Observations of the chemistry and concentrations of reactive Hg at locations with different ambient air chemistry

The Hg research community needs methods to more accurately measure atmospheric Hg concentrations and chemistry. The Reactive Mercury Active System (RMAS) uses cation exchange, nylon, and PTFE membranes to determine reactive mercury (RM), gaseous oxidized mercury, and particulate-bound mercury (PBM) concentrations and chemistry. New data for Atlanta, Georgia (NRGT) demonstrated that particulate-bound Hg was dominant and the chemistry was primarily N and S HgII compounds. At Great Salt Lake, Utah (GSL), RM was predominately PBM, with NS > organics > halogen > O HgII compounds. At Guadalupe Mountains National Park, Texas (GUMO), halogenated compound concentrations were lowest when air interacting with the site was primarily derived from the Midwest, and highest when the air was sourced from Mexico. At Amsterdam Island, Southern Indian Ocean, compounds were primarily halogenated with some N, S, and organic HgII compounds potentially associated with biological activity. The GEOS-Chem model was applied to see if it predicted measurements at five field sites. Model values were higher than observations at GSL, slightly lower at NRGT, and observations were an order of magnitude higher than modeled values for GUMO and Reno, Nevada. In general, data collected from 13 locations indicated that N, S, and organic RM compounds were associated with city and forest locations, halogenated compounds were sourced from the marine boundary layer, and O compounds were associated with long-range transport. Data being developed currently, and in the past, suggest there are multiple forms of RM that modelers must consider, and PBM is an important component of RM.

Over a decade of atmospheric mercury monitoring at Amsterdam Island in the French Southern and Antarctic Lands

The Minamata Convention, a global and legally binding treaty that entered into force in 2017, aims to protect human health and the environment from harmful mercury (Hg) effects by reducing anthropogenic Hg emissions and environmental levels. The Conference of the Parties is to periodically evaluate the Convention's effectiveness, starting in 2023, using existing monitoring data and observed trends. Monitoring atmospheric Hg levels has been proposed as a key indicator. However, data gaps exist, especially in the Southern Hemisphere. Here, we present over a decade of atmospheric Hg monitoring data at Amsterdam Island (37.80°S, 77.55°E), in the remote southern Indian Ocean. Datasets include gaseous elemental and oxidised Hg species ambient air concentrations from either active/continuous or passive/discrete acquisition methods, and annual total Hg wet deposition fluxes. These datasets are made available to the community to support policy-making and further scientific advancements.

Observation and quantification of aerosol outflow from southern Africa using spaceborne lidar

Biomass burning in Africa provides a prolific source of aerosols that are transported from the source region to distant areas, as far away as South America and Australia. Models have long predicted the primary outflow and transport routes. Over time, field studies have validated the basic production and dynamics that underlie these transport patterns. In more recent years, the advancement of spaceborne active remote-sensing techniques has allowed for more detailed verification of the models and, importantly,verification of the vertical distribution of the aerosols in the transport regions, particularly with respect to westerly transport over the Atlantic Ocean. The Cloud-Aerosol Transport System (CATS) lidar on the International Space Station has detection sensitivity that provides observations that support long-held theories of aerosol transport from the African subcontinent over the remote Indian Ocean and as far downstream as Australia.

Source apportionment of rainwater chemical composition to investigate the transport of lower atmospheric pollutants to the UTLS region

Efforts to understand the chemical composition of Asian Tropopause Aerosol Layer (ATAL) in the Upper Troposphere Lower Stratosphere (UTLS) region have revealed the dominance of nitrates in the samples collected from ATAL layer during the recent balloon campaigns. Potential sources have been thought to be in-situ formation, convective uplift and long-range transport. Rainwater chemical composition consists water-soluble chemical ions that are wet scavenged during rain events and gives an indirect indication of lower atmospheric pollutants. Keeping this in focus, total monsoon precipitation chemistry at Gadanki (13.5°N, 79.2°E) has been studied to understand the convective uplift possibilities to the UTLS region. About 32 rainwater samples collected during July to December 2017 were analysed for their chemical composition using Ion Chromatography. Total 16 ions comprising of 5 anions (F^-, Cl^-, NO₃^-, SO₄^2- and PO₄^3-), 6 cations (Na⁺, K⁺, Ca²⁺, Mg²⁺, Li⁺ and NH₄⁺) and 5 trace metals (Cd²⁺, Ni²⁺, Co²⁺, Mn²⁺ and Zn²⁺) have been detected in different rainwater samples. Rainwater chemical composition data has been subjected to the Principal Component Analysis (PCA) to understand the correlations between different chemical species and to identify the possible sources of origin qualitatively. It has been observed that the chemical composition of the rainwater is very different from the chemical composition of the ATAL layer indicating non-existence of convective transport of lower level pollutants to the UTLS region at Gadanki. This observation is also well supported by the vertical distribution of CALIPSO derived aerosol types and ERA interim vertical pressure velocities during the sampling period.

Chemical characterization of rainwater at a high-altitude site "Nainital" in the central Himalayas, India

The present study investigates the chemical composition of rainwater (RW) from a high-altitude site "Nainital" (1958 m above msl) in the central Himalaya region, to understand the influence of local, regional, and long-range transport of pollutants. A total of 55 (2 in pre-monsoon and 53 in monsoon) RW samples were collected during the study period (June-September 2012) and were analyzed for major anions and cations using an ion chromatograph. The pH of precipitation events ranged from 4.95 to 6.50 (average 5.6 ± 0.3) was observed during the monsoon period (near to the acidic), whereas during the pre-monsoon, the pH was 6.25 ± 0.49 (alkaline) over the study region; it is due the mixture of anthropogenic as well as the natural chemical constituents. The average ionic concentration (sum of measured chemical constituents) was ∼3 times higher during the pre-monsoon (986 ± 101 μeq/1) compared to that in the monsoon season (373 ± 37 μeq/1). This is mainly due to the presence of more natural aerosols in the pre-monsoon season which is also reflected in the pH of rainwater (average 6.25 ± 0.50) as well as ionic concentration. The chemical composition suggests that Ca²⁺ was the major contributor (34%) among cations, followed by Na⁺ (10%), K⁺ (8%), and Mg²⁺ (9%), whereas Cl^-, NO₃^-, and SO₄^2- contributed ∼13, 11, and 9%, respectively, among anions. The average ratio of acidic species (SO₄^2-/NO₃^-) is 1.56, suggesting 61 and 39% contribution of SO₄^2- and NO₃^-, respectively, which is very close to the estimated contribution of H₂SO₄ (60-70%) and HNO₃ (30-40%) in the precipitation samples. Neutralization factors for Ca²⁺, Mg²⁺, and NH₄⁺ in RW at Nainital are 4.94, 1.21, and 0.37, respectively, indicating their crucial role in neutralization of acidic species. The non-sea-salt (NSS) contribution to total Ca²⁺, K⁺, and Mg²⁺ is estimated to be ∼98, 97, and 74%, respectively, suggesting the dominance of crustal sources for cations. In contrast, the NSS contribution to the total Cl^- and SO₄^2- is 16 and 69% indicating their anthropogenic origin, respectively. Principle component analysis also suggests that the first factor (i.e., natural sources, mainly dust, and sea-salts) accounts for ∼33% variance, whereas the second factor (i.e., fossil fuel and biomass burning) accounts for ∼18% variance of the measured ionic composition. The remaining contributions are attributed to the mixed emission sources and transport of pollutants from Indo-Gangetic Plain (IGP) and western parts of India. The results of the present study reveal a significant contribution of crustal and anthropogenic sources in the RW and neutralization processes in the central Himalaya.