Air Mass Changes Research Articles

The role of snow in scavenging aerosol particles: A physical-chemical characterization

The below cloud scavenging of aerosols by snow has been analysed in León (NW Spain). Six snow events were registered over the course of one year of study. Ultrafine and accumulation aerosol particles were measured using a scanning mobility particle sizer spectrometer, while hydrometeors were characterized using a disdrometer. Furthermore, the chemical composition of the melted snow-water samples (soluble and insoluble fractions) was analysed.The scavenging coefficient (λ) showed a great variability among events. An effective washing of particles was observed during the first 30 min of snowfall. The mean change in the scavenging efficiency (%ΔC) of particle number concentration (PNC) and λ coefficient during this time interval were: i) nucleation mode: 36.3 % and 3.02 · 10−4 s−1; ii) Aitken mode: 30.4 % and 2.37 · 10−4 s−1 and iii) accumulation mode: 22.4 % and 1.77 · 10−4 s−1. The range of particle sizes that is less efficiently scavenged by snowfall was observed between 400 and 600 nm. When analyzing the whole snow event, an increase of PNC was observed. Two possible explanations underlie this behaviour: it could be caused by changes in air masses or by the resuspension of aerosol particles scavenged by snowflakes upon reaching the ground. A clear relationship was observed between Ca2+, SO42− and NO3− concentrations of aerosol particles before the snow event and the concentrations registered in the melted snow-water.The largest and smallest changes in aerosol number concentrations were caused by snowflakes of 3 and 6 mm in diameter, respectively. The particle size distributions (PSD) were fitted to log-normal distributions and the parameters were compared before and after snowfall.

When and How Can the Stratosphere Modify the Midlatitude Cold Air Outbreaks in Northern Winter: An Isentropic Meridional Mass Circulation View

AbstractCold air outbreaks (CAOs) in northern winter can be attributed to the variations in the lower‐tropospheric equatorward cold branch (CB) of the isentropic meridional mass circulation (IMMC) across the subpolar latitudes. Understanding when and how the variations of CB are related to the stratosphere is crucial for improving the extended‐range forecast skills of CAOs in winter. Investigation on the vertical coupling of the stratospheric and tropospheric branches of IMMC suggests that the CB (or CAOs) exhibits two major timescales: sub‐monthly (10–30 days) and monthly‐to‐seasonal (>30 days). The CB can be modified by the stratospheric variability that is represented by the stratospheric poleward warm branch (WB‐ST) of the IMMC at monthly‐to‐seasonal timescales via the two‐way stratosphere‐troposphere coupling. At sub‐monthly timescales, however, the upward one‐way coupling from the troposphere to the stratosphere dominates. While changes of the CB always precede those of the WB‐ST at both timescales, indicating the upward tropospheric wave forcing into the stratosphere, the downward impacts of the WB‐ST on the CB (particularly CAOs in North America) are observed only at monthly‐to‐seasonal timescales. Specifically, the accumulated effect of the monthly‐to‐seasonal mass transport by the poleward WB‐ST can be strong enough to dominate the Arctic total column airmass changes, producing not only a barotropic structure of the Northern Annular Mode but also direct changes to the low‐level ageostrophic zonal‐mean meridional flow. The anomalous low‐level ageostrophic flow, in turn, plays a greater role in intensifying (weakening) the equatorward CB following a stronger (weaker) WB‐ST than tropospheric wave activities.

The increase in intensity and frequency of surface air temperature extremes throughout the western South Atlantic coast

The climate is changing. At this stage, it is important to specify an ‘extreme’ climate and identify patterns that indicate its potential harm worldwide, including the coastal zones. Herein, we considered extremes based on the “Peaks Over Threshold” method from the “Extreme Value Theory”. We looked after geographical patterns of surface air temperature (SAT) extremes (e.g., Tmax, Tmin, daily temperature range (DTR), and inter-daily temperature range) over the last 40 years throughout the Brazilian coast. Overall, we found a trend increase in intensity and frequency, but the duration was barely affected. The latitudinal pattern of extremes and the temperatures considered extremes followed the settled perception that areas in higher latitudes will be more affected by the extent of warming. Additionally, the seasonal pattern of DTR demonstrated to be a good approach to make inferences about air mass changes, but joint analyses on extremes with other atmospheric variables are desirable. Given the potential effects of extreme climates on society and natural systems over the world, our study highlights the urge for action to mitigate the effects of the increase in SAT in coastal zones.

Seasonally and Diurnally Varying Cold Front Effects along the Minnesotan North Shore of Lake Superior

Cold fronts are typically associated with cooling, drying and a strengthening wind that shifts to have a northerly component. Cold front effects at a particular point, however, are dependent upon pre-existing air mass characteristics. Here, we examine 634 passages of synoptic-scale cold fronts in northeastern Minnesota from 2010 to 2018. While these fronts are associated with the expected effects in some areas, they are often associated with warming and enhanced drying in the region directly influenced by an air mass from Lake Superior (coastal sites). Coastal sites experience warming during more than half of cold frontal passages, in contrast to proximate inland sites out of the influence of the lake. This warming, combined with a removal of the moist lake air mass, often leads to a sharp post-front decrease in relative humidity. These relatively unusual local effects indicate a need to carefully consider characteristics of the lake air mass and likely changes during cold frontal passage when forecasting regional temperature and fire weather conditions.

Source apportionment of PM2.5 during haze episodes in Shanghai by the PMF model with PAHs

Positive matrix factorization (PMF) is a commonly used receptor model for PM2.5 source apportionment, relying on tracers to identify emission sources. Adding organic tracers such as polycyclic aromatic hydrocarbons (PAHs) to PMF model could enhance the performance of constraining emission sources. In this study, the PMF with PAHs was applied to 112 PM2.5 samples, including full-year and haze episodes during 2015–2016 in Shanghai. Compared to the original PMF, the PMF with PAHs newly identified the industry source and revised of the contributions of biomass burning and coal combustion. The PMF with PAHs were used to reconstruct the source apportionment of PM2.5 in four winter haze episodes. Taking the first two haze episodes (defined as P1and P2) as examples of frequent air mass changes, the P1 accumulation stage was successively affected by local and northwest sources. Vehicle (35.3%) and secondary particles (25.7%) were the primary contributors to PM2.5, due to local accumulation. Industry sources obtained a 25.9% increment under the effect from the northwest sources. P2 was successively controlled by the southwest and northwest sources from the clean stage to the dissipation stage. Significantly increased contributions of biomass burning and industry sources to PM2.5 were observed. Compared to other episodes, the P2 accumulation stage had the highest coal-combustion proportion (5.6%) under the influence of the northwest sources from Shaanxi, Shanxi, and Henan. Moreover, the toxicities of different emission sources were evaluated based on PAHs and BaPeq. Coal combustion, biomass burning, and vehicle sources were the three major contributors, especially coal combustion, contributing 17.1–66.8% to PAHs and 22.6–66.4% to BaPeq but less than 5.6% to PM2.5. We believe that these results will help policymakers by providing insights into the reduction in emission sources for regulation in urban environments and the mitigation of health risks.

Measurement report: Regional characteristics of seasonal and long-term variations in greenhouse gases at Nainital, India, and Comilla, Bangladesh

Abstract. Emissions of greenhouse gases (GHGs) from the Indian subcontinent have increased during the last 20 years along with rapid economic growth; however, there remains a paucity of GHG measurements for policy-relevant research. In northern India and Bangladesh, agricultural activities are considered to play an important role in GHG concentrations in the atmosphere. We performed weekly air sampling at Nainital (NTL) in northern India and Comilla (CLA) in Bangladesh from 2006 and 2012, respectively. Air samples were analyzed for dry-air gas mole fractions of CO2, CH4, CO, H2, N2O, and SF6 and carbon and oxygen isotopic ratios of CO2 (δ13C-CO2 and δ18O-CO2). Regional characteristics of these components over the Indo-Gangetic Plain are discussed compared to data from other Indian sites and Mauna Loa, Hawaii (MLO), which is representative of marine background air. We found that the CO2 mole fraction at CLA had two seasonal minima in February–March and September, corresponding to crop cultivation activities that depend on regional climatic conditions. Although NTL had only one clear minimum in September, the carbon isotopic signature suggested that photosynthetic CO2 absorption by crops cultivated in each season contributes differently to lower CO2 mole fractions at both sites. The CH4 mole fraction of NTL and CLA in August–October showed high values (i.e., sometimes over 4000 ppb at CLA), mainly due to the influence of CH4 emissions from the paddy fields. High CH4 mole fractions sustained over months at CLA were a characteristic feature on the Indo-Gangetic Plain, which were affected by both the local emission and air mass transport. The CO mole fractions at NTL were also high and showed peaks in May and October, while CLA had much higher peaks in October–March due to the influence of human activities such as emissions from biomass burning and brick production. The N2O mole fractions at NTL and CLA increased in June–August and November–February, which coincided with the application of nitrogen fertilizer and the burning of biomass such as the harvest residues and dung for domestic cooking. Based on H2 seasonal variation at both sites, it appeared that the emissions in this region were related to biomass burning in addition to production from the reaction of OH and CH4. The SF6 mole fraction was similar to that at MLO, suggesting that there were few anthropogenic SF6 emission sources in the district. The variability of the CO2 growth rate at NTL was different from the variability in the CO2 growth rate at MLO, which is more closely linked to the El Niño–Southern Oscillation (ENSO). In addition, the growth rates of the CH4 and SF6 mole fractions at NTL showed an anticorrelation with those at MLO, indicating that the frequency of southerly air masses strongly influenced these mole fractions. These findings showed that rather large regional climatic conditions considerably controlled interannual variations in GHGs, δ13C-CO2, and δ18O-CO2 through changes in precipitation and air mass.

Machine Learning-Based Front Detection in Central Europe

Extreme weather phenomena such as wind gusts, heavy precipitation, hail, thunderstorms, tornadoes, and many others usually occur when there is a change in air mass and the passing of a weather front over a certain region. The climatology of weather fronts is difficult, since they are usually drawn onto maps manually by forecasters; therefore, the data concerning them are limited and the process itself is very subjective in nature. In this article, we propose an objective method for determining the position of weather fronts based on the random forest machine learning technique, digitized fronts from the DWD database, and ERA5 meteorological reanalysis. Several aspects leading to the improvement of scores are presented, such as adding new fields or dates to the training database or using the gradients of fields.

Long-term trends and impacts of polar cold airmass in boreal summer

The northern polar region possesses the most extensive cold airmass on Northern Hemisphere. The generation of this polar cold airmass and its outflow to lower latitudes play an important role in the climate system in terms of mass and heat exchanges. However, long-term changes in the polar cold airmass, especially in summer, and the climatic effects on mid-high latitudes are still unclear. Using an isentropic approach, we quantitatively show that the polar cold airmass amount has decreased rapidly since the 1980s, with a decade lagging behind the global warming. The equatorward flux of the cold airmass has also weakened, trapping the cold airmass in its source region. These profound changes in the cold airmass coincide with a period characterized by rapid surface warming and increasingly frequent heat waves in recent three decades over the mid-high latitude continents. Owing to regional differences in the cold airmass reduction, Europe and North America have experienced a surface warming faster than the Northern Hemisphere mean. Furthermore, such a long-term trend in the polar cold airmass can be attributed to Arctic sea ice loss and internal decadal variability of sea surface temperature in high-latitude oceans. Our results highlight that the isentropic analysis of cold airmass may serve a good detection of the climate change at polar region and mid-high latitudes.

Future Reductions in Polar Cold Air Mass and Cold Air Outbreaks Revealed From Isentropic Analysis

AbstractThis study quantifies future changes in the polar cold air mass (PCAM) below a threshold potential temperature of 280 K and its horizontal fluxes, which are an indicator for the intensity of cold air outbreaks, by analyzing the outputs from five climate models from the Fifth Coupled Model Intercomparison Project. Under the business‐as‐usual scenario, the multimodel means of the wintertime total hemispheric PCAM decrease by 37% in the Northern Hemisphere and by 34% in the Southern Hemisphere by the end of the 21st century. The midlatitude equatorward PCAM fluxes are projected to decrease significantly by around 2050. This is primarily due to the reduction of the PCAM and to a lesser extent due to the weakening of the extratropical meridional circulation. Finally, intermodel differences in the total hemispheric PCAM reduction are shown to be linearly related to the climate sensitivity to the global mean surface air temperature.

Isotopic composition of oxygen in snow-and-firn thickness on the Eastern peak of Elbrus, the Caucasus

The isotopic composition of oxygen of the snow-and-firn and snow thickness on the Eastern peak of Elbrus had been studied. In 2006-2008, a number of snow samples were taken in the near-peak area of Elbrus to analyze the isotopic composition of them. The drilling was performed at the crater of the Eastern peak in 2006 (64 samples) while in 2017 the samples were taken from snow pit (25 samples). Another core to a depth of 23.8 m was extracted at the Western Plateau (118 samples) for the purpose to compare local 5 18 О values with samples from the Eastern peak. The δ 18 О values in the snow-and-firn thickness from the crater of the Eastern peak vary from -6.8 to -19.41 %o with the average value of -12.61 %o. It was revealed that snow layers with extremely low values of δ 18 О (down to -30 %) found on the eastern slope were absent on the western plateau. The loss of part of the annual isotope precipitation signal due to the winter extra light horizons could be caused by two reasons: wind drift of the freshly deposited snow, as well as the absence of part of the winter snowfalls with isotope-light precipitation at altitudes higher 5300 m. Seasonal variations of δ 18 О values, equal to 12 % and found in the snow-and-firn thickness on the Eastern peak, indicate that formation of the isotopic characteristics of snow is determined here by the equilibrium Rayleigh condensation and this is associated with the annual amplitude of the air temperature by a coefficient of 0.6 %o/°C. In the isotopic record obtained on the Western plateau of Elbrus, the relationship of values δ 18 О with the condensation temperature may be disturbed due to the frequent change of the main moisture-bearing air masses. This leads to significantly different δ 18 О values in precipitation at the same temperatures (the connection of seasonal δ 18 О values with the annual amplitude of air temperature varies from 0.3 to 1.12 % о /° С ).

The effects of stratospheric meridional circulation on surface pressure and tropospheric meridional circulation

Based on ERA-Interim and JRA-55 daily reanalysis datasets, connections among the variations in the Brewer–Dobson circulation (BDC) intensity, stratospheric air mass, surface pressure, and the tropospheric meridional circulation during the period from 1979 to 2015 are analyzed. The results show that the variations in the surface pressure, particularly at middle and high latitudes, have a close correlation with the variations in the stratospheric BDC intensity. When the upwelling at 450 K intensifies, the surface pressure increases in the high latitudes (poleward of 60°) and decreases in the adjacent mid-latitudes in both hemispheres. And these correlations are most significant during boreal winter in the Northern Hemisphere and during austral autumn in the Southern Hemisphere. It is found that the high latitude surface pressure changes follow the anomalous BDC by about 30 days. The increase in polar surface pressure associated with the stronger BDC is due to an increase in stratospheric air mass, while the decrease in mid-latitude surface pressure is related to a decrease in tropospheric air mass. These air mass changes are caused by the anomalous meridional transport of air mass by the residual mean circulation in the stratosphere and troposphere. In addition, we found a significant connection between the intensity of the BDC and the Ferrel cell, i.e., when the BDC strengthens (weakens), there is a weakened (strengthened) Ferrel cell with its position shifting equatorward (poleward).

Quantifying the Impacts of Cold Airmass on Aerosol Concentrations Over North China Using Isentropic Analysis

AbstractWinter air quality can vary markedly due to cold air activities. However, the impacts of the polar cold airmass on aerosol variations has not been clarified in a quantitative manner. Using an isentropic analysis method, we investigated changes in the cold airmass and sulfate aerosol distributions over North China during the winter of 2014/2015. The cold airmass and sulfate concentrations exhibited pronounced out‐of‐phase variations, with comparable amplitudes at subseasonal (30–60 days) and synoptic (4–6 days) scales. Subseasonal sulfate variations were closely associated with distribution of the polar cold airmass, as it shifted between zonal and meridional patterns, regulating cold airmass fluxes over North China. Typically, subseasonal surges in the cold airmass consisted of several synoptic disturbances, including one cold air outbreak event and 3–6 renewal/decay oscillations. These synoptic inflows of clean cold airmass repeatedly pushed the warm polluted airmass away from North China. Thus, spatiotemporal variations in sulfate better reflect cold airmass distributions and fluxes than northerly wind regimes. Using diagnostic equations derived herein, we estimated the contribution of various physical processes to aerosol variations during cold air anomalies. We found that the local changes in sulfate concentrations were mainly attributed to advection of warm airmasses, as well as advection and vertical displacement by horizontal convergence of the polar cold airmass. The latter two processes contributed most to sulfate decreases, leading to a relatively strong aerosol reductions during cold air outbreaks, while first two processes led to weaker aerosol reductions during cold air oscillations.

Numerical investigations on steam condensation in the presence of air on external surfaces of 3 × 3 tube bundles

Previous investigations on steam condensation in the presence of air have revealed general heat transfer characteristics in vertical plates and single tubes. However, there remains limited knowledge regarding the thermal-hydraulic phenomena and heat transfer properties of tube bundles. In order to evaluate steam condensation on the external surfaces of tube bundles, STAR-CCM + code with a diffusion boundary layer steam condensation model was employed in this work. In the assessments, 3 × 3 tube bundles with various tube pitches (from 1.5d to 5d) were investigated, and the results were compared to those of a single tube. The data analyses mainly focused on three aspects: (i) global and local condensation phenomena in tube bundles; (ii) heat transfer characteristics in tube bundles; and (iii) influences of tube pitches and global parameters (such as pressure and air mass fraction) on the condensation heat transfer. The phenomena analyses indicate that the axial field profiles are mainly determined by the development of a concentration boundary layer, while the radial field profiles are mostly affected by the tube pitches. Local heat flux analyses indicate that the condensation heat flux exhibits a significant reduction downwards along the vertical tube. In comparison, the condensation heat flux fluctuates in the tube peripheral direction. In the latter case, heat transfer enhancement and inhibition regions are identified and classified. For the tube average heat transfer, only the central tube is significantly affected by the tube pitches, and the degradation may be more than 30% at a tube pitch of 1.5d. The tube pitch effect is almost independent of the bulk air mass fraction and changes slightly with an increase in the wall sub-cooling and bulk pressure.

The instrument constant of sky radiometers (POM-02) – Part 2: Solid view angle

Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of aerosols using sky radiometers. In this study, errors related to the solid view angle (SVA) of sky radiometers, which are used by SKYNET, were investigated. The SVA is calculated using solar disk scan data, the measured radiances around the solar direction in 0.1×0.1∘ increments. These measurements include the scattered light from aerosol and air molecules, as well as the direct solar irradiance, causing errors in the SVA calculation. The influence of these errors was evaluated with simulations. From the results of these simulations if the aerosol optical depth (optical path length) is less than 0.5 (0.58) at 550 nm and the aerosol does not include large particles, such as desert dust particles, then its influence on the SVA calculation was less than 0.5 %. Problems with the software for the SVA calculation were also investigated. First, the data processing does not consider the change of airmass (solar zenith angle) during the solar disk scan measurement. In practice if a measurement is made in the period when the change in airmass is small, then the error is small. Second, before starting data processing, the minimum measured value is subtracted from the measured values, resulting in underestimation of the SVA by 1 % to 4 %. Thirdly, the values between 1.4 and 2.5∘ are not properly extrapolated, resulting in overestimation of the SVA by 0.6 % to 2.1 %. The second and third error sources partially cancel each other out, and the total error is an underestimation of 0.5 % to 1.9 % of the actual value. Furthermore, the annual trend in the SVA was examined. In both the visible and near-infrared regions (Si photodiode region) and in the shortwave-infrared region (InGaAs photodiode region), this trend cannot be seen in 4 and 8 years of data, respectively. The seasonal variation of the SVA was also examined, but no clear seasonal variation could be detected.

Thunderstorm induced changes in near-surface O3, NOx and CH4 and associated boundary layer meteorology over a tropical coastal station

The paper presents four cases of the response of near-surface O3, NOx and CH4 to thunderstorm activity at a tropical coastal site in India, occurred during pre-monsoon and post-monsoon seasons. The associated changes in boundary layer meteorology in terms of temperature, relative humidity (RH), wind and boundary layer height (BLH) are also examined. The meteorological parameters like temperature and RH respond very fast depicting changes during the initial phase of the thunderstorm activity, whereas a delayed response is observed in the wind direction (changing from the sea breeze to land breeze) and collapse of the BLH (1–1:30 h delay) at the mature phase of the activity. Associated with the thunderstorm activity, significant changes were observed in the mixing ratios of the near-surface O3, NOx and CH4. An increase in NOx (5.2–8.7 ppbv) and CH4 (36.7–134.6 ppbv) and reduction in O3 (9.9–18.8 ppbv) were recorded at the mature phase of the thunderstorm. The rise in NOx is attributed to the combined effect of thunderstorm activity and wind reversal whereas that in CH4 is attributed to the wind reversal and associated change in air mass. A post activity increase is observed in mixing ratio of O3 in all the four cases. This is attributed to the downdrafts characteristic of the dissipation phase of thunderstorm activity which brings in O3 rich upper air down.

A NONINVASIVE METHOD TO EVALUATE THE DEGREE OF UPPER AIRWAY STENOSIS

Purpose: This paper presented a new approach to noninvasively evaluate the upper airway stenosis for obstructive sleep apnea syndrome (OSAS) patients. Methods: In the proposed method, thoracic and abdominal movements were selected to calculate the respiratory movement (RM) and to indicate the change of lung volume. Due to the cumulative effect of thermal sensor, the oronasal-thermistor signal (T_flow) is applied to estimate the air mass change in lung. Based on the mathematical relationship, the “RM–T_flow curves”, drawn by RM and T_flow, together with the correlation coefficients ([Formula: see text] were used to analyze the upper airway stenosis. Results: This method was verified through portable monitoring (PM) based experiments, and numerical analysis of the polysomnography (PSG) data from 20 OSAS patients and 15 non-OSAS controls. Our results indicate the [Formula: see text] values decrease with the narrowing of the upper airway. At each sleep stage, the [Formula: see text] mean values of OSAS subjects are significantly ([Formula: see text]) smaller than those of the controls. These facts demonstrate that the [Formula: see text] value can be used to quantify the upper airway stenosis and the analysis of “RM–T_flow curves” is an efficient way to assess upper airway condition associated with the breathing phase. Conclusions: As this method can be used in spontaneous sleep and home sleep testing, we believe it will benefit the popularity of the diagnosis and evaluation of OSAS.

Evolution of extratropical cyclones during disturbed geomagnetic conditions

An analysis of temperature changes in warm and cold air masses of extratropical cyclones in both hemispheres with their movement during geomagnetic disturbances at the minimum of solar activity was performed. The location and movement of air masses of cyclones was determined by thermobaric maps at the 500 hPa level. In the conditions of a classical cyclogenesis, a warm air mass cools from day to day, while the cold air mass warms up. During geomagnetic disturbances, favorable conditions for increasing intensity and cyclone lifetime are formed, i.e., in a warm air mass, the temperature increases at heights lower than 300 hPa, while a cold air mass warms up more slowly. The distributions of the temperature of air masses of extratropical cyclones were shown to change due to the changes in geomagnetic activity.

Spectral Correction of CPV Modules Equipped with GaInP/GaInAs/Ge Solar Cells and Fresnel Lenses

Photovoltaic (PV) devices are spectrally selective, and their performance is influenced by unavoidable spectral variations. In addition, multijunction-based concentrating photovoltaic (CPV) devices show a strong spectral dependence due to the series connection of various junctions with different absorption bands, and also due to the use of concentrator optics. In this work, the accuracy of a new set of analytical equations that quantify the spectral impact caused by the changes in air mass (AM), aerosol optical depth (AOD) and precipitable water (PW) is discussed. Four different CPV devices based on lattice-matched and metamorphic triple-junction solar cells and a poly(methyl methacrylate) (PMMA) and silicon-on-glass (SoG) Fresnel lenses are considered. A long-term outdoor experimental campaign was carried out at the Centre for Advanced Studies on Energy and Environment (CEAEMA) of the University of Jaén, Spain. Results show a high accuracy in the estimations of the spectral factor (SF), with an average mean absolute percentage error (MAPE) within 0.91% and a mean relative error (MRE) within −0.32%.

Long-term quantitative field study of New Particle Formation (NPF) events as a source of Cloud Condensation Nuclei (CCN) in the urban background of Vienna

Many studies have indicated New Particle Formation (NPF) events as an important source of CCN in the atmosphere. However, most of these studies are based on indirect determination of CCN concentrations or were of rather short duration. In this study, we present long-term measurements of CCN concentrations and particle number size distributions performed in the urban background of Vienna, Austria, spanning a period of two years. NPF events occurred on 70 of the 539 measurement days. For 38 of these events, concurrent CCN concentrations (measured at 0.5% supersaturation) are available. Direct comparisons of absolute CCN concentrations before and after an event, however, are inconclusive as other influences can lead to changes in CCN concentrations. In this study, we developed criteria to take local emissions of ultrafine particles, atmospheric mixing conditions and changes in air mass into account. A new normalization procedure (CCN/PM2.5) is introduced to account for dilution and concentration effects due to changes in boundary layer height. Other possible influences, such as weather conditions and traffic emissions were accounted for by tracking wind direction and velocity, as well as black carbon (BC) mass concentrations. On 15 event days, all criteria to exclude other influences were met. On 14 days, NPF was found to be a source of CCN (at 0.5% supersaturation), with concentration increases by up to 143%.

Southerly nocturnal bores over northeastern Australia

Analyses of 1‐min data from automatic weather stations (AWSs) in the Gulf of Carpentaria region are used to study southerly nocturnal bores over northeastern Australia. The data cover the time period 1 August to 31 December in the three years 2012–2014. A total of 12 days on which a bore passed over the region of interest were found. Ten of the disturbances occurred between August and mid‐October in the presence of synoptic‐scale ridging across the continent behind a frontal trough. The other two occurred in early November in the presence of localized ridging behind an inland trough separate from a frontal trough. Six of the bores were followed some hours later by a clear air mass change. Four of the 12 cases are examined in detail wherein interpretations of the AWS data are aided by analysis fields from the European Centre for Medium‐range Weather Forecasts (ECMWF). The results indicate that the longevity of the bore is related to the time of day that the cold front interacts with the stable maritime layer to the north of it. The ECMWF analyses are able to capture the frontal troughs as well as the timing of their passage through the AWS network. In the analyses, the troughs are seen as narrow strips of elevated cyclonic vorticity at 950 mb. Vertical cross‐sections intersecting the troughs showed structural characteristics of a shallow cold front, but the horizontal resolution of the analyses is inadequate to capture the generation of bores ahead of the fronts.