Sulfate and Ammoniu Nitrate Replacement Protocols in the IMPROVE Database  

 

Sulfate Replacement Protocol:

Particle sulfur measurements from the Module A Teflon were underestimated at some Eastern US sites, primarily during the summers of 1992-95 on days with the highest sulfur loadings coinciding with high humidity. This problem was rectified by increasing the size of the Teflon filters at affected sites (Table 1). For derived values in the IMPROVE database the sulfate ion measurement from the Module B filter is used instead of the Sulfur*3 estimate of sulfate ion mass from module A for time periods before the Teflon filter size was increased at the sites listed below (Table 1).

Table 1. Monitoring sites and dates when the Teflon filter was increased from 2.2 sq. cm to 3.5 sq. cm :

19-Apr-95 - DOSO1,GRSM1,SHEN1,JEFF1
22-Apr-95 - SAGO1
26-Apr-95 - ACAD1,LYBR1,SHRO1,MACA1
03-May-95 - SIPS1,UPBU1
10-May-95 - BRIG1
27-May-98 - SEQU1
03-Jun-98 - PUSO1
29-Jul-98 - MOOS1
04-Aug-98 - CHAS1,OKEF1,ROMA1,EVER1
17-Apr-01 - CACR1
25-Apr-01 - HEGL1,GUMO1,BIBE1
16-May-01 - WIMO1

Note, there is no evidence of S underestimation at BIBE1, GUMO1, MOOS1, SAGO1 and SEQU1.

 

 

Ammonium Nitrate Replacement Protocol:

Due to a particle nitrate sampling protocol change in 1996 across the IMPROVE network, ammonium nitrate mass concentrations in all derived quantities which include NO3-, such as ammonium nitrate, RCFM, and optical parameters reconstructed from the aerosol data, have been replaced with a constant ammonium nitrate value prior to June of 1996. In general, this constant value is the annual average ammonium nitrate mass concentration obtained over the sampling period subsequent to the protocol change, or 1997 – 1999.  In the event a monitoring location did not have sufficient data during this period to form an annual average, values prior to the protocol change were used. In most cases the annual mean is derived from the mean of four valid quarterly means, however, if data are insufficient to form a seasonal weighted mean the annual mean is derived from available daily observations.

 

The intent of replacing ammonium nitrate mass concentrations with a constant value is to make trend analyses insensitive to the protocol change.  In our opinion it is preferable to keep an estimate of ammonium nitrate mass in derived quantities because in some cases particle nitrate constitutes a fairly large portion of the particle mass, and hence light extinction budget, at many monitoring sites.  Data users wishing to incorporate daily measured NO3- values in their analyses can make use of these data by downloading daily particle NO3- mass concentratinos from the database and converting these to ammonium nitrate and other derived variables.

 

A listing of the ammonium nitrate replacement values at 75 IMPROVE sites reporting particle NO3- data prior to Jun 1996 can be found in either NH4NO3_replacement.xls or NH4NO3_replacement.txt.  In these files the field NH4NO3_Replacement_Val is the annual mean ammonium nitrate concentration used as a constant replacement value for daily ammonium nitrate readings proir to June 1996.  N is the number of years in the seasonal weighted means or the number of daily observations used to form un-weighted means.  Start Yr and End Yr fields refer to the beginning and end years of input data used to form the annual mean values.

 

More information about IMPROVE nitrate issues can be found in the QA/QC section of the IMPROVE website: http://vista.cira.colostate.edu/improve/Data/QA_QC/issues.htm