February 26, 2014

Fires in Mississippi Valley and Florida;High NO2 over California and Northeastern US; Nitrate Particle Formation

Fire activity continues to be reported over Florida, Mississippi Valley and Plains States, as shown in the Google overlay of NOAA' HMS fire product and MODIS Aqua AOD. Smoke from these fires have a small impact on the AOD retrievals over this region, as values no greater than 0.2 were reported. The maximum PM2.5 AQI levels experienced today were Moderate (Code Yellow) along the western US and northeastern states. The next image (KNMI OMI), shows the retrieved NO2 column for the troposphere. This image shows a large amount of NO2 in California and the Northeastern states.The major component of particulate matter during winter are nitrate aerosols.

A simple description of the chemistry involved in the formation of PM2.5 during the winter months to understand the PM2.5/NO2 correlation that we often discuss inthe Smog Blog. First, nitrogen dioxide (NO2) is removed from the atmosphere by reaction with the hydroxyl free radical (OH·) forming nitric acid (HNO3):

NO2 + OH· →HNO3

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

OH· + HNO3 → H2O + NO3

or photochemically to produce NO2:

HNO3 + hν → OH· + NO2

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

HNO3 + NH3 → NH4NO3

For this reason during the wintertime the high NO2 tropospheric concentrations tend to correlate with the locations of higher PM2.5 concentrations.

Surface sulfate concentrations may have contributed to 1-2 ug/m3 to the total PM2.5 concentrations, as indicated by NRL NAAPS forecast. In the Southwestern US, AOD readings as high as 0.3 were observed. These AOD levels might be associated with light blowing dust light observed south of Colorado.

Posted by Ruben Delgado at February 26, 2014 11:58 PM
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