There are chemical differences between PM1 and PM2.5 which affect air pollution measurements and aerosol water content prediction.
Current air pollution studies largely rely upon aerosol mass spectrometers, most of which can only measure PM1 species – particulate matter with aerodynamic diameter less than 1 μm. In many studies, PM1 values are therefore assumed as PM2.5 values.
While a PM1 PM2.5 ratio of 1 indicates the assumption is correct, reported ratios actually vary largely from 0.7 to 1.2 depending on chemical environments. Ratios as low as 0.3 have even been registered during haze events, because a significant portion of the aerosol species exist in the 1 – 2.5 µm size range in severely polluted conditions. This means a large fraction of PM2.5 can be missed from measurements in air pollution studies.
To better understand when PM1 can be treated as PM2.5, a team from the Institute of Atmospheric Physics, Chinese Academy of Sciences, characterized the chemical differences of the two species in a highly polluted environment in north China in winter using a newly developed PM2.5 Time-of-Flight Aerosol Chemical Speciation Monitor. They found that the changes in PM1 PM2.5 ratios as a function of relative humidity (RH) were largely different for primary and secondary aerosol species.
“If organics is the dominant component – more than 50 percent – of particulate matter and RH is below 80 percent, the chemical species in PM1 would be highly correlated with those in PM2.5. PM1 can be representative of PM2.5,” said Professor Sun Yele, first and corresponding author of this study. “However, if sulfate, nitrate, and secondary organic aerosol are dominant components, there would be large chemical differences between PM1 and PM2.5 at RH greater than 60 percent. The major reason is that these secondary species have higher hygroscopicity and can uptake more water during higher RH periods.”
In the paper published in Geophysical Research Letters, the team also detailed the impacts of chemical differences between PM1 and PM2.5 on the predictions of pH and aerosol water content with thermodynamic modelling. “The chemical differences between PM1 and PM2.5 have negligible impacts on pH prediction, but a large impact on prediction of aerosol water content by up to 50-70 percent,” said Sun.
“Our findings are important because current air pollution studies in highly polluted environments, particularly during severe haze events with high RH, must consider the chemical differences between PM1 and PM2.5,” cautioned Sun. “Validation of model simulations in chemical transport models also need to consider such differences.” [APBN]