Postfrontal Air Research Articles

Synoptic-scale conditions and convection-resolving hindcast experiments of a cold-season derecho on 3 January 2014 in western Europe

Abstract. A major linear mesoscale convective system caused severe weather over northern France, Belgium, the Netherlands and northwestern Germany on 3 January 2014. The storm was classified as a cold-season derecho with widespread wind gusts exceeding 25 m s−1. While such derechos occasionally develop along cold fronts of extratropical cyclones, this system formed in a postfrontal air mass along a baroclinic surface pressure trough and was favoured by a strong large-scale air ascent induced by an intense mid-level jet. The lower-tropospheric environment was characterised by weak latent instability and strong vertical wind shear. Given the poor operational forecast of the storm, we analyse the role of initial and lateral boundary conditions to the storm's development by performing convection-resolving limited-area simulations with operational analysis and reanalysis datasets. The storm is best represented in simulations with high temporally and spatially resolved initial and lateral boundary conditions derived from ERA5, which provide the most realistic development of the essential surface pressure trough. Moreover, simulations at convection-resolving resolution enable a better representation of the observed derecho intensity. This case study is testimony to the usefulness of ensembles of convection-resolving simulations in overcoming the current shortcomings of forecasting cold-season convective storms, particularly for cases not associated with a cold front.

A Case Study in Low Aerosol Number Concentrations Over the Eastern North Atlantic: Implications for Pristine Conditions in the Remote Marine Boundary Layer

AbstractWe present a case study (20 September to 13 October 2015) of synergistic, multi‐instrument observations of aerosols, clouds, and the marine boundary layer (MBL) at the Eastern North Atlantic (ENA) Atmospheric Radiation Measurement site centered on a period of exceptionally low (20–50 cm−3) surface accumulation mode (0.1–1 μm) aerosol particle number concentrations. We divide the case study into three regimes (high, clean, and ultraclean) based on daily median number concentrations and compare finer resolution (hourly or less) observations between these regimes. The analysis focuses on the possibility of using these ultraclean events to study pristine conditions in the remote MBL, as well as examining evidence for a recently proposed conceptual model for the large‐scale depletion of cloud condensation nuclei‐sized particles in postfrontal air masses. Relative to the high and clean regimes, the ultraclean regime tends to exhibit significantly fewer particles between 0.1 and 0.4 μm in diameter and a relatively increased prevalence of larger accumulation mode particles. In addition, supermicron particles tend to dominate total scattering in the ultraclean regime, and there is little evidence for absorbing aerosol. These observations are more in‐line with a heavily scavenged but natural marine aerosol population and minimal contribution from continental sources such as anthropogenic pollution, biomass burning, or dust. The air masses with the consistently lowest accumulation mode aerosol number concentrations are largely dominated by heavily drizzling clouds with high liquid water path cores, deep decoupled boundary layers, open cellular organization, and notable surface forcing of subcloud turbulence, even at night.

Interaction of cold fronts with the Brazilian Plateau: a climatological analysis

ABSTRACTComplex orography of elevated terrain and numerous peaks can act as an important physical barrier to the atmospheric flow and can generate a range of meteorological phenomena. The objective of this study is to investigate the effect of the Brazilian Plateau on the spatial distribution of cold fronts and their climatology for the period between 1979 and 2010 using Climate Forecast System Reanalysis data. The cold fronts are detected and divided into three groups, according to their frequency and trajectory over South America: the continental fronts are referred to as the AN group; the fronts that pass over the coastal region of São Paulo are referred to as the SP group; and the fronts that pass over the two areas are referred to as the ANSP group. Composites of means and anomalies of atmospheric variables are constructed for each group. The investigation focuses on the orographic effects of the Brazilian Plateau on the climatology of the cold fronts. The Brazilian Plateau affects the spatial distribution of maximum and minimum frontal frequency. The number of cold fronts over the elevated regions of the Brazilian Plateau is significantly lower than in the surrounding regions. The maximum frontal frequency is found to the east of the Andes and a secondary maximum is found along the Southeast coast of Brazil. The main effect of the Brazilian Plateau on the mean and anomaly fields of meteorological variables is to block the dry and cool post‐frontal air at lower atmospheric levels. The most significant anomalies are found in the ANSP group of cold fronts, which are more intense and move more rapidly eastward. On the other hand, the AN cold front group is the most frequent, but the weakest and easily affected by the Brazilian Plateau orography.

Concentration of bacterial aerosols in response to synoptic weather and land‐sea breeze at a seaside site downwind of the Asian continent

AbstractAirborne bacteria are a major group of bioaerosols, but their temporal and spatial variations in the atmosphere are poorly understood due to a lack of accurate information at an appropriate temporal resolution. In this study, we quantified the concentration and viability of airborne bacteria at a seaside site on the southwestern coast of Japan in spring to investigate their association with synoptic weather, which was governed alternately by cyclones and anticyclones moving in the middle‐latitude westerly flow. The concentration of bacteria, on the order of 105–106 cells m−3, showed a small amplitude of variation, while the viability significantly differed in the range of 23–96% under different weather conditions. Cyclones transported bacteria from the Asian continent in their postfrontal air. Compared to bacteria under other weather conditions, the bacteria in postfrontal air had a large concentration, a low viability, and a close correlation with coarse aerosol particles. In contrast, bacterial concentration and viability in prefrontal air varied over small ranges, and no correlation between bacteria and aerosol particle number concentration was observed. During anticyclones, although bacterial concentration was similar to that in prefrontal air, bacterial viability was high, and the concentration had pulse increases of approximately 2 times the usual level when the wind changed to land/sea breezes. The increases were attributed to the accumulation of bacteria in stagnant air due to local sources. These results show the following two distinct patterns of bacterial concentration and viability at the seaside location of this study: (1) during postfrontal long‐distance dust plume transport related to a cyclone approaching from the Asian continent, the bacterial concentration is correlated with the coarse aerosol particle number concentration and the bacterial viability is low, and (2) pulse increases of viable bacteria are observed at anticyclone conditions corresponding to local sources and land‐sea breeze conditions.

Transport of bacterial cells toward the Pacific in Northern Hemisphere westerly winds

Viable and non-viable bacteria in the air were investigated on the southwestern Japan coast to outline the manner by which bacteria were transported and to quantify the bacterial abundance and viability in the Northern Hemisphere westerly winds. The observations were conducted when the weather was governed by cyclones or anticyclones associated with Asian continent outflow between 12 October 2011 and 7 April 2013. Bacterial concentration in thermodynamically different air parcels was in the same order but different ranges: 4.5 × 105–1.3 × 106 cells m−3 in postfrontal air, 2.4 × 105–1.4 × 106 cells m−3 in prefrontal air, 5.4 × 105–9.9 × 105 cells m−3 in the air between cyclones and anticyclones and 2.9 × 105–4.1 × 105 cells m−3 in anticyclone air. In postfrontal air, the concentration correlated closely with coarse aerosol particles (diameter > 1.0 μm). In contrast, bacteria did not show a correlation with coarse particles in prefrontal air and anticyclone air. Bacterial viability was approximately 70% on average of all samples. However, the viability in postfrontal air was smaller than 60% if cases of stationary fronts with stagnant air were excluded. These results indicate that air parcels following fast-moving cold fronts in the westerly wind flow constantly and efficiently conveyed airborne bacteria from the Asian continent toward northwestern Pacific and the bacteria were characterized by coarse particle-correlated high abundance and low viability. The bacteria in slowly moving anticyclone and prefrontal air, characterized by low abundance and high viability, were more likely a mixture of long-range transported ones and regionally or locally originated ones.

Structure of a Narrow Cold Front in the Boundary Layer: Observations versus Model Simulation

Abstract A narrow and shallow cold front that passed over Finland during the night 30–31 October 2007 is analyzed using model output and observations primarily from the Helsinki Testbed. The aim is to describe the structure of the front, especially within the planetary boundary layer, identify how this structure evolved, and determine the ability of a numerical model to correctly predict this structure. The front was shallow with a small (2.5–3 K) temperature decrease associated with it, which is attributed to the synoptic evolution of the cold front from a frontal wave on a mature, trailing cold front in a region of weak upper-level forcing and where the midtroposphere was strongly stratified. Within the boundary layer, the frontal surface was vertical and the frontal zone was narrow (<8 km). The small cross-front scale was probably a consequence of the weak frontolytical turbulent mixing occurring at night, at high latitudes, combined with strong, localized frontogenetic forcing driven by convergence. The model simulated the mesoscale evolution of the front well, but overestimated the width of the frontal zone. Within the boundary layer, the model adequately predicted the stratification and near-surface temperatures ahead of, and within, the frontal zone, but failed to correctly predict the thermal inversion that developed in the stably stratified postfrontal air mass. This case study highlights the complex structure of fronts both within the nocturnal boundary layer, and in a location far from regions of cyclogenesis, and hence the challenges that both forecasters and operational models face.

Soot particles at an elevated site in eastern China during the passage of a strong cyclone

Atmospheric particles larger than 0.2μm were collected at the top of Mt. Tai (36.25°N, 117.10°E, 1534m a.s.l.) in eastern China in May 2008 during the passage of a strong cyclone. The particles were analyzed with electron microscopes and characterized by morphology, equivalent diameter and elemental composition. Soot particles with coating (coated soot particles) and those without apparent coating (naked soot particles) were predominant in the diameter range smaller than 0.6μm in all samples. The number–size distribution of the relative abundance of naked soot particles in the prefrontal air was similar to that in the postfrontal air and their size modes were around 0.2–0.3μm. However, the distribution of inclusions of coated soot particles showed a mode in the range of 0.1–0.3μm. The coating degree of coated soot particles, which was defined by the ratio of the diameter of inclusion to the diameter of particle body, showed a mode around 0.5 with the range of 0.3–0.6. These results indicate that the status of soot particles in the prefrontal and postfrontal air was similar although air pollution levels were dramatically different. In addition, the relative abundance of accumulation mode particles increased with the decrease of soot particles after the front passage.

Episodic Particulate Sulfate and Sulfur Dioxide on the Southwestern Japan Coast in March and April 2010

Particulate sulfate in <TEX>$PM_{2.5}$</TEX>, sulfur dioxide (<TEX>$SO_2$</TEX>) and size-segregated aerosol particle number concentrations were measured at a site (<TEX>$32^{\circ}19'N$</TEX>, <TEX>$129^{\circ}59'E$</TEX>) on the southwestern Japan coast from 5 March to 10 April, 2010. Results show frequent episodic increases of sulfate and <TEX>$SO_2$</TEX>. Compared to the average concentration of sulfate <TEX>$4.4{\pm}2.7\;{\mu}g\;m^{-3}$</TEX> in the whole observation period, episodic sulfate reached <TEX>$10.5-20.1\;{\mu}g\;m^{-3}$</TEX>. The variation of sulfate always synchronized with aerosol particles in the size range of <TEX>$0.1-0.5 {\mu}m$</TEX>, indicating the episodic sulfate was a consequence of the increase of the sub-micron particles. <TEX>$SO_2$</TEX> did not have remarkable increase in any episodes of sulfate increase. During the passage of low pressure systems which loaded Asian dust in postfrontal air, concentrated sulfate appeared right behind the front but before dust arrival, suggesting the dominance of dust-free particulate sulfate. Weather and backward trajectory analyses revealed that air parcels with high sulfate passed eastern and northeastern China or Korean peninsula before arriving at the site. In contrast, those with high <TEX>$SO_2$</TEX> passed an active volcano, Mt. Sakurajima, about 100 km in the south, suggesting the <TEX>$SO_2$</TEX> was more likely from the volcanic emission. The ratio of sulfate to total sulfur compounds <TEX>$({SO_4}^{2-})/({SO_4}^{2-}+SO_2)$</TEX> was 0.31-0.89 in continentally originated air while was 0.25-0.43 in the air having passed the volcano, showing more efficient conversions of <TEX>$SO_2$</TEX> to sulfate in the air from the continent. The close dependence of the conversion on humidity in the continentally originated air was confirmed.

Carbon dioxide variability during cold front passages and fair weather days at a forested mountaintop site

Abstract This study describes temporal carbon dioxide (CO2) changes at a new meteorological site on a mountaintop in the Virginia Blue Ridge Mountains during the first year of measurements. Continental mountaintop locations are increasingly being used for CO2 monitoring, and investigations are needed to better understand measurements made at these locations. We focus on CO2 mixing ratio changes on days with cold front passages and on fair weather days. Changes in CO2 mixing ratios are largest during cold front passages outside the growing season and on clear, fair weather days in the growing season. 67% (60%) of the frontal passages during the non-growing (growing) season have larger postfrontal than prefrontal CO2 mixing ratios. The increase in CO2 mixing ratio around the frontal passage is short-lived and coincides with changes in CO and O3. The CO2 increase can therefore be used as an additional criterion to determine the timing of frontal passages at the mountaintop station. The CO2 increase can be explained by an accumulation of trace gases along frontal boundaries. The magnitude and duration of the CO2 increase is affected by the wind speed and direction that determine the source region of the postfrontal air. Southward-moving fronts result in the largest prolonged period of elevated CO2, consistent with the postfrontal advection of air from the Northeastern United States where anthropogenic contributions are relatively large compared to other areas in the footprint of the mountaintop station. These anthropogenic contributions to the CO2 changes are confirmed through concurrent CO measurements and output from NOAA’s CarbonTracker model.

Influences of the Sierra Nevada on Intermountain Cold-Front Evolution

Abstract Recent studies indicate that strong cold fronts develop frequently downstream of the Sierra Nevada over the Intermountain West. To help ascertain why, this paper examines the influence of the Sierra Nevada on the rapidly developing Intermountain cold front of 25 March 2006. Comparison of a Weather Research and Forecasting (WRF) model control simulation with a simulation in which the height of the Sierra Nevada is restricted to 1500 m (roughly the elevation of the valleys and basins of the Intermountain West) shows that the interaction of southwesterly prefrontal flow with the formidable southern High Sierra produces a leeward orographic warm anomaly that enhances the cross-front temperature contrast. Several processes generate this orographic warm anomaly, including flow modification by the Sierra Nevada (i.e., windward blocking of low-level Pacific air, leeward subsidence, and increased southerly flow from the Mojave Desert and lower Colorado River basin into the Intermountain West), diabatic heating and water vapor loss associated with orographic precipitation, and increased sensible heating and reduced subcloud diabatic cooling in the downstream cloud and precipitation shadow. In contrast, the postfrontal air mass experiences comparatively little orographic modification as it moves across the relatively low northern Sierra Nevada. These results show that the Sierra Nevada can enhance frontal development, which may contribute to the high frequency of strong cold-frontal passages over the Intermountain West.

Aged status of soot particles during the passage of a weak cyclone in Beijing

Atmospheric particles at an urban site in Beijing from Apr. 18 to 21, 2009, when a weak synoptic cyclone passed and the air quality was relatively fine, were investigated by using electron microscopes. Particles collected from different stages of the cyclone were characterized by their shape, size and elemental composition. Soot particles were predominant and concentrated in the size range of 0.2–1.0 μm, and out of this range were rarely detected. Most of the soot particles exhibited aged shape, i.e. shrunk clusters with liquid coating (the so-called core-shell structure), compared to the open spherule’s chain or agglomerate shape of fresh soot particles. Morphologies of the particles in the prefrontal and postfrontal air were compared. The mean equivalent diameter of the particles in the prefrontal air was 0.36–0.38 μm and in the postfrontal air was 0.35–0.41 μm. More than 50% of the particles had aspect ratios between 1.0 and 1.4, and the remains frequently smaller than 2.0. In terms of circularity factor, 57–64% particles in the prefrontal air and 41–51% in the postfrontal air appeared in the range of 0.6–1.0. These results indicate that the aged status of soot particles in the prefrontal and postfrontal air were not very different, although the quality of the postfrontal air was much better than that of the prefrontal air, suggesting that fresh soot particles in chain or aggregate shape could be modified into core-shell structure rapidly even in less polluted air. In addition, a primary estimate showed that aging had caused the decrease of the cross-section areas of soot parts in the particles. ► Soot particles during a cyclone passage in Beijing were studied. ► Aged status of soot in prefrontal and postfrontal air was not very different. ► Fresh soot particles could be modified rapidly in less polluted postfrontal air. ► Aging caused the decrease of cross-section areas of soot parts in the particles.

Factors Affecting the Predictability of Hurricane Humberto (2007)

Abstract This study uses ensemble Kalman filter analyses and short-range ensemble forecasts to study factors affecting the predictability of Hurricane Humberto, which made landfall along the Texas coast in 2007. Humberto is known for both its rapid intensification and extreme forecast uncertainty, which makes it an ideal case in which to examine the origins of tropical cyclone strength forecast error. Statistical correlation is used to determine why some ensemble members strengthen the incipient low into a hurricane and others do not. During the analysis period, it is found that variations in midlevel moisture, low-level convective instability, and strength of a front to the north of the cyclone likely lead to differences in net precipitation, which ultimately leads to storm strength spread. Stronger storms are favored when the atmosphere is more moist and unstable and when the front is weaker, possibly because some storms in the ensemble begin entraining cooler and drier postfrontal air during this period. Later during the free forecast, variable entrainment of postfrontal air becomes a leading cause of strength spread. Surface moisture differences are the primary contributor to intensity forecast differences, and convective instability differences play a secondary role. Eventually mature tropical cyclone dynamics and differences in landfall time result in very rapid growth of ensemble spread. These results are very similar to a previous study that investigated a 2004 Gulf of Mexico low with a different model and analysis technique, which gives confidence that they are relevant to tropical cyclone formation and intensification in general. Finally, the rapid increase in forecast uncertainty despite relatively modest differences in initial conditions highlights the need for ensembles and advanced data assimilation techniques.

Columnar modelling of nucleation burst evolution in the convective boundary layer – first results from a feasibility study Part III: Preliminary results on physicochemical model performance using two &amp;quot;clean air mass&amp;quot; reference scenarios

Abstract. In Paper I of four papers, a revised columnar high-order model to investigate gas-aerosol-turbulence interactions in the convective boundary layer (CBL) was proposed. In Paper II, the model capability to predict first-, second- and third-order moments of meteorological variables in the CBL was demonstrated using available observational data. In the present Paper III, the high-order modelling concept is extended to sulphur and ammonia chemistry as well as to aerosol dynamics. Based on the previous CBL simulation, a feasibility study is performed using two "clean air mass" scenarios with an emission source at the ground but low aerosol background concentration. Such scenarios synoptically correspond to the advection of fresh post-frontal air in an anthropogenically influenced region. The aim is to evaluate the time-height evolution of ultrafine condensation nuclei (UCNs) and to elucidate the interactions between meteorological and physicochemical variables in a CBL column. The scenarios differ in the treatment of new particle formation (NPF), whereas homogeneous nucleation according to the classical nucleation theory (CNT) is considered. The first scenario considers nucleation of a binary system consisting of water vapour and sulphuric acid (H2SO4) vapour, the second one nucleation of a ternary system additionally involving ammonia (NH3). Here, the two synthetic scenarios are discussed in detail, whereas special attention is payed to the role of turbulence in the formation of the typical UCN burst behaviour, that can often be observed in the surface layer. The intercomparison of the two scenarios reveals large differences in the evolution of the UCN number concentration in the surface layer as well as in the time-height cross-sections of first-order moments and double correlation terms. Although in both cases the occurrence of NPF bursts could be simulated, the burst characteristics and genesis of the bursts are completely different. It is demonstrated, that observations from the surface layer alone are not conclusive to elucidate the origin of newly formed particles. This is also true with respect to the interpretation of box modelling studies. The binary and ternary NPF bursts observed in the surface layer differ with respect to burst amplitude and phase. New particles simulated in the binary scenario are formed in the forenoon in the upper part of the growing CBL, followed by turbulence-induced top-down transport. Hence, with respect to the burst observation site in the surface layer, new particles are formed ex situ. In opposite to this, the ternary case reveals a much more complex pattern. Here, NPF is initiated in the early morning hours in the surface layer, when temperature (T) is low and relative humidity (RH), sulphur dioxide (SO2) and NH3 concentrations are high, hence new particles are formed in situ. Shortly after that, ex situ NPF in the free troposphere sets in, followed by entrainment and top-down diffusion of newly formed particles into the surface layer. Altogether, these processes mainly contribute to the formation of a strong burst in the morning hours in the ternary scenario. While the time-height cross-section of the binary nucleation rate resembles a "blob"-like evolution pattern, the ternary one resembles a "sucking tube"-like pattern. The time-height cross-sections of the flux pattern and double correlations could be plausibly interpreted in terms of CBL turbulence and entrainment/detrainment processes both in the binary and in the ternary case. Although the present approach is a pure conceptual one, it shows the feasibility to simulate gas-aerosol-turbulence interactions in the CBL. Prior to a dedicated verification/validation study, further attempts are necessary to consider a more advanced description of the formation and activation of thermodynamically stable clusters according to modern concepts proposed by Kulmala et al. (2000), Kulmala (2003) and Kulmala et al. (2004a).

Characterization of Asian Dust during ACE-Asia

ACE-Asia was a multi-national collaboration organized to investigate and understand the chemistry, radiative properties, and climatic effects of mineral dust and other aerosol particles in the East Asia/Northwest Pacific region. Studies conducted at the Gosan and Zhenbeitai surface supersites show striking variations in aerosol concentrations and properties that were affected by the occurrence and origins of the Asian dust storms, air mass pathways, and mixing during the transport. Investigations conducted on the R/V Ronald H. Brown (RHB) showed that dust had a pervasive influence on the chemical composition, size distribution, and optical properties of the aerosol. Analyses using an aerosol time-of-flight mass spectrometer on the RHB showed that most of the coarse-particle nitrate and sulfate in post-frontal air was associated with dust, and more remarkably, that competitive or exclusionary processes evidently are involved in the uptake or production of these substances. Studies conducted onboard the NCAR C-130 aircraft showed that coarse-mode dust was less absorbing and less hygroscopic than pollution aerosol and that there was little correlation in light scattering and absorption by the sub- vs. super-micrometer aerosol. Below ∼2 km, dust was commonly mixed with pollutants, and this had a stronger influence on the optical properties of the submicrometer particles than the coarse-mode dust; at higher altitudes, the dust was less affected by pollution. Single particle analyses of C-130 samples showed that the mixing of black carbon (BC) with dust was common, but only certain types of BC particles were aggregated. Models were used in the planning, execution and interpretative phases of ACE-Asia; and summaries of modeling results are presented to illustrate the progress being made in identifying new dust sources; in depicting the time-varying, three-dimensional structure of dust plumes; and in quantifying the production, transport, and deposition of Asian dust.

Separated status of the natural dust plume and polluted air masses in an Asian dust storm event at coastal areas of China

Asian dust particles usually refer to mineral particles which originate from arid and semiarid areas in the Asian continent and disperse eastward in a wide range in the atmosphere. Their appearances in the downstream marine areas are always accompanied by high concentrations of sulfate and nitrate, suggesting their significance as a medium for pollutant conversion and transportation. However, analysis of particles collected at a coastal site in east China before and after the cold front arrival of a heavy dust–loading low‐pressure system in this study revealed that air masses polluted by anthropogenic emissions from local or regional areas and the natural Asian dust plume from northwest China were in different air parcels. Anthropogenic pollutants were in the prefrontal air while the so‐called Asian dust particles were in the postfrontal air. There was a large amount of anthropogenic mineral particles in the polluted air masses, and the particles were abundant in sulfur, sodium, and calcium, while the so‐called Asian dust particles were very “clean” (meaning lack of anthropogenic pollution). Continuous records of PM10, SO2, and O3 concentrations indicated that the cold front was the boundary layer between the polluted air masses and the clean dust plume. Hence, at the coastal areas the Asian dust plume and polluted air masses did not mix with each other and were separated in two consecutive air parcels by the cold front although they were in the same low‐pressure system. In addition, the analysis also revealed that calcium was not a good indicator for Asian dust from natural sources because the anthropogenic mineral particles contained significant calcium.

Spatial distribution and size evolution of particles in Asian outflow: Significance of primary and secondary aerosols during ACE‐Asia and TRACE‐P

Quasi‐Lagrangian aircraft measurements above the Yellow Sea, East China Sea, and Sea of Japan revealed synoptic‐scale secondary aerosol formation and condensational growth during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) and Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment. This occurred in the presence of pollution and mineral dust aerosol surface areas as high as 1200 μm2 cm−3. Concentrations of sulfuric acid generally appeared insufficient for binary nucleation, but observations, models, and theory are consistent with a ternary nucleation mechanism involving H2SO4‐H2O‐NH3. Growth rates of ∼2 nm h−1 can be explained by the condensation of sulfuric acid at a rate of 2 ± 1 × 106 molecules cm−3 s−1. Aerosol volatility suggested increasing neutralization of the aerosol during growth. Size distribution measurements suggest that weak (mean condensation nuclei (CN) 3–13 nm ≈ 500 cm−3) new particle production was a common occurrence in the region. However, new particle production was enhanced by ∼1 order of magnitude (mean CN 3–13 nm ≈ 5000 cm−3) in postfrontal air masses associated with offshore flow during cloud‐free conditions. Fog and clouds appear to be regionally important in modulating nucleation events through scavenging of secondary aerosol and through depletion of gas‐phase precursors through enhanced heterogeneous chemistry. Our results indicate that only 10–30% of the total aerosol population consists of aged secondary aerosols after ∼2 days of transport from source regions. In spite of their high production during nucleation events, secondary aerosols advected out over the Pacific Ocean will have a small impact upon indirect forcing and a negligible impact upon direct forcing compared to primary aerosol emissions and the species that condense upon them.

Marine boundary layer dust and pollutant transport associated with the passage of a frontal system over eastern Asia

Aerosol chemical composition and number size distributions were measured aboard the R/V Ronald H. Brown during the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) from 14 March to 20 April 2001. This manuscript focuses on the prefrontal and postfrontal air masses sampled aboard the ship in the Sea of Japan between 6 and 15 April 2001 to illustrate the different chemical sources/mixtures off the coast of Asia resulting from the contrasting meteorological transport patterns. The prefrontal air masses had a dominant accumulation mode composed of pollution and volcanic aerosols. The aerosol was predominately ammonium sulfate and organic carbon. Minor amounts of dust were present in the marine boundary layer (MBL) as a result of subsidence from a pronounced Taklimakan dust aerosol layer aloft. The sea salt in both the submicron and supermicron modes was highly depleted in chloride from reaction with sulfuric and nitric acid vapors. The passage of a large low‐pressure center, surrounded by a widespread distribution of airborne dust, on 10 April brought elevated concentrations of submicron and supermicron Gobi desert dust to the ship. The supermicron dust particles contained high concentrations of sulfate, nitrate, organic, and elemental carbon. The MBL aerosol properties and controlling processes described here provide data to evaluate and refine chemical transport models.

Meteorological conditions and transport pathways during the Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment

The Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment was conducted between February and April 2001. It included extensive chemical sampling by two aircraft based primarily in Hong Kong and Yokota Air Base, Japan. TRACE‐P examined pathways for the outflow of chemically and radiatively important gases and aerosols and their precursors from eastern Asia to the western Pacific and explored the chemical evolution of Asian outflow. This paper describes meteorological conditions and transport pathways over the Pacific Basin during TRACE‐P. Meteorological conditions changed rapidly during the period due to the seasonal winter to spring transition and the decay of prolonged ENSO cold phase (La Nina) conditions. To document these changes, TRACE‐P was divided into two halves, and mean flow patterns during each half are presented and discussed. Important circulation features are the semipermanent Siberian anticyclone and transient middle latitude cyclones that form near eastern Asia and then move eastward over the northern Pacific. Five‐day backward trajectories from the various flight tracks show that air sampled by the aircraft had been transported from a variety of locations. Some parcels remained over the tropical western North Pacific during the entire period, while other important origins were Southeast Asia, Africa, and central Asia. Specifically, lower tropospheric flight segments out of Hong Kong sampled both prefrontal maritime air as well as postfrontal air from the Asian continent. Conversely, low‐level flight segments out of Yokota, Japan mostly sampled postfrontal Asian air. Southern portions of middle and upper tropospheric flight segments from Hong Kong sampled air previously in the deep tropics, while the more northerly flight segments sampled air that originated from the west (e.g., passing over central Africa and India). Most upper level flight segments from Yokota sampled air arriving from the west. Patterns of satellite‐derived precipitation and lightning are described. TRACE‐P occurs during a neutral to weak La Nina period of relatively cold sea surface temperatures in the tropical Pacific. Compared with climatology, the TRACE‐P period exhibits deep convection located west of its typical position; however, tropospheric flow patterns do not exhibit a strong La Nina signal. Circulation patterns during TRACE‐P are found to be generally similar to those during NASA's PEM WEST‐B mission that occurred in the same region during February–March 1994.

Observations Of A Progressive Warm-Fresh Oceanic Front and A Cool-Moist Atmospheric Front At Duck, North Carolina

This paper presents a unique set of observations of nearlycoincident and progressive oceanic and marine atmospheric boundary-layer (MABL) fronts in a coastal zone. The event was observed during the afternoon of 12 May 1996 at the United States Army Corps of Engineers, Coastal Engineering Research Center, Field Research Facility pier at Duck, North Carolina. The oceanic front was warm and fresh. Current variabilityaccompanied the oceanic front. A marked MABL front preceded the oceanic front by several minutes and had characteristics of a sea-breezefront. This MABL front separated warmer and dryer pre-frontal air from cooler and moister post-frontal air. Wind direction and wind speedvariability accompanied the MABL front. The sea-surface roughness signatures of both fronts were detected by an X-band pulsed Doppler radar. Supporting data are used to identify each front detected by the radar and to calculate each front's velocity. In an attempt to explain the sea-surface roughness variations associated with each front, the radar data are compared to corresponding variations in wind speed, wind direction, and air-sea temperature difference.

Transport of ozone and pollutants from North America to the North Atlantic Ocean during the 1996 Atmosphere/Ocean Chemistry Experiment (AEROCE) intensive

Research flights were conducted to investigate the episodic occurrence of enhanced ozone mixing ratios over the North Atlantic Ocean (NAO) during the spring as part of the Atmosphere/Ocean Chemistry Experiment (AEROCE). We measured meteorological variables, trace gases, and aerosol light scattering from the University of Wyoming King Air research aircraft during 16 research flights in April and early May 1996. The flights were conducted over eastern North America and over the NAO between the United States and Bermuda. On April 28, a cold front was located just west of Bermuda while a second system developed over the Midwest and central United States. In the midtroposphere, polluted layers with up to 250 ppbv CO, 150 pptv NO, and 1.8 ppbv NOy were observed within a deeper layer of enhanced O3 mixing ratios (up to 88 ppbv) over the NAO at around 32°N, 64–68°W. These aircraft observations, when coupled with ozone sondes, back trajectories, and satellite image loops, indicate two distinct meteorological mechanisms that combine to yield the observed chemical signature of the air. There is substantial meteorological evidence, supported by ozone sonde observations and earlier King Air flights, indicating that stratosphere/troposphere exchange associated with the upstream frontal system injects and advects dry, ozone‐rich air into midtropospheric regions over the continent. These subsiding air masses have the potential to result in deep layers of enhanced ozone in the offshore postfrontal area. Convection from the developing (upwind) system appears to lift air from the continental boundary layer into the area of subsidence in the mid to upper troposphere. The result is high concentrations of gaseous and particulate pollutants along with elevated quantities of ozone. Ozone mixing ratios exceed those attributable to boundary layer venting or in‐transit photochemical production. These meteorological processes lead to pollution and enhanced ozone from the stratosphere cooccurring in postfrontal air masses over the NAO.