Observations of rapid aerosol optical depth enhancements in the vicinity of polluted cumulus clouds

Abstract

Abstract. During the July 2011 Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field experiment in Maryland, significant enhancements in Aerosol Robotic Network (AERONET) sun-sky radiometer measured aerosol optical depth (AOD) were observed in the immediate vicinity of non-precipitating cumulus clouds on some days. Both measured Ångström exponents and aerosol size distribution retrievals made before, during and after cumulus development often suggest little change in fine mode particle size; therefore, implying possible new particle formation in addition to cloud processing and humidification of existing particles. In addition to sun-sky radiometer measurements of large enhancements of fine mode AOD, lidar measurements made from both ground-based and aircraftbased instruments during the experiment also measured large increases in aerosol signal at altitudes associated with the presence of fair weather cumulus clouds. These data show modifications of the aerosol vertical profile as a result of the aerosol enhancements at and below cloud altitudes. The airborne lidar data were utilized to estimate the spatial extent of these aerosol enhancements, finding increased AOD, backscatter and extinction out to 2.5 km distance from the cloud edge. Furthermore, in situ measurements made from aircraft vertical profiles over an AERONET site during the experiment also showed large increases in aerosol scattering and aerosol volume after cloud formation as compared to before. The 15-year AERONET database of AOD measurements at the Goddard Space Flight Center (GSFC), Maryland site, was investigated in order to obtain a climatological perspective of this phenomenon of AOD enhancement. Analysis of the diurnal cycle of AOD in summer showed significant increases in AOD from morning to late afternoon, corresponding to the diurnal cycle of cumulus development.

Long-term variability of aerosol optical thickness in Eastern Europe over 2001–2014 according to the measurements at the Moscow MSU MO AERONET site with additional cloud and NO<sub>2</sub> correction

“…In addition to the well-known process of hygroscopic particle growth, there is a mechanism of the gas-to-particle conversion that occurs more intensively in the aqueous phase in cloud droplets due to the oxidation of gases (SO 2 , NO x , SOA) (Eck et al, 2014). Due to this mechanism in the presence of convective cloudiness the formation of new aerosol particles may observe providing higher aerosol loading during the periods with higher cloud amount in the vicinity of clouds (Eck et al, 2014).…”

“…However, several recent studies indicated that clouds can have a real impact on AOT. These mechanisms of aerosolcloud interaction include aerosol hygroscopic growth, increasing aerosol concentration due to air convergence, and new particles formation in the presence of clouds (Su et al, 2008;Jeong and Li, 2010;Eck et al, 2012Eck et al, , 2014. In addition to the well-known process of hygroscopic particle growth, there is a mechanism of the gas-to-particle conversion that occurs more intensively in the aqueous phase in cloud droplets due to the oxidation of gases (SO 2 , NO x , SOA) (Eck et al, 2014).…”

“…The application of the 24 h Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) NOAA model backward trajectory analysis (Draxler and Hess, 1998) for all the cases in January has revealed for the NA < 3 sample the prevalence of the northern (north-west, north, north-east) advection (80 % of cases) characterized by low AOT (Chubarova, 2009), compared with 40 % of cases for partially cloudy conditions (3 < NA < 9). Hence, we should emphasize that quasiclear sky conditions NA < 3 data set may not reflect the typical aerosol features due to large-scale meteorological effects and possible changes in aerosol properties in the vicinity of clouds (Eck et al, 2014).…”

“…We should also mention that after applying this filter, which removes the data when solar disk was blocked by optically thin, homogeneous clouds, we do not remove any other cases with different cloud types, except those, which have been removed by the standard AERONET cloud-screening algorithm. However, it will be interesting to compare the results with the soon-to-be-released AERONET version 3 data set, where a modification of standard cloud-screening algo-320 N. Y. Chubarova et al: Long-term AERONET measurements with additional cloud and NO 2 correction rithm will be applied to the data according to the method mentioned in Eck et al, 2014. …”

“…These mechanisms of aerosolcloud interaction include aerosol hygroscopic growth, increasing aerosol concentration due to air convergence, and new particles formation in the presence of clouds (Su et al, 2008;Jeong and Li, 2010;Eck et al, 2012Eck et al, , 2014. In addition to the well-known process of hygroscopic particle growth, there is a mechanism of the gas-to-particle conversion that occurs more intensively in the aqueous phase in cloud droplets due to the oxidation of gases (SO 2 , NO x , SOA) (Eck et al, 2014). Due to this mechanism in the presence of convective cloudiness the formation of new aerosol particles may observe providing higher aerosol loading during the periods with higher cloud amount in the vicinity of clouds (Eck et al, 2014).…”