Professional Education

  • Doctor of Science, Johannes Gutenberg Universitat Mainz (2020)
  • PhD, Max Planck Institute for Chemistry, Germany, Atmospheric chemistry (2020)

Stanford Advisors

Community and International Work

  • Editorial Board Member, Communications Earth & Environment

    Ongoing Project


    Opportunities for Student Involvement


  • Blog Editor, European Geoscience Union

    Ongoing Project


    Opportunities for Student Involvement


  • Early Career Scientist Representative (2020-2022), European Geoscience Union

    Ongoing Project


    Opportunities for Student Involvement


Current Research and Scholarly Interests

atmospheric gases: trends and emissions, such as methane, volatile organic compounds.
atmospheric observations: ground, airborne, satellite remote sensing.
atmospheric measurement techniques.
atmospheric modeling.
indoor air chemistry and human emissions.
climate change.

All Publications

  • Carbonyl Sulfide (OCS) in the Upper Troposphere/Lowermost Stratosphere (UT/LMS) Region: Estimates of Lifetimes and Fluxes GEOPHYSICAL RESEARCH LETTERS Karu, E., Li, M., Ernle, L., Brenninkmeijer, C. M., Lelieveld, J., Williams, J. 2023; 50 (19)
  • Northern hemispheric atmospheric ethane trends in the upper troposphere and lower stratosphere (2006-2016) with reference to methane and propane EARTH SYSTEM SCIENCE DATA Li, M., Pozzer, A., Lelieveld, J., Williams, J. 2022; 14 (9): 4351-4364
  • The human oxidation field. Science (New York, N.Y.) Zannoni, N., Lakey, P. S., Won, Y., Shiraiwa, M., Rim, D., Weschler, C. J., Wang, N., Ernle, L., Li, M., Bekö, G., Wargocki, P., Williams, J. 2022; 377 (6610): 1071-1077


    Hydroxyl (OH) radicals are highly reactive species that can oxidize most pollutant gases. In this study, high concentrations of OH radicals were found when people were exposed to ozone in a climate-controlled chamber. OH concentrations calculated by two methods using measurements of total OH reactivity, speciated alkenes, and oxidation products were consistent with those obtained from a chemically explicit model. Key to establishing this human-induced oxidation field is 6-methyl-5-hepten-2-one (6-MHO), which forms when ozone reacts with the skin-oil squalene and subsequently generates OH efficiently through gas-phase reaction with ozone. A dynamic model was used to show the spatial extent of the human-generated OH oxidation field and its dependency on ozone influx through ventilation. This finding has implications for the oxidation, lifetime, and perception of chemicals indoors and, ultimately, human health.

    View details for DOI 10.1126/science.abn0340

    View details for PubMedID 36048928

  • Human metabolic emissions of carbon dioxide and methane and their implications for carbon emissions SCIENCE OF THE TOTAL ENVIRONMENT Li, M., Bekoe, G., Zannoni, N., Pugliese, G., Carrito, M., Cera, N., Moura, C., Wargocki, P., Vasconcelos, P., Nobre, P., Wang, N., Ernle, L., Williams, J. 2022; 833: 155241


    Carbon dioxide (CO2) and methane (CH4) are important greenhouse gases in the atmosphere and have large impacts on Earth's radiative forcing and climate. Their natural and anthropogenic emissions have often been in focus, while the role of human metabolic emissions has received less attention. In this study, exhaled, dermal and whole-body CO2 and CH4 emission rates from a total of 20 volunteers were quantified under various controlled environmental conditions in a climate chamber. The whole-body CO2 emissions increased with temperature. Individual differences were the most important factor for the whole-body CH4 emissions. Dermal emissions of CO2 and CH4 only contributed ~3.5% and ~5.5% to the whole-body emissions, respectively. Breath measurements conducted on 24 volunteers in a companion study identified one third of the volunteers as CH4 producers (exhaled CH4 exceeded 1 ppm above ambient level). The exhaled CH4 emission rate of these CH4 producers (4.03 ± 0.71 mg/h/person, mean ± one standard deviation) was ten times higher than that of the rest of the volunteers (non-CH4 producers; 0.41 ± 0.45 mg/h/person). With increasing global population and the expected large reduction in global anthropogenic carbon emissions in the next decades, metabolic emissions of CH4 (although not CO2) from humans may play an increasing role in regional and global carbon budgets.

    View details for DOI 10.1016/j.scitotenv.2022.155241

    View details for Web of Science ID 000805772900010

    View details for PubMedID 35421492

  • Ozone Initiates Human-Derived Emission of Nanocluster Aerosols. Environmental science & technology Yang, S., Licina, D., Weschler, C. J., Wang, N., Zannoni, N., Li, M., Vanhanen, J., Langer, S., Wargocki, P., Williams, J., Bekö, G. 2021; 55 (21): 14536-14545


    Nanocluster aerosols (NCAs, particles <3 nm) are important players in driving climate feedbacks and processes that impact human health. This study reports, for the first time, NCA formation when gas-phase ozone reacts with human surfaces. In an occupied climate-controlled chamber, we detected NCA only when ozone was present. NCA emissions were dependent on clothing coverage, occupant age, air temperature, and humidity. Ozone-initiated chemistry with human skin lipids (particularly their primary surface reaction products) is the key mechanism driving NCA emissions, as evidenced by positive correlations with squalene in human skin wipe samples and known gaseous products from ozonolysis of skin lipids. Oxidation by OH radicals, autoxidation reactions, and human-emitted NH3 may also play a role in NCA formation. Such chemical processes are anticipated to generate aerosols of the smallest size (1.18-1.55 nm), whereas larger clusters result from subsequent growth of the smaller aerosols. This study shows that whenever we encounter ozone indoors, where we spend most of our lives, NCAs will be produced in the air around us.

    View details for DOI 10.1021/acs.est.1c03379

    View details for PubMedID 34672572

  • Effect of Ozone, Clothing, Temperature, and Humidity on the Total OH Reactivity Emitted from Humans. Environmental science & technology Zannoni, N., Li, M., Wang, N., Ernle, L., Bekö, G., Wargocki, P., Langer, S., Weschler, C. J., Morrison, G., Williams, J. 2021; 55 (20): 13614-13624


    People influence indoor air chemistry through their chemical emissions via breath and skin. Previous studies showed that direct measurement of total OH reactivity of human emissions matched that calculated from parallel measurements of volatile organic compounds (VOCs) from breath, skin, and the whole body. In this study, we determined, with direct measurements from two independent groups of four adult volunteers, the effect of indoor temperature and humidity, clothing coverage (amount of exposed skin), and indoor ozone concentration on the total OH reactivity of gaseous human emissions. The results show that the measured concentrations of VOCs and ammonia adequately account for the measured total OH reactivity. The total OH reactivity of human emissions was primarily affected by ozone reactions with organic skin-oil constituents and increased with exposed skin surface, higher temperature, and higher humidity. Humans emitted a comparable total mixing ratio of VOCs and ammonia at elevated temperature-low humidity and elevated temperature-high humidity, with relatively low diversity in chemical classes. In contrast, the total OH reactivity increased with higher temperature and higher humidity, with a larger diversity in chemical classes compared to the total mixing ratio. Ozone present, carbonyl compounds were the dominant reactive compounds in all of the reported conditions.

    View details for DOI 10.1021/acs.est.1c01831

    View details for PubMedID 34591444

    View details for PubMedCentralID PMC8529706

  • Total OH Reactivity of Emissions from Humans: In Situ Measurement and Budget Analysis. Environmental science & technology Wang, N., Zannoni, N., Ernle, L., Bekö, G., Wargocki, P., Li, M., Weschler, C. J., Williams, J. 2021; 55 (1): 149-159


    Humans are a potent, mobile source of various volatile organic compounds (VOCs) in indoor environments. Such direct anthropogenic emissions are gaining importance, as those from furnishings and building materials have become better regulated and energy efficient homes may reduce ventilation. While previous studies have characterized human emissions in indoor environments, the question remains whether VOCs remain unidentified by current measuring techniques. In this study conducted in a climate chamber occupied by four people, the total OH reactivity of air was quantified, together with multiple VOCs measured by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) and fast gas chromatography-mass spectrometry (fast-GC-MS). Whole-body, breath, and dermal emissions were assessed. The comparison of directly measured OH reactivity and that of the summed reactivity of individually measured species revealed no significant shortfall. Ozone exposure (37 ppb) was found to have little influence on breath OH reactivity but enhanced dermal OH reactivity significantly. Without ozone, the whole-body OH reactivity was dominated by breath emissions, mostly isoprene (76%). With ozone present, OH reactivity nearly doubled, with the increase being mainly caused by dermal emissions of mostly carbonyl compounds (57%). No significant difference in total OH reactivity was observed for different age groups (teenagers/young adults/seniors) without ozone. With ozone present, the total OH reactivity decreased slightly with increasing age.

    View details for DOI 10.1021/acs.est.0c04206

    View details for PubMedID 33295177

    View details for PubMedCentralID PMC7788569

  • The Indoor Chemical Human Emissions and Reactivity (ICHEAR) project: Overview of experimental methodology and preliminary results. Indoor air Bekö, G., Wargocki, P., Wang, N., Li, M., Weschler, C. J., Morrison, G., Langer, S., Ernle, L., Licina, D., Yang, S., Zannoni, N., Williams, J. 2020; 30 (6): 1213-1228


    With the gradual reduction of emissions from building products, emissions from human occupants become more dominant indoors. The impact of human emissions on indoor air quality is inadequately understood. The aim of the Indoor Chemical Human Emissions and Reactivity (ICHEAR) project was to examine the impact on indoor air chemistry of whole-body, exhaled, and dermally emitted human bioeffluents under different conditions comprising human factors (t-shirts/shorts vs long-sleeve shirts/pants; age: teenagers, young adults, and seniors) and a variety of environmental factors (moderate vs high air temperature; low vs high relative humidity; presence vs absence of ozone). A series of human subject experiments were performed in a well-controlled stainless steel climate chamber. State-of-the-art measurement technologies were used to quantify the volatile organic compounds emitted by humans and their total OH reactivity; ammonia, nanoparticle, fluorescent biological aerosol particle (FBAP), and microbial emissions; and skin surface chemistry. This paper presents the design of the project, its methodologies, and preliminary results, comparing identical measurements performed with five groups, each composed of 4 volunteers (2 males and 2 females). The volunteers wore identical laundered new clothes and were asked to use the same set of fragrance-free personal care products. They occupied the ozone-free (<2 ppb) chamber for 3 hours (morning) and then left for a 10-min lunch break. Ozone (target concentration in occupied chamber ~35 ppb) was introduced 10 minutes after the volunteers returned to the chamber, and the measurements continued for another 2.5 hours. Under a given ozone condition, relatively small differences were observed in the steady-state concentrations of geranyl acetone, 6MHO, and 4OPA between the five groups. Larger variability was observed for acetone and isoprene. The absence or presence of ozone significantly influenced the steady-state concentrations of acetone, geranyl acetone, 6MHO, and 4OPA. Results of replicate experiments demonstrate the robustness of the experiments. Higher repeatability was achieved for dermally emitted compounds and their reaction products than for constituents of exhaled breath.

    View details for DOI 10.1111/ina.12687

    View details for PubMedID 32424858

  • Human Ammonia Emission Rates under Various Indoor Environmental Conditions. Environmental science & technology Li, M., Weschler, C. J., Bekö, G., Wargocki, P., Lucic, G., Williams, J. 2020; 54 (9): 5419-5428


    Ammonia (NH3) is typically present at higher concentrations in indoor air (∼10-70 ppb) than in outdoor air (∼50 ppt to 5 ppb). It is the dominant neutralizer of acidic species in indoor environments, strongly influencing the partitioning of gaseous acidic and basic species to aerosols, surface films, and bulk water. We have measured NH3 emissions from humans in an environmentally controlled chamber. A series of experiments, each with four volunteers, quantified NH3 emissions as a function of temperature (25.1-32.6 °C), clothing (long-sleeved shirts/pants or T-shirts/shorts), age (teenagers, adults, and seniors), relative humidity (low or high), and ozone (<2 ppb or ∼35 ppb). Higher temperature and more skin exposure (T-shirts/shorts) significantly increased emission rates. For adults and seniors (long clothing), NH3 emissions are estimated to be 0.4 mg h-1 person-1 at 25 °C, 0.8 mg h-1 person-1 at 27 °C, and 1.4 mg h-1 person-1 at 29 °C, based on the temperature relationship observed in this study. Human NH3 emissions are sufficient to neutralize the acidifying impacts of human CO2 emissions. Results from this study can be used to more accurately model indoor and inner-city outdoor NH3 concentrations and associated chemistry.

    View details for DOI 10.1021/acs.est.0c00094

    View details for PubMedID 32233434

  • Characteristics of atmospheric volatile organic compounds (VOCs) at a mountainous forest site and two urban sites in the southeast of China. The Science of the total environment Hong, Z., Li, M., Wang, H., Xu, L., Hong, Y., Chen, J., Chen, J., Zhang, H., Zhang, Y., Wu, X., Hu, B., Li, M. 2019; 657: 1491-1500


    Volatile organic compounds (VOCs) are important trace gases in the atmosphere, affecting air quality (e.g. ozone and secondary organic aerosol formation) and human health. To understand the emission, transport and chemistry of VOCs in the southeast of China (Fujian Province), a campaign was conducted in summer and winter of 2016 at three contrasting sites in close proximity. One measurement site (Mt. Wuyi) is a mountainous forest site (1139 m a.s.l.) located in a natural reserve, while the other two sites (Fuzhou, Xiamen) are coastal urban sites with high population and vehicle density. Comparison of VOCs at these three sites provides a valuable perspective on regional air pollution and transport. Many of the measured alkanes, alkenes and aromatics exhibited clear seasonal and diurnal patterns, driven by variations of hydroxyl (OH) radicals, which is the predominant oxidant of VOCs in the atmosphere. By examining tracer-tracer correlations for VOCs, variability-lifetime analysis and 36 h backward trajectories, strong emissions from vehicular exhaust, liquefied petroleum gas (LPG) and solvent usage were identified as key sources in Fuzhou and Xiamen, whereas at Mt. Wuyi the main emission sources were local emissions (e.g. biomass burning) in summer and long-range transport in winter. The results indicate that natural sites could be impacted strongly by surrounding urbanization. Isoprene and propylene in summer and propylene in winter contributed the most to ozone formation at the three sites. The data in this study provides a useful benchmark for future research on air quality monitoring and emission sources in the region.

    View details for DOI 10.1016/j.scitotenv.2018.12.132

    View details for PubMedID 30677915