Wet deposition of sub-micron aerosol particles in an urban area of the Amazon central

Abstract

Aerosol particles impact health, ecosystems, and climate, especially when in high concentrations in urban environments. Wet deposition is one of the most critical limiting mechanisms of particulate matter in the atmosphere. This mechanism captures particles inside clouds (rain-out) or below the cloud due to precipitation (washout). In the Amazon basin, the physical mechanisms and scavenging rates remain unknown in many regions. Several studies over the last decades have empirically ascertained the impact of wet deposition to develop local atmospheric models or to estimate the contribution of its effects. Here, we analyzed some of the physical and chemical properties of aerosols: nucleation mode (NU, 10-30 nm), Aitken (30-100 nm), accumulation mode (AC, 100-430 nm), total particle number (10-430 nm), Black Carbon equivalent (BCe), and chemical properties, such as organic aerosols (OA), sulfate (SO₄ ²⁻) and nitrate (NO₃ ⁻ ). We obtained the data set from the Intensive Observation Periods (IOPs) of the GoAmazon2014 experiment, Iranduba-AM (T2 sampling site), ~ 9 km NE of Manaus city. We conducted three analyzes from these data: (I) daily cycles on dry and rainy days; (II) scavenging rates (TS), i.e., the difference in the concentration of aerosol one hour before and the first hour of rainfall events, generally, during local thunderstorms or Mesoscale Convective Systems - MCS; and (III) scavenging coefficients (λ). We verified significant statistics decreases at both NU and AIT modes, as well SO₄ ²⁻ and NO₃ ⁻ . In TS analysis, we observed a similar decline (NU: -21%, AIT: -17%, AC: -22%), contributing to an overall removal of up to -13% (on average). The soluble fractions were removed easily (OA: -21%, SO₄ ²⁻: -16%, and NO₃ ⁻ : -16%) compared to insoluble fractions (BCe: -11%). In λ analysis, a substantial decrease in all size classes (NU: 2.0 × 10 ⁻ ⁴ s ⁻ ¹, AIT: 7.8 × 10⁻ ⁴ s ⁻ ¹, AC: 1.3 × 10⁻ ⁴ s ⁻ ¹, CN: 5.4 × 10⁻ ⁴ s ⁻ ¹) was also observed, with unexpected prominence to the AIT. We've attributed this result to introducing aerosol particles of 10-50 nm by deep convection, which may counteract the washout. Measurements of cloud properties might help confirm this hypothesis. Our preliminary findings are helpful for the local modeling of pollutant dynamics and provide evidence that wet deposition substantially removes sub-micron particles in the Amazon region. The wet deposition rates for other storms and clouds in the atmosphere, however, remain unknown.