Clean Air for Megacities Parrish, David D.; Zhu, Tong
Science (American Association for the Advancement of Science),
10/2009, Letnik:
326, Številka:
5953
Journal Article
Recenzirano
As of 2008, over half of humanity lives in cities. The number of megacities (with populations over 10 million) grew from 3 in 1975 to 19 in 2007, and is projected to increase to 27 in 2025 (1). These ...megacities are the engines of growing economies, but are also very large sources of air pollutants and climate-forcing agents. The growth of megacities greatly aggravates the health impacts of polluted air, yet it may also provide an opportunity to mitigate climate change, if implemented air quality policies are designed to also reduce global warming.
US Environmental Protection Agency estimates of on-road vehicle emissions are compared with ambient measurements and a fuel-based emission inventory. Several significant weaknesses and strengths are ...identified. (1) The emission estimates have varied considerably over the past 15 years and are not clearly converging to progressively more accurate and certain results. (2) The most recent emissions estimate accurately captures the rapid decrease in carbon monoxide (CO) and volatile organic compounds (VOC) emissions, but overestimates the magnitude of CO emissions by about a factor of two. (3) The oxides of nitrogen (NO
x
) emission estimates for the mid to late 1990s are reasonably accurate, but NO
x
emissions have increased through that decade rather than decreased as indicated in emission estimates. (4) The most recent emissions estimate more accurately apportions NO
x
emissions between diesel and gasoline fueled vehicles than did earlier reports. (5) The ratio of two specific VOC species that has been characterized by ambient measurements suggests that the inventory speciation of the VOCs is inaccurate by factors of 3–4. These tests lead to the derivation of “inferred emissions” for CO and NO
x
from the US on-road vehicle fleet that are consistent with all information used in these tests. Finally, it is shown that the international picture of US emissions has significant inaccuracies and inconsistencies.
Air quality progress in the North American megacities of Los Angeles, New York, and Mexico City is reviewed, compared, and contrasted. Enormous progress made in North America over the last 5 decades ...provides a template for other megacities of the world, especially in developing countries, attempting to achieve rapid economic growth without compromising air quality. While the progress to date has been impressive, many challenges remain including the need to improve air quality while simultaneously mitigating climate change. The impact of pollutant emissions from megacities is felt long distances away from the local sources but no policy mechanisms currently exist to mitigate air quality impacts resulting from such pollution transport.
► Air quality progress in three North American megacities is reviewed. ► Enormous progress has been made over the last 5 decades. ► Further progress is required to meet health-based standards. ► North American progress can provide a template for developing megacities. ► A remaining challenge is to improve air quality while mitigating climate change.
Challenges of a lowered U.S. ozone standard Cooper, Owen R; Langford, Andrew O; Parrish, David D ...
Science (American Association for the Advancement of Science),
06/2015, Letnik:
348, Številka:
6239
Journal Article
Recenzirano
At Earth's surface, ozone is an air pollutant that causes respiratory health effects in humans and impairs plant growth and productivity (1). The Clean Air Act (CAA) of 1970 mandates that the U.S. ...Environmental Protection Agency (EPA) assess the ozone standard every 5 years and revise when necessary to protect human health. With a decision expected in October 2015 as to whether the standard will be toughened, we discuss limitations of ozone and precursor observations that hinder the ability of state and local air pollution-control agencies to accurately attribute sources of ozone within their jurisdictions. Attaining a lower standard may be particularly challenging in high elevations of the western United States, which are more likely to be affected by ozone that has been transported long distances or that originated in the stratosphere.
Detailed airborne, surface, and subsurface chemical measurements, primarily obtained in May and June 2010, are used to quantify initial hydrocarbon compositions along different transport pathways ...(i.e., in deep subsurface plumes, in the initial surface slick, and in the atmosphere) during the Deepwater Horizon oil spill. Atmospheric measurements are consistent with a limited area of surfacing oil, with implications for leaked hydrocarbon mass transport and oil drop size distributions. The chemical data further suggest relatively little variation in leaking hydrocarbon composition over time. Although readily soluble hydrocarbons made up ~25% of the leaking mixture by mass, subsurface chemical data show these compounds made up ~69% of the deep plume mass; only ~31% of the deep plume mass was initially transported in the form of trapped oil droplets. Mass flows along individual transport pathways are also derived from atmospheric and subsurface chemical data. Subsurface hydrocarbon composition, dissolved oxygen, and dispersant data are used to assess release of hydrocarbons from the leaking well. We use the chemical measurements to estimate that (7.8 ± 1.9) × 10⁶ kg of hydrocarbons leaked on June 10, 2010, directly accounting for roughly three-quarters of the total leaked mass on that day. The average environmental release rate of (10.1 ± 2.0) × 10⁶ kg/d derived using atmospheric and subsurface chemical data agrees within uncertainties with the official average leak rate of (10.2 ± 1.0) × 10⁶ kg/d derived using physical and optical methods.
In the past decade, ozone (O3) pollution has become a severe
environmental problem in China's major cities. Here, based on available
observational records, we investigated the long-term trend of O3
...pollution in China during 2014–2020. The O3 concentrations were slightly
higher in urban areas than in non-urban areas. During these 7 years, the
highest O3 concentrations primarily occurred during summer in northern China,
and during autumn or spring in southern China. Although O3 precursors,
including nitrogen oxides (NOx) and carbon monoxide (CO), continuously
decreased, O3 concentrations generally increased throughout the 7
years with a slower increasing rate after 2017. The long-term trend of O3
concentrations differed across seasons, especially from 2019 to 2020, when
O3 concentrations decreased in summer and increased in winter. To analyse
the causes of this observed trend, a photochemical box model was used to
investigate the change in the O3 sensitivity regime in two representative
cities – Beijing and Shanghai. Our model simulations suggest that the
summertime O3 sensitivity regime in urban areas of China has changed from
a VOC-limited regime to a transition regime during 2014–2020. By 2020, the
urban photochemistry was in a transition regime in summer but in a
VOC-limited regime in winter. This study helps to understand the distinct
trends of O3 in China and provides insights into efficient future O3
control strategies in different regions and seasons.
Volatile organic compounds (VOCs) were measured online at an urban site in Beijing in August–September 2010. Diurnal variations of various VOC species indicate that VOCs concentrations were ...influenced by photochemical removal with OH radicals for reactive species and secondary formation for oxygenated VOCs (OVOCs). A photochemical age‐based parameterization method was applied to characterize VOCs chemistry. A large part of the variability in concentrations of both hydrocarbons and OVOCs was explained by this method. The determined emission ratios of hydrocarbons to acetylene agreed within a factor of two between 2005 and 2010 measurements. However, large differences were found for emission ratios of some alkanes and C8 aromatics between Beijing and northeastern United States secondary formation from anthropogenic VOCs generally contributed higher percentages to concentrations of reactive aldehydes than those of inert ketones and alcohols. Anthropogenic primary emissions accounted for the majority of ketones and alcohols concentrations. Positive matrix factorization (PMF) was also used to identify emission sources from this VOCs data set. The four resolved factors were three anthropogenic factors and a biogenic factor. However, the anthropogenic factors are attributed here to a common source at different stages of photochemical processing rather than three independent sources. Anthropogenic and biogenic sources of VOCs concentrations were not separated completely in PMF. This study indicates that photochemistry of VOCs in the atmosphere complicates the information about separated sources that can be extracted from PMF and the influence of photochemical processing must be carefully considered in the interpretation of source apportionment studies based upon PMF.
Key Points
Photochemistry significantly influenced VOC concentrations
VOC emission ratios and source allocations of OVOCs are determined
PMF factors represent different degrees of photochemical processing of VOCs
Airborne measurements of volatile organic compounds (VOCs) were performed during CalNex 2010 (California Research at the Nexus of Air Quality and Climate Change) in the Los Angeles (LA) basin in ...May–June 2010 and during ITCT2k2 (Intercontinental Transport and Chemical Transformation) in May 2002. While CO2 enhancements in the basin were similar between the two years, the ΔCO/ΔCO2 ratio had decreased by about a factor of two. The ΔVOC/ΔCO emission ratios stayed relatively constant between the two years. This indicates that, relative to CO2, VOCs in the LA basin also decreased by about a factor of two since 2002. These data are compared with the results from various previous field campaigns dating back as early as 1960 and from the extensive air quality monitoring system in the LA basin going back to 1980. The results show that the mixing ratios of VOCs and CO have decreased by almost two orders of magnitude during the past five decades at an average annual rate of about 7.5%. Exceptions to this trend are the small alkanes ethane and propane, which have decreased slower due to the use and production of natural gas. A comparison with trends in London, UK shows that, due to stricter regulations at the time, VOC mixing ratios in LA decreased earlier than in London, albeit at a slower rate, such that typical mixing ratios in both cities in 2008 were at about the same level.
Key Points
VOCs and CO have decreased by a large factor in LA since 1960s
VOC emission ratios have not changed
Rate of decrease in London is more rapid, but started later
In the past decade, average PM2.5 concentrations decreased rapidly
under the strong pollution control measures in major cities in China;
however, ozone (O3) pollution emerged as a significant ...problem. Here we examine a unique (for China) 12-year data set of ground-level O3 and precursor concentrations collected at an urban site in Beijing (PKUERS, campus of Peking University), where the maximum daily 8 h average (MDA8) O3 concentration and daytime Ox (O3+NO2) concentration in August increased by 2.3±1.2 ppbv (+3.3±1.8 %) yr−1 and 1.4±0.6
(+1.9±0.8 %) yr−1, respectively, from 2005 to 2016. In
contrast, daytime concentrations of nitrogen oxides (NOx) and the OH reactivity of volatile organic compounds (VOCs) both decreased
significantly. Over this same time, the decrease of particulate matter (and
thus the aerosol optical depth) led to enhanced solar radiation and
photolysis frequencies, with near-surface J(NO2) increasing at a rate of 3.6±0.8 % yr−1. We use an observation-based box model to analyze the combined effect of solar radiation and ozone precursor changes on ozone production rate, P(O3). The results indicate that the ratio of the rates of decrease of VOCs and NOx (about 1.1) is inefficient in reducing ozone production in Beijing. P(O3) increased during the decade due to more rapid atmospheric oxidation caused to a large extent by the decrease of particulate matter. This elevated ozone production was driven primarily by increased actinic flux due to PM2.5 decrease and to a lesser extent by reduced heterogeneous uptake of HO2. Therefore, the influence of PM2.5 on actinic flux and thus on the rate of oxidation of VOCs and NOx
to ozone and to secondary aerosol (i.e., the major contributor to
PM2.5) is important for determining the atmospheric effects of
controlling the emissions of the common precursors of PM2.5 and ozone
when attempting to control these two important air pollutants.
Elevated tropospheric ozone concentrations driven by anthropogenic precursor emissions are an environmental hazard scientifically similar to the depletion of the stratospheric ozone layer and global ...climate change; however, the tropospheric ozone issue lacks the generally accepted, international assessment efforts that have greatly informed our understanding of the other two. Here, we briefly review those successful science-into-policy approaches and outline the elements required to conduct a similar process for tropospheric ozone. Particular emphasis is placed on the need to establish a conceptual model to fully understand the underpinning science, useful policy metrics, and motivating international policy forums for regulating anthropogenic ozone production over the hemispheric and global scales, thereby expanding beyond the traditional regional, air basin approach that has dominated air quality regulatory philosophy to date.
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