Multi-method determination of the below-cloud wet scavenging coefficients of aerosols in Beijing, China

Abstract

Abstract. Wet scavenging is one of the most efficient processes for removing aerosols from the atmosphere. This process is not well constrained in chemical transport models (CTMs) due to a paucity of localized parameterization regarding the below-cloud wet scavenging coefficient (BWSC). Here we conducted field measurements of the BWSC during the Atmospheric Pollution and Human Health Beijing (APHH-Beijing) campaign of 2016. Notably, the observed BWSC values based on the updated aerosol mass balance agree well with another estimation technique, and they fall in a range of 10−5 s−1. The measurement in this winter campaign, combined with that in summer of 2014, supported an exponential power distribution of BWSCs with rainfall intensity. The observed parameters were also compared with both the theoretical calculations and modeling results. We found that the theoretical estimations can effectively characterize the observed BWSCs of aerosols with sizes smaller than 0.2 µm and larger than 2.5 µm. However, the theoretical estimations were an order of magnitude lower than observed BWSCs within 0.2–2.5 µm, a domain size range of urban aerosols. Such an underestimation of BWSC through a theoretical method has been confirmed not only in APHH-Beijing campaign but also in all the rainfall events in summer of 2014. Since the model calculations usually originated from the theoretical estimations with simplified scheme, the significantly lower BWSC could well explain the underprediction of wet depositions in polluted regions as reported by the Model Inter-Comparison Study for Asia (MICS-Asia) and the global assessment of the Task Force on Hemispheric Transport of Atmospheric Pollutants (TF-HTAP). The findings highlighted that the wet deposition module in the CTMs requires improvement based on field measurement estimation to construct a more reasonable simulation scheme for BWSC, especially in polluted regions.

Inter-annual variations of wet deposition in Beijing from 2014–2017: implications of below-cloud scavenging of inorganic aerosols

“…The measurement site is located on the roof of a two-floor building at the Institute of Atmospheric Physics tower site (IAP-tower; 39 • 58 28 N, 116 • 22 1 E) in northern Beijing. It is a typical urban site between the 3rd and 4th ring roads and lying close to the Badaling expressway (Xu et al, 2017(Xu et al, , 2019Sun et al, 2015). Four years of inter-annual observations of each rainfall event were conducted at this site.…”

“…However, more recent extensive research on wet scavenging has found that precipitation, even light rain, can remove 50 %-80 % of the number or mass concentration of belowcloud aerosols, and this is supported by both field measurements and semi-empirical parameterizations of below-cloud scavenging in models (Andronache, 2004b;Zhang et al, 2004;Wang et al, 2014). Xu et al (2017Xu et al ( , 2019 studied the below-cloud scavenging mechanism based on the simultaneous measurement of aerosol components in rainfall and in the air in Beijing. They found that below-cloud scavenging coefficients for PM 2.5 widely used in CTMs (∼ 10 −5 -10 −6 ) were 1-2 orders of magnitude lower than estimates from observations (at the range of 10 −4 -10 −5 for SO 2− 4 , NO − 3 , and NH + 4 .).…”

“…Previously, below-cloud scavenging was thought to be less important than in-cloud processes and was simplified or even ignored in many global and regional chemical transport models (CTMs) (Barth et al, 2000;Tang et al, 2005;ENVIRON.Inc, 2005;Textor et al, 2006;Bae et al, 2010). However, more recent extensive research on wet scavenging has found that precipitation, even light rain, can remove 50 %-80 % of the number or mass concentration of belowcloud aerosols, and this is supported by both field measurements and semi-empirical parameterizations of below-cloud scavenging in models (Andronache, 2004b;Zhang et al, 2004;Wang et al, 2014). Xu et al (2017Xu et al ( , 2019 studied the below-cloud scavenging mechanism based on the simultaneous measurement of aerosol components in rainfall and in the air in Beijing.…”

“…However, many studies have found that it is difficult to separate the two wet scavenging processes based on measurement methods alone (Huang et al, 1995;Wang and Wang, 1996;Goncalves et al, 2002;Bertrand et al, 2008;Xu et al, 2017). A commonly used approach to separating below-cloud scavenging from total wet deposition is through sequential sampling (Aikawa et al, 2014;Ge et al, 2016;Aikawa and Hiraki, 2009;Wang et al, 2009;Quyang et al, 2015;Xu et al, 2017). In this way, precipitation composition during different stages of a rainfall event can be investigated separately in the lab after sampling.…”

“…The chemical components in later increments of rainfall are thought to be less influenced by the below-cloud scavenging process than by the in-cloud scavenging process (Aikawa et al, 2014;Aikawa and Hiraki, 2009). Xu et al (2017) applied this approach to summer rainfall in Beijing in 2014 and found that more than 50 % of deposited sulfate, nitrate, and ammonium ions were from below-cloud scavenging. In this study, an innovated method based on exponential curve to chemical ions in rainfall by sequential sampling is developed and implemented to estimate the ratio of below-cloud to in-cloud wet deposition in Beijing over the four-year period between 2014 and 2017.…”