Cloud chemistry measurements and estimates of acidic deposition on an above cloudbase coniferous forest

Fog and rain water chemistry in a tea plantation of northern Taiwan

Tea plantations are expanding globally and many are in mountainous areas with frequent fog but few studies have examined fog chemistry in these areas. We examined chemical composition of fog and rain water at a tea plantation in northern Taiwan. Fog water was collected using a Kroneis passive cylindrical fog-water collector and rain water was collected using a 20-cm-diameter funnel. The most abundant ions were Cl- and Na+ in both fog and rain waters due to the proximity of the site to the coast. The order of abundance of other ions was NO3-  > Mg2+  > SO42-  > Ca2+  > NH4+  > K+  > H+ in fog water and SO42-  > K+  > NO3-  > NH4+  > Ca2+  > Mg2+  > H+ in rain water. The concentration enrichment ratio (fog to rain) ranged between 2.2 (K+) and 22 (Mg2+) lying between sites near major emission sources and sites in remote areas, possibly because the immediate surrounding landscape is covered with secondary forests although it is near large cities. Factor analysis highlights the influences of sea-salt aerosols on the variation of fog and rain water chemistry. Sea-salt corrections using Na+ as the sea salt tracer led to negative concentrations of Cl- and Mg2+ suggesting that assumptions involved in sea-salt corrections were not satisfied. Agriculture influence is identified as a unique factor for explaining variance of K+, NH4+, and dissolved organic nitrogen (DON) concentrations in fog water but not rain water. Ion concentrations in fog and rain water were generally higher in the weekly samples associated with air trajectories passing through the continental East Asia than those associated with oceanic trajectories pointing to the role of regional pollution sources in affecting local fog and rain water chemistry. Our study highlights greater effects of tea agriculture on fog than rain water chemistry.

Effects of Acidity on Reactive Oxygen Species Formation from Secondary Organic Aerosols

Reactive oxygen species (ROS) play a critical role in the chemical transformation of atmospheric secondary organic aerosols (SOA) and aerosol health effects by causing oxidative stress in vivo. Acidity is an important physicochemical property of atmospheric aerosols, but its effects on the ROS formation from SOA have been poorly characterized. By applying the electron paramagnetic resonance spin-trapping technique and the Diogenes chemiluminescence assay, we find highly distinct radical yields and composition at different pH values in the range of 1-7.4 from SOA generated by oxidation of isoprene, α-terpineol, α-pinene, β-pinene, toluene, and naphthalene. We observe that isoprene SOA has substantial hydroxyl radical (^•OH) and organic radical yields at neutral pH, which are 1.5-2 times higher compared to acidic conditions in total radical yields. Superoxide (O₂ ^•-) is found to be the dominant species generated by all types of SOAs at lower pH. At neutral pH, α-terpineol SOA exhibits a substantial yield of carbon-centered organic radicals, while no radical formation is observed by aromatic SOA. Further experiments with model compounds show that the decomposition of organic peroxide leading to radical formation may be suppressed at lower pH due to acid-catalyzed rearrangement of peroxides. We also observe 1.5-3 times higher molar yields of hydrogen peroxide (H₂O₂) in acidic conditions compared to neutral pH by biogenic and aromatic SOA, likely due to enhanced decomposition of α-hydroxyhydroperoxides and quinone redox cycling, respectively. These findings are critical to bridge the gap in understanding ROS formation mechanisms and kinetics in atmospheric and physiological environments.

Chemical compositions of fog and precipitation at Sejila Mountain in the southeast Tibetan Plateau, China

Chemical compositions of fog and rain water were measured between July 2017 and September 2018 at Sejila Mountain, southeast Tibet, where fog events frequently occurred in original fir forests at altitude 3950 m. Fog water samples were collected using a Caltech Active Strand Cloud Collector (CASCC), and rain samples were collected using a precipitation gauge. Differences were observed between fog water and rain composition for most analyzed ions. Ion abundance in fog water was Ca²⁺ > Cl^- > Na⁺ > SO₄^2- > Mg²⁺ > NH₄⁺ >K⁺ > NO₃^- whereas an order of Ca²⁺ > Na⁺ > Cl^- > Mg²⁺ > SO₄^2- > NO₃^- > K⁺ > NH₄⁺ was observed for rain water. All ion concentrations were higher in fog water than in rain water. Additionally, Ca²⁺ was the dominant cation in both fog and rain samples, accounting for more than half of all measured cations. NH₄⁺ and SO₄^2- concentrations were notable for being higher in fog than rain water when compared with other ions. For trace elements, Al, As, Mn and Se were the most abundant elements in fog water; only Al and As were detected in rain water. Seventy-two hour back-trajectory analysis showed that air masses during fog and/or rain events mainly came from the south of Sejila Mountain. Spearman correlation analysis and source contribution calculations indicated that both marine and terrestrial sources contributed to the observed ion concentrations. Considering the higher concentrations of NH₄⁺ and higher ratio of NH₄⁺/NO₃^- measured in fog than in rain, we suggest that quantification of fog nitrogen deposition and its ecological effect in this area should be given more attention.

The use of a common epiphytic lichen as a bioindicator of atmospheric inputs to two Venezuelan cloud forests

The chemistry of epiphytic lichens was determined in two Venezuelan cloud forests adjacent to the Caracas Valley at Altos de Pipe and El Avila National Park. This is the second component of a two-part study examining the chemistry of fogs and their effects on sensitive biota in these forests. The widespread lichen Parmotrema madagascariaceum was chosen as an indicator of trace metal input. Background lichen collections and transplant experiments were carried out to evaluate spatial trends in atmospheric depositon of trace metals at different altitudes and exposures of the two montane locations. Elevated concentratons of metals including Pb and Zn were seen in lichen tissue and associated bark. Lead levels in lichens reached 190 μg g-1dry wt. Lichens transplanted from relatively ‘clean’ to more polluted sites showed significant increases of metals, particularly Pb. Increases of up to 6.3 μg Pb g-1mo-1after 6 and 10 months were recorded relative to background andin situlichen material. In contrast, when lichsens were reciprocally transplanted in nylon net bags from less to more exposed sites there was a significant decrease in Pb and Zn concentrations after 6 and 10 months. With long-term exposure to elevated concentrations of these trace metals epiphytic lichens, as well as other sensitive cloud forest biota, may be at risk of decline.

Altitude-dependent wet, dry and occult nitrogen deposition in an Alpine region (Achenldrch, Austria, 920 m - 1758 m a.s.l.)

From November 1995 to October 1996 and from October 1997 to September 1998, samples of wet precipitation, cloud water as well as of reactive gases and particulate matter, were collected at three elevational levels (920 m, 1,280 m and 1,758 m a.s.l.) in Achenkirch, Austria. The samples were analysed for ammonium and nitrate in wet precipitation and in cloud water, for ammonia, nitric acid and nitrogen dioxide in the gas phase and for particulate ammonium and particulate nitrate in aerosol. Total nitrogen deposition was calculated combining measured concentrations in wet, dry and occult depositions with the corresponding deposition fluxes. Two multilayer deposition models were used for the calculation of dry and occult deposition. The total nitrogen input in 1995/96 was estimated to be 29 kg N ha(-1)a(-1) at the Christlumkopf station (1,758 m), 20 kg N ha(-1)a(-1) at the Christlumalm station (1,280 m) and 28 kg N ha(-1)a(-1) at the Talboden station (930 m). Respective data for the 1997/98 observation period were 31 kg N ha(-1)a(-1) at the Christlumkopf station (1,758 m) and 18 kg N ha(-1)a(-1) at the Mülhleggerköpfl station (920 m). Critical Loads of nitrogen for coniferous forests were exceeded significantly near-source regions represented by areas of intense agricultural use and at high elevation sites.

High frequency and large deposition of acid fog on high elevation forest

We have collected and analyzed fogwater on the mountainside of Mt. Oyama (1252 m) in the Tanzawa Mountains of Japan and observed the fog event frequency from the base of the mountain with a video camera. The fog event frequency increased with elevation and was observed to be present 46% of the year at the summit. The water deposition via throughfall increased with elevation because of the increase in fogwater interception and was about twice that via rain at the summit, where the air pollutant deposition via throughfall was several times that via rainwater. The dry deposition and the deposition via fogwater were dominant factors in the total ion deposition at high elevation sites. In a fog event, nitric acid, the major acid component on the mountain, is formed during the transport of the air mass from the base of the mountain along the mountainside, where gases including nitric acid deposit and are scavenged by fogwater. Therefore, high acidity caused by nitric acid and relatively low ion strength are observed in the fogwater at high elevation sites.

Direct damage to vegetation caused by acid rain and polluted cloud: definition of critical levels for forest trees

The concept of critical levels was developed in order to define short-term and long-term average concentrations of gaseous pollutants above which plants may be damaged. Although the usual way in which pollutants in precipitation (wet deposition) influence vegetation is by affecting soil processes, plant foliage exposed to fog and cloud, which often contain much greater concentrations of pollutant ions than rain, may be damaged directly. The idea of a critical level has been extended to define concentrations of pollutants in wet deposition above which direct damage to plants is likely. Concentrations of acidity and sulphate measured in mountain and coastal cloud are summarised. Vegetation at risk of injury is identified as montane forest growing close to the cloud base, where ion concentrations are highest. The direct effects of acidic precipitation on trees are reviewed, based on experimental exposure of plants to simulated acidic rain, fog or mist. Although most experiments have reported results in terms of pH (H(+) concentration), the accompanying anion is important, with sulphate being more damaging than nitrate. Both conifers and broadleaved tree seedlings showing subtle changes in the structural characteristics of leaf surfaces after exposure to mist or rain at or about pH 3.5, or sulphate concentration of 150 micromol litre(-1). Visible lesions on leaf surfaces occur at around pH 3 (500 micromol litre(-1) sulphate), broadleaved species tending to be more sensitive than conifers. Effects on photosynthesis and water relations, and interactions with other stresses (e.g. frost), have usually been observed only for treatments which have also caused visible injury to the leaf surface. Few experiments on the direct effects of polluted cloud have been conducted under field conditions with mature trees, which unlike seedlings in controlled conditions, may suffer a growth reduction in the absence of visible injury. Although leaching of cations (Ca(2+), Mg(2+), K(+)) is stimulated by acidic precipitation, amounts leached are small compared with root uptake, unless soils have been impoverished. This aspect of the potential effects of acidic precipitation is best considered in terms of the long-term critical-load of pollutants to the soil. Given the practical difficulties in monitoring cloud water composition, a method for defining critical levels is proposed, which uses climatological average data to identify the duration and frequency of hill cloud, and combines this information with measured or modelled concentrations of particulate sulphate in the atmosphere, to derive cloud water concentrations as a function of cloud liquid water content. For forests within 100 m of the cloud base the critical levels of particulate sulphate, corresponding to solution concentrations in the range 150-500 micromol litre(-1), are in the range 1-3.3 microg S m(-3). These concentrations are observed over much of central Europe, suggesting that many montane forests are at risk of direct effects of fossil-fuel-derived pollutants in cloud.

Chemical climatology of high elevation spruce-fir forests in the southern Appalachian mountains

The physical and chemical climatology of high elevation (> 1500 m) spruce-fir forests in the southern Appalachian mountains was studied by establishing a weather and atmospheric chemical observatory at Mt Mitchell State Park in North Carolina (35 degrees 44' 05" N, 82 degrees 17' 15"W). Data collected during the summer and autumn (May-October) of 1986, 1987, and 1988 are reported. All measurements were made on or near a 16.5 m walk-up tower extending 10 m above the forest canopy on Mt Gibbes (2006 m msl), which is located approximately 2 km SW of Mt Mitchell. The tower was equipped with standard meteorological instrumentation, a passive cloud water collector, and gas pollutant sensors for O3, SO2, NOx. The tower and nearby forest canopy were immersed in clouds 25 to 40% of the time. Non-precipitating clouds were very acidic (pH 2.5-4.5). Precipitating clouds were less acidic (pH 3.5-5.5). The dominant wind directions were WNW and ESE. Clouds from the most common wind direction (WNW) were more acidic (mean pH 3.5) than those from the next most common wind direction (ESE, mean pH 5.5). Cloud water acidity was related to the concentration of SO4(2-), and NO3- ions. Mean concentration of H+, NH4+, SO4(2-), and NO3- ions in the cloud water varied from 330-340, 150-200, 190-200 and 120-140 micromol litre(-1) respectively. The average and range of O3 were 50 (25-100) ppbv (109) in 1986, 51 (26-102) ppbv in 1987, and 66 (30-140) during the 1988 field seasons, respectively. The daily maximum, 1-h average, and 24-h average concentrations were all greatest during June through mid-August, suggesting a correlation with the seasonal temperature and solar intensity. Throughfall collectors near the tower were used to obtain a useful estimate of deposition to the forest canopy. Between 50-60% of the total deposition of SO4(2-) was due to cloud impact.