Synoptic-scale Air Mass Research Articles

Analysis of urban Heat Island intensity through air mass persistence: A case study of four United States cities

In conjunction with the ongoing rise in global air temperature, there is evidence to suggest that mid-latitude synoptic air masses have changed in frequency and persistence. The research presented here focuses on an analysis of urban heat island (UHI) intensity through the period of synoptic-scale air mass stagnation for four UHI-prone United States cities. It is theorized that the thermal inertia associated with a stagnant air mass conducive to UHI development causes an intensification of the urban-rural temperature difference through time. Results indicate that UHI intensification is responsive to more than just the thermal characteristics of the respective air mass in place. While the magnitude of UHI was found to be smallest under a cool, dry air mass, it generally intensifies with persistence. Conversely, commonly associated with the greatest UHI intensity, a warm, moist air mass exhibits little change in UHI intensity or even a de-intensification with persistence. The warm, moist air masses that are conducive to a stronger UHI appear to be increasing in frequency, partly due to greater persistence. However, somewhat encouraging is that the results here do not show UHI intensification through the period of their persistence.

Trends in synoptic heat events in four Minnesota urban areas through the 21st century

Extreme heat is often overlooked as a public health concern in Minnesota, where intraseasonal summer variability limits acclimatization to oppressive heat conditions. Specific categories of synoptic-scale air masses are linked to summer excess mortality and elevated health risk in the Midwestern United States, particularly within urban areas. Between 1948 and 2019, Minnesota's four largest urban areas have experienced decreased nighttime cooling, while warmer and more humid air masses have increased in frequency at the expense of cooler and drier ones. We used downscaled CMIP5 climate projections for 21st-century Minnesota, under RCP4.5 and RCP8.5 emissions scenarios, to generate daily synoptic classifications and evaluate projected frequency and character trends in the highest-risk air masses. Projections show dramatic increases in both the frequency and temperature of days within the Dry Tropical category, neither of which have changed significantly thus far across Minnesota's historical record. Frequency and duration of consecutive-day episodes of excess heat, as identified either by synoptic classifications or by the Excess Heat Factor, are likewise expected to increase more substantially in the future than they have in the past. Other projected trends, such as rising dew point temperatures and nighttime air temperatures, represent continuations of already existing historical trends.

Multi-year variation of near-surface ozone at Zhongshan Station, Antarctica

With the support of the Chinese National Antarctic Research Expedition, near-surface ozone (O3) was continuously monitored at Zhongshan Station (ZOS) (69°22′12″ S, 76°21′49″ E, 18.5 m above sea level) in East Antarctica from 2008 to 2020. The seasonal and diurnal variability of near-surface O3 at ZOS were investigated. O3 enhancement events (OEEs) were frequently observed in the warm season (OEEs in January accounted for 23.0% of all OEEs). The OEEs at ZOS were related to the photochemical reaction processes under the influences of O3 and solar radiation in the stratosphere and synoptic-scale air mass transport from coastal areas (Princess Elizabeth Land, Wilkes Land, and Queen Mary Land), as evidenced by the recorded wind speed, solar shortwave irradiance, and total column ozone data and the computed potential source contribution function and concentration-weighted trajectory models. The results computed by the tool Stratosphere-to-Troposphere Exchange Flux indicated that stratosphere-to-troposphere transport had no direct impact on OEEs at ZOS. Therefore, synoptic-scale air mass transport is the main cause of OEEs in Antarctica, which is consistent with previous studies. Unlike OEEs at inland Antarctic stations, which are mainly affected by air mass transport from inland plateaus, OEEs at ZOS, a coastal station, are mainly affected by air mass transport from coastal land in East Antarctica.

Year-round record of near-surface ozone and O3 enhancement events (OEEs) at Dome A, East Antarctica

Abstract. Dome A, the summit of the East Antarctic Ice Sheet, is an area challenging to access and is one of the harshest environments on Earth. Up until recently, long-term automated observations from Dome A (DA) were only possible with very low power instruments such as a basic meteorological station. To evaluate the characteristics of near-surface O3, continuous observations were carried out in 2016. Together with observations at the Amundsen–Scott Station (South Pole – SP) and Zhongshan Station (ZS, on the southeast coast of Prydz Bay), the seasonal and diurnal O3 variabilities were investigated. The results showed different patterns between coastal and inland Antarctic areas that were characterized by high concentrations in cold seasons and at night. The annual mean values at the three stations (DA, SP and ZS) were 29.2±7.5, 29.9±5.0 and 24.1±5.8 ppb, respectively. We investigated the effect of specific atmospheric processes on near-surface summer O3 variability, when O3 enhancement events (OEEs) are systematically observed at DA (average monthly frequency peaking at up to 64.5 % in December). As deduced by a statistical selection methodology, these O3 enhancement events (OEEs) are affected by significant interannual variability, both in their average O3 values and in their frequency. To explain part of this variability, we analyzed the OEEs as a function of specific atmospheric processes: (i) the role of synoptic-scale air mass transport over the Antarctic Plateau was explored using the Lagrangian back-trajectory analysis Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) method, and (ii) the occurrence of “deep” stratospheric intrusion events was investigated using the Lagrangian tool STEFLUX. The specific atmospheric processes, including synoptic-scale air mass transport, were analyzed by the HYSPLIT back-trajectory analysis and the potential source contribution function (PSCF) model. Short-range transport accounted for the O3 enhancement events (OEEs) during summer at DA, rather than efficient local production, which is consistent with previous studies of inland Antarctica. Moreover, the identification of recent (i.e., 4 d old) stratospheric-intrusion events by STEFLUX suggested that deep events only had a minor influence (up to 1.1 % of the period, in August) on deep events during the variability in near-surface summer O3 at DA. The deep events during the polar night were significantly higher than those during the polar day. This work provides unique data on ozone variation at DA and expands our knowledge of such events in Antarctica. Data are available at https://doi.org/10.5281/zenodo.3923517 (Ding and Tian, 2020).

The Influence of Synoptic-Scale Air Mass Conditions on Seasonal Precipitation Patterns over North Carolina

This paper characterizes the influence of synoptic-scale air mass conditions on the spatial and temporal patterns of precipitation in North Carolina over a 16-year period (2003–2018). National Center for Environmental Prediction Stage IV multi-sensor precipitation estimates were used to describe seasonal variations in precipitation in the context of prevailing air mass conditions classified using the spatial synoptic classification system. Spatial analyses identified significant clustering of high daily precipitation amounts distributed along the east side of the Appalachian Mountains and along the Coastal Plains. Significant and heterogeneous clustering was prevalent in summer months and tended to coincide with land cover boundaries and complex terrain. The summer months were dominated by maritime tropical air mass conditions, whereas dry moderate air mass conditions prevailed in the winter, spring, and fall. Between the three geographic regions of North Carolina, the highest precipitation amounts were received in western North Carolina during the winter and spring, and in eastern North Carolina in the summer and fall. Central North Carolina received the least amount of precipitation; however, there was substantial variability between regions due to prevailing air mass conditions. There was an observed shift toward warmer and more humid air mass conditions in the winter, spring, and fall months throughout the study period (2003–2018), indicating a shift toward air mass conditions conducive to higher daily average rain rates in North Carolina.

Trends in weather type frequencies across North America

While 20th century changes in many individual meteorological variables are well documented, the trends in multivariate synoptic-scale air masses—or weather types—largely remain unexplored. Utilizing a recently developed gridded weather typing classification system, this research investigates the changes in the frequency of weather types (WTs) across North America, 1979–2017. Averaged across the study domain as a whole, Humid Warm WTs are occurring 22 more days per year, while Dry Warm WTs have increased by 10 days/year. These increases are offset mostly by decreased frequency of Dry Cool (−17 days/year) and Cool WTs (−21 days/year). The largest absolute changes are in the Canadian Archipelago, where the Warm WT is occurring 42 more days/year and the Cool WT is occurring 48 fewer times per year. In western Canada all humid types are occurring more frequently, including a Humid Cool type that is occurring 16 more days/year. The Desert Southwest US and northern Mexico show significant increases in Dry Warm WTs (+33 to +40 days/year). Cold front and warm front passages show increases in most of the US and decreases in most of Canada. Describing these secular changes to the frequency of intuitive weather types may be an effective means of communicating these climate trends to policymakers and the general public, especially considering their large magnitude.

The Southeastern Aerosol Research and Characterization (SEARCH) study: Spatial variations and chemical climatology, 1999–2010

The Southeastern Aerosol Research and Characterization (SEARCH) study, which has been in continuous operation from 1999 to 2012, was implemented to investigate regional and urban air pollution in the southeastern United States. With complementary data from other networks, the SEARCH measurements provide key knowledge about long-term urban/nonurban pollution contrasts and regional climatology affecting inland locations and sites along the Gulf of Mexico coastline. Analytical approaches ranging from comparisons of mean concentrations to the application of air mass trajectories and principal component analysis provide insight into local and area-wide pollution. Gases (carbon monoxide, sulfur dioxide, nitrogen oxides, ozone, and ammonia), fine particle mass concentration, and fine particle species concentrations (including sulfate, elementary carbon, and organic carbon) are affected by a combination of regional conditions and local emission sources. Urban concentrations in excess of regional baselines and intraurban variations of concentrations depend on source proximity, topography, and local meteorological processes. Regional-scale pollution events (95th percentile concentrations) involving more than 6 of the 8 SEARCH sites are rare (< 2% of days), while subregional events affecting 4–6 sites occur on ˜10% of days. Regional and subregional events are characterized by widely coincident elevated concentrations of ozone, sulfate, and particulate organic carbon, driven by persistent synoptic-scale air mass stagnation and higher temperatures that favor formation of secondary species, mainly in the summer months. The meteorological conditions associated with regional stagnation do not favor long-range transport of polluted air masses during episodes. Regional and subregional pollution events frequently terminate with southward and eastward penetration of frontal systems, which may initially reduce air pollutant concentrations more inland than along the Gulf Coast. Implications: Regional distribution of emission sources and synoptic-scale meteorological influences favoring stagnation lead to high regionwide pollution levels. The regional influence is greatest with secondary species, including ozone (O3) particulate sulfate (SO4), and particulate organic matter, some of which is produced by atmospheric oxidation of volatile organic compounds (VOCs) from vegetation and anthropogenic sources. Other species, many of which are from primary emissions, are more influenced by local sources, especially within the Atlanta, GA, and Birmingham, AL, metropolitan areas. Limited measurements of modern and fossil total carbon point to the importance of biological and biogenic emissions in the Southeast.

Evaporation from Lake Superior: 2: Spatial distribution and variability

Evaporation is a critical component of the water balance of each of the Laurentian Great Lakes, and it is expected that because of their shear size, evaporation cannot be spatially or temporally uniform. Despite this, examples of spatially distributed estimates of evaporation in the scientific literature are rare for most of the lakes and non-existent for Lake Superior. Direct measurements of evaporation taken at an offshore site on Lake Superior from June 2008 to October 2010 were used with concurrent satellite and climate model data to extrapolate evaporation measurements across the entire lake. Evaporation rates, large scale forcing mechanisms, and spatial patterns and variability are presented. Spatial patterns of evaporation tend to follow synoptic-scale air masses traveling over the lake. While most evaporation occurs during relatively short term events, these episodes tend not to occur in isolated locations on the lake, but are spatially widespread. The exceptions to this rule are during periods of a stable atmosphere and low evaporation, and when a transient ice cover can limit evaporation from some areas.

Hygroscopic properties of atmospheric aerosol particles over the Eastern Mediterranean: implications for regional direct radiative forcing under clean and polluted conditions

Abstract. This work examines the effect of direct radiative forcing of aerosols in the eastern Mediterranean troposphere as a function of air mass composition, particle size distribution and hygroscopicity, and relative humidity (RH). During intensive field measurements on the island of Crete, Greece, the hygroscopic properties of atmospheric particles were determined using a Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) and a Hygroscopicity Differential Mobility Analyzer-Aerodynamic Particle Sizer (H-DMA-APS). Similar to former studies, the H-TDMA identified three hygroscopic sub-fractions of particles in the sub-μm range: a more hygroscopic group, a less hygroscopic group and a nearly hydrophobic particle group. The average hygroscopic particle growth factors at 90 % RH were a significant function of particle mobility diameter (Dp): 1.42 (± 0.05) at 30 nm compared to 1.63 (± 0.07) at 250 nm. The H-DMA-APS identified up to three hygroscopic sub-fractions at mobility diameters of 1.0 and 1.2 μm. The data recorded between 12 August and 20 October 2005 were classified into four distinct synoptic-scale air mass types distinguishing between different regions of origin (western Mediterranean vs. the Aegean Sea) as well as the degree of continental pollution (marine vs. continentally influenced). The hygroscopic properties of particles with diameter Dp≥150 nm showed the most pronounced dependency on air mass origin, with growth factors in marine air masses exceeding those in continentally influenced air masses. Particle size distributions and hygroscopic growth factors were used to calculate aerosol light scattering coefficients at ambient RH using a Mie model. A main result was the pronounced enhancement of particle scattering over the eastern Mediterranean due to hygroscopic growth, both in the marine and continentally influenced air masses. When RH reached its summer daytime values around 70–80 %, up to 50–70 % of the calculated visibility reduction was due to the hygroscopic growth of the particles by water compared to the effect of the dry particles alone. The estimated aerosol direct radiative forcings for both, marine and continentally influenced air masses were negative indicating a net cooling of the atmosphere due to the aerosol. The radiative forcing ΔFr was nevertheless governed by the total aerosol concentration most of the time: ΔFr was typically more negative for continentally influenced aerosols (ca. −4 W m−2) compared to rather clean marine aerosols (ca. −1.5 W m−2). When RH occasionally reached 90 % in marine air masses, ΔFr even reached values down to −7 W m−2. Our results emphasize, on the basis of explicit particle hygroscopicity measurements, the relevance of ambient RH for the radiative forcing of regional atmospheres.

Particle number size distributions in urban air before and after volatilisation

Abstract. Aerosol particle number size distributions (size range 0.003–10 μm) in the urban atmosphere of Augsburg (Germany) were examined with respect to the governing anthropogenic sources and meteorological factors. The two-year average particle number concentration between November 2004 and November 2006 was 12 200 cm−3, i.e. similar to previous observations in other European cities. A seasonal analysis yielded twice the total particle number concentrations in winter as compared to summer as consequence of more frequent inversion situations and enhanced particulate emissions. The diurnal variations of particle number were shaped by a remarkable maximum in the morning during the peak traffic hours. After a mid-day decrease along with the onset of vertical mixing, an evening concentration maximum could frequently be observed, suggesting a re-stratification of the urban atmosphere. Overall, the mixed layer height turned out to be the most influential meteorological parameter on the particle size distribution. Its influence was even greater than that of the geographical origin of the prevailing synoptic-scale air mass. Size distributions below 0.8 μm were also measured downstream of a thermodenuder (temperature: 300 °C), allowing to retrieve the volume concentration of non-volatile compounds. The balance of particle number upstream and downstream of the thermodenuder suggests that practically all particles &gt;12 nm contain a non-volatile core while additional nucleation of particles smaller than 6 nm could be observed after the thermodenuder as an interfering artifact of the method. The good correlation between the non-volatile volume concentration and an independent measurement of the aerosol absorption coefficient (R2=0.9) suggests a close correspondence of the refractory and light-absorbing particle fractions. Using the "summation method", an average diameter ratio of particles before and after volatilisation could be determined as a function of particle size. The results indicated that particles &gt;60 nm contain a significantly higher fraction of non-volatile compounds, most likely black carbon, than particles &lt;60 nm. The results are relevant for future health-related studies in that they explore the size distribution and time-dependent behaviour of the refractory component of the urban aerosol over an extended time period.

Atmospheric Mercury Concentrations Associated with Geologically and Anthropogenically Enriched Sites in Central Western Nevada

This paper documents the atmospheric mercury concentrations above anthropogenically contaminated and naturally enriched sites in central western Nevada. Atmospheric mercury concentrations were measured at five representative regional sites (1.2−7.5 ng/m3) and two anthropogenically contaminated areas (13−866 ng/m3) in the Carson River Superfund Site. The highest regional concentrations were measured at the Steamboat Geothermal area, where mercury mineralization occurs naturally. Concurrent with atmospheric sampling, environmental conditions were monitored to assess their covariance with mercury concentrations. Atmospheric mercury concentrations were influenced by multiple factors with dominance exerted by substrate mercury concentration, site surface characteristics, and local and synoptic scale air masses. A mercury flux of 5−125 (±50%) μg of mercury m-2 h-1 was estimated via modified K-theory for a contaminated location. This flux was scaled up to estimate the contribution of atmospheric mercury from min...