Reaction Rate Calculation

Hi all,

I need the reaction rates of the below reactions at different temperatures:

HO2 + HO2 → H2O2 + O2

RO2 + HO2 → ROOH + O2

OH + NO2 → HNO3

RO2 + NO2 → PANs

I searched the rates in the mcm_3-2.tar.gz file of the Master Chemical Mechanism (MCM Website), but it was so confusing for me. Also, there is a constant named M that I don’t know its value. As far as I understand, I should calculate the reaction rates using the formulas in the mcm_3-2.tar.gz file.

Could anyone help me to understand how to work with the Master Chemical Mechanism? And, is there an easier way to calculate the reaction rates?

Thank you for any help in advance.

Reza

Dear Reza,

These reactions are also included in CMAQ and are temperature dependent. You can calculate rate constants at different temperatures. Rate constants for most of these reactions use fall-off type expression.

For example, you can look at mech_cb6r5_ae7_aq and find the following reaction:
HO2 + HO2 = H2O2 %3 # 2.20E-13@-600 & 1.90E-33@-980;

In RXNS_FUNC_MODULE.F90 file, you will find:
! Reaction Label R19
RKI( NCELL, 19) = CFACT * FALLOFF_T09( INV_TEMP, CAIR, &
& 2.2000D-13, 6.0000D+02, 1.9000D-33, &
& 9.8000D+02 )

ARRHENUIS_T09 is defined as follows:
REAL( 8 ) FUNCTION FALLOFF_T09(INV_TEMP,CAIR,A1,C1,A2,C2)
! rate constant for CMAQ fall off reaction type 9
IMPLICIT NONE
REAL( 8 ), INTENT( IN ) :: INV_TEMP
REAL( 8 ), INTENT( IN ) :: CAIR
REAL( 8 ), INTENT( IN ) :: A1
REAL( 8 ), INTENT( IN ) :: C1
REAL( 8 ), INTENT( IN ) :: A2
REAL( 8 ), INTENT( IN ) :: C2
! Local:
REAL( 8 ) K1
REAL( 8 ) K2
INTRINSIC DEXP
K1 = A1 * DEXP( C1 * INV_TEMP )
K2 = A2 * DEXP( C2 * INV_TEMP )
FALLOFF_T09 = K1 + K2 * CAIR
RETURN
END FUNCTION FALLOFF_T09

INV_TEMP is the reciprocal of air temperature. CAIR (M) is the air number density [molecule/cm^3] and can be calculated using pressure and temperature. Similarly, you can find other reactions in mech_cb6r5_ae7_aq and corresponding rate constants in RXNS_FUNC_MODULE.F90. Please let me know if you need any other help.

Thanks,
Sarwar

1 Like