Emerging research needs in our understanding of urban and marine emissions, chemistry, and remote sensing capabilities

NYC skyline at sunrise
The largest US cities are on coasts (e.g., New York City), which allows for assessments of the interactions between megacity and marine environments.

Current and future urban air quality

Over half of the world's population lives in cities, and the number is anticipated to grow in all regions. Air pollution is the fifth largest human health risk factor globally, and a public health concern in megacities around the world. In addition to the emissions of short-lived air pollutants, cities are also estimated to account for ~70% of the global fossil carbon dioxide (CO2) emissions, and CO2 is the largest positive forcing on global climate.

In urban atmospheres in the U.S. and Europe, long-term reductions in emissions of volatile organic compounds (VOCs) from sources such as motor vehicles have made volatile chemical products (VCPs = personal care products, cleaning agents, coatings, adhesives, inks, etc.) the major VOC source in densely populated areas. The emissions and impacts of VCPs on atmospheric chemistry are not well understood. In the presence of nitrogen oxides (NOx), VOCs undergo chemistry that leads 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 2019), NOAA CSL field measurements in New York City revealed that VCPs account for over half of the anthropogenic VOC emissions, and enhance formation of ground-level ozone during a heatwave event. While VCPs emissions are included in the US National Emissions Inventory (NEI), and have been regulated for their impacts on ozone formation and air toxics, their emissions may be underestimated by a factor of 2-3. Over time the composition of VCPs has changed, shifting away from aromatics and chlorinated solvents towards oxygenated VOCs with the inclusion of fragranced components like d-limonene.

After decades of decline in ground-level ozone and fine particulate matter (PM2.5) in the U.S., the downward trends are 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 heat waves 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.

Improved understanding of chemistry in the marine atmosphere

Oceans cover ~70% of the surface area of the globe. Marine biogeochemical cycles, notably the emission and subsequent oxidation of dimethyl sulfide (DMS) from phytoplankton, strongly influence the natural climate system. DMS oxidation produces sulfate aerosol, which can serve as cloud condensation nuclei (CCN). Anthropogenic emissions strongly perturb the sulfur oxidation cycle, leading to large but unexplored differences in sulfur cycling between remote and urban-influenced environments. 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.

Biogenic sulfur oxidation products, mainly from oceanic dimethyl sulfide (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, sulfur dioxide (SO2), and carbonyl sulfide (OCS) 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.

AEROMMA will result in a significant step forward in our ability to explain the fate of marine sourced species and their impacts on aerosols and CCN, by bringing a comprehensive modern analytical suite to the NASA DC-8 and positioning it in representative locations in the marine atmosphere to observe the relevant chemistry and aerosol formation.

New capabilities for remote sensing of atmospheric composition

TEMPO field of regard

The NASA / SAO (Smithsonian Astrophysical Observatory) TEMPO (Tropospheric Emissions: Monitoring Pollution) instrument is a UV-visible spectrometer, and will be the first ever space-based instrument to monitor air pollutants at hourly time resolution across the North American continent during daytime. It will collect high spatial resolution measurements of ozone, nitrogen dioxide and other pollutants, data which will revolutionize air quality forecasts. TEMPO observations are from the geostationary vantage point, flying on a telecommunications host spacecraft with a planned launch in January of 2023.

NOAA's Geostationary Extended Observations (GeoXO) satellite system is the ground-breaking mission that will advance Earth observations from geostationary orbit. GeoXO will supply vital information to address major environmental challenges of the future in support of US weather, ocean, and climate operations. The GeoXO mission will continue and expand observations provided by the GOES-R Series as NOAA's next generation of geostationary satellites. GeoXO will bring new capabilities to address emerging environmental issues and challenges to foster the security and well-being of the Nation. NOAA is working to ensure these critical observations are in place by the early 2030s as the GOES-R Series nears the end of its operational lifetime.

In this new era of atmospheric composition measurements from space over the US, large-scale airborne missions are needed to conduct related science and satellite validation for TEMPO and to deliver value assessment and risk reduction for the future with GeoXO.

For further information, download the AEROMMA White Paper