Characterizing benthic macroinvertebrate and algal biological condition gradient models for California wadeable Streams, USA

The Biological Condition Gradient (BCG) is a conceptual model that describes changes in aquatic communities under increasing levels of anthropogenic stress. The BCG helps decision-makers connect narrative water quality goals (e.g., maintenance of natural structure and function) to quantitative measures of ecological condition by linking index thresholds based on statistical distributions (e.g., percentiles of reference distributions) to expert descriptions of changes in biological condition along disturbance gradients. As a result, the BCG may be more meaningful to managers and the public than indices alone. To develop a BCG model, biological response to stress is divided into 6 levels of condition, represented as changes in biological structure (abundance and diversity of pollution sensitive versus tolerant taxa) and function. We developed benthic macroinvertebrate (BMI) and algal BCG models for California perennial wadeable streams to support interpretation of percentiles of reference-based thresholds for bioassessment indices (i.e., the California Stream Condition Index [CSCI] for BMI and the Algal Stream Condition Index [ASCI] for diatoms and soft-bodied algae). Two panels (one of BMI ecologists and the other of algal ecologists) each calibrated a general BCG model to California wadeable streams by first assigning taxa to specific tolerance and sensitivity attributes, and then independently assigning test samples (264 BMI and 248 algae samples) to BCG Levels 1–6. Consensus on the assignments was developed within each assemblage panel using a modified Delphi method. Panels then developed detailed narratives of changes in BMI and algal taxa that correspond to the 6 BCG levels. Consensus among experts was high, with 81% and 82% expert agreement within 0.5 units of assigned BCG level for BMIs and algae, respectively. According to both BCG models, the 10th percentiles index scores at reference sites corresponded to a BCG Level 3, suggesting that this type of threshold would protect against moderate changes in structure and function while allowing loss of some sensitive taxa. The BCG provides a framework to interpret changes in aquatic biological condition along a gradient of stress. The resulting relationship between index scores and BCG levels and narratives can help decision-makers select thresholds and communicate how these values protect aquatic life use goals

Ground-based retrieval of continental and marine warm cloud microphysics

A technique for retrieving warm cloud microphysics using synergistic ground based remote sensing instruments is presented. The SYRSOC (SYnergistic Remote Sensing Of Cloud) technique utilises a K-a-band Doppler cloud RADAR, a LIDAR (or ceilometer) and a multichannel microwave radiometer. SYRSOC retrieves the main microphysical parameters such as cloud droplet number concentration (CDNC), droplets effective radius (r(eff)), cloud liquid water content (LWC), and the departure from adiabatic conditions within the cloud. Two retrievals are presented for continental and marine stratocumulus advected over the Mace Head Atmospheric Research Station. Whilst the continental case exhibited high CDCN ((N) over bar = 382 cm(-3); 10th-to-90th percentile [9.4-842.4] cm(-3)) and small mean effective radius ((r(eff)) over bar = 4.3; 10th-to-90th percentile [2.9-6.5] mu m), the marine case showed low CDNC and large mean effective radius ((N) over bar = 25 cm(-3), 10th-to-90th percentile [1.5-69] cm(-3); (r(eff)) over bar = 28.4 mu m, 10th-to-90th percentile [11.2-42.7] mu m) as expected since continental air at this location is typically more polluted than marine air. The mean LWC was comparable for the two cases (continental: 0.19 gm(-3); marine: 0.16 gm(-3)) but the 10th-90th percentile range was wider in marine air (continental: 0.11-0.22 gm(-3); marine: 0.01-0.38 gm(-3)). The calculated algorithm uncertainty for the continental and marine case for each variable was, respectively, sigma(N) = 161.58 cm(-3) and 12.2 cm(-3), sigma(reff) = 0.86 mu m and 5.6 mu m, sigma(LWC) = 0.03 gm(-3) and 0.04 gm(-3). The retrieved CDNC are compared to the cloud condensation nuclei concentrations and the best agreement is achieved for a supersaturation of 0.1% in the continental case and between 0.1 %-0.75% for the marine stratocumulus. The retrieved r(eff) at the top of the clouds are compared to the MODIS satellite r(eff): 7 mu m (MODIS) vs. 6.2 mu m (SYRSOC) and 16.3 mu m (MODIS) vs. 17 mu m (SYRSOC) for continental and marine cases, respectively. The combined analysis of the CDNC and the r(eff), for the marine case shows that the drizzle modifies the droplet size distribution and (r(eff)) over bar especially if compared to r(eff)(MOD). The study of the cloud subadiabaticity and the LWC shows the general sub-adiabatic character of both clouds with more pronounced departure from adiabatic conditions in the continental case than in the marine

Modeled optical thickness of sea-salt aerosol

We simulate the generation and microphysical evolution of sea-salt aerosol using a climatologically driven 3-D microphysical model for the year 2006. We then apply Mie theory to calculate the extinction and scattering efficiencies of our transported, size-resolved sea-salt aerosol, accounting for hygroscopic growth due to changes in ambient relative humidity. We calculate the column optical thickness of our modeled sea-salt aerosol for comparison to three previously published wind speed-dependent clean marine air optical thickness formulations. Variously derived from optical thickness measurements and retrievals taken from the Midway Island AERONET site, the satellite-based MODIS instruments, and the Global Atmospheric Watch (GAW) site at Mace Head, Ireland, the three formulations report similar background levels of clean marine AOT at zero wind speed but significantly different functional dependencies for nonzero wind speeds. We find that our modeled sea-salt aerosol optical thickness very closely depends on the square of surface wind speed under steady state conditions. This relationship is consistent across all latitudes. However, due to the fact that steady state winds are seldom maintained, the 24 h mean wind is more frequently applicable to calculations of sea-salt AOT, with only slightly diminished accuracy

Aerosol optical depth in clean marine and continental northeast atlantic air

The aerosol optical depth (AOD) and Angstrom exponent for the period 2002-2004 is evaluated for clean marine and continentally influenced air masses over the northeast Atlantic region. Measurements were carried out at the Mace Head atmospheric research station on the west coast of Ireland using a precision filter radiometer which measures the aerosol optical depth at four wavelengths centered at 368, 412, 500, and 862 nm. The clean marine AOD at 500 nm is characterized by a mean value of 0.14 +/- 0.06, exhibiting relatively small temporal variability. The Angstrom exponent was less than 1 for the majority of cases, having an average value of 0.40 +/- 0.29 in clean marine conditions. The latter is consistent with the presence of relatively large supermicron particles, such as sea salt dominating the marine aerosol size distribution. The Angstrom exponent shows a distinct seasonal cycle in clean marine air, with maximum values being derived in the summer months and minimum values in the winter. In continental air masses, while the range and standard deviation of the AOD is larger than in clean marine conditions, the overall mean AOD (tau(500) = 0.19 +/- 0.12) is comparable with the clean marine AOD. The continental Angstrom exponent is larger, having a mean value of 1.07 +/- 0.32. This is attributed to a dominating accumulation mode in the presence of urban-industrial aerosol particles originating mainly from continental Europe. These results demonstrate how the natural marine AOD can rival polluted AOD over the northeast Atlantic region and highlight the importance of the natural marine aerosol contribution over oceans

Effects of continental boundary layer evolution, convection, turbulence and entrainment, on aerosol formation

Aerosol nucleation events occurring in the continental boundary layer over the boreal forest region in Finland, during the BIOFOR experiment, have been examined to elucidate the role of micrometeorology in promoting such events. Invariably, during the spring campaign of 1999, nucleation events occurred in Arctic and polar air masses during cold air outbreaks. Under clear-sky conditions, typical of these synoptic meteorological patterns, the boundary layer evolution was characterized by the rapid growth of a mixed layer, convection and strong entrainment, first from the residual later and later from the free troposphere. It was found that the freshly nucleated particles were detected within two hours from the onset of strong turbulent kinetic energy, independent of how fast the boundary layer evolved. When considering the growth time from cluster size of approximate to 1 nm to detectable sizes of 3 nm, the nucleation and onset of strong turbulence coincided almost exactly. The most likely site for nucleation to take place was the mixed layer or the entrainment zone, while the forest canopy and the free troposphere could be excluded as the nucleation region. There are several possible explanations for the correlation between the onset of turbulence and nucleation: (1) new aerosols or clusters may have been entrained from the residual layer into the mixed layer where they then (in the case of clusters) underwent growth to detectable sizes; (2) two or more precursor gases may have been mixed with each other over the entrainment zone; (3) the adiabatic cooling in the rising convective plumes and the turbulent fluctuation in temperature and vapors by the entrainment flux may have enhanced aerosol formation; (4) a sudden decrease in preexisting aerosol due to dilution of the mixed layer aerosol by entrained air may have reduced the vapor sink enough to initiate nucleation. However, the lack of vertical profile measurements of nucleation mode aerosols, precursor vapors and turbulent fluctuations throughout and above the mixed-layer results in it remaining an open question as to which one of these processes dominates

Wind speed dependent size-resolved parameterization for the organic mass fraction of sea spray aerosol

For oceans to be a significant source of primary organic aerosol (POA), sea spray aerosol (SSA) must be highly enriched with organics relative to the bulk seawater. We propose that organic enrichment at the air-sea interface, chemical composition of seawater, and the aerosol size are three main parameters controlling the organic mass fraction of sea spray aerosol (OMSSA). To test this hypothesis, we developed a new marine POA emission function based on a conceptual relationship between the organic enrichment at the air-sea interface and surface wind speed. The resulting parameterization is explored using aerosol chemical composition and surface wind speed from Atlantic and Pacific coastal stations, and satellite-derived ocean concentrations of chlorophyll-a, dissolved organic carbon, and particulate organic carbon. Of all the parameters examined, a multi-variable logistic regression revealed that the combination of 10 m wind speed and surface chlorophyll-a concentration ([Chl-a]) are the most consistent predictors of OMSSA. This relationship, combined with the published aerosol size dependence of OMSSA, resulted in a new parameterization for the organic mass fraction of SSA. Global emissions of marine POA are investigated here by applying this newly-developed relationship to existing sea spray emission functions, satellite-derived [Chl-a], and modeled 10m winds. Analysis of model simulations shows that global annual sub-micron marine organic emission associated with sea spray is estimated to be from 2.8 to 5.6 TgC yr(-1). This study provides additional evidence that marine primary organic aerosols are a globally significant source of organics in the atmosphere

Do anthropogenic, continental or coastal aerosol sources impact on a marine aerosol signature at mace head?

Atmospheric aerosols have been sampled and characterised at the Mace Head north-east (NE) Atlantic atmospheric research station since 1958, with many interesting phenomena being discovered. However, with the range of new discoveries and scientific advances, there has been a range of concomitant criticisms challenging the representativeness of aerosol sampled at the station compared to that of aerosol over the pristine open-ocean. Two recurring criticisms relate to the lack of representativeness due to potentially enhanced coastal sources, possibly leading to artificially high values of aerosol concentrations, and to the influence of long-range transport of anthropogenic or continental aerosol and its potential dominance over, or perturbation of, a natural marine aerosol signal. Here, we review the results of previous experimental studies on marine aerosols over the NE Atlantic and at Mace Head with the aim of evaluating their representativeness relative to that of a pristine open-ocean aerosol, i.e. with negligible anthropogenic/continental influence. Particular focus is given to submicron organic matter (OM) aerosol. In summary, no correlation was found between OM and black carbon (BC) in marine air conforming to clean-air sampling criteria, either at BC levels of 0-15 or 15-50 ng m(-3), suggesting that OM concentrations, up to observed peak values of 3.8 mu g m(-3), are predominantly natural in origin. Sophisticated carbon isotope analysis and aerosol mass spectral finger printing techniques corroborate the conclusion that there is a predominant natural source of OM, with 80% biogenic source apportionment being observed for general clean-air conditions, rising to similar to 98% during specific primary marine organic plumes when peak OM mass concentrations > 3 mu g m(-3) are observed. Similarly, a maximum contribution of 20% OM mass coming from non-marine sources was established by dual carbon isotope analysis. Further, analysis of a series of experiments conducted at Mace Head conclude that negligible coastal, surf zone, or tidal effects are discernible in the secondary or primary aerosol mass residing in the submicron size range for sampling heights of 7m and above. The Mace Head marine-air criteria ensure anthropogenic and coastal effects are sufficiently minimised so as to guarantee a predominant, and sometimes overwhelming, natural marine aerosol contribution to the total aerosol population when the criteria are adhered to

Statistical characteristics and predictability of particle formation events at mace head

[ 1] The seasonal characteristics of coastal nucleation events at the Mace Head Atmospheric Research Station, resulting from exposure of macroalgae to the atmosphere, were analyzed for a 2-year period from August 2002 to July 2004. Nucleation events occurred on 58% of the days over the period. The seasonal variation of the number of event days and event duration show a clear cycle, with maximum values in spring and autumn and the minimum values in the winter season. The nucleation events typically start similar to 75 min prior to the occurrence of the low-tide mark and the event start time is correlated (r = 0.75) to the low-tide height. The intensity of the events, as determined by the peak particle concentrations achieved, is also positively correlated with the amount of tidal areas exposed to ambient air, as determined by the tidal height, and solar radiation. A nucleation potential index (NPI) was developed as a tool to provide a predictive capability for event prediction at Mace Head. The index was derived from normalized tidal height, solar radiation intensity, and wind direction and was compared with the occurrence of nucleation events from the database. The result shows that Mace Head particle formation events can be quite well predicted with a threshold probability of 50%

A statistical analysis of north east atlantic (submicron) aerosol size distributions

The Global Atmospheric Watch research station at Mace Head (Ireland) offers the possibility to sample some of the cleanest air masses being imported into Europe as well as some of the most polluted being exported out of Europe. We present a statistical cluster analysis of the physical characteristics of aerosol size distributions in air ranging from the cleanest to the most polluted for the year 2008. Data coverage achieved was 75% throughout the year. By applying the Hartigan-Wong k-Means method, 12 clusters were identified as systematically occurring. These 12 clusters could be further combined into 4 categories with similar characteristics, namely: coastal nucleation category (occurring 21.3% of the time), open ocean nucleation category (occurring 32.6% of the time), background clean marine category (occurring 26.1% of the time) and anthropogenic category (occurring 20% of the time) aerosol size distributions. The coastal nucleation category is characterised by a clear and dominant nucleation mode at sizes less than 10 nm while the open ocean nucleation category is characterised by a dominant Aitken mode between 15 nm and 50 nm. The background clean marine aerosol exhibited a clear bimodality in the sub-micron size distribution, with although it should be noted that either the Aitken mode or the accumulation mode may dominate the number concentration. However, peculiar background clean marine size distributions with coarser accumulation modes are also observed during winter months. By contrast, the continentally-influenced size distributions are generally more monomodal (accumulation), albeit with traces of bimodality. The open ocean category occurs more often during May, June and July, corresponding with the North East (NE) Atlantic high biological period. Combined with the relatively high percentage frequency of occurrence (32.6 %), this suggests that the marine biota is an important source of new nano aerosol particles in NE Atlantic Air