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Abstract. Nitrous oxide (N2O) is a potent greenhouse gas emitted by oceanic and terrestrial sources, with its biogeochemical cycle influenced by both natural processes and anthropogenic activities. Current atmospheric N2O monitoring networks, including tall-tower and flask measurements, often overlook major marine hotspots, such as the eastern tropical Pacific Ocean. We present the first 15 months of high-frequency continuous measurements of N2O and carbon monoxide from the newly established Galapagos Emissions Monitoring Station (GEMS) in this region. Over this period, N2O mole fractions vary by approximately 5 ppb, influenced by seasonal trade winds, local anthropogenic emissions, and air masses transported from marine N2O hotspots. Notably, between February and April 2024, we observe high variability linked to the southward shift of the intertropical convergence zone and weakened trade winds over the Galapagos Islands. Increased variability during this period is driven by stagnant local winds, which accumulate emissions, and the mixing of air masses with different N2O content from the northern and southern hemispheres. The remaining variability is primarily due to differences in air mass transport and heterogeneity in surface fluxes from the eastern tropical Pacific. Air masses passing over the Peruvian and Chilean upwelling systems— key sources of oceanic N2O efflux — show markedly higher N2O mole fractions at the GEMS station. 

Abstract. A portion of Alaska's Fairbanks North Star Borough was designated as nonattainment for the 2006 24 h fine particulate matter 2.5 µm or less in diameter (PM2.5) National Ambient Air Quality Standards (NAAQS) in 2009. PM2.5 NAAQS exceedances in Fairbanks mainly occur during dark and cold winters, when temperature inversions form and trap high emissions at the surface. Sulfate (SO42-), often the second-largest contributor to PM2.5 mass during these wintertime PM episodes, is underpredicted by atmospheric chemical transport models (CTMs). Most CTMs account for primary SO42- and secondary SO42- formed via gas-phase oxidation of sulfur dioxide (SO2) and in-cloud aqueous oxidation of dissolved S(IV). Dissolution and reaction of SO2 in aqueous aerosols are generally not included in CTMs but can be represented as heterogeneous reactive uptake and may help better represent the high SO42- concentrations observed during Fairbanks winters. In addition, hydroxymethanesulfonate (HMS), a particulate sulfur species sometimes misidentified as SO42-, is known to form during Fairbanks winters. Heterogeneous formation of SO42- and HMS in aerosol liquid water (ALW) was implemented in the Community Multiscale Air Quality (CMAQ) modeling system. CMAQ simulations were performed for wintertime PM episodes in Fairbanks (2008) as well as over the Northern Hemisphere and Contiguous United States (CONUS) for 2015–2016. The added heterogeneous sulfur chemistry reduced model mean sulfate bias by ∼ 0.6 µg m−3 during a cold winter PM episode in Fairbanks, AK. Improvements in model performance are also seen in Beijing during wintertime haze events (reducing model mean sulfate bias by ∼ 2.9 µg S m−3). This additional sulfur chemistry also improves modeled summertime SO42- bias in the southeastern US, with implications for future modeling of biogenic organosulfates. 

Abstract Air pollution in Africa is a significant public health issue responsible for 1.1 million premature deaths annually. Sub-Saharan Africa has the highest rate of population growth and urbanization of any region in the world, with substantial potential for future emission growth and worsening air quality. Accurate and extensive observations of meteorology and atmospheric composition have underpinned successful air pollution mitigation strategies in the Global North, yet Africa in general and East Africa in particular remain among the most sparsely observed regions in the world. This paper is based on the discussion of these issues during two international workshops, one held virtually in the United States in July 2021 and one in Kigali, Rwanda, in January 2023. The workshops were designed to develop a measurement, capacity building, and collaboration strategy to improve air quality-relevant measurements, modeling, and data availability in East Africa. This paper frames the relevant scientific needs and describes the requirements for training and infrastructure development for an integrated observing and modeling strategy that includes partnerships between East African scientists and organizations and their counterparts in the developed world. 

Abstract. Atmospheric aerosols are a significant public health hazard and havesubstantial impacts on the climate. Secondary organic aerosols (SOAs) havebeen shown to phase separate into a highly viscous organic outer layersurrounding an aqueous core. This phase separation can decrease thepartitioning of semi-volatile and low-volatile species to the organic phaseand alter the extent of acid-catalyzed reactions in the aqueous core. A newalgorithm that can determine SOA phase separation based on their glasstransition temperature (Tg), oxygen to carbon (O:C) ratio and organic massto sulfate ratio, and meteorological conditions was implemented into theCommunity Multiscale Air Quality Modeling (CMAQ) system version 5.2.1 andwas used to simulate the conditions in the continental United States for thesummer of 2013. SOA formed at the ground/surface level was predicted to bephase separated with core–shell morphology, i.e., aqueous inorganic coresurrounded by organic coating 65.4 % of the time during the 2013 SouthernOxidant and Aerosol Study (SOAS) on average in the isoprene-rich southeasternUnited States. Our estimate is in proximity to the previously reported∼70 % in literature. The phase states of organic coatingsswitched between semi-solid and liquid states, depending on theenvironmental conditions. The semi-solid shell occurring with lower aerosolliquid water content (western United States and at higher altitudes) has aviscosity that was predicted to be 102–1012 Pa s, whichresulted in organic mass being decreased due to diffusion limitation.Organic aerosol was primarily liquid where aerosol liquid water was dominant(eastern United States and at the surface), with a viscosity <102 Pa s.Phase separation while in a liquid phase state, i.e.,liquid–liquid phase separation (LLPS), also reduces reactive uptake ratesrelative to homogeneous internally mixed liquid morphology but was lowerthan aerosols with a thick viscous organic shell. The sensitivity casesperformed with different phase-separation parameterization and dissolutionrate of isoprene epoxydiol (IEPOX) into the particle phase in CMAQ can havevarying impact on fine particulate matter (PM2.5) organic mass, interms of bias and error compared to field data collected during the 2013 SOAS.This highlights the need to better constrain the parameters thatgovern phase state and morphology of SOA, as well as expand mechanisticrepresentation of multiphase chemistry for non-IEPOX SOA formation in modelsaided by novel experimental insights. 

Methyltetrol sulfates are unique tracers for secondary organic aerosols (SOA) formed from acid-driven multiphase chemistry of isoprene-derived epoxydiols. 2-Methyltetrol sulfate diastereomers (2-MTSs) are the dominant isomers and single most-abundant SOA tracers in atmospheric fine particulate matter (PM2.5), but their atmospheric sinks remain unknown. We investigated the oxidative aging of authentic 2-MTS aerosols by gas-phase hydroxyl radicals (•OH) at a relative humidity of 61 ± 1%. The effective rate constant for this heterogeneous reaction was determined as 4.9 ± 0.6 × 10–13 cm3 molecules–1 s–1, corresponding to an atmospheric lifetime of 16 ± 2 days (assuming an •OH concentration of 1.5 × 106 molecules cm–3). Chemical changes to 2-MTSs were monitored by hydrophilic interaction liquid chromatography interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Plausible reaction mechanisms are proposed for previously unknown OSs detected in atmospheric PM2.5 at mass-to-charge ratios (m/z) of 139 (C2H3O5S–), 155 (C2H3O6S–), 169 (C3H5O6S–), 171 (C3H7O6S–), 185 (C3H5O7S–), 199 (C4H7O7S–), 211 (C5H7O7S–), 213 (C5H9O7S–), 227 (C5H7O8S–), 229 (C5H9O8S–), and 231 (C5H11O8S–). Heterogeneous •OH oxidation of 2-MTSs redistributes the particulate sulfur speciation into more oxygenated/functionalized OSs, likely modifying the aerosol physicochemical properties of SOA containing 2-MTSs.