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Black carbon (BC) is the light absorbing component of particulate matter (PM) pollution emitted by biomass and fossil fuel combustion. Common sources of black carbon include wood heaters, internal combustion engines, and wildfires. Prolonged exposure to these combustion emissions increases the incidence of respiratory diseases and other adverse health impacts. Black carbon is also a short-lived but powerful climate forcer in the atmosphere: Only carbon dioxide contributes more to global radiative forcing. Monitoring black carbon emissions and mitigating black carbon pollution is critical to protecting human health and the environment.

Particulate matter (PM) is an umbrella term for all solid or liquid particles suspended in the atmosphere. PM is generated by both natural processes, such as the suspension of fine sand or sea salt, and anthropogenic sources, such as internal combustion engines. PM even forms directly in the atmosphere through secondary mechanisms, such as the nucleation of volatile organic compounds released by plants. Often, studies focus on measuring the total mass of PM suspended in a unit volume of air, as this is an important air quality indicator. However, it is difficult to pinpoint the contribution of relevant anthropogenic sources using these PM mass concentration data alone, since there are many other unrelated sources captured within the same measurement. Black carbon (BC) is a component of PM pollution only emitted by the incomplete combustion of fossil fuels and biomass. As a result, BC concentration measurements can be used to directly monitor local combustion sources, such as transportation or energy generation, and accurately quantify their impact on surrounding air quality. BC is a highly localized and short-lived pollutant that effectively illuminates spatiotemporal emission patterns on a fine scale. The figures below illustrate the basic utility of monitoring black carbon. Mass concentrations of BC and PM were monitored at two locations in the same neighborhood: a residential site and a site < 100 m from a major highway. The figure on the left shows that while particulate matter concentrations are effectively identical, black carbon concentrations are notably higher at the highway site, and clearly reveal the localized impact of nearby traffic emissions. Similarly, the plot on the right shows daily-average PM and concentrations at the highway site, normalized by the yearly average values. PM concentrations are stable throughout the week, but BC concentrations reflect the impact of increased highway traffic on weekdays. In this way, monitoring BC concentrations illuminates spatiotemporal emission patterns that are not captured by PM mass concentration data alone. This information regarding black carbon emissions can be used to develop more targeted and effective mitigation interventions to protect the air we share.

When measured alone, black carbon and gaseous pollutant concentrations are important air quality indicators that reveal potential impacts on the surrounding environment. By measuring all these air pollutants simultaneously in an integrated package, we can more effectively detect and characterize local pollution sources that drive air quality trends. For example, scientific instruments such as the ObservAir air quality monitoring station can be outfitted to measure BC, carbon monoxide (CO), and nitrogen dioxide (NO2). If high BC and CO concentrations are detected simultaneously, this could be an indication of nearby biomass combustion, while elevated BC and NO2 concentrations may point towards diesel engine emissions. Without all three pollution measurements, it would not be possible to make this distinction. Although these type of deductions are not universal (since they typically depend on the sampling context), the simultaneous measurement of BC and gaseous pollutants paints a more complete picture of local air quality to enable the characterization major air pollution drivers.

To determine black carbon concentrations, the ObservAir measures the optical absorption of particulate matter collected on the disposable filter tab. As the sensor draws in polluted air, light absorbing BC accumulates on the filter, and the intensity of light transmitted through the filter dims predictably over time. For example, if the ObservAir is drawing 100 ccm of air and the light signal dims by 10% over 1-minute, this correlates to an average BC concentration of 5 ug/m3 (not actual figures). However, if the light intensity dims too much, this mathematical relationship deteriorates, and the BC signal degrades (BC concentration measurements are lower than they should be). Therefore, the filter must be replaced periodically to maintain black carbon measurement performance. The filter on our ObservAir air quality measuring device should be replaced when the light intensity has dimmed to ~40% of its original value with a clean filter (optical attenuation value of ~80). Given this threshold, the filter replacement interval is a function of the average BC concentration and the ObservAir's sample flow rate. The higher the BC concentration and flow rate, the shorter the air filter's life. The table below shows recommended BC filter replacement intervals at various BC levels and flow rate settings. For example, at a concentration of 1 ug/m3 and 100 ccm flow rate, the filter will last about 3 days before requiring replacement. Setting the sample flow rate is a balance between maximizing filter life and maintaining BC measurement resolution - please see the ObservAir manual for detailed instructions/guidance.

The battery on our ObservAir air monitor device will last about 24 hours at a sample flow rate setting of 100 ccm. Higher flow rates diminish battery life, while lower flow rates extend it.

Our logo represents the peppered moth and its evolution in response to air pollution. Prior to the Industrial Revolution, peppered moths in England had white wings with black or gray spots, so as to camouflage against the local trees' light bark and lichens. During the 19th century, the area around Manchester became blanketed in air pollution from the coal-powered textile mills and factories that proliferated. As this sooty (black carbon) pollution deposited on the trees, they gradually grew darker. White moths now stood out against the dark bark, clearly visible to predators, and so darker moths naturally evolved in response. Our logo shows the dark wing of the peppered moth turning back to its natural, lighter color, as we hope our work eliminates the air pollution that drove this evolutionary response.