Characterizing Nitrogen Oxides and Fine Particulate Matter near Major Highways in the United States Using the National Near-Road Monitoring Network.

As part of the United States Environmental Protection Agency's 2010 Nitrogen Dioxide (NO2) National Ambient Air Quality Standards (NAAQS) review, a national network of near-road sites was established to characterize pollutant behavior, interaction, and dispersion in the ambient near-road environment. Using spatial interpolation to estimate the near-road concentration increments of NO2 and particulate matter with an aerodynamic diameter of 2.5 µm and less (PM2.5) relative to nearby non-near-road monitors, we found that the 2013-2018 national average increment is 6.9 ppb and 1.0 µg m-3 for NO2 and PM2.5, respectively. Analyses of the hourly near-road NO2, nitric oxide (NO), and PM2.5 increments showed distinct diurnal cycles; the NO2 increment peaks at ∼9 ppb during the early afternoon (2-4 pm local time) while the NO and PM2.5 increments peak during the morning rush hour (5-8 am local time) at 25 ppb and 1.8 µg m-3 for NO and PM2.5, respectively. Although long-term trends are not yet available for this network of sites, a similar analysis of the NO2 and PM2.5 increment at a quasi-near-road site outside of the official network in Elizabeth, NJ showed gradual decreases in the increment over time since the mid-2000s.

Seasonal and diurnal analysis of NO2 concentrations from a long-duration study conducted in Las Vegas, Nevada.

UNLABELLED: A study, conducted in Las Vegas, NV from mid-December 2008 to mid-December 2009 along an interstate highway, collected continuous and integrated samples for a wide variety of air pollutant species including NO2 and NO(x) associated with roadway traffic. This study examined long-term trends of NO2 and NO(x) in a near-road environment compared with previous near-road studies typically lasting only a few days to months. Study results revealed concentration gradients for NO2 and NO(x) with highest absolute and average concentrations at distances closest to the roadway throughout the year. Diurnal ambient temperature changes also influenced concentrations due to atmospheric chemistry activity as well as concentration changes due to seasonal effects. These concentration gradients were observed for all wind conditions; however under downwind conditions (winds from highway), the concentration gradients are more pronounced. Higher pollutant concentrations are generally observed during low wind speed conditions especially when those winds were from the highway. Understanding long-term, seasonal variability and levels of pollutant concentrations in the near-road environment is important to researchers and decision-makers evaluating exposures and risks for near-road populations; identifying locations for future near-road monitoring sites; and determining the viability and effectiveness of mitigation strategies. IMPLICATIONS: Population exposures to traffic emissions near roads have led to heightened public health concerns and awareness of the long-term levels and variability of these air pollutants. Epidemiological studies have lead to improved understanding of the associated risks and health effects of near road air pollutant emissions. While short-term studies provide insights on near-road air quality, longer-term trends need to be understood, especially for reactive pollutants such as NO2.

The national ambient air monitoring strategy: rethinking the role of national networks.

A current re-engineering of the United States routine ambient monitoring networks intended to improve the balance in addressing both regulatory and scientific objectives is addressed in this paper. Key attributes of these network modifications include the addition of collocated instruments to produce multiple pollutant characterizations across a range of representative urban and rural locations in a new network referred to as the National Core Monitoring Network (NCore). The NCore parameters include carbon monoxide (CO), sulfur dioxide (SO2), reactive nitrogen (NOy), ozone (O3), and ammonia (NH3) gases and the major fine particulate matter (PM2.5) aerosol components (ions, elemental and organic carbon fractions, and trace metals). The addition of trace gas instruments, deployed at existing chemical speciation sites and designed to capture concentrations well below levels of national air quality standards, is intended to support both long-term epidemiological studies and regional-scale air quality model evaluation. In addition to designing the multiple pollutant NCore network, steps were taken to assess the current networks on the basis of spatial coverage and redundancy criteria, and mechanisms were developed to facilitate incorporation of continuously operating particulate matter instruments.

Traffic and meteorological impacts on near-road air quality: summary of methods and trends from the Raleigh Near-Road Study.

A growing number of epidemiological studies conducted worldwide suggest an increase in the occurrence of adverse health effects in populations living, working, or going to school near major roadways. A study was designed to assess traffic emissions impacts on air quality and particle toxicity near a heavily traveled highway. In an attempt to describe the complex mixture of pollutants and atmospheric transport mechanisms affecting pollutant dispersion in this near-highway environment, several real-time and time-integrated sampling devices measured air quality concentrations at multiple distances and heights from the road. Pollutants analyzed included U.S. Environmental Protection Agency (EPA)-regulated gases, particulate matter (coarse, fine, and ultrafine), and air toxics. Pollutant measurements were synchronized with real-time traffic and meteorological monitoring devices to provide continuous and integrated assessments of the variation of near-road air pollutant concentrations and particle toxicity with changing traffic and environmental conditions, as well as distance from the road. Measurement results demonstrated the temporal and spatial impact of traffic emissions on near-road air quality. The distribution of mobile source emitted gas and particulate pollutants under all wind and traffic conditions indicated a higher proportion of elevated concentrations near the road, suggesting elevated exposures for populations spending significant amounts of time in this microenvironment. Diurnal variations in pollutant concentrations also demonstrated the impact of traffic activity and meteorology on near-road air quality. Time-resolved measurements of multiple pollutants demonstrated that traffic emissions produced a complex mixture of criteria and air toxic pollutants in this microenvironment. These results provide a foundation for future assessments of these data to identify the relationship of traffic activity and meteorology on air quality concentrations and population exposures.

A statistical comparison of active and passive ammonia measurements collected at Clean Air Status and Trends Network (CASTNET) sites.

Atmospheric concentrations of ammonia (NH3) are not well characterized in the United States due to the sparse number of monitors, the relatively short lifetime of NH3 in the atmosphere, and the difficulty in measuring non-point source emissions such as fertilized agricultural land. In this study, we compare measured weekly concentrations of NH3 collected by two denuder systems with a bi-weekly passive NH3 sampler used by the National Atmospheric Deposition Program's (NADP) Ammonia Monitoring Network (AMoN). The purpose of the study was to verify the passive samplers used by AMoN and characterize any uncertainties introduced when using a bi-weekly versus weekly sampling time period. The study was conducted for 1 year at five remote Clean Air Status and Trends Network (CASTNET) sites. Measured ambient NH3 concentrations ranged from 0.03 µg NH3 m(-3) to 4.64 µg NH3 m(-3) in upstate New York and northwest Texas, respectively, while dry deposition estimates ranged from 0.003 kg N ha(-1) wk(-1) to 0.47 kg N ha(-1) wk(-1). Results showed that the bi-weekly passive samplers performed well compared to annular denuder systems (ADS) deployed at each of the five CASTNET sites, while the MetOne Super SASS Mini-Parallel Plate Denuder System (MPPD) was biased low when compared to the ADS. The mean relative percent difference (MRPD) between the ADS and MPPD and the ADS and AMoN sampler was -38% and -9%, respectively. Precision of the ADS and MPPD was 5% and 13%, respectively, while the precision of the passive samplers was 5%. The results of this study demonstrate that the NH3 concentrations measured by AMoN are comparable to the ADS and may be used to supplement the high-time resolution measurements to gain information on spatial gradients of NH3, long-term trends and seasonal variations in NH3 concentrations.