Isoprene is the atmosphere's most important non-methane organic compound, with key impacts on atmospheric oxidation, ozone, and organic aerosols. In-situ isoprene measurements are sparse, and ...satellite-based constraints have employed an indirect approach using its oxidation product formaldehyde, which is affected by non-isoprene sources plus uncertainty and spatial smearing in the isoprene-formaldehyde relationship. Direct global isoprene measurements are therefore needed to better understand its sources, sinks, and atmospheric impacts. Here we show that the isoprene spectral signatures are detectable from space using the satellite-borne Cross-track Infrared Sounder (CrIS), develop a full-physics retrieval methodology for quantifying isoprene abundances from these spectral features, and apply the algorithm to CrIS measurements over Amazonia. The results are consistent with model output and in-situ data, and establish the feasibility of direct global space-based isoprene measurements. Finally, we demonstrate the potential for combining space-based measurements of isoprene and formaldehyde to constrain atmospheric oxidation over isoprene source regions.
Remote sensing may be a useful tool for exploring spatial variability of air pollution exposure within an urban area. To evaluate the extent to which satellite data from the Ozone Monitoring ...Instrument (OMI) can resolve urban-scale gradients in ground-level nitrogen dioxide (NO2) within a large urban area, we compared estimates of surface NO2 concentrations derived from OMI measurements and US EPA ambient monitoring stations. OMI, aboard NASA's Aura satellite, provides daily afternoon (∼13:30 local time) measurements of NO2 tropospheric column abundance. We used scaling factors (surface-to-column ratios) to relate satellite column measurements to ground-level concentrations. We compared 4138 sets of paired data for 25 monitoring stations in the South Coast Air Basin of California for all of 2005. OMI measurements include more data gaps than the ground monitors (60% versus 5% of available data, respectively), owing to cloud contamination and imposed limits on pixel size. The spatial correlation between OMI columns and corrected in situ measurements is strong (r=0.93 for annual average data), indicating that the within-urban spatial signature of surface NO2 is well resolved by the satellite sensor. Satellite-based surface estimates employing scaling factors from an urban model provide a reliable measure (annual mean bias: −13%; seasonal mean bias: <1% spring to −22% fall) of fine-scale surface NO2. We also find that OMI provides good spatial density in the study region (average area km2 per measurement: 730 for the satellite sensor vs. 1100 for the monitors). Our findings indicate that satellite observations of NO2 from the OMI sensor provide a reliable measure of spatial variability in ground-level NO2 exposure for a large urban area.
► We estimate satellite-based surface NO2 concentrations from the OMI sensor. ► We compare OMI estimates with ground-based in situ measurements in a large urban area. ► Within-urban spatial signature of surface NO2 is well resolved by OMI column measurements. ► OMI provides a useful dataset for exploring the epidemiological impact of urban air pollution. ► OMI measurements may be a useful tool for exploring NO2 variability between urban locations.
Wildfires are increasing across the USA. While smoke events affect human exposure and air quality, wildfire smoke effects on ecosystem‐atmosphere interactions are poorly understood. We investigate ...smoke effects on biogenic volatile organic compound (VOC) emissions and photosynthesis for ponderosa pines. During several wildfire smoke events, we observed photosynthetic reduction with evidence for stomatal plugging and changes in leaf‐level uptake and emission of both biogenic and wildfire VOCs. During intense smoke events, photosynthesis and VOC emissions were almost entirely suppressed, but increased dramatically upon stomatal opening. We propose four types of VOC responses to this burst in stomatal opening: post‐burst emissions, pulsed emissions, surge emissions, and post‐burst uptake. Our observations suggest that wildfire smoke can affect plant physiology and leaf‐atmosphere gas exchange.
Plain Language Summary
Wildfires are becoming more prevalent in the United States. Wildfire smoke affects human health and air quality, but we know little about how smoke interacts with plants, or the subsequent feedback with the atmosphere. We present results from a field experiment during which smoke repeatedly spread across a pine forest. We measured smoke‐induced changes in plant physiology (namely photosynthesis) and a suite of chemical emissions. During prolonged periods of intense smoke, we found that both photosynthesis and leaf emissions of chemical compounds decreased. When we induced photosynthesis, we observed a burst of different chemicals and identified several different pathways for these chemical releases. Further analyses suggest that smoke intensity and exposure may change the known relationship between emissions and temperature.
Key Points
Wildfire smoke interacts with stomates and leaf surfaces, suppressing photosynthesis and stomatal conductance
Intense smoke can alter emissions and uptake of biogenic and other volatile organic compounds through three mechanisms
The effects of wildfires on photosynthetic capacity and leaf emissions appear proportional to the intensity and duration of smoke events
Formaldehyde (HCHO) is the most important carcinogen in outdoor air among the 187 hazardous air pollutants (HAPs) identified by the U.S. Environmental Protection Agency (EPA), not including ozone and ...particulate matter. However, surface observations of HCHO are sparse and the EPA monitoring network could be prone to positive interferences. Here we use 2005–2016 summertime HCHO column data from the OMI satellite instrument, validated with high-quality aircraft data and oversampled on a 5 × 5 km2 grid, to map surface air HCHO concentrations across the contiguous U.S. OMI-derived summertime HCHO values are converted to annual averages using the GEOS-Chem chemical transport model. Results are in good agreement with high-quality summertime observations from urban sites (−2% bias, r = 0.95) but a factor of 1.9 lower than annual means from the EPA network. We thus estimate that up to 6600–12 500 people in the U.S. will develop cancer over their lifetimes by exposure to outdoor HCHO. The main HCHO source in the U.S. is atmospheric oxidation of biogenic isoprene, but the corresponding HCHO yield decreases as the concentration of nitrogen oxides (NO x ≡ NO + NO2) decreases. A GEOS-Chem sensitivity simulation indicates that HCHO levels would decrease by 20–30% in the absence of U.S. anthropogenic NO x emissions. Thus, NO x emission controls to improve ozone air quality have a significant cobenefit in reducing HCHO-related cancer risks.
The layout of an urban area can impact air pollution via changes in emissions and their spatial distribution. Here, we explore relationships between air quality and urban form based on ...cross-sectional observations for 111 U.S. urban areas. We employ stepwise linear regression to quantify how long-term population-weighted outdoor concentrations of ozone, fine particulate matter (PM2.5), and other criteria pollutants measured by the U.S. Environmental Protection Agency depend on urban form, climate, transportation, city size, income, and region. Aspects of urban form evaluated here include city shape, road density, jobs-housing imbalance, population density, and population centrality. We find that population density is associated with higher population-weighted PM2.5 concentrations (p < 0.01); population centrality is associated with lower population-weighted ozone and PM2.5 concentrations (p < 0.01); and transit supply is associated with lower population-weighted PM2.5 concentrations (p < 0.1). Among pollutants, interquartile range changes in urban form variables are associated with 4%-12% changes in population-weighted concentrationsamounts comparable, for example, to changes in climatic factors. Our empirical findings are consistent with prior modeling research and suggest that urban form could potentially play a modest but important role in achieving (or not achieving) long-term air quality goals.
Modifying urban form may be a strategy to mitigate urban air pollution. For example, evidence suggests that urban form can affect motor vehicle usage, a major contributor to urban air pollution. We ...use satellite-based measurements of urban form and nitrogen dioxide (NO2) to explore relationships between urban form and air pollution for a global data set of 1274 cities. Three of the urban form metrics studied (contiguity, circularity, and vegetation) have a statistically significant relationship with urban NO2; their combined effect could be substantial. As illustration, if findings presented here are causal, that would suggest that if Christchurch, New Zealand (a city at the 75th percentile for all three urban-form metrics, and with a network of buses, trams, and bicycle facilities) was transformed to match the urban form of Indio - Cathedral City, California, United States (a city at the 25th percentile for those same metrics, and exhibiting sprawl-like suburban development), our models suggest that Christchurch’s NO2 concentrations would be ∼60% higher than its current level. We also find that the combined effect of urban form on NO2 is larger for small cities (β × IQR = −0.46 for cities < ∼300 000 people, versus −0.22 for all cities), an important finding given that cities less than 500 000 people contain a majority of the urban population and are where much of the future urban growth is expected to occur. This work highlights the need for future study of how changes in urban form and related land use and transportation policies impact urban air pollution, especially for small cities.
Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity. Atmospheric models currently account for less than ...half the observed, globally averaged formic acid loading. Here we report that acetaldehyde photo-tautomerizes to vinyl alcohol under atmospherically relevant pressures of nitrogen, in the actinic wavelength range, λ = 300-330 nm, with measured quantum yields of 2-25%. Recent theoretical kinetics studies show hydroxyl-initiated oxidation of vinyl alcohol produces formic acid. Adding these pathways to an atmospheric chemistry box model (Master Chemical Mechanism) demonstrates increased formic acid concentrations by a factor of ~1.7 in the polluted troposphere and a factor of ~3 under pristine conditions. Incorporating this mechanism into the GEOS-Chem 3D global chemical transport model reveals an estimated 7% contribution to worldwide formic acid production, with up to 60% of the total modeled formic acid production over oceans arising from photo-tautomerization.
Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a ...mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above‐canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above‐canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15× underestimated in a state‐of‐science atmospheric model (GEOS‐Chem). Gradient and soil chamber measurements identify the canopy layer as the controlling source of HCOOH or its precursors to the forest environment; below‐canopy sources were minor. A correlation analysis using an ensemble of marker volatile organic compounds suggests that secondary formation, not direct emission, is the major source driving ambient HCOOH.
Plain Language Summary
Formic acid (HCOOH) is one of the most abundant acids in the atmosphere and affects the acidity of precipitation. A number of recent studies have shown that the atmospheric abundance of HCOOH is much higher than predicted, implying some unknown or underrepresented source. Here we present new measurements of HCOOH and related species above, within, and below a mixed forest canopy and use the results to investigate its sources and sinks in this ecosystem. We find that the forest is simultaneously a source and a sink of atmospheric HCOOH, and vertically resolved measurements identify the canopy layer as the major HCOOH source for this environment. Soils have been shown to be a source of HCOOH in some cases, but we show that their influence is unimportant for this ecosystem. The magnitude of the gross HCOOH source from this forest is 15 times higher than predicted in a current atmospheric model. A correlation analysis suggests that the main HCOOH source from this forest is oxidation of other compounds rather than direct emissions from vegetation.
Key Points
Bidirectional exchange of HCOOH was observed over a mixed forest, with a canopy‐level source; soil and below‐canopy sources were minor
The gross canopy HCOOH source is 3% and 38% of isoprene and monoterpene C emissions and is underestimated fifteenfold in a current model
Correlation analysis points to secondary formation, not direct emission, as the major HCOOH source in this forest ecosystem
India is currently experiencing degraded air quality, and future economic development will lead to challenges for air quality management. Scenarios of sectoral emissions of fine particulate matter ...and its precursors were developed and evaluated for 2015–2050, under specific pathways of diffusion of cleaner and more energy-efficient technologies. The impacts of individual source sectors on PM2.5 concentrations were assessed through systematic simulations of spatially and temporally resolved particulate matter concentrations, using the GEOS-Chem model, followed by population-weighted aggregation to national and state levels. We find that PM2.5 pollution is a pan-India problem, with a regional character, and is not limited to urban areas or megacities. Under present-day emissions, levels in most states exceeded the national PM2.5 annual standard (40 µg m-3). Sources related to human activities were responsible for the largest proportion of the present-day population exposure to PM2.5 in India. About 60 % of India's mean population-weighted PM2.5 concentrations come from anthropogenic source sectors, while the remainder are from “other” sources, windblown dust and extra-regional sources. Leading contributors are residential biomass combustion, power plant and industrial coal combustion and anthropogenic dust (including coal fly ash, fugitive road dust and waste burning). Transportation, brick production and distributed diesel were other contributors to PM2.5. Future evolution of emissions under regulations set at current levels and promulgated levels caused further deterioration of air quality in 2030 and 2050. Under an ambitious prospective policy scenario, promoting very large shifts away from traditional biomass technologies and coal-based electricity generation, significant reductions in PM2.5 levels are achievable in 2030 and 2050. Effective mitigation of future air pollution in India requires adoption of aggressive prospective regulation, currently not formulated, for a three-pronged switch away from (i) biomass-fuelled traditional technologies, (ii) industrial coal-burning and (iii) open burning of agricultural residue. Future air pollution is dominated by industrial process emissions, reflecting larger expansion in industrial, rather than residential energy demand. However, even under the most active reductions envisioned, the 2050 mean exposure, excluding any impact from windblown mineral dust, is estimated to be nearly 3 times higher than the WHO Air Quality Guideline.
Urban air pollution is among the top 15 causes of death and disease worldwide, and a problem of growing importance with a majority of the global population living in cities. A important question for ...sustainable development is to what extent urban design can improve or degrade the environment and public health. We investigate relationships between satellite-derived estimates of nitrogen dioxide concentration (NO2, a key component of urban air pollution) and urban form for 83 cities globally. We find a parsimonious yet powerful relationship (model R 2 = 0.63), using as predictors population, income, urban contiguity, and meteorology. Cities with highly contiguous built-up areas have, on average, lower urban NO2 concentrations (a one standard deviation increase in contiguity is associated with a 24% decrease in average NO2 concentration). More-populous cities tend to have worse air quality, but the increase in NO2 associated with a population increase of 10% may be offset by a moderate increase (4%) in urban contiguity. Urban circularity (“compactness”) is not a statistically significant predictor of NO2 concentration. Although many factors contribute to urban air pollution, our findings suggest that antileapfrogging policies may improve air quality. We find that urban NO2 levels vary nonlinearly with income (Gross Domestic Product), following an “environmental Kuznets curve”; we estimate that if high-income countries followed urban pollution-per-income trends observed for low-income countries, NO2 concentrations in high-income cities would be ∼10× larger than observed levels.
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