The Asia Pacific Mercury Monitoring Network (APMMN) cooperatively measures mercury in precipitation in a network of sites operating in Asia and the Western Pacific region. The network addresses significant data gaps in a region where mercury emission estimates are the highest globally, and available measurement data are limited. The reduction of mercury emissions under the Minamata Convention on Mercury also justifies the need for continent-wide and consistent observations that can help determine the magnitude of the problem and assess the efficacy of reductions over time. The APMMN’s primary objectives are to monitor wet deposition and atmospheric concentrations of mercury and assist partners in developing their own monitoring capabilities. Network planning began in 2012 with wet deposition sampling starting in 2014. Currently, eight network sites measure mercury in precipitation following standardized procedures adapted from the National Atmospheric Deposition Program. The network also has a common regional analytical laboratory (Taiwan), and quality assurance and data flagging procedures, which ensure the network makes scientifically valid and consistent measurements. Results from our ongoing analytical and field quality assurance measurements show minimal contamination in the network and accurate analytical analyses. We are continuing to monitor a potential concentration and precipitation volume bias under certain conditions. The average mercury concentration in precipitation was 11.3 (+9.6) ng L⁻¹ for 139 network samples in 2018. Concentrations for individual sites vary widely. Low averages compare to the low concentrations observed on the U.S. West Coast; while other sites have average concentrations similar to the high values reported from many urban areas in China. Future APMMN goals are to (1) foster new network partnerships, (2) continue to collect, quality assure, and distribute results on the APMMN website, (3) provide training and share best monitoring practices, and (4) establish a gaseous concentration network for estimating dry deposition.

Black carbon (BC) aerosols emitted from biomass, fossil fuel, and waste combustion contribute to the radiation budget imbalance and are transported over extensive distances in the Earth’s atmosphere. These aerosols undergo physical and chemical modifications with co-existing aerosols (e.g., nitrate, sulfate, ammonium) through aging processes during long-range transport and are primarily removed from the troposphere by wet deposition. Using precipitation samples collected in North America between 26 October and 1 December 2020 by the National Atmospheric Deposition Program (NADP), we investigated the relationships between BC and both water-soluble ions and water-soluble organic carbon (WSOC) using Spearman’s rank coefficients. We then attempted to identify the sources of BC in the wet deposition using factor analysis (FA) and satellite data of fire smoke. BC showed a very strong correlation with nitrate (ρ = 0.83). Strong correlations were also found with WSOC, ammonium, calcium, and sulfate ions (ρ = 0.78, 0.74, 0.74, and 0.67, respectively). FA showed that BC was in the same factor as nitrate, ammonium, sulfate, and WSOC, indicating that BC could originate from secondary aerosol formation and biomass burning. Supported by satellite data of fire and smoke, BC and other correlated pollutants were believed to be associated with wildfire outbreaks in several states in the United States (US) during November 2020.

Air pollution has long posed a significant environmental challenge in the North American region, including both the USA and Mexico. Among its impacts, acid rain, which is characterized by a pH lower than 5.6, negatively affects ecosystems and biodiversity. This study evaluates and compares the physicochemical characteristics of precipitation in the urban areas of Denver, New York City, Los Angeles, and the Mexico City Metropolitan Area (MCMA) during the period 2003–2019. Long-term analysis revealed a substantial decline in sulfate concentrations in New York City after 2009, accompanied by a gradual rise in precipitation pH. In contrast, sulfate levels in the MCMA remained elevated and variable, while Los Angeles exhibited persistently acidic precipitation despite relatively low sulfate concentrations. Nitrate trends were weak or irregular across cities, and ammonium increased only in New York City. Results indicated the presence of acid rain in New York City, Los Angeles, and the MCMA, but not in Denver, where precipitation in 2017 did not exhibit acidic pH values. The highest concentrations of SO₄²⁻ and NO₃⁻ in precipitation were observed in the MCMA, likely due to SO₂ emissions from the Tula–Vito–Apasco industrial corridor and high NO_x emissions from mobile sources. The SO₄²⁻/NO₃⁻ ratio was used to determine the predominant ion influencing precipitation acidity. SO₄²⁻ was dominant in New York City and the MCMA, whereas NO₃⁻ was more prevalent in Los Angeles and Denver. Strategies implemented in the USA, such as the transition to cleaner fuels, policies for monitoring emission sources, and the expansion of atmospheric deposition networks, could inform efforts to reduce emissions of acid rain precursors in the MCMA.