigate AOD: background marine, marine, mixed, and continental. AOD was similar for all types, but α368–862 was distinctly lower (0.43) for background marine and higher (1.07) for continental air masses, illustrating the presence of coarse mode and anthropogenic aerosols, respectively. Trajectory cluster analysis of 5 day back trajectories confirmed/augmented this classification. AOD for background marine and marine air mass types were consistent with ship‐based (Maritime Aerosol Network) and island (AErosol RObotic NETwork) measurements, suggesting that CPT is a suitable site to monitor pristine conditions in the South Atlantic and Southern Oceans when 222Rn concentrations are < 100 mBq m−3.

Disaggregating the contribution of local dispersion and long-range transport to the high PM10 values measured in a Mediterranean urban environment

The EU daily PM10 limit-value of 50μgm−3 is frequently breached in the center of Athens, Greece. A total of 852 daily exceedances were recorded in the city-center during the 6-year period 2001–2006. These exceedances were more frequent in winter, followed by spring and autumn. For the needs of the study, the PM10 episodes (i.e., concentrations 30% above the median value) were grouped in two categories: (a) the City-Center episodes and (b) the Wider Metropolitan Area episodes. It was assumed that City-Center episodes occurred when elevated PM10 values were measured in the city-center exclusively, whereas Wider Metropolitan Area episodes occurred when high PM10 concentrations were also measured in the suburbs. Then back-trajectory cluster analysis was performed in an attempt to associate high PM10 levels with local dispersion or long-range transport. The City-Center episodes were associated according to the origin of air parcels with six types of air-masses (slow and fast moving from northern, northeasterly and southern directions, or stagnated around Athens) and were mainly due to traffic or other local sources, whereas the Wider Metropolitan Area episodes were mainly associated with air-masses coming from southern directions and were linked to long-range transport. On the whole, the analysis provided evidence of Sahara-dust events and sea-spray transportation.

An Analysis of Air Masses Associated with Federal Ozone Exceedances Across St. Louis, Missouri: 2001–2010

The St. Louis, Missouri metropolitan region has a history of poor air quality. Although some studies have examined the surface and atmospheric conditions associated with federal ozone exceedances across St. Louis, little research has been done on the air mass types associated with ozone exceedances in this region. The research is a first step in that direction. Days with ozone exceedances from 2001 to 2010 were classified according to the accompanying air mass type that dominated the region on each of those days. This study found that over 70 percent of ozone exceedances occurred on days with maritime tropical air masses, and over 20 percent occurred on days with dry tropical masses. Exceedances also occurred with dry moderate air masses in the region. It is hoped that recognizing air masses associated with ozone exceedances will allow us to better alert the residents of these dangerous events.

Chemical characteristics of long-range-transported fine particulate matter at Gosan, Jeju Island, in the spring and fall of 2008, 2009, 2011, and 2012

Carbonaceous species (organic carbon [OC] and elemental carbon [EC]) and inorganic ions of particulate matter less than 2.5 μm (PM2.5) were measured to investigate the chemical characteristics of long-range-transported (LTP) PM2.5 at Gosan, Jeju Island, in Korea in the spring and fall of 2008–2012 (excluding 2010). On average, the non-sea-salt (nss) sulfate (4.2 µg/m3) was the most dominant species in the spring, followed by OC (2.6 µg/m3), nitrate (2.1 µg/m3), ammonium (1.7 µg/m3), and EC (0.6 µg/m3). In the fall, the nss-sulfate (4.7 µg/m3) was also the most dominant species, followed by OC (4.0 µg/m3), ammonium (1.7 µg/m3), nitrate (1.1 µg/m3), and EC (0.7 µg/m3). Both sulfate and OC were higher in the fall than in the spring, possibly due to more common northwest air masses (i.e., coming from China and Korea polluted areas) and more frequent biomass burnings in the fall. There was no clear difference in the EC between the spring and fall. The correlation between OC and EC was not strong; thus, the OC measured at Gosan was likely transported across a long distance and was not necessarily produced in a manner similar to the EC. Distinct types of LTP events (i.e., sulfate-dominant LTP versus OC-dominant LTP) were observed. In the sulfate-dominant LTP events, air masses directly arrived at Gosan without passing over the Korean Peninsula from the industrial area of China within 48 hr. During these events, the aerosol optical depth (AOD) increased to 1.63. Ionic balance data suggest that the long-range transported aerosols are acidic. In the OC-dominant LTP event, a higher residence time of air masses in Korea was observed (the air masses departing from the mainland of China arrived at the sampling site after passing Korea within 60–80 hr).Implications: In Northeast Asia, various natural and anthropogenic sources contribute to the complex chemical components and affect local/regional air quality and climate change. Chemical characteristics of long-range-transported (LTP) PM2.5 were investigated during spring and fall of 2008, 2009, 2011, and 2012. Based on air mass types, sulfate-dominant LTP and OC-dominant LTP were observed. A long-term variation and chemical characteristics of PM2.5 along with air mass and satellite data are required to better understand long-range-transported aerosols.

Analysis and interpretation of 18 years of mercury observations since 1996 at Mace Head, Ireland

Observations of atmospheric mercury at the Mace Head Atmospheric Research Station on the Atlantic coast of Ireland made from February 1996 to December 2013 are analyzed. Using meteorological analysis and a sophisticated Lagrangian dispersion model, the hourly averaged mercury concentrations were attributed to four different air mass types: baseline, local, European polluted, and sub-tropical maritime. Monthly median Hg concentrations of all types decreased over the analyzed period but the trend for sub-tropical maritime air masses was with −0.016 ± 0.002 ng m−3 yr−1 in absolute terms significantly smaller than the trends for all other classes which varied between −0.021 and −0.023 ng m−3 yr−1. The seasonal variation for sub-tropical maritime air masses is also shallower than for all other classes. This is most likely due to shallower seasonal variation of oxidant concentrations at lower latitudes. The north-south gradient of the trend is qualitatively consistent with the GEOS-Chem model predictions based on decrease of mercury concentrations in surface waters of the North Atlantic but the trends are smaller than predicted. Tests for temporal change of the trends indicate that the decreasing trends of mercury concentrations are leveling off for all air masses with possible exception of the sub-tropical maritime air mass. Quantitative assessment of the trend changes, however, will require a longer time series of the mercury measurements at Mace Head.

Air Mass Frequency During Precipitation Events in the Northern Plains of the United States

Since 1980, numerous billion-dollar disasters have affected the Northern Plains of the United States, including nine droughts and four floods. The atmospheric environment present during precipitation events can largely be described by the presiding air mass conditions since air masses characterize a multitude of meteorological variables at one time over a large region. The goal of this research is to add knowledge to current understandings of the factors responsible for precipitation in the Northern Plains through an assessment of synoptic air mass conditions. The Spatial Synoptic Classification is used to categorize 30 years of daily surface air mass types across the region alongside precipitation from the United States Historical Climatological Network. Annual and seasonal air mass frequencies are examined for all precipitation events. Precipitation days are also examined by intensity. Results indicate that the Transitional air mass, associated with changing air mass conditions commonly related to passing fronts, is not the leading producer of rainfall in the region. All moist air mass varieties are generally more dominant during precipitation events and the Moist Moderate (MM) and Moist Polar (MP) air masses are frequently responsible for half of all rainfall in the region. MM and MP tend to be particularly prominent during the winter season. The MM and Moist Tropical air masses dominate around 65% of summer precipitation events. Interestingly, there is a tendency for precipitation while dry air masses are present to the north and west within the study region.

Hygroscopic properties and mixing state of aerosol measured at the high-altitude site Puy de Dôme (1465 m a.s.l.), France

Abstract. A Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) was used to evaluate the hygroscopic properties of aerosol particles measured at the Puy de Dôme research station in central France, periodically from September 2008 to January 2010, and almost continuously from October 2010 to December 2012. This high-altitude site is ideally situated to allow for both the upper part of the planetary boundary layer and the lower free troposphere to be sampled. The aim of the study is to investigate both the influence of year-to-year, seasonal and diurnal cycles, as well as the influence of air mass type on particle hygroscopicity and mixing state. Results show that particle hygroscopicity increases with particle size and depends both on air mass type and on season. Average growth factor values, GFs, are lowest in winter (1.21 ± 0.13, 1.23 ± 0.18 and 1.38 ± 0.25 for 25, 50 and 165 nm particles, respectively) and highest in autumn (1.27 ± 0.11, 1.32 ± 0.12 and 1.49 ± 0.15 for 25, 50 and 165 nm particles, respectively). Particles are generally more hygroscopic at night than during the day. The seasonal and diurnal variations are likely to be strongly influenced by boundary layer dynamics. Furthermore, particles originating from oceanic and continental regions tend to be more hygroscopic than those measured in African and local air masses. The high hygroscopicity of oceanic aerosol can be explained by large proportions of inorganic aerosol and sea salts. Aerosols measured at the Puy de Dôme display a high degree of external mixing, and hygroscopic growth spectra can be divided into three different hygroscopic modes: a less-hygroscopic mode (GF < 1.3), a hygroscopic mode (GF~1.3–1.7) and a more-hygroscopic mode (GF > 1.7). The majority of particles measured can be classified as being in either the less-hygroscopic mode or the hygroscopic mode, and only few of them have more-hygroscopic properties. The degree of external mixing, evaluated as the fraction of time when the aerosol is found with two or more aerosol populations with different hygroscopic properties, increases with particle size (average yearly values are 20, 28 and 45 {%} for 25, 50, and 165 nm particles, respectively). The degree of external mixing is more sensitive to season than to air mass type, and it is higher in the cold seasons than in the warm seasons. With more than two years of nearly continuous measurements, this study gathers the results from one of the longest data sets of hygroscopic growth factor measurements to date, allowing a statistically relevant hygroscopic growth parameterization to be determined as a function of both air mass type and season.