Regional secondary PM Likely
Daily PM2.5 peaked at 60.8 µg/m³ near Louisville/Jefferson County across 1 covered metro home to 1.4 million people. 1 metro placed a top-ten day in their record.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 2.2 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 3.1 million cigarettes. An awareness illustration of outdoor exposure at 22 µg/m³ per cigarette (Berkeley Earth), counting only the event's ranked days; not a medical dose.
Why we think this happened
Louisville's exact-date CSN sample was 84% secondary inorganic aerosol and overwhelmingly ammonium sulfate. A peer-reviewed study independently identifies August 1–2 as a sulfate-heavy regional high-PM2.5 episode, while the Louisville airport reported haze for 21 hours under predominantly light winds.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
Pittsburgh Air Quality Study · Jun 1, 2004 corroborates
The peer-reviewed Pittsburgh Air Quality Study identifies August 1–2, 2001 as one of its high-PM2.5 episodes and reports that the episode had higher-than-average sulfate and lower-than-average organic carbon.
NOAA/NWS surface observations via Iowa Environmental Mesonet · Aug 2, 2001 hypothesis
Louisville Intl, 10.3 km from the target PM2.5 monitor, reported haze during 21 observed hours. Winds were at or below 3 knots during 11 representative hours.
U.S. EPA AirData Chemical Speciation Network hypothesis
EPA's exact-date CSN sample reconstructed a median 31.9 µg/m³ of PM2.5. Ammonium sulfate and ammonium nitrate contributed a median 26.8 µg/m³, or 84.1% of reconstructed mass; every usable site exceeded 50%.
Inference, not direct attribution
Same-day particle composition: EPA speciation observations on the peak date show a majority secondary-inorganic aerosol signature. This supports a regional secondary-particle mechanism, not a single emitting source.
Evidence path
A cause is more convincing when the record connects three things: something happened, the air mass moved toward the region, and local instruments responded. Here is how much of that chain is documented for this event.
Source or mechanism
The peer-reviewed Pittsburgh Air Quality Study identifies August 1–2, 2001 as one of its high-PM2.5 episodes and reports that the episode had higher-than-average sulfate and lower-than-average organic carbon.
View supporting source ↗Air-mass movement
NOAA HYSPLIT modeled 12 backward paths across multiple arrival times and heights. They show where the air traveled, not which source polluted it.
Explore the modeled paths ↓Local response
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 2.8 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓Event autopsy
Before, during, and after the episode: nearby pollutant monitors and surface-weather stations show what changed. This context tests the “Regional secondary PM” explanation; it does not assign the cause by itself.
Louisville/Jefferson County, KY-IN
Monitor comparison window 2001-07-30 to 2001-08-05 · PM2.5 target: Algonquin Parkway
67% above nearby days
About the same as surrounding days
67% higher than surrounding days
About the same as surrounding days
5% lower than surrounding days
20% higher than surrounding days
26% higher than surrounding days
About the same as surrounding days
About the same as surrounding days
About the same as surrounding days
Source: U.S. EPA AirData daily observations and, where available, nearby ASOS/AWOS surface observations. Values are descriptive medians over the retained monitor set; missing measurements remain missing. Transport and source evidence remain separate layers because this instrument pattern does not assign a cause by itself.
Air-mass reconstruction
NOAA HYSPLIT traces the air arriving at each metro backward for 72 hours. Multiple arrival times and heights show whether the broad path is stable—or sensitive to assumptions about when and where the polluted layer arrived.
Air arriving near Louisville/Jefferson County
Peak-date arrivals · 2001-08-02
Each line begins 72 hours before arrival and points toward the metro. Color represents arrival height; repeated lines represent 00, 06, 12, and 18 UTC arrival assumptions. A path can show where the air traveled, but it cannot prove which source added the PM2.5.
Episode peak
-- cigarettes
The smoke map's scale, replayed over this event: the surface interpolates between covered metros' daily records, framed to the affected region. Hover for a reading, click a metro to pin it to the card. Days a monitor did not report draw nothing.
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Louisville/Jefferson County, KY-IN | 60.8 µg/m³ | 2.8 | Aug 2, 2001 | #5 of its top days |
"Cigarettes, whole event" totals a metro's ranked event days at 22 µg/m³ per cigarette: what a person outdoors there breathed across the event, as the illustration. "All-time standing" is where this episode's days sit in each metro's ranked record (worst-area measure, since 1999). Full local context is on each metro's Air Records page.
Public evidence library
Each card states what a source supports—and what it does not. Official records, research, reporting, community observations, and transparent inferences remain visibly different kinds of evidence.
NOAA/NWS surface observations via Iowa Environmental Mesonet
Louisville Intl, 10.3 km from the target PM2.5 monitor, reported haze during 21 observed hours. Winds were at or below 3 knots during 11 representative hours.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
The airport and regulatory pollution monitors are nearby but not co-located.
U.S. EPA AirData Chemical Speciation Network
EPA's exact-date CSN sample reconstructed a median 31.9 µg/m³ of PM2.5. Ammonium sulfate and ammonium nitrate contributed a median 26.8 µg/m³, or 84.1% of reconstructed mass; every usable site exceeded 50%.
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Chemical composition identifies the measured aerosol mechanism, not a particular emitting facility or upwind source region.
Pittsburgh Air Quality Study
The peer-reviewed Pittsburgh Air Quality Study identifies August 1–2, 2001 as one of its high-PM2.5 episodes and reports that the episode had higher-than-average sulfate and lower-than-average organic carbon.
A retained source supports the attributed cause for the stated place and dates.
The research site was in Pittsburgh rather than Louisville, so it establishes a same-window regional sulfate episode rather than local source apportionment for Louisville.
U.S. EPA Chemical Speciation Network
On the affected place peak date, reconstructed ammonium sulfate plus ammonium nitrate represented a majority of EPA reconstructed mass at every usable sampled site. louisville: median 84% (minimum 84% across 1 site).
Measured composition consistent with secondary-inorganic-dominated PM2.5 at sampled monitors on the local peak date.
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
IAQng deterministic inference policy
Under csn_secondary_inorganic_majority_on_peak_date.v1, complete affected-place peak-date coverage with a secondary-inorganic majority creates a possible regional-secondary-PM hypothesis.
A possible regional secondary PM hypothesis; this is a reproducible inference, not direct source attribution.
A component majority describes aerosol composition at sampled monitors; it does not identify an emitting source.
Likely Applicable reporting or a supported inference makes this the best current causal explanation.
This episode was grouped from the monitor record by the catalog pipeline and given its cause from the evidence listed above. It has not been individually reviewed and signed off, so its boundaries, cause, and name may change as evidence accumulates. Grouping policy common_sense_root_cause_grouping.v2. The cause label reflects the assembled evidence above, not an IAQng detection formula. If it is wrong, tell us.