Observed Compound Heat–particulate Pollution Events in an Amazonian city during the Extreme 2023 Drought: Evidence from a Low-cost Monitoring Network

Heatwaves are intensifying globally under anthropogenic climate change and increasingly co-occur with degraded air quality, forming compound environmental hazards with amplified impacts on human health. While such heat–pollution interactions have been widely documented in mid-latitude regions, observational evidence from tropical rainforest environments remains scarce. This study examines the relationship between heatwaves and particulate matter (PM₂.₅ and PM₁₀) in Santarém, an urban center in the central Amazon, using continuous observations from a low-cost monitoring network and meteorological data collected throughout 2023. Heatwaves were identified using the CTX90pct index based on long-term maximum temperature records, and daily particulate concentrations were analyzed through descriptive statistics, categorical classification, and regression modeling. Both PM fractions exhibited substantially higher concentrations during heatwave periods, particularly in the late dry season under severe drought and biomass-burning conditions. Mean PM₁₀ increased from 17.1 to 25.4 µg m⁻³ and PM₂.₅ from 9.5 to 15.8 µg m⁻³ on heatwave days, accompanied by a shift toward higher pollution categories. Multiple linear regression identified air temperature as the dominant driver of particulate variability, while relative humidity exerted a mitigating effect. Heatwave occurrence was significantly associated with increased PM₁₀ concentrations, whereas PM₂.₅ responded mainly through enhanced extremes. Logistic regression further indicated that elevated PM₁₀ concentrations on the preceding day increased the probability of heatwave onset, suggesting that coarse particles act as indicators of pre-heatwave atmospheric stagnation Overall, the results demonstrate that heatwaves in the Amazon are embedded within broader periods of atmospheric degradation characterized by dry, stagnant conditions that promote particulate accumulation. These findings provide the first observational evidence of compound heat–pollution events in an Amazonian city and highlight the need for integrated heat and air-quality monitoring and early-warning strategies to reduce public-health risks in tropical regions.