Current air pollution studies largely rely upon aerosol mass spectrometers, most of which can only measure submicron aerosol (PM₁) species - particulate matter with aerodynamic diameter less than 1 μm.

In many studies, PM₁ aerosol species are therefore used to validate those of PM_2.5 (particulate matter with aerodynamic diameter less than 2.5 μm) in chemical transport models, and estimate particle acidity (pH) and aerosol water content which are key parameters in studying heterogeneous reactions.

However, are there chemical differences between PM₁ and PM_2.5? Will the differences bring uncertainties into air pollution studies, especially in highly polluted environment?

Prof. SUN Yele and his team from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences tried to answer these questions by characterizing the chemical differences between PM₁ and PM_2.5 in a highly polluted environment in north China in winter using a newly developed PM_2.5 Time-of-Flight Aerosol Chemical Speciation Monitor. Their study was published in Geophys. Res. Lett.

They found that the changes in PM₁/PM_2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary aerosol species.

"If organics is the dominant component (> 50%) of particulate matter and RH is below 80%, the chemical species in PM₁ would be highly correlated with those in PM_2.5. PM₁ can be representative of PM_2.5," said SUN, the first and corresponding author of this study.

"However, if sulfate, nitrate, and secondary organic aerosol that are formed from secondary formation are dominant components, there would be large chemical differences between PM₁ and PM_2.5 at RH > 60%. The major reason is that these secondary species have higher hygroscopicity and can uptake more water during higher RH periods," he said.

A fog event in Gucheng, Hebei province in winter 2018. (Image by ZHAO Xiaoxuan) 

SUN also evaluated the impacts of chemical differences between PM₁ and PM_2.5 on the predictions of pH and aerosol water content with thermodynamic modeling. According to the study, the chemical differences between PM₁ and PM_2.5 have negligible impacts on pH prediction, but have a large impact on prediction of aerosol water content by up to 50-70%.

"Our findings are important because current air pollution studies in highly polluted environment, particularly during severe haze events with high RH must consider the chemical differences between PM₁ and PM_2.5," said SUN, "Validation of model simulations in chemical transport models also need to consider such differences."