Over half of the world's population lives in cities, and the number is anticipated to grow in all regions. Cities are estimated to account for ~70% of the global fossil carbon dioxide (CO2) emissions, and CO2 is the largest positive forcing on global climate. Air pollution is the fifth largest human health risk factor globally , and a public health concern in megacities around the world.
Oceans cover ~70% of the surface area of the globe and impact climate, including through the emissions of dimethyl sulfide (DMS) from phytoplankton. DMS oxidizes in the marine atmosphere to form sulfate aerosol, which can serve as cloud condensation nuclei (CCN). Many of the U.S.'s largest cities are located on or near coastlines, providing an opportunity to assess interactions of anthropogenic and marine emissions, and atmospheric chemistry affecting both climate and air quality.
Recent research from the NOAA Chemical Sciences Laboratory (CSL) reveal major gaps in our understanding of both urban and marine chemistry. In urban atmospheres, volatile chemical products (VCPs = coatings, adhesives, inks, personal care products, cleaning agents, etc.) are emerging as a major source of volatile organic compounds (VOCs). The emissions and impacts of VCPs on atmospheric chemistry are not well understood. In the presence of nitrogen oxides (NOx), VOCs undergo chemistry that lead to the formation of ground-level ozone and aerosols. In a pilot study performed in conjunction with the Long Island Sound Tropospheric Ozone Study (LISTOS), NOAA CSL field measurements in New York City revealed that fragranced consumer products and other VCPs account for over half of the anthropogenic VOC emissions, and enhance formation of ground-level ozone during a heatwave event.
After decades of decline in ground-level ozone and fine particulate matter (PM2.5), the downward trends may be slowing in the most recent years. This could be a result of unanticipated trends in emissions, increasing influence of regional background sources, long-range transport, changes in atmospheric chemistry, and/or a consequence of a changing climate with heatwaves in the US becoming more frequent, longer in duration, and more intense. Many US metropolitan areas violate the 8-hour ozone standard as regulated under the Clean Air Act, which is of concern to environmental managers. In addition to air quality, many cities and states are developing plans to reduce their carbon footprint, including for CO2 and methane (CH4). Such efforts will impact future emissions of VOCs and NOx with potential co-benefits on air quality.
Biogenic sulfur oxidation products, mainly from oceanic DMS (CH3SCH3) emissions, are the primary driver of particulate sulfur formation in the remote atmosphere. The DMS oxidation mechanism is not fully characterized, and many of the key intermediates affecting aerosol and sulfur dioxide (SO2) yields have only been theorized. Accurate representation of both the DMS oxidation product branching fractions and timescales in chemical transport models is critical to establishing a quantitative relationship between oceanic DMS emissions and atmospheric particle number and CCN concentrations in the marine boundary layer (MBL). Recent developments in the understanding of this system, mainly the discovery of hydroperoxymethyl thioformate (HPMTF), highlight the degree to which global models inaccurately parameterize this chemistry. These recent advances motivate a reexamination of several decades of research assessing the role of DMS derived CCN relative to other sources of marine CCN, such as sea-spray aerosol, long-range transport of terrestrial particles, and secondary marine aerosol produced from non-DMS precursors, in both pre-industrial and present-day atmospheres.
To improve our understanding of emissions and chemical reactions that affect climate and air quality, the NOAA Chemical Sciences Laboratory proposes the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) field campaign to collect new observations from megacities to marine environments. The largest US cities are on coasts (e.g., Los Angeles and New York City), which allows for assessments of the interactions between megacity and marine environments. The chemical instrumentation that will be used on the WP-3D NOAA aircraft will address gaps in both urban and marine chemistry. It is anticipated that the field observations will:
Both major science questions, on VCPs and biogenic sulfur, are current "hot topics" in the atmospheric science community that NOAA CSL has initiated with prior research. Due to our position at the forefront of this research, NOAA CSL, its collaborators, and stakeholders have an unparalleled opportunity to lead in the efforts to (1) refine understanding of the biogenic sulfur oxidation system and elucidate its impacts on Earth's radiative budget and to (2) improve urban emission inventories to assess their influence on trends of ozone and aerosol precursors, as well as (3) quantifying associated co-benefits between air quality and greenhouse gases.
For further information, download the AEROMMA White Paper