January 20, 2011

What drives the PM2.5 formation during the winter?; Moderate PM2.5 AQI Observed Throughout US

Moderate PM2.5 concentrations were experienced along the west coast, the Great Lakes, Northeast states, and Missouri. Snow covered surface and clouds are present in the MODIS Aqua, "true color/rgb image" retrieval.

Wintertime pollution episodes associated to PM2.5 tend to occur to a higher frequency of high pressure systems (stagnant air). Also, snow-covered surfaces, do not allow the incoming solar radiation to break temperature inversions. The major component of particulate matter are nitrate aerosols. Below, a simple description of the chemistry involved in the formation of PM2.5 during the winter months. First, nitrogen dioxide (NO2) is removed from the atmosphere by reaction with the hydroxyl free radical (OH·) and forming nitric acid (HNO3) according to the following reaction:

NO2 + OH· →HNO3 (Reaction 1)

Nitric acid is a temporary sink for NO2 since it can react with OH radicals to produce nitrate

OH· + HNO3 → H2O + NO3 (Reaction 2)

or photochemically to produce NO2:

HNO3 + hν → OH· + NO2 (Reaction 3)

Nitric acid is removed from the atmosphere by precipitation (acid rain), or by reaction with
ammonia to produce ammonium nitrate (NH4NO3) :

HNO3 + NH3 → NH4NO3 (Reaction 4). Cold temperatures and higher humidity strongly favors the formation of fine NH4NO3 aerosol particles in Reaction 4. For this reason during the wintertime the high NO2 tropospheric concentrations tend to correlate with the locations of higher PM2.5 concentrations, as it seems to be the case in the last two days. Below the OMI retrieval for today's tropospheric NO2 shows that the higher concentrations are in those locations where Moderate (Code Yellow) PM2.5 concentrations were experienced.

Lidar observations in Baltimore show the boundary layer extending up to 1.5 km and cloud capped in the early morning hours. Clouds, at heights between 2-6 km advected over UMBC after 12:00 UTC (7:00 AM local time).

Posted by Ruben Delgado at January 20, 2011 10:02 PM
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