Clean Marine Air Masses Research Articles

Wintertime fine aerosol particles composition and its evolution in two megacities of southern and northern China

Study on fine aerosols composition can help understand the particles formation and is crucial for improving the accuracy of model simulations. Based on field data measured by a Q-ACSM (Quadrupole-Aerosol Chemical Speciation Monitor), we have comprehensively compared the characteristics, evolution, and potential formation mechanisms of the components in NR-PM2.5 during wintertime at two megacities (Beijing and Guangzhou) of southern and northern China. We show that as PM pollution intensifies, the mass fraction of the primary aerosols (e.g., COA, HOA) in PM2.5 in Guangzhou increased, along with a slight decline in proportion of both the secondary organic (SOA) and inorganic (SIA) aerosols; In contrast, in Beijing, the proportion of the SIA ramped up from 28 % to 53 % with the pollution evolution; and the fraction of SOA in total OA also increased due to a substantial increment in the proportion of MO-OOA (from 29 % to 48 %), suggesting a significance of the secondary processes in worsening aerosols pollution in Beijing. Our further analysis demonstrates a leading role of aqueous pathway in the secondary formation of aerosols at the Beijing site, presenting an exponential rising of SIA and SOA with the relative humidity (RH) increase. Compared to Beijing, however, we find that the photochemical oxidation other than aqueous process in Guangzhou plays a more critical role in those secondary aerosols formation. Combined with the Hysplit trajectory model, we identify the high humid conditions in Guangzhou are typically affected by clean marine air masses, explaining the slower response of secondary components to the RH changes. Moreover, the particles in Guangzhou were observed less hygroscopic that is adverse to the aerosol aqueous chemistry. The results provide basis for the precise control of PM pollution in different regions across China and would be helpful in improving model simulations.

Decoupling in the vertical shape of HCHO during a sea breeze event: The effect on trace gas satellite retrievals and column-to-surface translation

The effect of sea breeze circulation on stratification of the vertical formaldehyde (HCHO) concentration vertical profiles is explored using a regional atmospheric chemical transport model (CTM) for three synoptically stagnant days focused on the east coast of the U.S in June 2018. During this event, a significant thermal contrast between the Atlantic Ocean and the terrestrial regions (12–17 °C), observed by moderate resolution imaging spectroradiometer (MODIS) and well-captured by the WRF-CMAQ model (15–18 °C), is conducive to monsoon-like flow, perpendicular to the shorelines, carrying clean marine air masses over the land within a few hundreds of meters above the surface. In contrast, the westerly continental polluted air masses prevail in higher altitudes. These two conflicting flows result in atypical vertical shapes of HCHO concentrations increasing with altitude. This decoupling pattern is so pronounced that we observe total column HCHO negatively correlate with surface concentrations. Comparisons of an accredited global model, GEOS-CF, to surface wind measurements and MODIS skin temperature indicate its poor representation of the sea breeze timing and strength, resulting in GEOS-CF HCHO vertical shapes being drastically different from the WRF-CMAQ. Based on radiative transfer calculations, the differences in the vertical distribution of HCHO between the WRF-CMAQ and that of GEOS-CF in the first 3 km are sufficient to induce a 20–30% error in air mass factors (thus total vertical HCHO column abundances). Through an experiment involving converting HCHO total columns to surface mixing ratios, we demonstrate that GEOS-CF allocates noticeably more HCHO molecules (40–150%) to the surface layer due to the misrepresentation of the vertical shape of HCHO during the sea breeze event. It is known that a significant fraction of the human population lives in coastal areas prone to detrimental effects caused by air pollution, and elevated pollutant concentrations usually occur in synoptically stagnant atmospheric conditions when local circulation patterns come into play; accordingly, our experiments emphasize the importance of the effect a priori profiles can have on satellite-derived applications under such conditions. To ensure that the quantitative representation of satellite-based trace gas retrievals on a daily basis is trustworthy and useable for air quality applications, atmospheric models providing a priori profiles for satellite retrievals should be well-tuned to reproduce complex local circulation such as sea-land breezes.

Seasonal Trends of Aerosol Hygroscopicity and Mixing State in Clean Marine and Polluted Continental Air Masses Over the Northeast Atlantic

AbstractFive years of nearly continuous measurements of aerosol hygroscopicity by a Humidified Tandem Differential Mobility Analyzer (HTDMA) at Mace Head research station are presented, making it the longest aerosol hygroscopicity data set in marine environment and Northeast Atlantic region in particular. The main goal of this work was to examine seasonal patterns of aerosol hygroscopicity and mixing state in a variety of air masses. The entire data set was categorized into clean marine and polluted continental sectors to reveal contrasting physicochemical features. Distinct seasonal patterns were revealed where hygroscopicity of the clean sector aerosol was significantly higher than that of the polluted sector aerosol. The median growth factors (GF) of clean and polluted sectors were 1.67 and 1.31, respectively. Clean sector aerosol median GF reached the lowest values in summer and the highest in winter pointing at an impact of marine biological activity and wind speed driven sea spray production to aerosol hygroscopicity. Conversely, polluted sector aerosol reached the highest median GF of all sizes in summer due to the decrease of anthropogenic emission, enhanced photochemical aging, and secondary inorganic aerosol formation. Wintertime aerosols in both sectors were largely externally mixed, while summertime aerosols were relatively internally mixed. A distinct subset of near‐hydrophobic Aitken mode particles smaller than 50 nm in diameter was observed in the wintertime clean sector which most likely originated in the clean marine atmosphere based on the evolution of air mass back trajectory clustering.

Exploratory investigation on spatiotemporal variation and source identification of atmospheric speciated mercury surrounding the Taiwan Strait

This pioneering study explored the spatiotemporal variation, gas-solid partition, and potential sources of atmospheric speciated mercury surrounding the Taiwan Strait. Total gaseous mercury (TGM) and particle bound mercury (Hgp) were sampled simultaneously at six coastal and island sites located on both sides and in the middle of the Taiwan Strait. Field measurement results showed that the campaign average concentrations of TGM and Hgp at all sites were 4.56 ± 0.35 and 0.17 ± 0.02 ng/m3, with the range of 3.22–5.84 and 0.06–0.25 ng/m3, indicating that TGM and Hgp partitioned as 96.09–97.02% and 2.98–3.91%, respectively, The spatial distribution results showed that the highest and lowest TGM and Hgp concentrations were mostly observed at Xiang-An site in the suburban of Xiamen City and at Sandiaojiao site in the remote coast of New Taipei City, respectively. The seasonal variation of TGM at six sampling sites was in the order as: spring > winter > fall > summer, while that of Hgp was in the order as: winter > spring > fall > summer. Regarding the levels of TGM and Hgp, they increased gradually and consistantly from late fall to early spring when polluted air masses were transported from the Northeast Asia to the Taiwan Strait by Asian Northeastern Monsoons (ANEMs). On the contrary, the lowest concentrations of TGM and Hgp were observed commonly in summer as clean marine air masses transported from the seas toward the Taiwan Strait. It was inferred that atmospheric speciated mercury surrounding the Taiwan Strait was mainly attributed from both local sources and long-range trasport (LRT) from the Northeast Asia.

Changes in aerosol size distributions over the Indian Ocean during different meteorological conditions

Aerosol emissions in South Asia are large. The emitted aerosols can travel significant distances and, during the Asian southwest monsoon especially, are prone to modification through cloud processing and wet scavenging while being transported. The scale of emissions and transport means that the global climate impact of these aerosols are sensitive to modification en route, but the process-level understanding is still largely lacking. In this study, we analyse long-term aerosol data measured at an observatory established in Hanimaadhoo, Republic of Maldives, to investigate the long-term properties of aerosols over the Indian Ocean as well as to understand the effect of precipitation on the aerosol particle size distribution during long-range transport. The observatory location is ideal because it is a receptor site with little local influence, and, depending on the season, receives either polluted air masses coming from the Indian subcontinent or clean marine air masses from the Indian Ocean. We analysed the sub-micron particle number size distribution measured during the years 2004–2008, and 2014–2017, and this is the first inter-seasonal long-term study of the sub-micron aerosol features in the region. The aerosol origin and its relative exposure to wet scavenging during long-range transport were analysed using back-trajectory analysis from HYSPLIT. By comparing aerosol measurements to precipitation along its transport, this study shows that there is a substantial change in particle number size distributions and concentrations depending on the amount of rainfall during transport. During the southwest monsoon season, the aerosol size distribution was notably bi-modal and total particle concentrations clearly reduced in comparison with the prevailing aerosol size distribution during the northeast monsoon season. Precipitation during transport usually corresponded with a greater reduction in accumulation mode concentrations than for smaller sizes, and the shape of the median size distribution showed a clear dependence on the trajectory origin and route taken.

Volatile halocarbon measurements in the marine boundary layer at Cape Point, South Africa

Volatile organic halocarbons play a significant role in the biogeochemical cycling of halogens in the atmosphere. High frequency, in situ measurements of methyl iodide (CH3I), bromoform (CHBr3), dibromomethane (CH2Br2) and methyl bromide (CH3Br) were measured in the marine boundary layer at Cape Point, South Africa by an automated Absorption Desorption System-Gas Chromatography-Mass Spectrometer (ADS-GC-MS) between January and November 2017. These are the first multi-halocarbon measurements recorded at Cape Point and represent the longest time series to date. Baseline wind conditions were established using both wind direction and radon (222Rn) concentration as markers of clean air. The baseline mixing ratios observed were 0.61 ± 0.29, 2.02 ± 0.89, 1.08 ± 0.17 and 6.09 ± 0.50 ppt, for CH3I, CHBr3, CH2Br2, and CH3Br, respectively. A statistically significant difference in short-lived halocarbon occurrence was established between anthropogenically affected and clean marine air masses for CH3Br at the Cape Point site. The CHBr3 and CH2Br2 mixing ratios were not statistically different (p > 0.05) when comparing the anthropogenically affected and clean marine air masses. The mixing ratios of CH3I suggested a strong seasonal variation with higher production in austral Summer-Autumn months than in the austral Winter-Spring months. A general chemical decay line was calculated for baseline CH2Br2 and CHBr3 measurements at Cape Point. An analysis of the ln([CH2Br2]/[CHBr3]):ln([CHBr3]) ratio for the Cape Point dataset exhibited a slight deflection to the right of the general chemical decay line, suggesting the influence of greater [CHBr3]:[CH2Br2] source ratios rather than their dilution.

Substantial Seasonal Contribution of Observed Biogenic Sulfate Particles to Cloud Condensation Nuclei

Biogenic sources contribute to cloud condensation nuclei (CCN) in the clean marine atmosphere, but few measurements exist to constrain climate model simulations of their importance. The chemical composition of individual atmospheric aerosol particles showed two types of sulfate-containing particles in clean marine air masses in addition to mass-based Estimated Salt particles. Both types of sulfate particles lack combustion tracers and correlate, for some conditions, to atmospheric or seawater dimethyl sulfide (DMS) concentrations, which means their source was largely biogenic. The first type is identified as New Sulfate because their large sulfate mass fraction (63% sulfate) and association with entrainment conditions means they could have formed by nucleation in the free troposphere. The second type is Added Sulfate particles (38% sulfate), because they are preexisting particles onto which additional sulfate condensed. New Sulfate particles accounted for 31% (7 cm−3) and 33% (36 cm−3) CCN at 0.1% supersaturation in late-autumn and late-spring, respectively, whereas sea spray provided 55% (13 cm−3) in late-autumn but only 4% (4 cm−3) in late-spring. Our results show a clear seasonal difference in the marine CCN budget, which illustrates how important phytoplankton-produced DMS emissions are for CCN in the North Atlantic.

Contribution of Regional Transport to Surface Ozone at an Island Site of Eastern China

The surface ozone (O3) in non-urban areas can be strongly influenced by transported regional air pollutants from distant urban agglomerations. Comprehensively using the polar plot graphical technique, and backward trajectory and potential source contribution analyses, the variations of surface O3 and its potential source regions at an island site of Zhejiang, China, were analyzed based on data from June 2013 to October 2016. Relatively high hourly O3 levels (max.: 154.0 ppb) were often observed, with the total number of hourly O3 exceeding the national standard (75 ppb) being 583, indicating high health risks from O3 exposure. An obvious time-lag for the diurnal O3 peak was observed, with the daily maxima often occurring at 16:00. A decreased O3 level was observed in summer, which was probably due to the comprehensive influence of intense rainfall, high relative humidity and clean marine airmasses during this season. The transport of external air pollutants played a dominant role in affecting local O3, with relatively high O3 concentrations (> 50 ppb) being observed under high wind speeds (> 5 m s–1). Based on all the source identification analyses, the major regions contributing to the surface O3 lay to the north and northwest of the study area. Long-range transported air pollutants from the coastal provinces of the North China Plain may have significantly enhanced surface O3 at the study site, while contributions from local areas and areas to the south of the study site were rather small. A high incidence (64.3%) of O3 pollution was accounted for by the inflowing airmasses from outside of Zhejiang, which quantitatively confirmed the significant background transport of O3 to this province. Coordinated inter-regional control on pollutant emissions should be carried out to reduce O3 in Zhejiang Province.

Overview and preliminary results of the Surface Ocean Aerosol Production (SOAP) campaign

Abstract. Establishing the relationship between marine boundary layer (MBL) aerosols and surface water biogeochemistry is required to understand aerosol and cloud production processes over the remote ocean and represent them more accurately in earth system models and global climate projections. This was addressed by the SOAP (Surface Ocean Aerosol Production) campaign, which examined air–sea interaction over biologically productive frontal waters east of New Zealand. This overview details the objectives, regional context, sampling strategy and provisional findings of a pilot study, PreSOAP, in austral summer 2011 and the following SOAP voyage in late austral summer 2012. Both voyages characterized surface water and MBL composition in three phytoplankton blooms of differing species composition and biogeochemistry, with significant regional correlation observed between chlorophyll a and DMSsw. Surface seawater dimethylsulfide (DMSsw) and associated air–sea DMS flux showed spatial variation during the SOAP voyage, with maxima of 25 nmol L−1 and 100 µmol m−2 d−1, respectively, recorded in a dinoflagellate bloom. Inclusion of SOAP data in a regional DMSsw compilation indicates that the current climatological mean is an underestimate for this region of the southwest Pacific. Estimation of the DMS gas transfer velocity (kDMS) by independent techniques of eddy covariance and gradient flux showed good agreement, although both exhibited periodic deviations from model estimates. Flux anomalies were related to surface warming and sea surface microlayer enrichment and also reflected the heterogeneous distribution of DMSsw and the associated flux footprint. Other aerosol precursors measured included the halides and various volatile organic carbon compounds, with first measurements of the short-lived gases glyoxal and methylglyoxal in pristine Southern Ocean marine air indicating an unidentified local source. The application of a real-time clean sector, contaminant markers and a common aerosol inlet facilitated multi-sensor measurement of uncontaminated air. Aerosol characterization identified variable Aitken mode and consistent submicron-sized accumulation and coarse modes. Submicron aerosol mass was dominated by secondary particles containing ammonium sulfate/bisulfate under light winds, with an increase in sea salt under higher wind speeds. MBL measurements and chamber experiments identified a significant organic component in primary and secondary aerosols. Comparison of SOAP aerosol number and size distributions reveals an underprediction in GLOMAP (GLObal Model of Aerosol Processes)-mode aerosol number in clean marine air masses, suggesting a missing marine aerosol source in the model. The SOAP data will be further examined for evidence of nucleation events and also to identify relationships between MBL composition and surface ocean biogeochemistry that may provide potential proxies for aerosol precursors and production.

Regional severe particle pollution and its association with synoptic weather patterns in the Yangtze River Delta region, China

Abstract. Regional air pollution is significantly associated with dominant weather systems. In this study, the relationship between the particle pollution over the Yangtze River Delta (YRD) region and weather patterns is investigated. First, the pollution characteristics of particles in the YRD are studied using in situ monitoring data (PM2.5 and PM10) in 16 cities and Terra/MODIS AOD (aerosol optical depth) products collected from December 2013 to November 2014. The results show that the regional mean value of AOD is high in the YRD, with an annual mean value of 0.71±0.57. The annual mean particle concentrations in the cities of Jiangsu Province all exceed the national air quality standard. The pollution level is higher in inland areas, and the highest concentrations of PM2.5 and PM10 are 79 and 130 µg m−3, respectively, in Nanjing. The PM2.5 : PM10 ratios are typically high, thus indicating that PM2.5 is the overwhelmingly dominant particle pollutant in the YRD. The wintertime peak of particle concentrations is tightly linked to the increased emissions during the heating season as well as adverse meteorological conditions. Second, based on NCEP (National Center for Environmental Prediction) reanalysis data, synoptic weather classification is conducted and five typical synoptic patterns are objectively identified. Finally, the synthetic analysis of meteorological fields and backward trajectories are applied to further clarify how these patterns impact particle concentrations. It is demonstrated that air pollution is more or less influenced by high-pressure systems. The relative position of the YRD to the anti-cyclonic circulation exerts significant effects on the air quality of the YRD. The YRD is largely influenced by polluted air masses from the northern and the southern inland areas when it is located at the rear of the East Asian major trough. The significant downward motion of air masses results in stable weather conditions, thereby hindering the diffusion of air pollutants. Thus, this pattern is quite favorable for the accumulation of pollutants in the YRD, resulting in higher regional mean PM10 (116.5 ± 66.9 µg m−3), PM2.5 (75.9 ± 49.9 µg m−3), and AOD (0.74) values. Moreover, this pattern is also responsible for the occurrence of most large-scale regional PM2.5 (70.4 %) and PM10 (78.3 %) pollution episodes. High wind speed and clean marine air masses may also play important roles in the mitigation of pollution in the YRD. Especially when the clean marine air masses account for a large proportion of all trajectories (i.e., when the YRD is affected by the cyclonic system or oceanic circulation), the air in the YRD has a lesser chance of being polluted. The observed correlation between weather patterns and particle pollution can provide valuable insight into making decisions about pollution control and mitigation strategies.

Aerosol contributions at an urban background site in Eastern Mediterranean – Potential source regions of PAHs in PM10 mass

In this paper, two backward air mass trajectory-based models (Potential Source Contribution Function [PSCF] and Concentration Weighted Trajectory [CWT]) were combined, aiming to identify sources and factors defining the load of PM in the city of Limassol (Cyprus). The study also focused on the determination of atmospheric pathways enriching the aerosol phase of four carcinogenic Polycyclic Aromatic Hydrocarbons (PAHs): Benzo(a)pyrene (BaP), Benzo(a)anthracene (BaA), Benzo(b)fluoranthene (BbF) and Benzo(k)fluoranthene (BkF), in PM10 mass. The analysis was performed on a 0.5°·0.5° resolution grid for the two-year period 2011–2012. During cold seasons, regional airflows triggered the accumulation of locally produced PM2.5, while the impact of dust plumes originated from deserts in NE Africa, Syria and the Middle East, was apparent on PM2.5 and principally on PMCOARSE levels. On the contrary, within warm seasons, weaker dust PMCOARSE contributions were detected in Limassol from areas in Egypt and Libya. Raised particulate-phase PAH concentrations in Limassol were clearly related to air parcels reaching Cyprus via continental areas. The use of outdated technologies for heating and transportation in Turkey and Syria, and fire events in central Turkey, are possible sources of exogenous PAHs throughout cold and warm period respectively. The influence of clean marine air masses dropped the levels of PAH compounds in all seasons.

The Impact of Iodide-Mediated Ozone Deposition and Halogen Chemistry on Surface Ozone Concentrations Across the Continental United States.

The air quality of many large coastal areas in the United States is affected by the confluence of polluted urban and relatively clean marine airmasses, each with distinct atmospheric chemistry. In this context, the role of iodide-mediated ozone (O3) deposition over seawater and marine halogen chemistry accounted for in both the lateral boundary conditions and coastal waters surrounding the continental U.S. is examined using the Community Multiscale Air Quality (CMAQ) model. Several nested simulations are conducted in which these halogen processes are implemented separately in the continental U.S. and hemispheric CMAQ domains, the latter providing lateral boundary conditions for the former. Overall, it is the combination of these processes within both the continental U.S. domain and from lateral boundary conditions that lead to the largest reductions in modeled surface O3 concentrations. Predicted reductions in surface O3 concentrations occur mainly along the coast where CMAQ typically has large overpredictions. These results suggest that a realistic representation of halogen processes in marine regions can improve model prediction of O3 concentrations near the coast.

Chemical composition, structural properties, and source apportionment of organic macromolecules in atmospheric PM10 in a coastal city of Southeast China.

Particulate matter (PM10) associated with the fractions of organic macromolecules, including humic acid (HA), kerogen + black carbon (KB), and black carbon (BC), was determined during summer and winter at urban and suburban sites in a coastal city of southeast China. The organic macromolecules were characterized by elemental analysis (EA), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR), and their sources were identified by using stable carbon/nitrogen isotope (δ13C/δ15N) and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model. The results showed that HA, kerogen (K), and BC accounted for the range of 3.89 to 4.55% in PM10, while they were the dominant fractions of total organic carbon (TOC), ranging from 64.70 to 84.99%. SEM analysis indicated that BC particles were porous/nonporous and consisted of spherical and non-spherical (i.e., cylindrical and elongate) structures. The FTIR spectra of HA, KB, and BC exhibited similar functional groups, but the difference of various sites and seasons was observed. HA in PM10 contained a higher fraction of aliphatic structures, such as long-chain fatty and carbohydrates with a carboxylic extremity. The C/N ratio, SEM, and δ13C/δ15N values provided reliable indicators of the sources of HA, K, and BC in PM10. The results suggested that HA and K majorly originated from terrestrial plants, and BC came from the mixture of combustion of terrestrial plants, fossil fuel, and charcoal. The air masses in winter originated from Mongolia (4%), the northern area of China (48%), and northern adjacent cities (48%), suggesting the influence of anthropogenic sources through long-range transport, while the air masses for the summer period came from South China Sea (34%) and Western Pacific Sea (66%), representing clean marine air masses with low concentrations of organic macromolecules.

Development of a Combined Heart-Cut and Comprehensive Two-Dimensional Gas Chromatography System to Extend the Carbon Range of Volatile Organic Compounds Analysis in a Single Instrument

The majority of atmospheric measurements of volatile organic compounds (VOCs) are usually limited to a small range, either in volatility or time resolution. A combined heart-cut gas chromatography (GC) with comprehensive two-dimensional GC (GC×GC) instrument was developed, specifically to increase the number of VOCs analysed using a single instrument. The system uses valve based modulation and was fully automated, making it suitable for use in the field. A laboratory comparison to an existing dual-channel GC (DC-GC) instrument demonstrated that this new GC-GC×GC can accurately measure atmospheric mixing ratios of C 5 -C 13 VOC species with a wide range of functionalities. Approximately hourly field measurements were conducted at a remote marine atmospheric research station in Bachok, Malaysia. This region was shown to be influenced by clean marine air masses, local anthropogenic and biogenic emission sources and aged emissions transported from highly polluted South East Asian regions. A dramatic shift in air mass direction was observed each day associated with the development of a sea breeze, which influenced the diurnal profiles of species measured at the Bachok site. A proton-transfer-reaction mass spectrometer (PTR-MS) was also deployed at Bachok and compared to the new GC-GC×GC instrument. Overall, the GC-GC×GC instrument has been shown to perform well in lab comparisons and during field observations. This represents a good compromise between volatility and high complexity o n l i n e measurements of VOCs.

Molecular Markers of Secondary Organic Aerosol in Mumbai, India.

Biogenic secondary organic aerosols (SOA) are generally considered to be more abundant in summer than in winter. Here, polar organic marker compounds in urban background aerosols from Mumbai were measured using gas chromatography-mass spectrometry. Surprisingly, we found that concentrations of biogenic SOA tracers at Mumbai were several times lower in summer (8-14 June 2006; wet season; n = 14) than in winter (13-18 February 2007; dry season; n = 10). Although samples from less than 10% of the season are extrapolated to the full season, such seasonality may be explained by the predominance of the southwest summer monsoon, which brings clean marine air masses to Mumbai. While heavy rains are an important contributor to aerosol removal during the monsoon season, meteorological data (relative humidity and T) suggest no heavy rains occurred during our sampling period. However, in winter, high levels of SOA and their day/night differences suggest significant contributions of continental aerosols through long-range transport together with local sources. The winter/summer pattern of SOA loadings was further supported by results from chemical transport models (NAQPMS and GEOS-Chem). Furthermore, our study suggests that monoterpene- and sesquiterpene-derived secondary organic carbon (SOC) were more significant than those of isoprene- and toluene-SOC at Mumbai.

CCN and Vertical Velocity Influences on Droplet Concentrations and Supersaturations in Clean and Polluted Stratus Clouds

Abstract Cloud microphysics and cloud condensation nuclei (CCN) measurements from two marine stratus cloud projects are presented and analyzed. Results show that the increase of cloud droplet concentrations Nc with CCN concentrations NCCN rolls off for NCCN at 1% supersaturation (S)N1% above 400 cm−3. Moreover, at such high concentrations Nc was not so well correlated with NCCN but tended to be more closely related to vertical velocity W or variations of W (σw). This changeover from predominate Nc dependence on NCCN to Nc dependence on W or σw is due to the higher slope k of CCN spectra at lower S, which is made more relevant by the lower cloud S that is forced by higher NCCN. Higher k makes greater influence of W or σw variations than NCCN variations on Nc. This changeover at high NCCN thus seems to limit the indirect aerosol effect (IAE). On the other hand, in clean-air stratus cloud S often exceeded 1% and decreased to slightly less than 0.1% in polluted conditions. This means that smaller CCN [those with higher critical S (Sc)], which are generally more numerous than larger CCN (lower Sc), are capable of producing stratus cloud droplets, especially when they are advected into clean marine air masses where they can induce IAE. Positive correlations between turbulence σw and NCCN are attributed to greater differential latent heat exchange of smaller more numerous cloud droplets that evaporate more readily. Such apparent CCN influences on cloud dynamics tend to support trends that oppose conventional IAE, that is, less rather than greater cloudiness in polluted environments.

Total OH reactivity measurements in Paris during the 2010 MEGAPOLI winter campaign

Abstract. Hydroxyl radicals play a central role in the troposphere as they control the lifetime of many trace gases. Measurement of OH reactivity (OH loss rate) is important to better constrain the OH budget and also to evaluate the completeness of measured VOC budget. Total atmospheric OH reactivity was measured for the first time in an European Megacity: Paris and its surrounding areas with 12 million inhabitants, during the MEGAPOLI winter campaign 2010. The method deployed was the Comparative Reactivity Method (CRM). The measured dataset contains both measured and calculated OH reactivity from CO, NOx and VOCs measured via PTR-MS, GC-FID and GC-MS instruments. The reactivities observed in Paris covered a range from 10 s−1 to 130 s−1, indicating a large loading of chemical reactants. The present study showed that, when clean marine air masses influenced Paris, the purely local OH reactivity (20 s−1) is well explained by the measured species. Nevertheless, when there is a continental import of air masses, high levels of OH reactivity were obtained (120–130 s−1) and the missing OH reactivity measured in this case jumped to 75%. Using covariations of the missing OH reactivity to secondary inorganic species in fine aerosols, we suggest that the missing OH reactants were most likely highly oxidized compounds issued from photochemically processed air masses of anthropogenic origin.

Seasonal comparisons of single-particle chemical mixing state in Riverside, CA

The Study of Organic Aerosols in Riverside, CA (SOAR) field campaign, conducted in the summer and fall of 2005, focused on developing a stronger understanding of seasonal influences on the sources and processes contributing to some of the highest levels of particulate matter in the United States. On-line single-particle mass spectrometry measurements showed that carbonaceous particles generally dominated the submicron size range (>75% by number), compared to primarily aged sea salt and dust in the supermicron size range. During periods with high PM2.5 (particulate matter <2.5 μm) mass and number concentrations, hygroscopic water-containing carbonaceous particles internally mixed with sulfate and nitrate extended up into the supermicron size mode. The mixing state of carbonaceous particles changed with season. In the summer, carbonaceous particles were primarily mixed with secondary (oxidized) organic carbon and sulfate; whereas in the fall, they were mixed mostly with ammonium nitrate. During each season, different processes contributed to the highest pollution levels. In the summer, photochemical production of highly aged secondary organic carbon resulted in daily peaks in both number and mass particle concentrations, followed by removal in the afternoon by the arrival of the clean marine air masses from the coast. In contrast, cooler, more stagnant meteorological conditions in the fall season resulted in the buildup over several days of submicron carbonaceous particles mixed with semivolatile ammonium, nitrate, and amines. In addition, Santa Ana wind events in the fall led to dust and biomass burning particles dominating both the submicron and supermicron size ranges.

Seasonal Variation of the Aerosol Light Scattering Coefficient in Marine Air of the Northeast Atlantic

Aerosol light scattering measurements were carried out using a TSI 3563 Nephelometer at the Mace Head Atmospheric Research Station, on the west coast of Ireland from year 2001–2010. A strong seasonal trend in the aerosol light scattering coefficient at 550 nm (), for clean marine air masses, is observed with a high value, [average (geometric mean)] of 35.3 Mm−1(29.5 Mm−1), in January and a low value of 13.7 Mm−1(10.2 Mm−1), in July. This near threefold increase in the value during the winter season is because of the large contribution of wind-speed generated sea-salt particles in the marine boundary layer. A high positive correlation coefficient of 0.82 was found between the percentage occurrence of relatively large Ångström exponent (Å) values (&gt;1.2) and the percentage occurrence of lower values (5–15 Mm−1) in the summer season. and wind-speed have a high positive correlation coefficient of 0.88 whereas Å and wind-speed have a negative correlation coefficient of −0.89. Å values during the summer months indicate the dominance of sub-m particles thus indicating the contribution of non-sea-salt sulphate and organics towards the as these species show an enhanced concentration during the summer months.

Comparison between simulated and observed chemical composition of fine aerosols in Paris (France) during springtime: contribution of regional versus continental emissions

Abstract. Hourly concentrations of inorganic salts (ions) and carbonaceous material in fine aerosols (aerodynamic diameter, A.D. &lt;2.5 μm) have been determined experimentally from fast measurements performed for a 3-week period in spring 2007 in Paris (France). The sum of these two chemical components (ions and carbonaceous aerosols) has shown to account for most of the fine aerosol mass (PM2.5). This time-resolved dataset allowed investigating the factors controlling the levels of PM2.5 in Paris and showed that polluted periods with PM2.5 &gt; 15 μg m−3 were characterized by air masses of continental (North-Western Europe) origin and chemical composition made by 75% of ions. By contrast, periods with clean marine air masses have shown the lowest PM2.5 concentrations (typically of about 10 μg m−3); carbonaceous aerosols contributing for most of this mass (typically 75%). In order to better discriminate between local and continental contributions to the observed chemical composition and concentrations of PM2.5 over Paris, a comparative study was performed between this time-resolved dataset and the outputs of a chemistry transport model (CHIMERE), showing a relatively good capability of the model to reproduce the time-limited intense maxima observed in the field for PM2.5 and ion species. Different model scenarios were then investigated switching off local and European (North-Western and Central) emissions. Results of these scenarios have clearly shown that most of the ions observed over Paris during polluted periods, were either transported or formed in-situ from gas precursors transported from Northern Europe. On the opposite, long-range transport from Europe appeared to weakly contribute to the levels of carbonaceous aerosols observed over Paris. The model failed to properly account for the concentration levels and variability of secondary organic aerosols (SOA) determined experimentally by the EC-tracer method. The abundance of SOA (relatively to organic aerosol, OA) was as much as 75%, showing a weak dependence on air masses origin. Elevated SOA/OA ratios were also observed for air masses having residence time above ground of less than 10 h, suggesting intense emissions and/or photochemical processes leading to rapid formation of secondary organic aerosols.