Differences in Model Performance and Source Sensitivities for Sulfate Aerosol Resulting from Updates of the Aqueous- and Gas-Phase Oxidation Pathways for a Winter Pollution Episode in Tokyo, Japan

During the Japanese intercomparison study, Japan’s Study for Reference Air Quality Modeling (J-STREAM), it was found that wintertime SO42– concentrations were underestimated over Japan with the Community Multiscale Air Quality (CMAQ) modeling system. Previously, following two development phases, model performance was improved by refining the Fe- and Mn-catalyzed oxidation pathways and by including an additional aqueous-phase pathway via NO2 oxidation. In a third phase, we examined a winter haze period in December 2016, involving a gas-phase oxidation pathway whereby three stabilized Criegee intermediates (SCI) were incorporated into the model. We also included options for a kinetic mass transfer aqueous-phase calculation. According to statistical analysis, simulations compared well with hourly SO42– observations in Tokyo. Source sensitivities for four domestic emission sources (transportation, stationary combustion, fugitive VOC, and agricultural NH3) were investigated. During the haze period, contributions from other sources (overseas and volcanic emissions) dominated, while domestic sources, including transportation and fuel combustion, played a role in enhancing SO42– concentrations around Tokyo Bay. Updating the aqueous phase metal catalyzed and NO2 oxidation pathways lead to increase contribution from other sources, and the additional gas phase SCI chemistry provided a link between fugitive VOC emission and SO42– concentration via changes in O3 concentration.

Modeling oceanic nitrate and nitrite concentrations and isotopes using a 3-D inverse N cycle model

Nitrite (NO2-) is a key intermediate in the marine nitrogen (N) cycle and a substrate in nitrification, which produces nitrate (NO3-), as well as water column N loss processes denitrification and anammox. In models of the marine N cycle, NO2- is often not considered as a separate state variable, since NO3- occurs in much higher concentrations in the ocean. In oxygen deficient zones (ODZs), however, NO2- represents a substantial fraction of the bioavailable N, and modeling its production and consumption is important to understand the N cycle processes occurring there, especially those where bioavailable N is lost from or retained within the water column. Improving N cycle models by including NO2- is important in order to better quantify N cycling rates in ODZs, particularly N loss rates. Here we present the expansion of a global 3-D inverse N cycle model to include NO2- as a reactive intermediate as well as the processes that produce and consume NO2- in marine ODZs. NO2- accumulation in ODZs is accurately represented by the model involving NO3- reduction, NO2- reduction, NO2- oxidation, and anammox. We model both 14N and 15N and use a compilation of oceanographic measurements of NO3- and NO2- concentrations and isotopes to place a better constraint on the N cycle processes occurring. The model is optimized using a range of isotope effects for denitrification and NO2- oxidation, and we find that the larger (more negative) inverse isotope effects for NO2- oxidation, along with relatively high rates of NO2-, oxidation give a better simulation of NO3- and NO2- concentrations and isotopes in marine ODZs.

Modeling oceanic nitrite concentrations and isotopes using a 3D inverse N cycle model

Nitrite (NO2−) is a key intermediate in the marine nitrogen (N) cycle and a substrate in nitrification, which produces nitrate (NO3−), as well as water column N loss processes, denitrification and anammox. In models of the marine N cycle, NO2− is often not considered as a separate state variable, since NO3− occurs in much higher concentrations in the ocean. In oxygen deficient zones (ODZs), however, NO2− represents a substantial fraction of the bioavailable N, and modeling its production and consumption is important to understanding the N cycle processes occurring there, especially those where bioavailable N is lost from or retained within the water column. Here we present the expansion of a global 3D inverse N cycle model to include NO2− as a reactive intermediate as well as the processes that produce and consume NO2− in marine ODZs. NO2− accumulation in ODZs is accurately represented by the model involving NO3− reduction, NO2− reduction, NO2− oxidation, and anammox. We model both 14N and 15N and use a compilation of oceanographic measurements of NO3− and NO2− concentrations and isotopes to place a better constraint on the N cycle processes occurring. The model is optimized using a range of isotope effects for denitrification and NO2− oxidation, and we find that the larger (more negative) inverse isotope effects for NO2− oxidation along with relatively high rates of NO2− oxidation give a better simulation of NO3− and NO2− concentrations and isotopes in marine ODZs.

Grafted iron(iii) ions significantly enhance NO2 oxidation rate and selectivity of TiO2 for photocatalytic NOx abatement

By grafting small amounts of iron ions onto TiO2, the rate of photocatalytic oxidation of NO2 is increased by a factor of 9.

Nitrification and ammonium dynamics in Lake Taihu, China: seasonal competition for ammonium between nitrifiers and cyanobacteria

Taihu Lake is hypereutrophic and experiences seasonal, cyanobacterial harmful algal blooms. These Microcystis blooms produce microcystin, a potent liver toxin, and are linked to anthropogenic nitrogen (N) and phosphorus (P) loads to lakes. Microcystis spp. cannot fix atmospheric N and must compete with ammonia-oxidizing and other organisms for ammonium (NH4+). We measured NH4+ regeneration and potential uptake rates and total nitrification using stable isotope techniques. Nitrification studies included separate NH4+ and nitrite (NO2−) oxidation rates and abundance of the functional gene for NH4+ oxidation, amoA, for ammonia-oxidizing archaea (AOA) and bacteria (AOB). Potential NH4+ uptake rates ranged from 0.02–6.80 µmol L−1 hr−1 in the light and 0.05–3.33 µmol L−1 hr−1 in the dark, and NH4+ regeneration rates ranged from 0.03–2.37 µmol L−1 hr−1. Nitrification rates exceeded previously reported rates in most freshwater systems. Total nitrification often exceeded 200 nmol L−1 d−1 and exceeded 1000 nmol L−1 d−1 at one station near a river discharge. In Meiliang Bay and the open lake, average NO2− oxidation rates (248 ± 39.0 nmol L−1 d−1) exceeded NH4+ oxidation rates (22.0 ± 6.00 nmol L−1 d−1; p