Aerosol composition, sources and processes during wintertime in Beijing, China

Look at a photograph of Beijing in January, and the skyline just disappears. The buildings fade into a brownish-grey wall maybe one kilometer away. You see the haze. What you cannot see is what it's actually made of — whether that murk is mostly traffic exhaust, coal smoke, cooking fumes, or chemistry that happened in the atmosphere itself. For decades, the tools for answering that question provided an average over an entire day, maybe two days, smeared across a filter. By the time you analyzed it, the atmosphere had already moved on. Sun and colleagues set out to change that. From November twenty-first, 2011, to January twentieth, 2012, the team deployed an Aerodyne Aerosol Chemical Speciation Monitor, known as an ACSM, at the Institute of Atmospheric Physics in Beijing, positioned between the city's third and fourth ring roads. The ACSM was developed from the Aerodyne Aerosol Mass Spectrometer framework and built specifically for routine, long-term field work. It measures the non-refractory submicron aerosol, also referred to as NR-PM1. This means all the chemically reactive particles smaller than one micrometer that aren't black carbon or dust. It does this in real time, continuously, providing data on organics, sulfate, nitrate, ammonium, and chloride as they actually change hour by hour. Alongside it, gas analyzers for carbon monoxide, sulfur dioxide, nitrogen oxides, and ozone operated, plus a tapered element oscillating microbalance instrument that tracked total PM2.5 mass. The point was simple: stop averaging and start watching. What they observed was striking. Average NR-PM1 over the two months was sixty-seven micrograms per cubic meter, but that average hides fluctuations between clean periods below fifteen micrograms and polluted episodes exceeding one hundred micrograms, with those episodes able to persist for more than a week. The aerosol was not sitting still; it was lurching between states depending on what the wind was doing. Organics dominated the mixture, making up fifty-two percent of NR-PM1 in winter. Compare that to summer when organics account for about forty percent, and you get a sense of how dramatically the heating season reshapes the aerosol. Chloride tells the same seasonal story even more sharply: roughly one percent of NR-PM1 in summer jumps to five percent in winter. The authors attribute that chloride enhancement to intensified coal-combustion emissions during the heating season. Nitrate averaged about ten point nine micrograms per cubic meter and contributed sixteen percent of NR-PM1. Its diurnal pattern is unexpected: rather than peaking at night through simple gas-to-particle partitioning of nitric acid, nitrate rises gradually from early morning through the afternoon and tracks closely with secondary organic aerosol. Sun and colleagues interpret this as evidence that photochemical, sunlight-driven production is driving wintertime nitrate formation — which means even in Beijing's grey January skies, daytime chemistry still matters. But the real analytical power came from decomposing the organic fraction. The team applied positive matrix factorization, known as PMF, a statistical technique that unmixes mass spectra to identify distinct source fingerprints, to the ACSM organic data. Four factors emerged: hydrocarbon-like organic aerosol, or HOA, associated with traffic and fossil fuel combustion; cooking organic aerosol, known as COA; coal-combustion organic aerosol, called CCOA; and oxygenated organic aerosol, or OOA, which represents secondary aerosol formed in the atmosphere itself rather than emitted directly from a source. The numbers partition out like this. HOA averaged five point eight micrograms per cubic meter, or seventeen percent of organic aerosol. COA averaged six point six micrograms, or nineteen percent. CCOA averaged eleven point three micrograms, or thirty-three percent. And OOA averaged ten point seven micrograms, or thirty-one percent. Adding up the three primary factors, you get primary organic aerosol at sixty-nine percent of total organics, with secondary organic aerosol at thirty-one percent. Primary sources dominate Beijing's winter air. That is the headline finding. CCOA is the lead character. It is the single largest primary source of organic aerosol, averaging thirty-three percent of organic aerosol and seventeen percent of total NR-PM1. Its diurnal cycle is unmistakable: concentration climbs to roughly eighteen micrograms per cubic meter at midnight, driven by residential coal heating, then falls to around seven micrograms by mid-morning. Its tight correlation with chloride, showing an r-squared value of zero point eighty-one, and its resemblance to pulverized coal combustion spectra confirm the attribution. Crucially, CCOA was only observed during the heating season. It disappears from the analysis in summer, because the source disappears. HOA and COA follow similar logic but different rhythms. HOA peaks at night — seven point one micrograms after dark versus three point six during the day — consistent with evening traffic and reduced boundary layer mixing. COA shows lunchtime and dinner peaks, reflecting the chemical signature of cooking activity. All three primary factors share one pattern: nighttime concentrations roughly double daytime values. OOA runs opposite, rising from around nine micrograms at eight a.m. to roughly fifteen micrograms by six p.m., with its daytime build-up consistent with photochemical production. Two distinct regimes operate simultaneously in the same city air. Now here is where the story gets genuinely counterintuitive. As pollution gets worse, not all sources scale the same way. CCOA's share of NR-PM1 grows with total loading — from roughly five percent when NR-PM1 is around ten micrograms per cubic meter, up to about twenty-five percent when NR-PM1 exceeds two hundred micrograms. Coal combustion doesn't just add mass to the pile; it increasingly dominates the pile as conditions worsen. OOA does the opposite. Its fractional contribution falls from about twenty-five percent at low NR-PM1 down to roughly ten percent at the highest loadings. Above about one hundred forty micrograms per cubic meter total NR-PM1, OOA mass stops growing and levels off. Secondary aerosol formation appears to hit a ceiling during the worst pollution episodes. The worst days are not chemistry days. They are combustion days. Meteorology decides when those worst days arrive. Sun and colleagues tracked two levers in particular. The first is wind: when ground-level wind speed exceeds about three meters per second, all aerosol species drop rapidly to below roughly two micrograms per cubic meter. Modest winds are enough to flush the city. The second lever is humidity. During high-humidity periods, especially fog events, sulfate mass jumps sharply. The team attributes this to aqueous-phase oxidation — sulfur dioxide dissolving into fog droplets and being chemically converted to sulfate there, rather than through gas-phase reactions. This is a different mechanism than what drives sulfate in summer, and it is far more efficient under the high sulfur dioxide concentrations Beijing experiences from coal burning. Humidity also shifts nitrate behavior between seasons, with summer relative humidity favoring nitrate through ammonium nitrate partitioning, while winter humidity more strongly enhances sulfate. Temperature matters too: lower winter temperatures are linked to increased primary traffic emissions, adding another layer to the compounding of pollution. Put it all together, and the picture is coherent and sobering. Beijing's worst winter pollution episodes are driven by coal combustion, amplified by calm winds and humid, foggy air. CCOA rises linearly with total particulate matter loading. Secondary aerosol, meanwhile, fails to keep pace — its formation appears suppressed when the air is already thick with primary particles. The interaction of meteorology and the mix of sources conspire: the heating season raises CCOA emissions, stagnant air lets them accumulate, fog converts sulfur dioxide to sulfate, and the result is a chemical regime fundamentally different from summer. The policy implication is direct. If you want to cut the peak wintertime particulate matter episodes, you target the heating sector. That is where the mass is coming from, and that is where the share grows fastest as conditions deteriorate. One question the paper raises but doesn't fully resolve is why secondary organic aerosol formation shuts down during the heaviest pollution. Is it competition for oxidants? Aqueous partitioning that sequesters precursors? Suppressed photochemistry under thick aerosol layers? Sun and colleagues identify the pattern — OOA plateauing above one hundred forty micrograms per cubic meter — but the mechanism is left open. That question turned out to be a productive one. The field kept exploring it. The continuous, chemically resolved measurement approach demonstrated in this study became the template for how to investigate this — watching the aerosol change in real time as the city breathes. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.