On the opposite seasonality of MODIS AOD and surface PM2.5 over the Northern China plain

Aerosol Vertical Structure (AVS) retrieved from CALIOP was used to resolve contradictory seasonality between MODIS aerosol optical depth (AOD) data and surface PM2.5 over the Northern China Plain (NCP). CALIOP level 2 data was used to determine seasonally averaged day and night time AVS during 2011-2015 at wavelengths of 532 and 1064 nm. Results showed that the AVS differed markedly between seasons. It was the most complex during summer days at 532 nm, when a nose like peak was apparent between 500 and 1000 m. This structure was attributed to strong aerosol hygroscopic growth, and accounted for approximately 40% of column AOD. The lowest section of AVS at 532 nm which is important for surface PM2.5 showed the highest during winter, an improved seasonality compared to column AOD; however, discrepancies were apparent during other seasons. Conversely, AVS at 1064 tun was virtually identical to observed PM2.5, implying that coarse particulates are more representative of PM2.5 in the NCP as compared to fine particulates. Thus, MODIS AOD at 550 nm may not be a good proxy for surface PM2.5 retrieval. Using CALIOP active detection, day and night time AOD were obtained, and showed that seasonality was more reasonable at night as compared to during the day. Comparisons of reconstructed daytime CALIOP AOD with MODIS AOD and AERONET AOD implied that winter AOD over the NCP was not as low as indicated by MODIS AOD.