Total aerosol mass deposition flux
Particulate mass wet deposition flux – Wet deposition samples are collected using a passive MTX ARS 1010 automatic deposition sampler (MTX Italia SPA, Modane, Italy). The collector is made of two buckets (30 cm diameter) and a humidity sensor activated an aluminum lid to cover or uncover the buckets. The samples are weekly or after each precipitation event collected.
MTX ARS 1010
Particulate mass total deposition flux – Total aerosol deposition is collected with a passive collector of an inverted « frisbee » shape (Hall and Waters, 1986; Hall and Upton, 1988), equipped with a flow deflector ring, as tested by Wiggs et al. (2002) and full with marbles to limit dust resuspension and rain splashing.The samples are weekly collected, except in the wet season where they are also collected when the wet deposition samples are collected.
FRISBEE passive collector
For both total and wet deposition samples, the collection procedure is decomposed into three main phases ((1) rinsing with a large volume of water, (2) decantation during more than 6 hours and siphoning of excess water; (3) drying in an oven) . The collected dust is finally weighted with a precision balance.
Wet deposition – Precipitation chemical composition
Event sampling is carried out at each station using an automatic precipitation collector specifically designed for the INDAAF network.
The automatic precipitation collectors function with a high degree of cleanliness, with a separate polyethylene bag being used for each precipitation event. A detector automatically opens the aperture, whose area of collection is 225 cm2 when rainfall occurs and hermetically seals the collector when precipitation stops. This prevents aerosols from being deposited in the collector outside of precipitation events. After each precipitation event, 50 cm3 of the collected precipitation is sampled in 50ml Greiner tubes. Then, samples are stored in a deep freeze environment from collection to analysis.
INDAAF monitoring rain network follows the GAW WMO recommendations guidelines n°160.
Samples are analyzed for pH, Conductivity, major Inorganic and Organic ions (Cl– , NO3– , SO42-, HCO3– , CO32-, Na+, NH4+, K+, Mg2+, Ca2+, HCOO–, CH3COO–, C2H5COO– ).
The annual volume weighted mean concentration (VWM) of each ion was calculated using the following relationship:
where Ci is the ion concentration for each element in µeq.l-1, Pi the precipitation amount for each rainfall event in mm, and N the total number of samples. The annual wet deposition (WD) expressed in mmol.m-2.yr-1 is calculated by multiplying the VWM concentrations by the annual average rainfall amount.