Most environmental monitoring studies center around two key questions: what are the pollutant concentrations in the area of interest, and from where did the pollution come?
To assess the pollutant concentrations in the area of interest, it is necessary to collect samples under a variety of conditions, and depending on the size of the area of interest, possibly in multiple locations within that area. Variables could include time of day, day of the week, season, wind speed, wind direction, humidity, precipitation, temperature, nearby landscape features, etc.
To attempt to discern the source of pollution using general methods, it is necessary to utilize conditions that can help distinguish input from one site or process versus that of another. Generally, assessing the influence of a particular industrial site is accomplished by utilizing wind or water flow conditions that allow for sample collection at a location that is upwind of the site, which should not be influenced by that site, and sample collection downwind of the site, which should likely be influenced by the site. When selecting locations, it is important to assess what other sources may be upwind of the sampling locations too. An ideal scenario is one in which the only difference in potential sources to a sampling location is the site of interest, though that situation is rare. To discern the influence of a given process, it is important to document when that process is and is not occurring, and sample during both active and inactive times. Depending on the pollutant of interest, effects of the process may be immediate (such as for visible emissions), or may be delayed (such as smog), and the appropriate sampling time interval will depend on expected pollutant reactions.
In every study, technique accuracy and sample precision should be assessed. To understand the accuracy of a technique before using it for a study, it would be ideal to co-locate the tool with a known high-quality instrument for sample collection and analysis, and use that information to help interpret data in the subsequent study. For studies where multiple tools will be deployed to collect samples, co-locating multiple monitors at a single site before deploying them at different sites would be useful to understand instrument variability. If a single instrument will be used for the entire study, that style of co-locating is not necessary. For each study, an assessment of sample precision, or local variation, is important. This entails taking multiple samples under the same conditions and using the same instrument, which is usually achieved taking samples in rapid succession. The longer the sample time (e.g. 24 hour integrated sample versus 3 minute sample), the less susceptible it is to ephemeral local variation. Including a set of replicate samples for at least 10% of the studies’ samples is generally the best practice.
The following sections provide general guidelines and tips for ways to structure environmental monitoring studies with different objectives.
A study to determine the general ambient airborne particulate matter concentration can easily become a long and large study. For example, to discern or determine compliance with the particulate matter national ambient air quality standards (NAAQS), state and federal environmental agencies collect data every six days for three consecutive years. While that longevity may be outside the scope of a community project, obtaining data under a variety of conditions is important.
Spatial and temporal resolution are important when estimating the ambient particulate matter concentration for a given area. If possible, multiple monitoring sites within the boundaries of the area of interest would be ideal, especially if there is any topographical, land use, or foliage diversity. If there is topographical diversity, establishing monitoring sites at leeward, pinnacle, and windward locations could be advantageous. Monitoring sites downwind of various land uses or amidst various dominant ecosystems would be advantageous as well.
For any given site, it is important to monitor during a variety of wind and weather conditions. Monitoring at least three times under at least three different general wind speeds, with additional monitoring during the dominant wind speed conditions would be beneficial. Similarly, monitoring at least three times during conditions with different wind directions, with extra sampling during conditions with the dominant wind directions would be ideal.
Many pollutants can have large seasonal variations, so it is important to plan a study that incorporates data from different seasons and dominant seasonal weather conditions if the goal of the study is to determine annual ambient estimates. If the scope of the project is shorter, perhaps for a single season, then collecting samples throughout the timeframe of interest, would be best.
For samples that take less than 24 hours to collect, it’s important to collect samples at different times of day in order to incorporate common daily variations in pollutant concentration. The atmospheric boundary layer is lowest just before dawn due to radiative cooling overnight. This means that there is a smaller volume of air for pollutants to mix into, so pollutant concentrations are often higher in the early morning. Societal and industrial processes schedules also contribute to different pollutant concentrations at various points throughout the day and on different days of the week. For example, morning rush hour can yield very high particulate matter, carbon monoxide, and nitrous oxide concentrations due to the combination of the increase in vehicle traffic and the lower atmospheric boundary layer. Those pollutant concentrations are generally higher during the week than on the weekends due to traffic patterns. To gather information about general ambient air quality, samples should be collected on different days of the week and during different times of day.
For studies with a goal of characterizing effects of episodic events or conditions, it is important to first clarify the boundaries and details of those episodes. For example, if the goal is to assess the air quality impacts of fugitive emissions from a sand mining location during high winds, clearly defining the wind categories, other relevant weather criteria, emissions sources, and location and type of impacts assessed are important.
The most poignant aspect of assessing the impacts of an episodic event is the comparison between samples collected during the event and those collected under alternative, likely “background” conditions. Depending on the research question of interest, the best practice is often to collect samples before, during, and after episodes. This may most easily be achieved for episodes that are regular or expected, such as a known locomotive engine startup sequence, or scheduled blasting events. In situations where unanticipated conditions yield a research question, such as a breached retaining wall or a fire, it may be necessary to collect data only shortly after an event and then substantially after an event, when conditions may have returned to pre-event characteristics.
To characterize impacts of the event, conduct either an ambient conditions study or an industry influence study, depending on the driving research questions.
Influence of specific mine
The basic primary principle of assessing the influence of a particular site, such as a sand mine, is to gather data that is upwind and downwind (or upstream and downstream if considering a water-related study) of the site. Setting up monitors in a radial distribution around the mine, and at various distances away from the mine, would yield the most information about the influence of the mine on air quality conditions. If possible, it would be idea to set up at least six monitors, with three relatively upwind and three relatively downwind, but a minimum of two sites with one upwind and one downwind could be reasonable. To assess relative impacts of different sources on ambient air quality, it would be ideal to monitor upwind and downwind of each potential source.
Depending on the research question, it could be useful to measure downwind of the site at at least two distances away, though the further downwind, the higher potential for confounding factors such as other pollutant sources. Emissions height, wind speed, and particle size or reaction chemistry all influence the distance a pollutant is likely to travel from the point of emissions. Topography can also have a significant influence on whether pollutants may disperse widely or accumulate in certain locations. It may be worthwhile to collect samples on the windward side of a hill that is downwind of the mine of interest, and in any valleys that may be downwind of the mine. Particulate matter travels in accordance with its size, with larger particles depositing closer to their emission source, though most inhalable particles can travel tens or hundreds of miles too. Inhalable particulate matter that is less than 10 micrometers in diameter (PM10) often deposits within 400 meters of a sand or gravel mine, but can also travel several miles. Particulate matter that is less than 2.5 micrometers in diameter (PM2.5) can become well mixed in the atmosphere and travel tens or hundreds of miles, though also can deposit within a mile. Gaseous pollutants like formaldehyde or nitrous oxide undergo chemical reactions that dictate their airborne lifespan and the distance from their source they travel. It is important to gather background information about typical travel distances for the pollutant of interest prior to designing a field study to monitor it.
In order to assess the influence of a mine and attempt to decouple its influence from other potential pollution sources, it would be best to collect samples in a radial array around the mine under a variety of wind conditions. If possible, collecting samples on days with different dominant wind directions and dominant wind speeds would be ideal. Collecting samples during at least three different dominant wind directions would be useful if possible. Collecting samples at various distances from the mine during calm conditions could be an important study component and provide information to guide interpretation of samples collected under windy conditions.
To assess the influence of different aspects of the mine operation, it could be useful to monitor during a variety of mine activities and during periods of inactivity. It could be very useful to couple this activity-specific monitoring with documentation of the ongoing activities, such as by using a digital or video camera. It may also be useful to conduct visible emissions monitoring methods (e.g. Method 9 or Method 22 -- insert links) to compare to measured PM10 or PM2.5 data.
Influence of specific process
To assess the impact of a specific industrial process, it is important to monitor during periods of activity and inactivity in locations that are likely to be affected and in another location of interest (e.g. a nearby neighborhood). To document the industrial process, it may be useful to use a digital camera or video camera demonstrating the time and location of the process occurring. Information regarding the timing of certain industrial processes may be gleaned through observation of the site and sometimes through operating permits and contracts. With carefully timed sample collection intervals, it may be possible to discern the influence from multiple processes if they do not occur simultaneously. If the process occurs on a routine schedule, it would be the best practice to collect a sample before the process were to begin, then during the process or operation, and again following the process. Note that this is only possible for sample collection periods that are shorter than the anticipated process period. For example, to assess the influence of an industrial process that usually occurs for two hours, it would be ideal to have a sample collection time of two hours or less, rather than something like an 8 hour or 24 hour integrated sample that would also be sampling during time when the process of interest were not occurring.
Depending on the size and spatial distribution of the various processes on the plant, and the pollutant of interest, it might also be possible though challenging to discern the influence of a process on a very local spatial area, even if process are occurring simultaneously. This may be accomplished by collecting near-field samples upwind and downwind of the different processes (such as blasting and sorting), if the wind is mostly perpendicular to the line of spatial relationship between the two process sites. For particulate matter, a visual assessment of fugitive emissions from various processes, such as by conducting EPA Method 22 (link), might be more effective at discerning the source of particulate matter than other air-sample based approaches.
As with other types of monitoring, such as to assess the influence of a mining site regardless of processes, it is important to collect samples under a variety of wind conditions. This is important both for assessing the impacts to ambient air under a variety of conditions, and also to discern the potential source of emissions by creating a variety of upwind and downwind pair locations. Ideally, sample collection would occur under at least three different wind directions and three different wind speeds, though that may not be possible if there is a dominant wind pattern in an area.
If part of the goal of the study is to make industry recommendations about specific processes due to their influence on air quality, it may be necessary to collect data from multiple sites that utilize that process. The same study design should be applied to each site, to the extent practicable. Compiling data from multiple sites to then be presented to management and director personnel within the industry, or in a regulatory policy position, may be worthwhile to improve industry practices. The number of sites assessed would be influenced by the level of policy or practice advocacy of interest (e.g. national industry-wide, or operations within a given municipality or county). If advocacy would focus on a county or municipality scale, it may be advantageous to monitor each site conducting the process of interest within that county.