AEROMMA Motivation

North American Megacity VOC Emissions, Chemistry, and Trends

Volatile organic compounds (VOCs) emitted into the urban atmosphere are one of the needed ingredients for ozone and aerosol formation and therefore have the potential to impact air quality. For decades, fossil fuel usage has been considered to be the primary source of urban VOCs in megacities around the world, such as Los Angeles. However, as tighter emission regulations in the US and Europe have led to sharp reductions in transportation VOCs, new sources of pollution have emerged as potentially important precursors to ozone and aerosol formation. Studies have shown that VCPs are a major, under-studied source of urban VOCs that potentially play a significant role in urban air quality and human health. In industrialized cities, VCPs may make up as much as 50% of the total petrochemical VOC emissions and, consequently, could be responsible for as much as 50% of the mass associated with fossil-derived secondary organic aerosol (SOA) formation.

Unlike emissions from vehicles and energy production, VCPs are emitted from a wide range of human activities over a dispersed area. Quantifying the chemical fingerprint and emission strength is challenging. For instance, a significant fraction of VCPs likely originates from use in residential or commercial buildings, and emitted via building exhaust, though it is not well understood what fraction of VCP emissions could also occur outside or at waste facilities (e.g., landfills and wastewater treatment plants). These "indoor" VCPs, which include cleaning and personal care products, constitute approximately 50% of VCP emissions and are composed of oxygenated molecules that could form SOA efficiently. In addition to VCPs, there could be other indoor sources of VOCs from cooking and building materials, and which can contribute primary emissions of reactive aldehydes to the atmosphere.

A recent pilot study in New York City and other major urban areas has shown that there is a clear signature of VCP emissions, such as D5-siloxanes from personal care products and anthropogenic monoterpenes from fragrances in personal care and cleaning products. VOCs such as monoterpenes from VCPs might be efficient at producing ozone and SOA in an urban environment. D5 siloxanes and monoterpenes show the largest enhancements in the most densely populated areas around Manhattan and are well correlated with population density. Speciation measurements by GC-MS found that limonene was the dominant monoterpene isomer in downtown NYC during both winter and summer campaigns, which is the most commonly used monoterpene in fragrances, compared to a- and b-pinene from biogenic emissions in New Jersey and Long Island. The monoterpene speciation, the high wintertime mixing ratios, and the correlation with population density clearly prove anthropogenic emissions of monoterpenes, particularly limonene.

North American Megacity NOx Emissions, Chemistry, and Trends

NOx is the second needed ingredient for ozone and secondary aerosol formation and has been decreasing since 1960. For example, For example, NOx emissions in Los Angeles have been reducing at a rate of about 2.6% per year. Similar decreases in NO2 are observed nationwide and satellite retrievals of NO2 columns from the Ozone Monitoring Instrument (OMI) indicate a plateau since 2011. A variety of hypotheses have been suggested for why this trend is slowing, including: (i) a decrease in the rate of decline in anthropogenic NOx emissions, (ii) the growing influence of background and free tropospheric NO2, and (iii) changes in NOx lifetime.

A recent study suggested that agricultural soils are a dominant source of NOx pollution in California, with especially high soil NOx emissions from the state's Central Valley region. This large, overlooked NOx source from cropland soil could increase the NOx budget by 20 to 51%. Fertilizer application also results in nitrous oxide (N2O) emissions, a potent greenhouse gas, and emissions are strongest in the Midwestern corn/soy belt and in spring/early summer. It is possible that the increasing importance of soil NOx emissions could be contributing to the slowdown observed in satellite NO2 columns, and consistent with NOAA CSL modeling of ozone over the Eastern US, which also suggested an under-accounted soil NOx source in the Upper Midwest.

In addition to uncertain trends, recent studies have also suggested significant uncertainties in current vehicle emission models of mobile source NOx based on a variety of field campaigns. It is critical to improve inventories of NOx for accurate model predictions of ozone and aerosol chemistry. With the launch of the Sentinel-5P Tropospheric Ozone Monitoring Instrument (TROPOMI) and enhanced spatial resolution of its satellite products (3.5 km x 7 km), satellite NO2 and formaldehyde data are becoming an increasingly useful tool for evaluating and constraining emission inventories, including of NOx and VOCs. Satellite capabilities are expected to increase with the anticipated launch of the geostationary Tropospheric Emissions: Monitoring Pollution (TEMPO) satellite in 2022.

Tropospheric Ozone Formation and Trends

Overall, summertime levels of tropospheric ozone have been trending downward from 2000-2014 together with its precursors. It was also shown recently that ozone maxima decreased in proportion with NOx emissions in the southeast US, and there was an observed shift from the NO2 regeneration product (PAN) to the radical termination product (HNO3). Prior to the 2008 recession, large emission reductions have been observed from space due to U.S. regulatory efforts to control NOx from power plants and transportation.

In contrast to downward trends in average tropospheric ozone levels in North America, the worst days quantified as ozone design values (ODVs) (i.e., the 3-year average of the 4th annual maxima, as regulated under the Clean Air Act) have been decreasing exponentially until recently. If those trends were to continue, it will take ~35 years for the Los Angeles region to be in attainment for ozone. For California, Los Angeles is the air basin with the greatest exceedance of the 8-hour ozone NAAQS in the US, especially in the downwind "Inland Empire" parts of the basin. Currently there are only very few sites in the U.S. where declines in ODVs have slowed from the exponential trend. In the South Coast Air Basin (SOCAB) the improvement of ozone and PM2.5 concentrations has slowed in recent years as can be seen in the increasing trend of the 8-hour ozone design value over the past several years. The national trend has also showed a slowdown and is flattening since 2010. It is currently unclear if this trend could be caused by changes in NOx emission sources, VOCs from VCPs, or unusual meteorology associated with the frequency of heatwaves.

Aerosol Formation and Trends

Nationally, organic aerosol (OA) comprises around half of the fine particulate matter (PM2.5) mass, and US concentrations have been decreasing since 1990. Studies attribute the decreases in atmospheric concentrations of OA to reductions in transportation and residential fuel burning emissions, including directly emitted particles and VOC precursors. Similarly, studies found that OA concentrations were decreasing in Los Angeles due to reductions in tailpipe emissions of primary and secondary OA. Although, the decreases were not as large as expected from the observed reductions in mobile source emissions. The study suggested that other VOC sources could be contributing to the slower-than-expected decreases in OA concentrations. Studies later found that the slow OA decrease is likely due to the emerging importance of VCPs.

Additionally, ammonium nitrate accounted for around ~1/3 of the summertime submicron PM in Los Angeles during CalNex 2010. Heavy-duty diesel trucks have become the leading source of NOx in the Los Angeles basin, and in cities over the Eastern US. Starting in 2010, new heavy-duty diesel trucks are required to install selective catalytic reduction (SCR) systems. Under highway driving conditions, the SCR systems are effective at reducing NOx emissions. However, under urban driving, the SCR systems are ineffective and result in significantly elevated emissions of NOx. The reagent used to reduce NOx within the SCR is urea, and if slippage occurs, could also result in a local urban source of ammonia (NH3).

The San Joaquin Valley has the most severe PM2.5 problems in the nation, which is also one of the most agriculturally productive regions of the US. In this region, it is expected that aerosol nitrates could form from both precursor emissions of NH3 and NOx resulting from agriculture. Additionally, the region is a major conduit of interstate long-haul truck traffic. Recently, studies suggested that VOCs also play a role in the formation of ammonium nitrate, similar to how NOx and VOCs affect ozone chemistry. While researchers investigated wintertime ammonium nitrate formation in Salt Lake City, another modeling study over California found that increasing VCP emissions by a factor of ~3 had non-linear effects on PM2.5 via SOA and ammonium nitrate formation. During winter, the CMAQ model predicted increased PM2.5 universally across California, mainly through formation of ammonium nitrate from increased VCP emissions. However, in summer, CMAQ predictions of PM2.5 were largely unchanged due to increased SOA offset by reduced ammonium nitrate formation from increased VCP emissions. More measurements on the interplay of VOCs, NOx, and NH3 on secondary aerosol chemistry are needed to better predict urban PM2.5.

Greenhouse Gases and Air Quality Co-Benefits

Globally, cities account for ~50% of the world population and at least 70% of the CO2 emissions. Yet a robust urban carbon monitoring system for CO2 and CH4 has not been established to track trends in greenhouse gas emissions, though urban testbeds have been established in Indianapolis (INFLUX), Los Angeles (Megacities Carbon Project), and Baltimore/Washington, DC (Northeast Corridor Urban Test Bed) with support from NIST.

In most US cities, buildings and transportation comprise the two largest sources of CO2 emissions. These sources also are the two main NOx sources in cities and of VOCs to the urban atmosphere contributing to ozone and PM2.5. Fugitive leaks of methane have been shown to occur from the oil and natural gas production and supply infrastructure and landfills. Refineries remain an important source of reactive VOCs and oil and natural gas infrastructure are a source of light alkanes. Relatively little attention has been paid to VOC emissions from landfills, though they could be a potential emission pathway by which VCPs are emitted into the atmosphere. Given the overlap in emission sources that contribute to both air pollutants and greenhouse gases, there are potential synergies for cities to optimize management of air quality and the carbon cycle.

The role of Heatwaves, Meteorology, and Long-Range Transport on Urban Air Quality

Over the last 50-60 years, US heatwaves have become more frequent, last longer, and are more intense. Heatwaves have been associated with enhanced levels of ozone, as well as human mortality. In New York City, heatwaves can significantly enhance ozone well in exceedance of national ambient air quality standards. However, the impacts of meteorology on ozone and aerosol formation are complex through dependencies on temperature, sunlight, precipitation, and effects on dynamical and physical processes. For example, warmer temperatures are expected to result in a higher planetary boundary layer which enhances dilution and lowers air pollutant concentrations. This can be offset if a heatwave results in stagnant wind conditions and/or recirculation of air masses that allow for build-up of air pollution.

It is well established from prior NOAA CSL field campaigns that coastal dynamics affect the transport of ozone and other air pollutants, including during NEAQS 2002, ICARTT 2004, TEXAQS 2000, TEXAQS 2006, and CALNEX 2010. Chemical transport modeling from ICARTT 2004 shows that ozone can be transported from Washington, DC through New York City up to the Gulf of Maine, which is driven by mesoscale meteorology along the coast. There is a lack of high-quality wind profiler and thermodynamic measurements over water surfaces, which inhibits evaluation of numerical weather prediction models. Ship or airborne lidars fill a critical measurement gap in providing data that can improve models of the marine boundary layer, land-sea breeze recirculation, vertical mixing, as well as characterizing offshore wind energy resources.

In addition to coastal dynamics, urbanization results in the modification of land surfaces that alter the surface energy balance. Urban heat islands (UHI) result when asphalt or concrete with low albedo replace soils and vegetation and inhibit evapotranspiration. The magnitude of UHI can be modeled as a function of population and precipitation, with the effect stronger in the most populated cities. Accounting for the urban canopy in chemical transport models was shown to improve model predictions of ozone in New York City during the ICARTT 2004 field campaign. Using high-resolution vegetation maps and albedo maps was shown to improve the WRF model performance of urban meteorology in Los Angeles. The effects of urbanization are an additional factor to consider on meteorology in coastal cities. A key question is to what extent does the urban canopy need to be parameterized and represented in chemical transport models? Often the urban canopy is overlooked in operational weather forecasts due to computational cost.

Ozone and aerosols can be transported on regional to continental scales. Global contributions to background ozone are estimated to be ~30 ppb over the Eastern US and have a higher contribution in the Western US of ~40 ppb at high-altitude sites. With the lowering of the 8-hour US ozone standard to 70 ppb, and generally increasing trends of global background ozone, it is becoming more challenging for cities to meet national ambient air quality standards. Additionally, ozone can also be transported from the stratosphere to the troposphere, especially in the intermountain west. Terrain can result in lofting of air pollution from Los Angeles, which lowers concentrations in Los Angeles and elevates pollutants transported over long distances to other states. Similarly, for aerosols, wildfire smoke from Alberta, Canada has been shown to transport over North America and contribute to increased surface PM2.5 by 5-30 μg/m3 in New York City. The vertical mixing and transport between the stratosphere, free troposphere, and planetary boundary layer is an additional consideration on coastal meteorology that can affect urban air quality.

DMS oxidation in the marine atmosphere

DMS is naturally emitted from the oceans and is the most abundant biological source of sulfur to the marine atmosphere. Once in the atmosphere, DMS undergoes radical-initiated oxidation by hydroxyl (OH), halogen radicals (e.g. chlorine, Cl, and bromine oxide, BrO), and the nitrate radical (NO3) to form a suite of species including SO2 and methane sulfonic acid (MSA, CH3SO3H). Gas phase SO2 can be oxidized further to form sulfuric acid (H2SO4), a key precursor to new particles formed via homogeneous nucleation in airmasses where the existing condensation sink is small. These newly formed particles may grow by further condensation and coagulation to sizes large enough to serve as cloud condensation nuclei (CCN), thus affecting cloud optical properties and climate. In addition, SO2 can partition to aerosol liquid water to form non-sea salt sulfate (nss-SO42-). Gas-phase MSA contributes to particle growth via condensation onto existing particles.

Understanding the global impact of DMS in a changing climate and demands that models faithfully represent DMS oxidation chemistry. Yet, parameterizations of the complex DMS oxidation pathways are insufficient to capture how product distributions will respond to physical uptake and varying chemical environments. Studies of DMS oxidation have focused almost exclusively on the yields and fate of the terminal products SO2 and MSA and their impact on the concentration of CCN. Many of the proposed intermediates in the DMS oxidation scheme have not been directly observed, thus creating uncertainty in the DMS product branching ratios and oxidation timescales. Direct pathways to form sulfate from DMS that bypass SO2 exist, yet remain incompletely understood. The most recent example of such is the identification of hydroperoxymethyl thioformate (HPMTF), a DMS oxidation product, that has been revealed to be a major reservoir of marine sulfur.

Accurate representation of both the DMS oxidation product branching fractions and timescales in chemical transport models is critical to establishing a relationship between oceanic DMS emissions, atmospheric particle number and mass, and CCN concentrations in the marine boundary layer (MBL). Understanding the importance of these 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 requires complete understanding of the chemical and physical processes affecting DMS oxidation products. The atmospheric fate of HPMTF, however, remains currently unknown and therefore limits our ability to model the temporal and geographical distribution of the contribution of this newly identified oxidation pathway on global SO2 and sulfate. This lack of knowledge limits our ability to accurately assess the role of DMS in marine sulfur chemistry and its impact on new particle formation and growth, the global distribution of CCN, and their effects on Earth's radiative balance. In current global models, the oxidation of DMS is parameterized to produce SO2 and MSA with fixed yields typically totaling 100%. However, the existence of HPMTF and observations of its solubility demonstrate instead that the majority of DMS oxidation may not result in the formation of gas phase sulfur species at all. If a large fraction of the sulfur emitted from the ocean as DMS is returned to the aqueous phase by uptake onto the sea surface or marine clouds, the availability of condensable gas-phase sulfur will be significantly less than previously thought. This could severely reduce the modeled CCN concentrations in both the present and pre-industrial atmosphere.

For further information, download the AEROMMA White Paper